Variety in chemical zonation of garnet in eclogite from Nové Dvory, Czech Republic

Czech Academy of Sciences Publication Activity Database

Nakamura, D.; Hirajima, T.; Svojtka, Martin

2005-01-01

Roč. 150, - (2005) ISSN 1609-0144. [International eclogite conference /7./. 03.07.2005-09.07.2005, Graz] Institutional research plan: CEZ:AV0Z30130516 Keywords : garnet * zonation * eclogite Subject RIV: DB - Geology ; Mineralogy

First report of garnet corundum rocks from southern India: Implications for prograde high-pressure (eclogite-facies?) metamorphism

Science.gov (United States)

Shimpo, Makoto; Tsunogae, Toshiaki; Santosh, M.

2006-02-01

We report here for the first time the occurrence of garnet and corundum in Mg-Al-rich rocks at Sevitturangampatti (Namakkal district) in the Palghat-Cauvery Shear Zone System (PCSS), southern India. The rocks contain several rare mineral assemblages such as garnet-corundum-sillimanite-cordierite-sapphirine-spinel-Mg-rich staurolite, garnet-corundum-sodic gedrite-cordierite-sillimanite/kyanite, garnet-Mg-rich staurolite-sillimanite/kyanite, sodic gedrite-Mg-rich staurolite-corundum-sapphirine, biotite-corundum-sapphirine and sodic gedrite-sapphirine-spinel-cordierite. Both garnet and corundum in these rocks occur as coarse-grained (1 mm to 10 cm) porphyroblasts in the matrix of sillimanite, cordierite and gedrite. Kyanite is common as inclusions in garnet, but matrix aluminosilicates are mainly sillimanite. The presence of rare garnet + corundum, which has so far been reported from kimberlite xenoliths, aluminous eclogites and ultrahigh-pressure metamorphic rocks as well as in high-pressure experiments, suggests that the assemblage is an indicator of an unusually high-pressure event, which has not been recorded in previous studies from southern India. Phase analysis of quartz-absent MAS system also suggests high-pressure stability of the assemblage. The inference of high pressure metamorphism is also supported by the presence of Mg-rich [Mg/(Fe + Mg) = 0.51] staurolite, which has been reported from high-pressure rocks, included from cores of coarse-grained garnet and gedrite. Porphyroblastic occurrence of garnet + corundum as well as staurolite and kyanite inclusions suggests that the area underwent prograde high-pressure metamorphism, probably in the eclogite field. The rocks subsequently underwent continuous heating at 940 to 990 °C, suggesting ultrahigh-temperature (UHT) metamorphism along a clockwise trajectory. Sapphirine + cordierite and spinel + cordierite symplectites between garnet and sillimanite suggest near isothermal decompression after the peak event

Petrology, isotopic and fluid inclusion studies of eclogites from Sujiahe, NW Dabie Shan (China), July 1 2002

NARCIS (Netherlands)

Fu, B.; Zheng, Y.-F.; Touret, J.L.R.

2002-01-01

In addition to the Triassic Hong'an low-T-high-P eclogite and the Xinxian coesite-bearing kyanite-glaucophane eclogite, Silurian coesite-free amphibole eclogites occur in the Sujiahe region, NW Dabie Shan of central China. A comprehensive study of petrology, Nd-Sr, O-H isotopes and fluid inclusions

Preliminary data on the Bramberg (Hassberge, Bavaria, Germany) mantle xenoliths

Science.gov (United States)

Kukuła, Anna; Puziewicz, Jacek; Ntaflos, Theodoros; Matusiak-Małek, Magdalena

2015-04-01

Bramberg hill is a well preserved basaltic volcanic cone (494 m asl), situated 2 km north-west of Bramberg village (Hassberge, Bavaria, Germany). Bramberg basalt belongs to Heldburger Ganschar subset of the Central European Volcanic Province and contains 5 - 8 cm peridotitic xenoliths, which are the aim of our study, based on characterization of 7 xenoliths. Two groups of spinel peridotite xenoliths occur in the Bramberg basalt. Group A spinel peridotite (6 xenoliths) is characterized by protogranular texture with typical grain size of 2 - 3 mm (max 8 mm). It consists of olivine (89.9 - 91.1 % Fo, 0.32 - 0.44 wt. % NiO), orthopyroxene (mg# 0.90 - 0.92, Al 0.10 - 0.18 a. pfu), clinopyroxene (mg# 0.90 - 0.92, Al 0.13 - 0.26 a pfu) and spinel (cr# 0.13 - 0.39, mg# 0.58 - 0.75). LA-ICP-MS trace element analyses (xenolith 3150) show that orthopyroxene is depleted relative to primitive mantle. Rare earth element patterns exhibit two trends. The first (U-shaped, LaN/YbN = 0.1) probably reflects orthopyroxene with thin clinopyroxene lamellae, while pure orthopyroxene is characterized by constant depletion from Lu to La (LaN/YbN = 1.1). In the primitive mantle normalized trace element diagram negative Ti, Hf, Sr anomalies occur. Clinopyroxene is enriched in trace elements relative primitive mantle. REE normalized diagram is characterized by constant enrichment from Lu to La (LaN/YbN = 10.7). Strong negative Ti, Hf, Nd and Pb anomalies occur in the trace element pattern. The group B (1 xenolith) contains olivine of forsterite content 87.3 - 88.2 mole % and containing 0.35 - 0.39 wt. % NiO. Orthopyroxene is characterized by mg# 0.88 - 0.89 and variable content of Al 0.07 - 0.14 atoms pfu. Clinopyroxene (mg# 0.88 - 0.90) contains 0.10 - 0.26 atoms Al pfu. Spinel is characterized by low mg# (0.50 - 0.52) and high cr# (0.49) relative to group A xenoliths. The group B xenolith is characterized by presence of carbonates. Carbonates (up to 0.5 mm grains) are located in interstices

Xenoliths from Bunyaruguru volcanic field: Some insights into lithology of East African Rift upper mantle

Science.gov (United States)

Muravyeva, N. S.; Senin, V. G.

2018-01-01

The mineral composition of mantle xenoliths from kamafugites of the Bunyaruguru volcanic field has been determined. The major and some trace elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, Cr, Ni, Ba, Sr, La, Ce, Nd, Nb) has been analyzed in olivine, clinopyroxene, phlogopite, Cr-spinel, titanomagnetite, perovskite and carbonates of xenoliths and their host lavas. Bunyaruguru is one of three (Katwe-Kikorongo, Fort Portal and Bunyaruguru) volcanic fields included in the Toro-Ankole province located on the North end of the West Branch of the East African Rift. The xenoliths from three craters within the Bunyaruguru volcanic field revealed the different character of metasomatic alteration, reflecting the heterogeneity of the mantle on the kilometer scale. The most unusual finding was composite glimmerite-wehrlite xenolith from the crater Kazimiro, which contains the fresh primary high-Mg olivine with inclusions of Cr-spinel that had not been previously identified in this area. The different composition of phenocryst and xenolith minerals indicates that the studied xenoliths are not cumulus of enclosing magma, but the composition of xenoliths characterizes the lithology of the upper mantle of the area. The carbonate melt inclusions in olivine Fo90 demonstrate the existence of primary carbonatitic magmas in Bunyaruguru upper mantle. The results of texture and chemical investigation of the xenolith minerals indicate the time sequence of metasomatic alteration of Bunyaruguru upper mantle: MARID metasomatism at the first stage followed by carbonate metasomatism. The abundances of REE in perovskites from kamafugite are 2-4 times higher than similar values for xenolith. Therefore the kamafugite magma was been generated from a more enriched mantle source than the source of the xenoliths. The evaluation of P-T conditions formation of clinopyroxene xenolith revealed the range of pressure 20-65 kbar and the temperatures range 830-1040 °C. The pressure of clinopyroxene phenocryst

Petrographical, geochemical and petrological study of the xenoliths associating the basalt of (Southwest, Syria)

International Nuclear Information System (INIS)

Safarjalani, A.; Nasir, S.

1998-01-01

Alkali basalt spread northeast part of Shamah volcanic field (Southwest of Syria) belonging to the Neogene and Quaternary ages, which are coexisted with a great quantity of mafic and ultramafic xenoliths and megacrysts. Field observations and data of geochemical and petrographical studies results, for xenoliths coexisted with alkali basalt speared over the northwest part of the arabian plate (Syria) indicate availability of a proper environment where various kinds of xenoliths of lower crustal and upper mantle were formed, this indicates that these xenoliths have been formed under different thermo barometric conditions. The study of available mineral para genesis and geothermobarometrics on coexisting minerals suggests equilibration conditions, ranging between 6-8 kba for pressure and 850-920 Centigrade for temperature, and that is for xenoliths of gabbroic nature formed in the lower crustal between 20-27 km depth. With regard to the formation conditions of the xenoliths formed in the upper mantle (Pyroxenite and Lherzolite); they rang between 13.5 - 14.5 kba for pressure and 950-1060 Centigrade for temperature. (Author)

Petrogenesis of ultramafic xenoliths from Hawaii inferred from Sr, Nd, and Pb isotopes

Science.gov (United States)

Okano, Osamu; Tatsumoto, Mitsunobu

Isotopic compositions of Nd, Sr, and Pb in xenoliths in the Honolulu volcanic series from the Salt Lake Crater (H-type) are similar to those of the host post-erosional basalts, but are distinct from the magma sources of Koolau shield tholeiites and MORB. In contrast, one spinel Iherzolite (K-type) has isotopic compositions of Nd and Sr that are close to those of Koolau tholeiite rather than to the other Hawaiian basalts. Previous studies have shown that Sr isotopic composition of the xenoliths and the host basalt and that trace element concentrations in minerals of garnet Iherzolites from Honolulu basalt were nearly in equilibrium with the host magma, indicating that Honolulu volcanics were derived from garnet Iherzolite or similar material. However, differences exist among the isotopic compositions (especially Nd) of the xenoliths indicating that they are accidental inclusions from upper layers. The similarity in isotopic compositions between xenoliths and Honolulu basalt suggests that the source areas in the mantle are chemically similar. Correlation of 238U/204Pb vs. 206Pb/204Pb of chrome diopside separated from the H-type spinel Iherzolites indicates that the xenoliths are 80±36 Ma, which corresponds to the lithosphere age of the Hawaiian site. This age is consistent with petrological studies [e.g., Sen and Leeman, 1991] which have found that the spinel Iherzolite inclusions are derived from the lithosphere wall rocks. The ɛNd = ˜+8 of the H-xenoliths is slightly lower than that for the East Pacific Rise MORB indicating that the xenoliths are derived from a trace element depleted source similar to the MORB residue. If the garnet Iherzolite xenoliths are derived from mixture of spinel Iherzolite with intrusive pyroxenite, then the source of the pyroxenite contained little plume component. The one exceptional spinel Iherzolite xenolith may be a residue of Koolau-like tholeiitic magma or may have been metasomatized by Koolau volcanism in the deep lithosphere

A novel chemical scheme for flotation of rutile from eclogite tailing

Directory of Open Access Journals (Sweden)

Bo Xu

Full Text Available A novel chemical scheme for the flotation of rutile from eclogite tailings has been developed in this work. It consists of lead ion as the activator, sodium fluorosilicate (SF as the depressant, and styryl phosphonic acid (SPA and n-octyl alcohol (OCT as the collector. By using the proposed scheme to treat a feed ore of 4.5% TiO2, a rougher concentrate of grade 84.47% TiO2 was achieved with the recovery of 61.5%. Also, the scheme made a high flotation rate for rutile. The scheme was applied to closed-circuit flotation (one-stage rougher flotation, two-stage scavenger flotation and two-stage cleaner flotation, produced a concentrate of 92% TiO2 with the recovery of 70%. It is shown that the new chemical scheme would be a potential one for the effective separation of rutile from eclogite ores. Keywords: Rutile, Flotation, Reagents, Eclogite tailings

The diamondiferous roots of our wandering continent

International Nuclear Information System (INIS)

Gurney, J.J.

1990-01-01

Mantle xenoliths and minerals, including diamonds, found in the mainly Cretaceous kimberlites on the Kalahari craton provide evidence for the stabilization of a thick pool predominantly peridotitic continental nucleus in the Archaean. This was followed by widespread formation of lesser amounts of eclogite added to the lithosphere throughout the Proterozoic. Most diamonds were derived from the Archaean peridotite and Proterozoic eclogite and were liberated into kimberlite by xenolith disaggregation. The mantle lithosphere is heterogeneous and has undergone considerable modification by such processes as intrusive igneous activity, metasomatism of different styles, subsolidus metamorphism, deformation and probably the recycling of oceanic crust throughout much of its history. Diamond formation and preservation is similarly complex. The kimberlite-borne mantle sample provides a unique window of information about continental evolution in which the diamonds and their occasional encapsulated inclusions play a key role. 88 refs., 16 figs., 1 tab

Pressure-temperature evolution of eclogites from the Kechros complex in the Eastern Rhodope (NE Greece)

Science.gov (United States)

Mposkos, E.; Baziotis, I.; Proyer, A.

2012-06-01

The Rhodope Domain in NE Greece consists of different tectonometamorphic complexes involved in the Alpine collisional history between the Eurasian and African plates. In the Kechros Complex, which is the lowermost tectonic unit in the East Rhodope, a lense of kyanite eclogite occurs within orthogneiss and common eclogites are found between serpentinized peridotite and underlying pelitic gneisses. In kyanite eclogite, the high-pressure (HP) mineral assemblage is Grt + Omp (Jd35-55) + Ky + Ph + Qz + Rt + (indirectly inferred Tlc + Law); a Na-rich tremolite and zoisite formed at or near peak metamorphic conditions. In common eclogites, the HP mineral assemblage is Grt + Omp (Jd29-41) + Rt and, with less certainty, Amp (Gln-rich + Brs + Wnc + Hbl) ± Czo. The inclusions in garnet are glaucophane, actinolite, barroisite, hornblende, omphacite, clinozoisite, titanite, rutile and rarely paragonite and albite. In kyanite eclogite, peak P- T conditions are constrained at 2.2 GPa and 615°C using garnet-omphacite-phengite geothermobarometry and very similar values of 585 ± 32°C and 2.17 ± 0.11 GPa with the average P- T method, by which conditions of formation could also be narrowed down for the common eclogite (619 ± 53°C and 1.69 ± 0.17 GPa) and for a retrogressed eclogite (534 ± 36°C and 0.77 ± 0.11 GPa). Ages for the HP metamorphism in the Kechros Complex are not yet available. A Rb-Sr white mica age of 37 Ma from orthogneiss records a stage of the exhumation. The HP event may be coeval with the Eocene HP metamorphism (49-55 Ma) recorded in the Nestos Shear Zone in Central Rhodope and in the Attic-Cycladic crystalline belt, where it is interpreted as the result of subduction and final closure of the Axios/Vardar ocean and subsequent subduction of the Apulian continental crust (a promontory of the Africa continent) under the southern margin of the European continent in the late Cretaceous and early Tertiary.

Millennia of magmatism recorded in crustal xenoliths from alkaline provinces in Southwest Greenland

DEFF Research Database (Denmark)

Smit, Matthijs; Waight, Tod Earle; Nielsen, Troels

2016-01-01

Neoproterozoic alkaline provinces in West Greenland: 1)Sarfartôq, which overlies Archean ultra-depleted SCLM and yielded ultra-deep mineral indicators, and 2)Sisimiut, where the SCLM is refertilized and deep xenoliths (>120km) are lacking. We focused on the rare and understudied crustal xenoliths, which preserve...

Intermediate-depth earthquakes facilitated by eclogitization-related stresses

Science.gov (United States)

Nakajima, Junichi; Uchida, Naoki; Shiina, Takahiro; Hasegawa, Akira; Hacker, Bradley R.; Kirby, Stephen H.

2013-01-01

Eclogitization of the basaltic and gabbroic layer in the oceanic crust involves a volume reduction of 10%–15%. One consequence of the negative volume change is the formation of a paired stress field as a result of strain compatibility across the reaction front. Here we use waveform analysis of a tiny seismic cluster in the lower crust of the downgoing Pacific plate and reveal new evidence in favor of this mechanism: tensional earthquakes lying 1 km above compressional earthquakes, and earthquakes with highly similar waveforms lying on well-defined planes with complementary rupture areas. The tensional stress is interpreted to be caused by the dimensional mismatch between crust transformed to eclogite and underlying untransformed crust, and the earthquakes are probably facilitated by reactivation of fossil faults extant in the subducting plate. These observations provide seismic evidence for the role of volume change–related stresses and, possibly, fluid-related embrittlement as viable processes for nucleating earthquakes in downgoing oceanic lithosphere.

Cognate xenoliths in Mt. Etna lavas: witnesses of the high-velocity body beneath the volcano

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Corsaro, Rosa Anna; Rotolo, Silvio Giuseppe; Cocina, Ornella; Tumbarello, Gianvito

2014-01-01

Various xenoliths have been found in lavas of the 1763 ("La Montagnola"), 2001, and 2002-03 eruptions at Mt. Etna whose petrographic evidence and mineral chemistry exclude a mantle origin and clearly point to a cognate nature. Consequently, cognate xenoliths might represent a proxy to infer the nature of the high-velocity body (HVB) imaged beneath the volcano by seismic tomography. Petrography allows us to group the cognate xenoliths as follows: i) gabbros with amphibole and amphibole-bearing mela-gabbros, ii) olivine-bearing leuco-gabbros, iii) leuco-gabbros with amphibole, and iv) Plg-rich leuco gabbros. Geobarometry estimates the crystallization pressure of the cognate xenoliths between 1.9 and 4.1 kbar. The bulk density of the cognate xenoliths varies from 2.6 to 3.0 g/cm3. P wave velocities (V P ), calculated in relation to xenolith density, range from 4.9 to 6.1 km/s. The integration of mineralogical, compositional, geobarometric data, and density-dependent V P with recent literature data on 3D V P seismic tomography enabled us to formulate the first hypothesis about the nature of the HVB which, in the depth range of 3-13 km b.s.l., is likely made of intrusive gabbroic rocks. These are believed to have formed at the "solidification front", a marginal zone that encompasses a deep region (>5 km b.s.l.) of Mt. Etna's plumbing system, within which magma crystallization takes place. The intrusive rocks were afterwards fragmented and transported as cognate xenoliths by the volatile-rich and fast-ascending magmas of the 1763 "La Montagnola", 2001 and 2002-03 eruptions.

Sr-Nd isotope systematics of xenoliths in Cenozoic volcanic rocks from SW Japan

International Nuclear Information System (INIS)

Kagami, Hiroo; Iwata, Masatoshi; Iizumi, Shigeru; Nureki, Terukazu.

1993-01-01

Based on new and previously published Sr and Nd isotope data, we examined the petrogenetic relationship between deep crust- and upper mantle-derived xenoliths contained in Cenozoic volcanic rocks and Cretaceous-Paleogene granitoid rocks in SW Japan. The deep crust- and upper mantle-derived mafic to ultramafic xenoliths contained in Cenozoic volcanic rocks from SW Japan have comparable initial Sr and Nd isotope ratios to the Cretaceous-Paleogene granitoid rocks in their respective districts. This may suggest that these xenoliths were genetically related to the Cretaceous-Paleogene granitoid rocks in SW Japan, and that regional variations in Sr and Nd isotope ratios observed in the granitoid rocks are attributed to differences in the geochemistry of the magma sources. (author)

Deep subduction of hot young oceanic slab required by the Syros eclogites

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Flemetakis, Stamatis; Moulas, Evangelos; Kostopoulos, Dimitrios; Chatzitheodoridis, Elias

2014-05-01

The Cycladic islands of Syros and Siphnos, Aegean Sea, Greece, represent subducted IAT and BABB remnants of the Neotethyan Pindos Ocean. Garnet porphyroblasts (Ø=1mm) in a glaucophane-zoisite eclogite from Kini locality on Syros are compositionally zoned and display a unique prograde heating path from a high-pressure greenschist-facies core with high XSps and low Mg# via a blueschist-facies mantle with moderate XSps and Mg# to an eclogite-facies rim with low XSps and high Mg#. The outermost 35 μm of the garnet rims show flat XSps with rapidly increasing outwards Mg#. Na-Act-Chl-Ph rimmed by Gln mark the greenschist-blueschist facies transition, whereas Pg rimmed by Omp and the incoming of Rt at the expense of Ttn signify the blueschist-eclogite facies transition. Raman barometry of quartz inclusions in the eclogitic garnet rims coupled with elastic modelling of the garnet host [1], and Zr-in-Rt and Grt-Cpx-Ph thermobarometry revealed near-UHP P-T conditions of the order of 2.6 GPa/660°C (maximum residual pressure was 0.8-0.9GPa). By contrast, the greenschist-blueschist transition lies at ~0.75 GPa/355°C. This pressure is in excellent agreement with the position of the albite = jadeite + quartz boundary calculated at 350°C using the observed omphacite composition corrected for jadeite activity (Koons & Thompson, 1985) [2]. As a result, Cpx inclusions in garnet core signify the early entrance of garnet in the subduction zone history of the slab. Furthermore, the early growth of garnet (in lower pressures) observed in eclogites from Syros lies in great agreement with published slab-geotherms that indicate hot subduction and show a precocious garnet growth (Baxter and Caddick, 2013) [3]. The complete absence of lawsonite and the great abundance of zoisite crystals, based on the stability fields of both minerals (Poli et al., 2009) [4], further constrain the P-T trajectory of the slab. Our new P-T estimates match published T distributions on the slab surface

Orthopyroxene-enrichment in the lherzolite-websterite xenolith suite from Paleogene alkali basalts of the Poiana Ruscă Mountains (Romania

Directory of Open Access Journals (Sweden)

Nédli Zsuzsanna

2015-12-01

Full Text Available In this paper we present the petrography and geochemistry of a recently collected lherzolite-websterite xenolith series and of clinopyroxene xenocrysts, hosted in Upper Cretaceous–Paleogene basanites of Poiana Ruscă (Romania, whose xenoliths show notable orthopyroxene-enrichment. In the series a slightly deformed porphyroclastic-equigranular textured series could represent the early mantle characteristics, and in many cases notable orthopyroxene growth and poikilitic texture formation was observed. The most abundant mantle lithology, Type A xenoliths have high Al and Na-contents but low mg# of the pyroxenes and low cr# of spinel suggesting a low degree (< 10 % of mafic melt removal. They are also generally poor in overall REE-s (rare earth elements and have flat REY (rare earth elements+ Y patterns with slight LREE-depletion. The geochemistry of the Type A xenoliths and calculated melt composition in equilibrium with the xenolith clinopyroxenes suggests that the percolating melt causing the poikilitization can be linked to a mafic, Al-Na-rich, volatile-poor melt and show similarity with the Late Cretaceous–Paleogene (66–72 Ma subduction-related andesitic magmatism of Poiana Ruscă. Type B xenoliths, with their slightly different chemistry, suggest that, after the ancient depletion, the mantle went through a slight metasomatic event. A subsequent passage of mafic melts in the mantle, with similar compositions to the older andesitic magmatism of Poiana Ruscă, is recorded in the pyroxenites (Fe-rich xenoliths, whereas the megacrysts seem to be cogenetic with the host basanite. The Poiana Ruscă xenoliths differ from the orthopyroxene-enriched mantle xenoliths described previously from the Carpathian-Pannonian Region and from the Dacia block.

Silicate Veining Above an Ascending Mantle Plume - Evidence from New Ethiopian Xenolith Localities

Science.gov (United States)

Rooney, T. O.; Furman, T.; Ayalew, D.; Yirgu, G.

2004-12-01

Quaternary basaltic eruptions in the Debre Zeyit (Bishoftu) and Butajira regions of the Main Ethiopian Rift host Al-augite, norite and rare lherzolite xenoliths, xenocrysts and megacrysts. These explosive basaltic eruptions are located 20 km to the west of the main rift axis and are characterized by cinder cones and maars. The host basalt was generated as a small degree partial melt of fertile peridotite between 15 and 25 kb and host abundant Al-augite (Type II) xenoliths derived from pressures up to 10 kb. The central Main Ethiopian Rift lies in a transitional zone between the continental rifting of East Africa and the sea floor spreading associated with the Red Sea. Lithospheric and sub-lithospheric processes that occur during the transition from continental to oceanic magmatism may be investigated using these xenolith-bearing basalts. Neither carbonatitic nor hydrous (amphibole + phlogopite) metasomatism is evident in either the xenoliths or host basalts, suggesting that infiltration of silicate melts that produced Al-augite veining dominates the regional lower crust and lithospheric mantle. These veins are significantly hotter (200 - 300 ° C) than the lherzolite wall rock they intrude suggesting the thermal influence of the Afar plume. Recent geophysical tomography indicates that this veining is pervasive and segmented, supporting the association of these Al-augite veins with the formation of a proto-ridge axis. Al-augite xenoliths and megacrysts have been observed in other continental rift settings such as Durango (Luhr, 2001) and Lake Baikal (Litasov, 2000), indicating Al-augite silicate melt metasomatism is a fundamental process associated with continental rift development.

Possibilities for rutile extraction from Norwegian eclogite by flotation

OpenAIRE

Gaydardzhiev, Stoyan; Sandvik, Knut

1993-01-01

Preliminary flotations tests towards rutile extraction from Norwegian eclogites (Verving deposit) have been carried out. Various sulfonate type flotation reagents (produced by Hoechts) have been tested. Selectivity between rutile/pyroxene has been targeted. Rutile separation from pyrite and garnet has been further identified as principal problem.

The parent magma of xenoliths in shergottite EETA79001: Bulk and trace element composition inferred from magmatic inclusions

Science.gov (United States)

Treiman, Allan H.; Lindstrom, David J.; Martinez, Rene R.

1994-01-01

The SNC meteorites are samples of the Martian crust, so inferences about their origins and parent magmas are of wide planetologic significance. The EETA79001 shergottite, a basalt, contains xenoliths of pyroxene-olivine cumulate rocks which are possibly related to the ALHA77005 and LEW88516 SNC lherzolites. Olivines in the xenoliths contain magmatic inclusions, relics of magma trapped within the growing crystals. The magmatic inclusions allow a parent magma composition to be retrieved; it is similar to the composition reconstructed from xenolith pyroxenes by element distribution coefficients. The xenolith parent magma is similar but not identical to parent magmas for the shergottite lherzolites.

Eclogitic breccia from the Monviso ophiolite complex: new field and petrographic data

Science.gov (United States)

Locatelli, Michele; Verlaguet, Anne; Federico, Laura; Agard, Philippe

2015-04-01

The Monviso meta-ophiolite complex (Northern Italy, Western Alps) represents a coherent portion of oceanic lithosphere metamorphosed under eclogite facies conditions during the Alpine orogeny (2.6 GPa - 550°C, Lago Superiore Unit, Angiboust et al., 2011), and exhibits from bottom to top a thick serpentinite sole locally capped by metasediments, Mg-Al-rich metagabbros, then Fe-Ti-metagabbros capped by metabasalts. This section is disrupted by three main shear zones. Our study focusses on the Lower Shear Zone (LSZ), situated between the serpentinite sole (to the East) and the Mg-metagabbro bodies (to the West), and composed of blocks of both Fe-Ti and Mg-Al metagabbros embedded in a talc and tremolite-rich serpentinite matrix. Among these blocks, some were described as eclogitic breccias and interpreted as the result of a seismic rupture plane (Angiboust et al., 2012). These breccias correspond to blocks of Fe-Ti-metagabbros that were brecciated in eclogitic facies conditions (as attested by the omphacite + garnet ± lawsonite cement of the breccia) in a fluid-rich environment, as suggested by the abundance of lawsonite in the cement. Here we present new field data on the distribution and petrographic characterization of these eclogitic blocks in the LSZ. The aim of this work is twofold: (I) detailed mapping of the eclogitic block distribution along the LSZ, in order to determine precisely the extent and representativity of the breccias and (II) characterization of the brecciated blocks, at the outcrop scale, to explore the brecciation processes and structures. Between Pian del Re and Colle di Luca localities, the occurrence of eclogite blocks is uniform along the strike of the shear-zone, resulting in a 16 km-long belt of outcropping eclogitic bodies embedded in serpentinite matrix. The shear-zone width, by contrast, varies from 1.3 km to 0.8 km. Three types of eclogitic blocks can be distinguished: (1) intact (i.e., not brecciated) blocks of Fe

Nd-isotopes in selected mantle-derived rocks and minerals and their implications for mantle evolution

Science.gov (United States)

Basu, A.R.; Tatsumoto, M.

1980-01-01

The Sm-Nd systematics in a variety of mantle-derived samples including kimberlites, alnoite, carbonatite, pyroxene and amphibole inclusions in alkali basalts and xenolithic eclogites, granulites and a pyroxene megacryst in kimberlites are reported. The additional data on kimberlites strengthen our earlier conclusion that kimberlites are derived from a relatively undifferentiated chondritic mantle source. This conclusion is based on the observation that the e{open}Nd values of most of the kimberlites are near zero. In contrast with the kimberlites, their garnet lherzolite inclusions show both time-averaged Nd enrichment and depletion with respect to Sm. Separated clinopyroxenes in eclogite xenoliths from the Roberts Victor kimberlite pipe show both positive and negative e{open}Nd values suggesting different genetic history. A whole rock lower crustal scapolite granulite xenolith from the Matsoku kimberlite pipe shows a negative e{open}Nd value of -4.2, possibly representative of the base of the crust in Lesotho. It appears that all inclusions, mafic and ultramafic, in kimberlites are unrelated to their kimberlite host. The above data and additional Sm-Nd data on xenoliths in alkali basalts, alpine peridotite and alnoite-carbonatites are used to construct a model for the upper 200 km of the earth's mantle - both oceanic and continental. The essential feature of this model is the increasing degree of fertility of the mantle with depth. The kimberlite's source at depths below 200 km in the subcontinental mantle is the most primitive in this model, and this primitive layer is also extended to the suboceanic mantle. However, it is clear from the Nd-isotopic data in the xenoliths of the continental kimberlites that above 200 km the continental mantle is distinctly different from their suboceanic counterpart. ?? 1980 Springer-Verlag.

Eclogitization-induced mechanical instanility in granulite: Implications for deep seismicity in southern Tibet

Science.gov (United States)

Wang, Y.; Shi, F.; Yu, T.; Zhu, L.; Zhang, J.; Gasc, J.; Incel, S.; Schubnel, A.; Li, Z.; Liu, W.; Jin, Z.

2017-12-01

Southern Tibet is the most active orogenic region on Earth where the Indian plate thrusts under the Eurasian continent, pushing the Moho to unusual depths of 80 km. Seismicity is wide spread, reaching 100 km depth. Mechanisms of these deep earthquakes remain enigmatic. Here we examine the hypothesis of metamorphism induced mechanical instability in granulite-facies rocks, which are the dominant constituent in subducted Indian lower crust. We conducted deformation experiments on natural and nominally dry granulite in a DDIA apparatus within the stability fields of both granulite and eclogite. The system is interfaced with an acoustic emission (AE) monitoring system, allowing in-situ detection of mechanical instability along with the progress of eclogitization. We found that granulite deformed within its own stability field behaved in a ductile fashion without any AE activity. In contrast, numerous AE events were observed during deformation of metastable granulite in the eclogite field. The observed AE activities were episodic. Correlating closely to the AE burst episodes, measured differential stresses rose and fell during deformation, suggesting unstable fault slip. Microstructural observation shows that strain is highly localized around grain boundaries, which are decorated by eclogitization products. Time-resolved event location analysis showed large episodes corresponded to the growth of branches of macroscopic faults in recovered samples. It appears that ruptures originate from weakened grain boundaries, propagate through grains, and self-organize into macroscopic fault zones. No melting is required in the fault zones to facilitate brittle failure. This process may be responsible for the deep crustal seismicity in Southern Tibet and other continental-continental subduction regions.

Geochronology of Zircon in Eclogite Reveals Imbrication of the Ultrahigh-Pressure Western Gneiss Region of Norway.

Science.gov (United States)

Young, D. J.; Kylander-Clark, A. R.; Root, D. B.

2014-12-01

Eclogite provides the only record of kinematic events at the deepest levels of orogens. Integrating the U-Pb geochronology and trace element chemistry of zircon in eclogite reveals the most complete view of the PTt history, yet low concentrations of uranium and zirconium and drier compositions that hinder zircon growth at peak conditions render it a challenging rocktype for this approach. The iconic Western Gneiss Region (WGR) in Norway is one of the largest terranes of deeply subducted continental rocks in the world, and contains many indicators of ultrahigh-pressure metamorphic conditions (P>2.8 GPa) that developed during the Siluro-Devonian Caledonian Orogeny. A metamorphic transition from amphibolite-facies to ultrahigh-pressure eclogite facies broadly coincides with a km-scale shear zone that underlies the majority of the WGR. A critical unknown is the timing of movement on this feature, which emplaced allochthonous units above the Baltica basement, but might also have accommodated late-orogenic exhumation of the WGR from mantle depths. We carried out laser ablation split-stream ICPMS (LASS) and selected multigrain TIMS analyses of zircons from eleven eclogites across the southern WGR, of which eight are located within or above the shear zone. LASS spots on polished grains mostly yield weakly discordant Proterozoic intrusive ages, and often minimal indication of a Caledonian (U)HP metamorphic overprint. Direct ablation into unpolished zircon reveals thin rims of Caledonian age in some cases. Overall, the dataset shows that all samples began zircon growth at approximately the same time (ca. 430-420 Ma). Eclogite from lower levels of the shear zone does not contain any dates younger than ca. 410 Ma, however, while eclogite from higher levels continued growth until ca. 400 Ma. We interpret this to result from thrusting of the WGR above cooler basement after 410 Ma, terminating new zircon crystallization within the shear zone but allowing limited further growth in

Sr and Nd isotope composition of the metamorphic, sedimentary and ultramafic xenoliths of Lanzarote (Canary Islands): Implications for magma sources

Science.gov (United States)

Aparicio, Alfredo; Tassinari, Colombo C. G.; García, Roberto; Araña, Vicente

2010-01-01

The lavas produced by the Timanfaya eruption of 1730-1736 (Lanzarote, Canary Islands) contain a great many sedimentary and metamorphic (metasedimentary), and mafic and ultramafic plutonic xenoliths. Among the metamorphosed carbonate rocks (calc-silicate rocks [CSRs]) are monomineral rocks with forsterite or wollastonite, as well as rocks containing olivine ± orthopyroxene ± clinopyroxene ± plagioclase; their mineralogical compositions are identical to those of the mafic (gabbros) and ultramafic (dunite, wherlite and lherzolite) xenoliths. The 87Sr/ 86Sr (around 0.703) and 143Nd/ 144Nd (around 0.512) isotope ratios of the ultramafic and metasedimentary xenoliths are similar, while the 147Sm/ 144Nd ratios show crustal values (0.13-0.16) in the ultramafic xenoliths and mantle values (0.18-0.25) in some CSRs. The apparent isotopic anomaly of the metamorphic xenoliths can be explained in terms of the heat source (basaltic intrusion) inducing strong isotopic exchange ( 87Sr/ 86Sr and 143Nd/ 144Nd) between metasedimentary and basaltic rocks. Petrofabric analysis also showed a possible relationship between the ultramafic and metamorphic xenoliths.

Do the eclogites of the Variscan Malpica-Tuy shear zone in NW Spain point to continental subduction?

Science.gov (United States)

Li, Botao; Massonne, Hans-Joachim

2014-05-01

High-pressure (HP: P > 10 kbar) rocks such as eclogite typically occur in suture zones of collided continental plates in Phanerozoic times. In case of an extended area at the surface of a denuded orogen with HP and even ultrahigh pressure (UHP) metamorphic rocks, they are often interpreted as the result of continental subduction. We have tested this idea for the HP-UHP area of the Malpica-Tuy zone of the Variscan orogen, which was formed by the collision of Gondwana and Laurussia. For the test, we have initially studied an eclogite and its surrounding gneiss of this zone in detail. The eclogite contains the assemblage garnet, omphacite, amphibole, rutile, ilmenite, clinozoisite/epidote, quartz, and phengite with Si-contents as high as 3.45 per formula unit (pfu) in inner portions and 3.27-3.35 pfu in rims. Garnet exhibits chemical zonation with Gro25Alm55Pyr15Spe5, Gro29Alm57Pyr13Spe1, and Gro23Alm56Pyr20Spe1 as inner core, mantle, and outermost rim compositions, respectively. The gneiss is a former medium-grained granite now composed of quartz, plagioclase, K-feldspar, biotite, phengite, garnet, clinozoisite/epidote, titanite, apatite and ilmenite. Phengite shows Si contents between 3.40 (core) and 3.00 (rim) pfu. Garnet is chemically zoned with Gro69.6Alm27Pyr0.4Spe3, Gro65.5Alm32.5Pyr0.5Spe1.5, Gro65.7Alm31.7Pyr0.6Spe2, and Gro56.6Alm41.6Pyr1.2Spe0.6 as core, mantle, rim and outermost rim compositions, respectively. P-T pseudosections were calculated with the PERPLEX computer program in the system Na2O-K2O-CaO-FeO-O2-MnO-MgO-Al2O3-SiO2-TiO2-H2O for the bulk-rock compositions of the studied eclogite and gneiss. These pseudosections were contoured by isopleths of various parameters such as molar fractions of garnet components. Based on this contouring a P-T path was derived that starts at HP conditions for both lithologies. Garnet began to form at 22 kbar and 565°C in the eclogite. Subsequently, the temperatures increased to 585°C and the pressure decreased to

Petrochemistry of a xenolith-bearing Neogene alkali olivine basalt from northeastern Iran

Science.gov (United States)

Saadat, Saeed; Stern, Charles R.

2012-05-01

A small isolated Neogene, possibly Quaternary, monogenetic alkali olivine basalt cone in northeastern Iran contains both mantle peridotite and crustal gabbroic xenoliths, as well as plagioclase megacrysts. The basaltic magma rose to the surface along pathways associated with local extension at the junction between the N-S right-lateral and E-W left-lateral strike slip faults that form the northeastern boundary of the Lut microcontinental block. This basalt is enriched in LREE relative to HREE, and has trace-element ratios similar to that of oceanic island basalts (OIB). Its 87Sr/86Sr (0.705013 to 0.705252), 143Nd/144Nd (0.512735 to 0.512738), and Pb isotopic compositions all fall in the field of OIB derived from enriched (EM-2) mantle. It formed by mixing of small melt fractions from both garnet-bearing asthenospheric and spinel-facies lithospheric mantle. Plagioclase (An26-32) megacrysts, up to 4 cm in length, have euhedral crystal faces and show no evidence of reaction with the host basalt. Their trace-element concentrations suggest that these megacrysts are co-genetic with the basalt host, although their 87Sr/86Sr (0.704796) and 143Nd/144Nd (0.512687) ratios are different than this basalt. Round to angular, medium-grained granoblastic meta-igneous gabbroic xenoliths, ranging from ~ 1 to 6 cm in dimension, are derived from the lower continental crust. Spinel-peridotite xenoliths equilibrated in the subcontinental lithosphere at depths of 30 to 60 km and temperatures of 965 °C to 1065 °C. These xenoliths do not preserve evidence of extensive metasomatic enrichment as has been inferred for the mantle below the Damavand volcano further to the west in north-central Iran, and clinopyroxenes separated from two different mantle xenoliths have 87Sr/86Sr (0.704309 and 0.704593) and 143Nd/144Nd (0.512798) ratios which are less radiogenic than either their host alkali basalt or Damavand basalts, implying significant regional variations in the composition and extent of

Petrogenesis of pelitic xenoliths at the Babbitt CuNi deposit, Duluth Complex, Minnesota, U.S.A.

Science.gov (United States)

Ripley, Edward M.; Alawi, Jomaah A.

1988-02-01

The Babbitt deposit consists of disseminated CuFeNi sulfides found within mafic rocks of the Duluth Complex, generally near contacts with underlying metasedimentary rock types. Host rocks for the deposit include troctolites, olivine gabbros, gabbronorites, norites, and occasionally country rock hornfels. Xenoliths of country rocks are abundant in the deposit, and suggest a relationship between sulfide mineralization and country rock contamination. Country rocks in the Babbitt area include those of the middle Precambrian Biwabik Iron Formation, and both calcareous and non-calcareous pelites of the Virginia Formation. Xenoliths contain the assemblage cordierite-plagioclase-biotite-orthopyroxene, and are thought to have been derived from Virginia Formation protoliths. Comparison of protoliths and xenoliths using composition-volume, element ratio and mass-balance techniques suggests that xenoliths have been strongly depleted in volatiles, alkalis and Si. Footwall rocks show only a depletion in volatiles. Neither fluid-phase transport nor diffusion through an intergranular fluid can account for the mass of material transferred. Extensive partial melting of xenoliths, with residual enrichment of FeO, MgO and Al 2O 3, is the most viable transfer process. The lack of SiO 2 concentration gradients around xenoliths and anomalous igneous rock compositions suggest that desilicification occurred at a time when physical mixing of extracted partial melt and host magma was possible, and prior to final emplacement. Sulfide saturation may have been initiated due to Si assimilation with an auxiliary magma chamber. However, the composition of ores in the Babbitt deposit is consistent with saturation being achieved by addition of sediment-derived volatile sulfur, independent of major-element assimilation.

Coupling of Oceanic and Continental Crust During Eocene Eclogite-Facies Metamorphism: Evidence From the Monte Rosa Nappe, Western Alps, Italy

Science.gov (United States)

Lapen, T. J.; Johnson, C. M.; Baumgartner, L. P.; Skora, S.; Mahlen, N. J.; Beard, B. L.

2006-12-01

Subduction of continental crust to HP-UHP metamorphic conditions requires overcoming density contrasts that are unfavorable to deep burial, whereas exhumation of these rocks can be reasonably explained through buoyancy-assisted transport in the subduction channel to more shallow depths. In the western Alps, both continental and oceanic lithosphere has been subducted to eclogite-facies metamorphic conditions. The burial and exhumation histories of these sections of lithosphere bear directly on the dynamics of subduction and the stacking of units within the subduction channel. We address the burial history of the continental crust with high precision U-Pb rutile and Lu-Hf garnet geochronology of the eclogite-facies Monte Rosa nappe (MR), western Alps, Italy. U-Pb rutile ages from quartz-carbonate-white mica-rutile veins that are hosted within eclogite and schist of the MR, Gressoney Valley, Italy, indicate that it was at eclogite-facies metamorphic conditions at 42.6 +/- 0.6 Ma. The sample area (Indren glacier, Furgg zone; Dal Piaz, 2001) consists of eclogite boudins that are surrounded by micaceous schist. Associated with the eclogite and schist are quartz-carbonate-white mica-rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins occurred at eclogite-facies metamorphic conditions (480-570°C, >1.3-1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. Lu-Hf geochronology of garnet from a chloritoid-talc-garnet-phengite-quartz-calcite-pyrite - chalcopyrite bearing boudin within talc-chloritoid whiteschists of the MR, Val d'Ayas, Italy (Chopin and Monie, 1984; Pawlig, 2001) yields an age of 40.54 +/- 0.36 Ma. The talc-chloritoid whiteschists from the area record pressures and temperatures of 1.6-2.4 GPa and 500-530°C (Chopin and Monie, 1984; Le Bayon et al., 2006) indicating near UHP metamorphic conditions. Based on the age, P-T, and textural

Fluid-induced transition from banded kyanite- to bimineralic eclogite and implications for the evolution of cratons

Science.gov (United States)

Sommer, H.; Jacob, D. E.; Stern, R. A.; Petts, D.; Mattey, D. P.; Pearson, D. G.

2017-06-01

Heterogeneous, modally banded kyanite-bearing and bimineralic eclogites from the lithospheric mantle, collected at the Roberts Victor Diamond mine (South Africa), show a reaction texture in which kyanite is consumed. Geothermobarometric calculations using measured mineral compositions in Perple_X allowed the construction of a P-T path showing a steep, cool prograde metamorphic gradient of 2 °C/km to reach peak conditions of 5.8 GPa and 890 °C for the kyanite eclogite. The kyanite-out reaction formed bimineralic eclogite and is probably an integral part of the mineralogical evolution of most archetypal bimineralic eclogites at Roberts Victor and potentially elsewhere. The kyanite-out reaction occured at close to peak pressure (5.3 GPa) and was associated with a rise in temperature to 1380 °C. Mass balance calculations show that upon breakdown, the kyanite component is fully accommodated in garnet and omphacite via a reaction system with low water fugacity that required restricted fluid influx from metasomatic sources. The δ18O values of garnets are consistently higher than normal mantle values. Each sample has its characteristic trend of δ18O variance between garnets in the kyanite-bearing sections and those in the bimineralic parts covering a range between 5.1‰ and 6.8‰. No systematic change in O-isotope signature exists across the sample population. Differences in garnet trace element signatures between differing lithologies in the eclogites are significant. Grossular-rich garnets coexisting with kyanite have strong positive Eu-anomalies and low Gd/Yb ratios, while more pyrope-rich garnets in the bimineralic sections have lost their positive Eu-anomaly, have higher Gd/Yb ratios and generally higher heavy rare earth element contents. Garnets in the original kyanite-bearing portions thus reflect the provenance of the rocks as metamorphosed gabbros/troctolites. The kyanite-out reaction was most likely triggered by a heating event in the subcratonic

An eclogite-bearing continental tectonic slice in the Zermatt-Saas high-pressure ophiolites at Trockener Steg (Zermatt, Swiss Western Alps)

Science.gov (United States)

Weber, Sebastian; Bucher, Kurt

2015-09-01

The Theodul Glacier Unit (TGU) at "Trockener Steg" represents a continental slice, embedded within the ophiolitic Zermatt-Saas Zone. The Zermatt-Saas Zone is the remnant of the Piemonte-Liguria oceanic lithosphere, formed in the middle Jurassic and subducted up to eclogite facies conditions in the Early Tertiary. The close spatial association of the TGU to the Zermatt-Saas Zone permits a comparison of the metamorphic evolution of the units by detailed field mapping and a petrological investigation of eclogites. The eclogites from both tectono-metamorphic units can be clearly distinguished by their textures, mineral assemblages and by mineral and bulk-rock composition. Geothermobarometry and computed assemblage stability diagrams for the TGU eclogites indicate P-T conditions of 2.2 ± 0.1 GPa and 580 ± 50 °C. These derived P-T conditions must be considered as minimum peak metamorphic conditions the rocks achieved during subduction. The P-T data are different from those derived for eclogites of Zermatt-Saas Zone adjacent to the Theodul Glacier Unit, that reached maximal burial depths at 2.3-2.4 GPa and 500 ± 50 °C. While the estimates of the eclogites of Zermatt-Saas Zone are in good agreement with some of the previous studies, the contrasting P-T estimates for the TGU eclogites suggest that the Zermatt-Saas complex must be subdivided into several tectonic subunits. The non-uniform peak conditions over the "Trockener Steg" area and the maximum pressures conditions reported from ultra-high pressure localities within Zermatt-Saas Zone suggest, that individual tectonic slices have been assembled after detachment from the slab at the return-point, i.e. along the exhumation path. Detached packages of rocks may range from small tectonic slices up to several kilometer-sized fragments. The TGU is separated from the surrounding rocks of the ophiolite unit by two major tectonic contacts. In addition, the formation of biotite-rich crusts along the basal contact of the TGU

Retrogressive hydration of calc-silicate xenoliths in the eastern Bushveld complex: evidence for late magmatic fluid movement

Science.gov (United States)

Wallmach, T.; Hatton, C. J.; De Waal, S. A.; Gibson, R. L.

1995-11-01

Two calc-silicate xenoliths in the Upper Zone of the Bushveld complex contain mineral assemblages which permit delineation of the metamorphic path followed after incorporation of the xenoliths into the magma. Peak metamorphism in these xenoliths occurred at T=1100-1200°C and P <1.5 kbar. Retrograde metamorphism, probably coinciding with the late magmatic stage, is characterized by the breakdown of akermanite to monticellite and wollastonite at 700°C and the growth of vesuvianite from melilite. The latter implies that water-rich fluids (X CO 2 <0.2) were present and probably circulating through the cooling magmatic pile. In contrast, calc-silicate xenoliths within the lower zones of the Bushveld complex, namely in the Marginal and Critical Zones, also contain melilite, monticellite and additional periclase with only rare development of vesuvianite. This suggests that the Upper Zone cumulate pile was much 'wetter' in the late-magmatic stage than the earlier-formed Critical and Marginal Zone cumulate piles.

The role of eclogite in the mantle heterogeneity at Cape Verde

DEFF Research Database (Denmark)

Barker, Abigail Katrine; Holm, Paul Martin; Troll, Valentin R.

2014-01-01

The Cape Verde hotspot, like many other Ocean Island Basalt provinces, demonstrates isotopic heterogeneity on a 100–200 km scale. The heterogeneity is represented by the appearance of an EM1-like component at several of the southern islands and with a HIMU-like component present throughout...... have been limited. We apply the minor elements in olivine approach (Sobolev et al. in Nature 434:590–597, 2005; Science, doi:10.1126/science.1138113, 2007), to determine and quantify the contributions of peridotite, pyroxenite and eclogite melts to the mantle heterogeneity observed at Cape Verde. Cores...... of olivine phenocrysts of the Cape Verde volcanics have low Mn/FeO and low Ni*FeO/MgO that deviate from the negative trend of the global array. The global array is defined by mixing between peridotite and pyroxenite, whereas the Cape Verde volcanics indicate contribution of an additional eclogite source...

Formation of Atoll Garnets in the UHP Eclogites of the Tso Morari ...

Indian Academy of Sciences (India)

7

across the studied atoll grains show elemental variations with higher concentrations of Ca and. 10 ...... from a single eclogite outcrop in the complex and probably represent a case of accidental. 28 ... Textural and compositional relations in the.

Evolution of the volcanic plumbing systemof Alicudi (Aeolian Islands - Italy: evidence from fluid and melt inclusionsin quartz xenoliths

Directory of Open Access Journals (Sweden)

A. Peccerillo

2004-06-01

Full Text Available Quartz-rich xenoliths in lavas (basalts to andesites; 90-30 ka from Alicudi contain abundant melt and fluid inclusions. Two generations of CO2-rich fluid inclusions are present in quartz-rich xenolith grains: early (Type I inclusions related to partial melting of the host xenoliths, and late Type II inclusions related to the fluid trapping during xenolith ascent. Homogenisation temperatures of fluid inclusions correspond to two density intervals: 0.93-0.68 g/cm3 (Type I and 0.47-0.26 g/cm3 (Type II. Early Type I fluid inclusions indicate trapping pressures around 6 kbar, which are representative for the levels of partial melting of crustal rocks and xenolith formation. Late Type II fluid inclusions show lower trapping pressures, between 1.7 kbar and 0.2 kbar, indicative for shallow magma rest and accumulation during ascent to the surface. Data suggest the presence of two magma reservoirs: the first is located at lower crustal depths (about 24 km, site of fractional crystallization, mixing with source derived magma, and various degrees of crustal assimilation. The second magma reservoir is located at shallow crustal depths (about 6 km, the site where magma rested for a short time before erupting.

On the Preservation of Intergranular Coesite in UHP Eclogite at Yangkou Bay, Sulu belt of eastern China

Science.gov (United States)

Wang, L.; Wang, S.; Brown, M.

2016-12-01

In contrast to coesite that occurs as inclusions in zircon and rock-forming minerals, intergranular coesite is preserved in UHP eclogite at Yangkou in the Sulu belt. The survival of intergranular coesite is intriguing because the eclogite experienced phengite growth and partial melting during exhumation. The coesite eclogite occurs as rootless isoclinal fold noses within quartz-rich schist which contains 10-20 vol% phengite, whereas phengite is absent from coesite eclogite in the fold noses. To evaluate the factors that control preservation of intergranular coesite, four samples representative of different stages along the retrograde P-T path were selected for study. For each sample we determined the number of intergranular coesite grains per cm2 and the OH content of garnet and omphacite. As the number of coesite grains decreases, the bulk rock OH content increases from transformation to quartz of intergranular coesite outside of the fold noses. The fluid is inferred to have been a supercritical fluid probably residual from prograde dehydration but also derived by dissolution of nominally anhydrous minerals. Post-metamorphic-peak deformation combined with fluid percolation along sheared fold limbs induced phengite growth during initial exhumation and then facilitated partial melting. In contrast, fold hinges in competent layers are unfavourable sites for fluid penetration. At Yangkou, the intergranular coesite is preserved in the fold noses where it was protected from both penetrative deformation and fluid ingress. Therefore, the fold noses maintained a relatively dry environment that allowed preservation of the intergranular coesite. Thus, deformation partitioning and strain localization impose local controls on fluid distribution and migration in UHP eclogite. This study informs our understanding of variations in fluid regime during exhumation of deeply subducted continental crust.

Petrology of spinel lherzolite xenoliths in alkali basalts from Liri ...

African Journals Online (AJOL)

Al2O3), and Al-rich spinel occur in alkali basalts from Liri, South of the ... these spinel lherzolite xenoliths are reported, along with the analyses of ...... erupted in the Liri region. .... and temperatures with controlled activities of water, carbon.

U-Pb, Nd isotope and REE geochemistry in eclogites from the Cabo Ortegal Complex, Galicia, Spain: an example of REE immobility conserving MORB-like patterns during high-grade metamorphism

International Nuclear Information System (INIS)

Bernard-Griffiths, J.; Peucat, J.-J.; Cornichet, J.; Iglesias Ponce de Leon, M.; Gil Ibarguchi, J.I.

1985-01-01

REE abundances and Nd isotopic compositions were determined on representative samples of eclogite from the Cabo Ortegal Complex of northern Spain. Zircons were also separated from a whole-rock eclogite and analysed by the U-Pb radiometric method. Results indicate that eclogite facies metamorphism occurred between 480 and 420 Ma ago, but no precise constraint can be placed on the protolith age. The REE patterns observed suggest that there has been no significant alteration of the protolith whole-rock systems and that high-grade metamorphism has had little effect on the more mobile LREE. The eclogite protoliths were probably derived from ancient mantle sources with geochemical characteristics very similar to present-day MORB sources. This implies that LREE-depleted (N-type) tholeiites have been erupted at constructive plate margins since at least the early Palaeozoic and possibly long before. The Cabo Ortegal eclogites are allochthonous. They have been thrust up on to the continent and thus they can be compared to other eclogites which also show MORB-like characteristics (e.g., 90% of the eclogites of Vendee area of western France). (orig.)

Water in the oceanic lithosphere: Salt Lake Crater xenoliths, Oahu, Hawaii

Science.gov (United States)

Peslier, A. H.; Bizimis, M.

2010-12-01

Water can be present in nominally anhydrous minerals of peridotites in the form of hydrogen bonded to structural oxygen. Such water in the oceanic upper mantle could have a significant effect on its physical and chemical properties. However, the water content of the MORB source has been inferred indirectly from the compositions of basalts. Direct determinations on abyssal peridotites are scarce because they have been heavily hydrothermally altered. Here we present the first water analyses of minerals from spinel peridotite xenoliths of Salt Lake Crater, Oahu, Hawaii, which are exceptionally fresh. These peridotites are thought to represent fragments of the Pacific oceanic lithosphere that was refertilized by alkalic Hawaiian melts. A few have unradiogenic Os and radiogenic Hf isotopes and may be fragments of an ancient ( 2 Ga) depleted and recycled lithosphere. Water contents in olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx) were determined by FTIR spectrometry. Preliminary H_{2}O contents show ranges of 8-10 ppm for Ol, 151-277 ppm for Opx, and 337-603 ppm for Cpx. Reconstructed bulk rock H_{2}O contents range from 88-131 ppm overlapping estimates for the MORB source. Water contents between Ol minerals of the same xenolith are heterogeneous and individual OH infrared bands vary within a mineral with lower 3230 cm^{-1} and higher 3650-3400 cm^{-1} band heights from core to edge. This observation suggests disturbance of the hydrogen in Ol likely occurring during xenolith entrainment to the surface. Pyroxene water contents are higher than most water contents in pyroxenes from continental peridotite xenoliths and higher than those of abyssal peridotites. Cpx water contents decrease with increasing degree of depletion (e.g. increasing Fo in Ol and Cr# in spinel) consistent with an incompatible behavior of water. However Cpx water contents also show a positive correlation with LREE/HREE ratios and LREE concentrations consistent with refertilization. Opx

Water in the Oceanic Lithosphere: Salt Lake Crater Xenoliths, Oahu, Hawaii

Science.gov (United States)

Peslier, Anne H.; Bizimis, Michael

2010-01-01

Water can be present in nominally anhydrous minerals of peridotites in the form of hydrogen bonded to structural oxygen. Such water in the oceanic upper mantle could have a significant effect on its physical and chemical properties. However, the water content of the MORB source has been inferred indirectly from the compositions of basalts. Direct determinations on abyssal peridotites are scarce because they have been heavily hydrothermally altered. Here we present the first water analyses of minerals from spinel peridotite xenoliths of Salt Lake Crater, Oahu, Hawaii, which are exceptionally fresh. These peridotites are thought to represent fragments of the Pacific oceanic lithosphere that was refertilized by alkalic Hawaiian melts. A few have unradiogenic Os and radiogenic Hf isotopes and may be fragments of an ancient (2 Ga) depleted and recycled lithosphere. Water contents in olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx) were determined by FTIR spectrometry. Preliminary H_{2}O contents show ranges of 8-10 ppm for Ol, 151-277 ppm for Opx, and 337-603 ppm for Cpx. Reconstructed bulk rock H_{2}O contents range from 88-131 ppm overlapping estimates for the MORB source. Water contents between Ol minerals of the same xenolith are heterogeneous and individual OH infrared bands vary within a mineral with lower 3230 cm^{-1} and higher 3650-3400 cm^{-1} band heights from core to edge. This observation suggests disturbance of the hydrogen in Ol likely occurring during xenolith entrainment to the surface. Pyroxene water contents are higher than most water contents in pyroxenes from continental peridotite xenoliths and higher than those of abyssal peridotites. Cpx water contents decrease with increasing degree of depletion (e.g. increasing Fo in Ol and Cr# in spinel) consistent with an incompatible behavior of water. However Cpx water contents also show a positive correlation with LREE/HREE ratios and LREE concentrations consistent with refertilization. Opx water

Felsic granulite with layers of eclogite facies rocks in the Bohemian Massif; did they share a common metamorphic history?

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Jedlicka, Radim; Faryad, Shah Wali

2017-08-01

High pressure granulite and granulite gneiss from the Rychleby Mountains in the East Sudetes form an approximately 7 km long and 0.8 km wide body, which is enclosed by amphibolite facies orthogneiss with a steep foliation. Well preserved felsic granulite is located in the central part of the body, where several small bodies of mafic granulite are also present. In comparison to other high pressure granulites in the Bohemian Massif, which show strong mineral and textural re-equilibration under granulite facies conditions, the mafic granulite samples preserve eclogite facies minerals (garnet, omphacite, kyanite, rutile and phengite) and their field and textural relations indicate that both mafic and felsic granulites shared common metamorphic history during prograde eclogite facies and subsequent granulite facies events. Garnet from both granulite varieties shows prograde compositional zoning and contains inclusions of phengite. Yttrium and REEs in garnet show typical bell-shaped distributions with no annular peaks near the grain rims. Investigation of major and trace elements zoning, including REEs distribution in garnet, was combined with thermodynamic modelling to constrain the early eclogite facies metamorphism and to estimate pressure-temperature conditions of the subsequent granulite facies overprint. The first (U)HP metamorphism occurred along a low geothermal gradient in a subduction-related environment from its initial stage at 0.8 GPa/460 °C and reached pressures up to 2.5 GPa at 550 °C. The subsequent granulite facies overprint (1.6-1.8 GPa/800-880 °C) affected the rocks only partially; by replacement of omphacite into diopside + plagioclase symplectite and by compositional modification of garnet rims. The mineral textures and the preservation of the eclogite facies prograde compositional zoning in garnet cores confirm that the granulite facies overprint was either too short or too faint to cause recrystallisation and homogenisation of the eclogite

40Ar-39Ar laser probe dating of individual clinopyroxene inclusions in Premier eclogitic diamonds

International Nuclear Information System (INIS)

Burgess, R.; Turner, G.; Laurenzi, M.; Harris, J.W.

1989-01-01

The ages of seven individual clinopyroxene inclusions in Premier diamonds of eclogitic association have been determined using the 40 Ar- 39 Ar dating technique. Syngenetic inclusions weighing between 10 and 130 μg were exposed on cleaved surfaces of the diamonds and analysed using a laser probe. The inclusion ages were found to be in the range 1111±35 to 1254±38 Ma with an average of 1185±94 Ma. The ages obtained are in good agreement with previous determinations made on aggregates of eclogitic inclusions from Premier diamonds and demonstrate the applicability of the laser probe to dating individual diamond inclusions. (orig.)

Orthopyroxene-enrichment in the lherzolite-websterite xenolith suite from Paleogene alkali basalts of the Poiana Ruscă Mountains (Romania)

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Nédli, Zsuzsanna; Szabó, Csaba; Dégi, Júlia

2015-12-01

In this paper we present the petrography and geochemistry of a recently collected lherzolite-websterite xenolith series and of clinopyroxene xenocrysts, hosted in Upper Cretaceous-Paleogene basanites of Poiana Ruscă (Romania), whose xenoliths show notable orthopyroxene-enrichment. In the series a slightly deformed porphyroclastic-equigranular textured series could represent the early mantle characteristics, and in many cases notable orthopyroxene growth and poikilitic texture formation was observed. The most abundant mantle lithology, Type A xenoliths have high Al and Na-contents but low mg# of the pyroxenes and low cr# of spinel suggesting a low degree (Dacia block.

The evolution and ascent paths of mantle xenolith-bearing magma: Observations and insights from Cenozoic basalts in Southeast China

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Sun, Pu; Niu, Yaoling; Guo, Pengyuan; Cui, Huixia; Ye, Lei; Liu, Jinju

2018-06-01

Studies have shown that mantle xenolith-bearing magmas must ascend rapidly to carry mantle xenoliths to the surface. It has thus been inferred inadvertently that such rapid ascending melt must have undergone little crystallization or evolution. However, this inference is apparently inconsistent with the widespread observation that xenolith-bearing alkali basalts are variably evolved with Mg# ≤72. In this paper, we discuss this important, yet overlooked, petrological problem and offer new perspectives with evidence. We analyzed the Cenozoic mantle xenolith-bearing alkali basalts from several locations in Southeast China that have experienced varying degrees of fractional crystallization (Mg# = 48-67). The variably evolved composition of host alkali basalts is not in contradiction with rapid ascent, but rather reflects inevitability of crystallization during ascent. Thermometry calculations for clinopyroxene (Cpx) megacrysts give equilibrium temperatures of 1238-1390 °C, which is consistent with the effect of conductive cooling and melt crystallization during ascent because TMelt > TLithosphere. The equilibrium pressure (18-27 kbar) of these Cpx megacrysts suggests that the crystallization takes place under lithospheric mantle conditions. The host melt must have experienced limited low-pressure residence in the shallower levels of lithospheric mantle and crust. This is in fact consistent with the rapid ascent of the host melt to bring mantle xenoliths to the surface.

Asphaltene-bearing mantle xenoliths from Hyblean diatremes, Sicily

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Scirè, Salvatore; Ciliberto, Enrico; Crisafulli, Carmelo; Scribano, Vittorio; Bellatreccia, Fabio; Ventura, Giancarlo Della

2011-08-01

Microscopic blebs of sulfur-bearing organic matter (OM) commonly occur between the secondary calcite grains and fibrous phyllosilicates in extensively serpentinized and carbonated mantle-derived ultramafic xenoliths from Hyblean nephelinite diatremes, Sicily, Italy. Rarely, coarse bituminous patches give the rock a blackish color. Micro Fourier transform infrared spectra (μ-FTIR) point to asphaltene-like structures in the OM, due to partially condensed aromatic rings with aliphatic tails consisting of a few C atoms. X-ray photoelectron spectroscopy (XPS) analysis indicates the occurrence of minor S═O (either sulphonyl or sulphoxide) functional groups in the OM. Solubility tests in toluene, thermo-gravimetric (TGA) and differential thermal (DTA) analyses confirm the presence of asphaltene structures. It is proposed that asphaltenes derive from the in situ aromatization (with decrease in H/C ratio) of previous light aliphatic hydrocarbons. Field evidence excludes that hydrocarbon from an external source percolated through the xenolith bearing tuff-breccia. The discriminating presence of hydrocarbon in a particular type of xenolith only and the lack of hydrocarbon in the host breccia matrix, are also inconsistent with an interaction between the ascending eruptive system and a supposed deep-seated oil reservoir. Assuming that the Hyblean unexposed basement consists of mantle ultramafics and mafic intrusive rocks having hosted an early abyssal-type hydrothermal system, one can put forward the hypothesis that the hydrocarbon production was related to hydrothermal activity in a serpentinite system. Although a bacteriogenesis or thermogenesis cannot be ruled out, the coexisting serpentine, Ni-Fe ores and hydrocarbon strongly suggest a Fischer-Tropsch-type (FTT) synthesis. Subsequent variations in the chemical and physical conditions of the system, for example an increase in the water/rock ratio, gave rise to partial oxidation and late carbonation of the serpentinite

Two types of gabbroic xenoliths from rhyolite dominated Niijima volcano, northern part of Izu-Bonin arc: petrological and geochemical constraints

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Arakawa, Yoji; Endo, Daisuke; Ikehata, Kei; Oshika, Junya; Shinmura, Taro; Mori, Yasushi

2017-03-01

We examined the petrography, petrology, and geochemistry of two types of gabbroic xenoliths (A- and B-type xenoliths) in olivine basalt and biotite rhyolite units among the dominantly rhyolitic rocks in Niijima volcano, northern Izu-Bonin volcanic arc, central Japan. A-type gabbroic xenoliths consisting of plagioclase, clinopyroxene, and orthopyroxene with an adcumulate texture were found in both olivine basalt and biotite rhyolite units, and B-type gabbroic xenoliths consisting of plagioclase and amphibole with an orthocumulate texture were found only in biotite rhyolite units. Geothermal- and barometricmodelling based on mineral chemistry indicated that the A-type gabbro formed at higher temperatures (899-955°C) and pressures (3.6-5.9 kbar) than the B-type gabbro (687-824°C and 0.8-3.6 kbar). These findings and whole-rock chemistry suggest different parental magmas for the two types of gabbro. The A-type gabbro was likely formed from basaltic magma, whereas the B-type gabbro was likely formed from an intermediate (andesitic) magma. The gabbroic xenoliths in erupted products at Niijima volcano indicate the presence of mafic to intermediate cumulate bodies of different origins at relatively shallower levels beneath the dominantly rhyolitic volcano.

Seismic properties of lawsonite eclogites from the southern Motagua fault zone, Guatemala

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Kim, Daeyeong; Wallis, Simon; Endo, Shunsuke; Ree, Jin-Han

2016-05-01

We present new data on the crystal preferred orientation (CPO) and seismic properties of omphacite and lawsonite in extremely fresh eclogite from the southern Motagua fault zone, Guatemala, to discuss the seismic anisotropy of subducting oceanic crust. The CPO of omphacite is characterized by (010)[001], and it shows P-wave seismic anisotropies (AVP) of 1.4%-3.2% and S-wave seismic anisotropies (AVS) of 1.4%-2.7%. Lawsonite exhibits (001) planes parallel to the foliation and [010] axes parallel to the lineation, and seismic anisotropies of 1.7%-6.6% AVP and 3.4%-14.7% AVS. The seismic anisotropy of a rock mass consisting solely of omphacite and lawsonite is 1.2%-4.1% AVP and 1.8%-6.8% AVS. For events that propagate more or less parallel to the maximum extension direction, X, the fast S-wave velocity (VS) polarization is parallel to the Z in the Y-Z section (rotated from the X-Z section), causing trench-normal seismic anisotropy for orthogonal subduction. Based on the high modal abundance and strong fabric of lawsonite, the AVS of eclogites is estimated as ~ 11.7% in the case that lawsonite makes up ~ 75% of the rock mass. On this basis, we suggest that lawsonite in both blueschist and eclogite may play important roles in the formation of complex pattern of seismic anisotropy observed in NE Japan: weak trench-parallel anisotropy in the forearc basin domains and trench-normal anisotropy in the backarc region.

Eclogites and garnet clinopyroxenites in the Anrakhai complex, Central Asian Orogenic Belt, Southern Kazakhstan: P-T evolution, protoliths and some geodynamic implications

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Pilitsyna, Anfisa V.; Tretyakov, Andrey A.; Degtyarev, Kirill E.; Cuthbert, Simon J.; Batanova, Valentina G.; Kovalchuk, Elena V.

2018-03-01

The Anrakhai Metamorphic Complex (AMC), located in the SE part of the Chu-Ili Mountains of Southern Kazakhstan in the western part of Central Asian Orogenic Belt, exhibits occurrences of HP metamorphic rocks in the form of eclogites and garnet clinopyroxenites with peak metamorphic conditions of 750-850° and 15-19 kbar estimated with both conventional geothermobarometric methods and phase diagram modeling. P-T estimates as well as intimate field relations evidently imply a common metamorphic history for eclogites and garnet clinopyroxenites of the AMC. These high-pressure, medium temperature eclogite facies P-T conditions are indicative of a collision or subduction tectonic setting. Major and trace element geochemistry suggests that they probably had a common magmatic origin as part of a suite of differentiated tholeiitic intrusions. Furthermore, distinctive mineral and chemical compositions of these eclogites and garnet clinopyroxenites correspond to the Fe-Ti type of ultramafic rocks suggesting that they may have been derivatives of intraplate tholeiitic melts, introduced into continental crust before HP metamorphism.

Power law olivine crystal size distributions in lithospheric mantle xenoliths

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Armienti, P.; Tarquini, S.

2002-12-01

Olivine crystal size distributions (CSDs) have been measured in three suites of spinel- and garnet-bearing harzburgites and lherzolites found as xenoliths in alkaline basalts from Canary Islands, Africa; Victoria Land, Antarctica; and Pali Aike, South America. The xenoliths derive from lithospheric mantle, from depths ranging from 80 to 20 km. Their textures vary from coarse to porphyroclastic and mosaic-porphyroclastic up to cataclastic. Data have been collected by processing digital images acquired optically from standard petrographic thin sections. The acquisition method is based on a high-resolution colour scanner that allows image capturing of a whole thin section. Image processing was performed using the VISILOG 5.2 package, resolving crystals larger than about 150 μm and applying stereological corrections based on the Schwartz-Saltykov algorithm. Taking account of truncation effects due to resolution limits and thin section size, all samples show scale invariance of crystal size distributions over almost three orders of magnitude (0.2-25 mm). Power law relations show fractal dimensions varying between 2.4 and 3.8, a range of values observed for distributions of fragment sizes in a variety of other geological contexts. A fragmentation model can reproduce the fractal dimensions around 2.6, which correspond to well-equilibrated granoblastic textures. Fractal dimensions >3 are typical of porphyroclastic and cataclastic samples. Slight bends in some linear arrays suggest selective tectonic crushing of crystals with size larger than 1 mm. The scale invariance shown by lithospheric mantle xenoliths in a variety of tectonic settings forms distant geographic regions, which indicate that this is a common characteristic of the upper mantle and should be taken into account in rheological models and evaluation of metasomatic models.

Two types of gabbroic xenoliths from rhyolite dominated Niijima volcano, northern part of Izu-Bonin arc: petrological and geochemical constraints

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Arakawa Yoji

2017-03-01

Full Text Available We examined the petrography, petrology, and geochemistry of two types of gabbroic xenoliths (A- and B-type xenoliths in olivine basalt and biotite rhyolite units among the dominantly rhyolitic rocks in Niijima volcano, northern Izu-Bonin volcanic arc, central Japan. A-type gabbroic xenoliths consisting of plagioclase, clinopyroxene, and orthopyroxene with an adcumulate texture were found in both olivine basalt and biotite rhyolite units, and B-type gabbroic xenoliths consisting of plagioclase and amphibole with an orthocumulate texture were found only in biotite rhyolite units. Geothermal- and barometricmodelling based on mineral chemistry indicated that the A-type gabbro formed at higher temperatures (899–955°C and pressures (3.6–5.9 kbar than the B-type gabbro (687–824°C and 0.8–3.6 kbar. These findings and whole-rock chemistry suggest different parental magmas for the two types of gabbro. The A-type gabbro was likely formed from basaltic magma, whereas the B-type gabbro was likely formed from an intermediate (andesitic magma. The gabbroic xenoliths in erupted products at Niijima volcano indicate the presence of mafic to intermediate cumulate bodies of different origins at relatively shallower levels beneath the dominantly rhyolitic volcano.

Intermediate-Depth Subduction Earthquakes Recorded by Pseudotachylyte in Dry Eclogite-Facies Oceanic Lithosphere from the Alps

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Scambelluri, M.; Pennacchioni, G.; Gilio, M.; Bestmann, M.

2016-12-01

While geophysical studies and laboratory experiments provide much information on subduction earthquakes, field studies identifying the rock types for earthquake development and the deep seismogenic environments are still scarce. To date, fluid overpressure and volume decrease during hydrous mineral breakdown the widely favoured trigger of subduction earthquakes in serpentinized lithospheric mantle and hydrated low-velocity layers atop slabs. Here we document up to 40 cm-thick pseudotachylyte (PST) in Alpine oceanic gabbro and peridotite (2-2.5 GPa-550-620°C), the analogue of a modern cold subducting oceanic lithosphere. These rocks mostly remained unaltered dry systems; only very minor domains (<1%) record partial hydration and static eclogitic metamorphism. Meta-peridotite shows high-pressure olivine + antigorite (garnet + zoisite + chlorite after mantle plagioclase); meta-gabbro develops omphacite + zoisite + talc + chloritoid + garnet. Abundant syn-eclogitic pseudotachylyte cut the dry gabbro-peridotite and the eclogitized domains. In meta-peridotite, PST shows olivine, orthopyroxene, spinel microliths and clasts of high-pressure olivine + antigorite and garnet + zoisite + chlorite aggregates. In metagabbro, microfaults in damage zones near PST cut brecciated igneous pyroxene cemented by omphacite. In unaltered gabbro, glassy PST contains micron-scale garnet replacing plagioclase microliths during, or soon after, PST cooling. In the host rock, garnet coronas between igneous olivine and plagioclase only occur near PST and between closely spaced PST veins. Absence of garnet away from PST indicates that garnet growth was triggered by mineral seeds and by heat released by PST. The above evidence shows that pseudotachylyte formed at eclogite-facies conditions. In such setting, strong, dry, metastable gabbro-peridotite concentrate stress to generate large intermediate depth subduction earthquakes without much involvement of free fluid.

An eclogitic diamond from Mir pipe (Yakutia), recording two growth events from different isotopic sources

NARCIS (Netherlands)

Bulanova, G.P.; Wiggers de Vries, D.F.; Pearson, D.G.; Beard, A.; Mikhail, S.; Smelov, A.P.; Davies, G.R.

2014-01-01

An eclogitic octahedral macrodiamond from the Mir kimberlite (Yakutia) has a complex growth structure with distinctive core, intermediate and rim zones. Carbon isotope ratios change abruptly from depleted δ

Is the X-discontinuity really related to the presence of eclogite bodies in the mantle?

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Woodland, Alan; Knapp, Nadia; Klimm, Kevin

2013-04-01

A local seismic feature observed at ~300 km depth is referred to as the X-discontinuity (X-disc, e.g. Revenaugh & Jordan 1991). Several petrological explanations have been proposed for this discontinuity, but Pushcharovsky & Pushcharovsky (2012) attribute it to the formation of stishovite in eclogitic bodies, based upon the suggestion of Williams & Revenaugh (2005). If this link between the X-disc and the presence of eclogite is valid, it could have important geodynamic implications. In their model, stishovite appears in the eclogitic assemblage either through the transformation of previously existing free coesite or by exsolution of "excess" SiO2 from Ca-Eskola-bearing clinopyroxene (Ca0.50.5Si2O6). Essential to this model is if the amount of free SiO2 is enough to produce the observed seismic impedance contrast or not. To test whether exsolution of stishovite from Ca-Eskola-bearing clinopyroxene is a feasible mechanism, we have undertaken high-pressure experiments to determine the maximum Ca-Eskola component that can be incorporated in clinopyroxene over a range of P-T conditions, both shallower and deeper than that corresponding to the position of the X-disc. One series of experiments were performed in the simplified CaO-MgO-Al2O3-SiO2±Na2O system and one with 3 "natural" analog eclogite compositions (K2O-Na2O-CaO-MgO-FeO-Al2O3-SiO2). For the CMAS-experiments, all samples have the typical eclogitic assemblage of clinopyroxene + garnet ± SiO2 ± kyanite. With increasing pressure, the amount of garnet increases at the expense of clinopyroxene. Maximizing the Ca-Eskola content of clinopyroxene requires coexistence with a free SiO2 phase and an elevated Al2O3 content, but not necessarily the presence of kyanite. Ca-Eskola contents of ~20 mol % are obtained at 4 GPa, but decrease steadily with increasing pressure so that ˜ 4 mol % is present at pressures corresponding to the depth of the X-disc. Experiments in natural analog eclogite compositions produced even

Geochronology and geochemistry of deep-seated crustal xenoliths in the northern North China Craton: Implications for the evolution and structure of the lower crust

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Su, Yuping; Zheng, Jianping; Griffin, William L.; Huang, Yan; Wei, Ying; Ping, Xianquan

2017-11-01

The age and composition of the lower crust are critical in understanding the processes of continental formation and evolution, and deep-seated granulite xenoliths can offer direct information on the lower crust. Here, we report mineral chemistry, whole-rock major and trace elements, Sr-Nd isotopes and zircon U-Pb-Hf results for a suite of deep-seated crustal xenoliths, recently discovered in the Cenozoic basalts of the Nangaoya area in the northern part of the North China Craton (NCC). Based on the P-T estimates, these xenoliths including mafic, intermediate and felsic granulites and hornblendites were sampled from different levels of the lower crust. While a hornblendite has a flat REE pattern, all other xenoliths display LREE enrichment and depletion of Nb, Ta, Th and Ti. The mafic granulite xenolith has relatively high whole-rock εNd(t) value of - 13.37, and yields Mesozoic (188-59 Ma) zircons ages with high εHf(t) values from - 15.3 to - 9.2. The garnet-bearing intermediate granulite-facies rocks show low εNd(t) values from - 16.92 to - 17.48, and reveal both Paleoproterozoic (1948 Ma) and Mesozoic (222-63 Ma) zircon U-Pb ages. Their Mesozoic zircons have lower εHf(t) values (from - 18.4 to - 13.8) than those from the mafic xenolith. The remaining intermediate to felsic xenoliths show Paleoproterozoic zircon ages, and the lowest εNd(t) values (from - 20.78 to - 24.03). The mafic-intermediate granulites with Mesozoic zircons originated from the interaction of lower crust-derived magmas with mantle melts, with higher proportions of mantle magmas involved in the generation of mafic granulite, whereas intermediate to felsic xenoliths without Mesozoic zircons represent ancient Paleoproterozoic to Neoarchean deep crust. These deep-seated xenoliths reveal complicated crustal evolution processes, including crustal growth during Neoarchean (2.5-2.7 Ga), middle Paleoproterozoic (2.2-2.1 Ga) and Mesozoic, and reworking during early Paleoproterozoic, late

Eclogitic metatrondhjemites from metaophiolites of the Western Alps

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Martin, Silvana; Tartarotti, Paola; Meyzen, Chrstine; Benciolini, Luca; Toffolo, Luca

2016-04-01

Eclogitic metatrondhjemites from metaophiolites of the Western Alps Martin S.**, Tartarotti P.*, Meyzen C. **, Benciolini L.***, Toffolo L. ** *Dipartimento di Scienze della Terra, Università degli Studi di Milano ** Dipartimento di Geoscienze, Università di Padova *** Dipartimento di Chimica, Fisica e Ambiente, Università di Udine In the Urtier valley (southern Aosta Valley, Italy), the Piemonte metaophiolites mainly consist of serpentinized peridotites including pods and boudinaged layers of Fe-metagabbro and trondhjemite transposed in the main eclogitic foliation. The contact between serpentinized peridotites and Fe-metagabbro/trondhjemite is locally lined by chloriteschist and rodingite. The high pressure parageneses in the Fe-metagabbro are omphacite-garnet-rutile-glaucophane-phengite, and in the metatrondhjemite plagioclase-quartz-phengite-clinozoisite-epidote-garnet, respectively. Bulk-rock major and trace elements in addition to O isotope analyses were performed in both rock types. Fe-metagabbros are characterized by MgO wt% ranging between 6.11 and 9.63%, ∑REE= 20-101 ppm, (La/Yb)N = 0.22-0.91; trondhjemites have SiO2 43%, Al2O3 ranging between 21 and 24%, CaO ranging between 17 and 20%, ∑REE = 172 - 272 ppm, (La/Yb)N ranging between 7.78 and 13.70. The δ18O is 5.9 ‰ in a Fe-metagabbro sample and 7.4 ‰ in a trondhjemite sample, suggesting that these rocks have been affected by a weak oceanic low temperature alteration. The high CaO content may indicate a metasomatic process which could have occurred during the oceanic stage or at high pressure conditions.

Eclogitization of the Subducted Oceanic Crust and Its Implications for the Mechanism of Slow Earthquakes

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Wang, Xinyang; Zhao, Dapeng; Suzuki, Haruhiko; Li, Jiabiao; Ruan, Aiguo

2017-12-01

The generating mechanism and process of slow earthquakes can help us to better understand the seismogenic process and the petrological evolution of the subduction system, but they are still not very clear. In this work we present robust P and S wave tomography and Poisson's ratio images of the subducting Philippine Sea Plate beneath the Kii peninsula in Southwest Japan. Our results clearly reveal the spatial extent and variation of a low-velocity and high Poisson's ratio layer which is interpreted as the remnant of the subducted oceanic crust. The low-velocity layer disappears at depths >50 km, which is attributed to crustal eclogitization and consumption of fluids. The crustal eclogitization and destruction of the impermeable seal play a key role in the generation of slow earthquakes. The Moho depth of the overlying plate is an important factor affecting the depth range of slow earthquakes in warm subduction zones due to the transition of interface permeability from low to high there. The possible mechanism of the deep slow earthquakes is the dehydrated oceanic crustal rupture and shear slip at the transition zone in response to the crustal eclogitization and the temporal stress/strain field. A potential cause of the slow event gap existing beneath easternmost Shikoku and the Kii channel is the premature rupture of the subducted oceanic crust due to the large tensional force.

Lead isotopes in mantle derived xenoliths from Japan and South Africa

International Nuclear Information System (INIS)

Morioka, Masana; Kigoshi, Kunihiko.

1978-01-01

The lead isotopic compositions and the uranium, thorium and lead contents of eclogite and harzburgite inclusions from South African Kimberlite-pipes and lherzolite inclusions from Itinomegata crater, Japan and of their main mineral fractions have been determined. The observed 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 238 U/ 204 Pb in lherzolite samples from Itinomegata range from 17.9 to 18.2, from 15.1 to 15.5 and from 1.9 to 23, respectively. The results suggest that the lherzolites were of recent crystallization. The observed data on eclogite inclusions show no consistent pattern and suggest that they have a complicated history. The observed lead isotopic compositions of harzburgite inclusions from kimberlite-pipes are 206 Pb/ 204 Pb = 17.3 -- 19.0 and 207 Pb/ 204 Pb = 15.3 -- 15.6. These results are distributed in a wide range on two regression lines, suggesting that they had been formed from the same source material but with different ages or have been formed recently by mixing of two different source materials and host rock of kimberlite. (author)

Water Content of the Oceanic Lithosphere at Hawaii from FTIR Analysis of Peridotite Xenoliths

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Peslier, Anne H.; Bizmis, Michael

2013-01-01

Although water in the mantle is mostly present as trace H dissolved in minerals, it has a large influence on its melting and rheological properties. The water content of the mantle lithosphere beneath continents is better constrained by abundant mantle xenolith data than beneath oceans where it is mainly inferred from MORB glass analysis. Using Fourier transform infrared (FTIR) spectrometry, we determined the water content of olivine (Ol), clinopyroxene (Cpx) and orthopyroxene (Opx) in spinel peridotite xenoliths from Salt Lake Crater, Oahu, Hawaii, which are thought to represent fragments of the Pacific oceanic lithosphere that was refertilized by alkalic Hawaiian melts. Only Ol exhibits H diffusion profiles, evidence of limited H loss during xenolith transport to the surface. Water concentrations (Ol: 9-28 ppm H2O, Cpx: 246-566 ppm H2O, Opx: 116-224 ppm H2O) are within the range of those from continental settings but higher than those from Gakkel ridge abyssal peridotites. The Opx H2O contents are similar to those of abyssal peridotites from Atlantic ridge Leg 153 (170-230 ppm) but higher than those from Leg 209 (10- 14 ppm). The calculated bulk peridotite water contents (94 to 144 ppm H2O) are in agreement with MORB mantle source water estimates and lower than estimates for the source of Hawaiian rejuvenated volcanism (approx 540 ppm H2O) . The water content of Cpx and most Opx correlates negatively with spinel Cr#, and positively with pyroxene Al and HREE contents. This is qualitatively consistent with the partitioning of H into the melt during partial melting, but the water contents are too high for the degree of melting these peridotites experienced. Melts in equilibrium with xenolith minerals have H2O/Ce ratios similar to those of OIB

Long distance transport of eclogite and blueschist during early Pacific Ocean subduction rollback

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Tamblyn, Renee; Hand, Martin; Kelsey, David; Phillips, Glen; Anczkiewicz, Robert

2017-04-01

The Tasmanides in eastern Australia represent a period of continental crustal growth on the western margin of the Pacific Ocean associated with slab rollback from the Cambrian until the Triassic. During rollback numerical models predict that subduction products can become trapped in the forearc (Geyra et al., 2002), and can migrate with the trench as it retreats. In a long-lived subduction controlled regime such as the Tasmanides, this should result in an accumulation of subduction products with protracted geochronological and metamorphic histories. U-Pb, Lu-Hf, Sm-Nd and Ar-Ar geochronology and phase equilibria modelling of lawsonite-eclogite and garnet blueschist in the Southern New England Fold Belt in Australia demonstrate that high-P low-T rocks remained within a subduction setting for c. 40 Ma, from c. 500 to 460 Ma. High-P metamorphic rocks initially formed close to the Australian cratonic margin during the late Cambrian, and were subsequently transported over 1500 Ma oceanward, during which time subducted material continued to accumulate, resulting in the development of complex mélange which records eclogite and blueschist metamorphism and partial exhumation over 40 Ma. The duration of refrigerated metamorphism approximates the extensional evolution of the upper plate which culminated in the development of the Lachlan Fold Belt. The protracted record of eclogite and blueschist metamorphism indicates that rapid exhumation is not necessarily required for preservation of high-pressure metamorphic rocks from subduction systems. Reference: Gerya, T. V., Stockhert, B., & Perchuk, A. L. (2002). Exhumation of high-pressure metamorphic rocks in a subduction channel: A numerical simulation. Tectonics, 21(6), 6-1-6-19. doi:10.1029/2002tc001406

Petrology of an eclogite- and pyrigarnite-bearing polymetamorphic rock complex at Cabo Ortegal, NW Spain

NARCIS (Netherlands)

Vogel, D.E.

1967-01-01

At Cabo Ortegal, paragneisses are found in association with amphibolites, metagabbros, amphibolized eclogites, amphibolized (plagio) pyrigarnites, and serpentinized ultrabasic rocks. On the basis of petrographical and chemical evidence, their geological history was reconstructed as follows:

Origin of garnet and clinopyroxene in Kaapvaal low-T peridotite xenoliths

NARCIS (Netherlands)

Simon, N.S.C.; Irvine, G.J.; Davies, G.R.; Pearson, D.G.; Carlson, R.W.

2003-01-01

A detailed petrographic, major and trace element and isotope (Re-Os) study is presented on 18 xenoliths from Northern Lesotho kimberlites. The samples represent typical coarse, low-temperature garnet and spinel peridotites and span a P-T range from ∼60 to 150 km depth. With the exception of one

Experimental determination of dissolved CO2 content in nominally anhydrous andesitic melts at graphite/diamond saturation - Remobilization of deeply subducted reduced carbon via partial melts of MORB-like eclogite

Science.gov (United States)

Eguchi, J.; Dasgupta, R.

2015-12-01

Experimental phase relations of carbonated lithologies [1] and geochemistry of deep diamonds [2] suggest that deep recycling of carbon has likely been efficient for a significant portion of Earth's history. Both carbonates and organic carbon subduct into the mantle, but with gradual decrease of fO2 with depth [3] most carbon in deep mantle rocks including eclogite could be diamond/graphite [4]. Previous studies investigated the transfer of CO2 from subducted eclogite to the ambient mantle by partial melting in the presence of carbonates, i.e., by generation of carbonate-rich melts [5]. However, the transfer of carbon from subducted eclogite to the mantle can also happen, perhaps more commonly, by extraction of silicate partial melt in the presence of reduced carbon; yet, CO2 solubility in eclogite-derived andesitic melt at graphite/diamond saturation remains unconstrained. CO2content of eclogite melts is also critical as geochemistry of many ocean island basalts suggest the presence of C and eclogite in their source regions [6]. In the present study we determine CO2 concentration in a model andesitic melt [7] at graphite/diamond saturation at conditions relevant for partial melting of eclogite in the convecting upper mantle. Piston cylinder and multi anvil experiments were conducted at 1-6 GPa and 1375-1550 °C using Pt/Gr double capsules. Oxygen fugacity was monitored with Pt-Fe sensors in the starting mix. Completed experiments at 1-3 GPa show that CO2 concentration increases with increasing P, T, and fO2 up to ~0.3 wt%. Results were used to develop empirical and thermodynamic models to predict CO2 concentration in partial melts of graphite saturated eclogite. This allowed us to quantify the extent to which CO2 can mobilize from eclogitic heterogeneities at graphite/diamond saturated conditions. With estimates of eclogite contribution to erupted basaltic lavas, the models developed here allow us to put constraints on the flux of CO2 to mantle source regions

A Dynamic study of Mantle processes applying In-situ Methods to Compound Xenoliths: implications for small to intermediate scale heterogeneity

Science.gov (United States)

Baziotis, Ioannis; Asimow, Paul; Koroneos, Antonios; Ntaflos, Theodoros; Poli, Giampero

2013-04-01

The mantle is the major geochemical reservoir of most rock-forming elements in the Earth. Convection and plate-tectonic driven processes act to generate local and regional heterogeneity within the mantle, which in turn through thermal and chemical interactions modulates ongoing geophysical processes; this feedback shapes the dynamics of the deep interior. Consequently, these processes contribute to the evolution of the earth throughout its geological history. Up to now, the heterogeneity of the mantle has been extensively studied in terms of conventional methods using basalt chemistry, bulk rock and mineral major and trace element analysis of isolated xenolith specimens of varying lithology, and massif exposures. The milestone of the present study, part of an ongoing research project, is the application of in-situ analytical methods such as microprobe, LA-ICP-MS and high resolution SEM in order to provide high quality major and trace element analyses as well as elemental distribution of the coexisting phases in the preserved intra-mantle lithologies, Particularly, in the context of the current study we used selected compound xenoliths from San Carlos (Arizona, USA), Kilbourne Hole (New Mexico, USA), Cima Dome and Dish Hill suites (California, USA), San Quintin (Baja California, Mexico) and Chino Valley (Arizona, USA), from the Howard Wilshire collection archived at the Smithsonian Institution. The selection of these compound xenoliths was based upon freshness and integrity of specimens, maximum distance on both sides of lithologic contacts, and rock types thought most likely to represent subsolidus juxtaposition of different lithologies that later partially melted in contact. The San Carlos samples comprise composite xenoliths with websterite, lherzolite and clinopyroxenite layers or clinopyroxenite veins surrounded by lherzolite or orthopyroxenite-rich rims. The Kilbourne Hole suite comprises spinel-(olivine) clinopyroxenite and orthopyroxenite dikes cutting

Application of nuclear analytical techniques to trace elements in cenozoic basalt and their mantle xenoliths from Aershan area in Inner Mongolia, China

International Nuclear Information System (INIS)

Yu Fusheng; Han Song; Huang Yuying; He Wei; Cao Jie; Wang Hongyue

2005-01-01

Basaltic samples from different locations in Aershan area determined by instrumental neutron activation analysis (INAA) have the same distribution patterns of REE and trace elements. The similar REE contents of the same minerals without inclusions from different xenolith suggest that the mantle source region under different active volcanoes have the same composition. The REE content differences between the same minerals with and without melt inclusions selected from the same mantle xenolith indicate that the melt inclusions are rich in REE. The same patterns of trace elements of inclusions and host minerals from different xenolith analyzed by SRXRF suggest that the mantle fluid has no notable heterogeneity in Aershan area. (authors)

Emplacement of Xenolith Nodules in the Kaupulehu Lava Flow, Hualalai Volcano, Hawaii

Science.gov (United States)

Guest, J. E.; Spudis, P. D.; Greeley, R.; Taylor, G. J.; Baloga, S. M.

1995-01-01

The basaltic Kaupulehu 1800-1801 lava flow of Hualalai Volcano, Hawaii contains abundant ultramafic xenoliths. Many of these xenoliths occur as bedded layers of semi-rounded nodules, each thinly coated with a veneer (typically 1 mm thick) of lava. The nodule beds are analogous to cobble deposits of fluvial sedimentary systems. Although several mechanisms have been proposed for the formation of the nodule beds, it was found that, at more than one locality, the nodule beds are overbank levee deposits. The geological occurrence of the nodules, certain diagnostic aspects of the flow morphology and consideration of the inferred emplacement process indicate that the Kaupulehu flow had an exceptionally low viscosity on eruption and that the flow of the lava stream was extremely rapid, with flow velocities of at least 10 m/s (more than 40 km/h. This flow is the youngest on Hualalai Volcano and future eruptions of a similar type would pose considerable hazard to life as well as property.

Eclogite-high-pressure granulite metamorphism records early collision in West Gondwana: new data from the Southern Brasilia Belt, Brazil

DEFF Research Database (Denmark)

Reno II, Barry Len; Brown, Michael; Kobayashi, Katsura

2009-01-01

constrain the age of. (1) retrograded eclogite from a block along the tectonic contact beneath the uppermost nappe in a stack of passive margin-derived nappes; (2) high-pressure granulite-facies metamorphism in the uppermost passive margin-derived nappe; (3) high-pressure granulite-facies metamorphism...... in the overlying arc-derived nappe. Rare zircons from a retrograded eclogite yield a Pb-206/U-238 age of 678 +/- 29 Ma. which we interpret as most likely to (late close-to-peak-P metamorphism and to provide a minimum age for detachment of the overlying passive margin-derived nappe from the subducting plate. Zircon...

Pb and O isotope systematics in granulite facies xenoliths, French Massif Central: Implications for crustal processes

International Nuclear Information System (INIS)

Downes, H.; Kempton, P.D.; Harmon, R.S.; Briot, D.; Leyreloup, A.F.

1991-01-01

Pb and O isotope data are represented for a suite of granulite facies xenoliths found within Tertiary alkaline volcanic rocks of the Massif Central, France. The suite consists of ultramafic and mafic cumulates, metagabbros which are considered to represent basic liquids, felsic meta-igneous lithologies (charnockites) and metasediments. Ranges of δ 18 O values are +6.9 to +9.8per mille for mafic xenoliths, +9.3 to +10.2per mille for felsic meta-igneous samples and +6.1 to +11.8per mille for the metasediments. By comparison, δ 18 O values for mantle peridotites from the same region range from +5.1 to +6.9per mille, whilst local Hercynian granitoids vary from +8.6 to +12.0per mille. The 206 Pb/ 204 Pb ratios of the granulite xenoliths are between 17.77 and 19.19, 207 Pb/ 204 Pb ratios vary from 15.51 to 15.69, and 208 Pb/ 204 Pb ratios range from 38.07 to 40.07. In general, metasedimentary granulites have the more radiogenic Pb isotope compositions, whereas mafic meta-igneous samples are less radiogenic. These isotopic characteristics can be explained as the result of the interaction of mafic magmas with the metasedimentary crust into which they intruded. The release of heat also provoked melting of the more fusible parts of the lower crust and led to the formation of late-orogenic Hercynian granitoids. However, an additional component which provides less radiogenic Pb is also needed in the source of the granitoids; this may be the felsic meta-igneous xenoliths or middle/upper crustal gneisses. (orig.)

Geochemistry, mineralogy, and zircon U-Pb-Hf isotopes in peraluminous A-type granite xenoliths in Pliocene-Pleistocene basalts of northern Pannonian Basin (Slovakia)

Science.gov (United States)

Huraiová, Monika; Paquette, Jean-Louis; Konečný, Patrik; Gannoun, Abdel-Mouhcine; Hurai, Vratislav

2017-08-01

Anorogenic granite xenoliths occur in alkali basalts coeval with the Pliocene-Pleistocene continental rifting of the Pannonian Basin. Observed granite varieties include peraluminous, calcic to peralkalic, magnesian to ferroan types. Quartz and feldspars are dominant rock-forming minerals, accompanied by minor early ilmenite and late magnetite-ulvöspinel. Zircon and Nb-U-REE minerals (oxycalciopyrochlore, fergusonite, columbite) are locally abundant accessory phases in calc-alkalic types. Absence of OH-bearing Fe, Mg-silicates and presence of single homogeneous feldspars (plagioclase in calcic types, anorthoclase in calc-alkalic types, ferrian Na-sanidine to anorthoclase in alkalic types) indicate water-deficient, hypersolvus crystallization conditions. Variable volumes of interstitial glass, absence of exsolutions, and lacking deuteric hydrothermal alteration and/or metamorphic/metasomatic overprint are diagnostic of rapid quenching from hypersolidus temperatures. U-Pb zircon ages determined in calcic and calc-alkalic granite xenoliths correspond to a time interval between 5.7 and 5.2 Ma. Positive ɛHf values (14.2 ± 3.9) in zircons from a 5.2-Ma-old calc-alkalic granite xenolith indicate mantle-derived magmas largely unaffected by the assimilation of crustal material. This is in accordance with abundances of diagnostic trace elements (Rb, Y, Nb, Ta), indicating A1-type, OIB-like source magmas. Increased accumulations of Nb-U-REE minerals in these granites indicate higher degree of the magmatic differentiation reflected in Rb-enrichment, contrasting with Ba-enrichment in barren xenoliths. Incipient charnockitization, i.e. orthopyroxene and ilmenite crystallization from interstitial silicate melt, was observed in many granite xenoliths. Thermodynamic modeling using pseudosections showed that the orthopyroxene growth may have been triggered by water exsolution from the melt during ascent of xenoliths in basaltic magma. Euhedral-to-skeletal orthopyroxene growth

Influence of loading and heating processes on elastic and geomechanical properties of eclogites and granulites

Directory of Open Access Journals (Sweden)

Hem Bahadur Motra

2018-02-01

Full Text Available Increased knowledge of the elastic and geomechnical properties of rocks is important for numerous engineering and geoscience applications (e.g. petroleum geoscience, underground waste repositories, geothermal energy, earthquake studies, and hydrocarbon exploration. To assess the effect of pressure and temperature on seismic velocities and their anisotropy, laboratory experiments were conducted on metamorphic rocks. P- (Vp and S-wave (Vs velocities were determined on cubic samples of granulites and eclogites with an edge length of 43 mm in a triaxial multianvil apparatus using the ultrasonic pulse emission technique in dependence of changes in pressure and temperature. At successive isotropic pressure states up to 600 MPa and temperatures up to 600 °C, measurements were performed related to the sample coordinates given by the three principal fabric directions (x, y, z representing the foliation (xy-plane, the normal to the foliation (z-direction, and the lineation direction (x-direction. Progressive volumetric strain was logged by the discrete piston displacements. Cumulative errors in Vp and Vs are estimated to be <1%. Microcrack closure significantly contributes to the increase in seismic velocities and decrease in anisotropies for pressures up to 200–250 MPa. Characteristic P-wave anisotropies of about 10% are obtained for eclogite and 3–4% in a strongly retrogressed eclogite as well as granulites. The wave velocities were used to calculate the geomechanical properties (e.g. density, Poisson's ratio, volumetric strain, and elastic moduli at different pressure and temperature conditions. These results contribute to the reliable estimate of geomechanical properties of rocks.

Crystallographic preferred orientations of exhumed subduction channel rocks from the Eclogite Zone of the Tauern Window (Eastern Alps, Austria), and implications on rock elastic anisotropies at great depths

Czech Academy of Sciences Publication Activity Database

Keppler, R.; Ullemeyer, K.; Behrmann, J. H.; Stipp, M.; Kurzawski, R. M.; Lokajíček, Tomáš

647/648, April (2015), s. 89-104 ISSN 0040-1951 Institutional support: RVO:67985831 Keywords : crystallographic preferred orientation * eclogite Zone * elastic properties * P-wave anisotropy * retrogression of eclogites * subduction channel Subject RIV: DC - Siesmology, Volcanology, Earth Structure Impact factor: 2.650, year: 2015

Flow in the western Mediterranean shallow mantle: Insights from xenoliths in Pliocene alkali basalts from SE Iberia (eastern Betics, Spain)

Science.gov (United States)

Hidas, Károly; Konc, Zoltán.; Garrido, Carlos J.; Tommasi, Andréa.; Vauchez, Alain; Padrón-Navarta, José Alberto; Marchesi, Claudio; Booth-Rea, Guillermo; Acosta-Vigil, Antonio; Szabó, Csaba; Varas-Reus, María. Isabel; Gervilla, Fernando

2016-11-01

Mantle xenoliths in Pliocene alkali basalts of the eastern Betics (SE Iberia, Spain) are spinel ± plagioclase lherzolite, with minor harzburgite and wehrlite, displaying porphyroclastic or equigranular textures. Equigranular peridotites have olivine crystal preferred orientation (CPO) patterns similar to those of porphyroclastic xenoliths but slightly more dispersed. Olivine CPO shows [100]-fiber patterns characterized by strong alignment of [100]-axes subparallel to the stretching lineation and a girdle distribution of [010]-axes normal to it. This pattern is consistent with simple shear or transtensional deformation accommodated by dislocation creep. One xenolith provides evidence for synkinematic reactive percolation of subduction-related Si-rich melts/fluids that resulted in oriented crystallization of orthopyroxene. Despite a seemingly undeformed microstructure, the CPO in orthopyroxenite veins in composite xenoliths is identical to those of pyroxenes in the host peridotite, suggesting late-kinematic crystallization. Based on these observations, we propose that the annealing producing the equigranular microstructures was triggered by melt percolation in the shallow subcontinental lithospheric mantle coeval to the late Neogene formation of veins in composite xenoliths. Calculated seismic properties are characterized by fast propagation of P waves and polarization of fast S waves parallel to olivine [100]-axis (stretching lineation). These data are compatible with present-day seismic anisotropy observations in SE Iberia if the foliations in the lithospheric mantle are steeply dipping and lineations are subhorizontal with ENE strike, implying dominantly horizontal mantle flow in the ENE-WSW direction within vertical planes, that is, subparallel to the paleo-Iberian margin. The measured anisotropy could thus reflect a lithospheric fabric due to strike-slip deformation in the late Miocene in the context of WSW tearing of the subducted south Iberian margin

Compositional trends among Kaapvaal Craton garnet peridotite xenoliths and their effects on seismic velocity and density

DEFF Research Database (Denmark)

Schutt, Derek; Lesher, Charles

2010-01-01

garnet and clinopyroxene enrichment. Using the parameterization of Schutt and Lesher (2006) we show that at cratonic mantle temperatures and pressures, orthopyroxene enrichment results in little change in bulk density (ρbulk) and shear-wave velocity (VS), but decreases compressional wave velocities (VP......We examine the modes and compositions of garnet-bearing peridotite xenoliths from the Kaapvaal Craton to quantify factors governing density and seismic velocity variations within metasomatically altered cratonic mantle. Three distinct compositional trends are resolved by principal component...... analysis. The first reflects differences in residue composition resulting from partial melting. The second is associated with orthopyroxene (opx) enrichment, possibly due to silica addition by subduction zone fluids in the source region of the xenoliths. The third principal component reflects garnet...

Short wavelength lateral variability of lithospheric mantle beneath the Middle Atlas (Morocco) as recorded by mantle xenoliths

Science.gov (United States)

El Messbahi, Hicham; Bodinier, Jean-Louis; Vauchez, Alain; Dautria, Jean-Marie; Ouali, Houssa; Garrido, Carlos J.

2015-05-01

The Middle Atlas is a region where xenolith-bearing volcanism roughly coincides with the maximum of lithospheric thinning beneath continental Morocco. It is therefore a key area to study the mechanisms of lithospheric thinning and constrain the component of mantle buoyancy that is required to explain the Moroccan topography. Samples from the two main xenolith localities, the Bou Ibalghatene and Tafraoute maars, have been investigated for their mineralogy, microstructures, crystallographic preferred orientation, and whole-rock and mineral compositions. While Bou Ibalghatene belongs to the main Middle Atlas volcanic field, in the 'tabular' Middle Atlas, Tafraoute is situated about 45 km away, on the North Middle Atlas Fault that separates the 'folded' Middle Atlas, to the South-East, from the 'tabular' Middle Atlas, to the North-West. Both xenolith suites record infiltration of sub-lithospheric melts that are akin to the Middle Atlas volcanism but were differentiated to variable degrees as a result of interactions with lithospheric mantle. However, while the Bou Ibalghatene mantle was densely traversed by high melt fractions, mostly focused in melt conduits, the Tafraoute suite records heterogeneous infiltration of smaller melt fractions that migrated diffusively, by intergranular porous flow. As a consequence the lithospheric mantle beneath Bou Ibalghaten was strongly modified by melt-rock interactions in the Cenozoic whereas the Tafraoute mantle preserves the record of extensional lithospheric thinning, most likely related to Mesozoic rifting. The two xenolith suites illustrate distinct mechanisms of lithospheric thinning: extensional thinning in Tafraoute, where hydrous incongruent melting triggered by decompression probably played a key role in favouring strain localisation, vs. thermal erosion in Bou Ibalghatene, favoured and guided by a dense network of melt conduits. Our results lend support to the suggestion that lithospheric thinning beneath the Atlas

Petrological and geochemical studies of mantle xenoliths from La Palma, Canary Islands

Science.gov (United States)

Janisch, Astrid; Ntaflos, Theodoros

2015-04-01

La Palma is the second youngest island, after El Hierro, of the Canary archipelago. The archipelago consists of seven large islands, forming an east-west-trending island chain, and several seamounts. All together they form a volcanic belt of around 800 km length and 450 km width, which presumably comprises roughly the Canary hotspot. The islands are located off the western coast of Morocco, Africa. The distance ranges from 100 km to 500 km. Concurrently with the distance, subaerial volcanism age progresses from the oldest lava in the east to the youngest in the west of the archipelago. Presently, La Palma is in the shield building stage of growth (alongside with El Hierro and Tenerife) and is furthermore the fastest growing island of the Canary archipelago. Historical volcanic eruptions are restricted on the younger islands, La Palma and El Hierro, with the last eruption at the south end of La Palma in 1971. Mantle xenoliths described in this work were collected at the slopes of San Antonio Volcano, Fuencaliente, brought to the surface during the 1677/1678 eruption. The mantle xenolith collection comprises sp-lherzolites, sp-harzburgites and pyroxenites. The texture can be distinguished between coarse-grained matrix and fine-grained veins in various thicknesses, mostly with olivine and pyroxene but also with amphibole, phlogopite as well as apatite. Mineral analyses reveal the existence of primary and secondary ol, cpx and opx. Primary ol has Fo contents of 89.2 to 91.7 and NiO ranging from 0.3 to 0.45 wt.%, whereas secondary ol show Fo values of 78.4 to 91.9 but with NiO below 0.3 wt.%. Primary cpx are predominantly Cr-Diopsides with En48.7-51.9-Wo43.5-44.3-Fs4.1-4.9 and Mg# of 91.5 to 92.4. Secondary cpx, primarily Ti-Augit, display En36.7-44.4-Wo47.7-49.6-Fs6.7-13.0 and Mg# of 75.3 to 90.8. Primary opx compositions are in range of En89.3-90.6-Wo1.3-1.5-Fs8.1-9.3 with Mg# between 90.7 and 92.0. Secondary opx exhibit En88.7-89.2-Wo1.7-1.9-Fs9.1-9.5 and Mg# of 90

Geochemistry and petrology of pyroxenite xenoliths from Cenozoic alkaline basalts, Bohemian Massif

Czech Academy of Sciences Publication Activity Database

Ackerman, Lukáš; Špaček, Petr; Medaris Jr., G.; Hegner, E.; Svojtka, Martin; Ulrych, Jaromír

2012-01-01

Roč. 57, č. 4 (2012), s. 199-219 ISSN 1802-6222 R&D Projects: GA ČR(CZ) GA205/09/1170 Institutional research plan: CEZ:AV0Z30130516; CEZ:AV0Z30120515 Institutional support: RVO:67985831 ; RVO:67985530 Keywords : pyroxenite * xenolith * Cenozoic * basalt * Sr-Nd isotopes * geothermobarometry Subject RIV: DD - Geochemistry Impact factor: 0.804, year: 2012

Facilitation of intermediate-depth earthquakes by eclogitization-related stresses and H2O

Science.gov (United States)

Nakajima, J.; Uchida, N.; Hasegawa, A.; Shiina, T.; Hacker, B. R.; Kirby, S. H.

2012-12-01

Generation of intermediate-depth earthquakes is an ongoing enigma because high lithostatic pressures render ordinary dry frictional failure unlikely. A popular hypothesis to solve this conundrum is fluid-related embrittlement (e.g., Kirby et al., 1996; Preston et al., 2003), which is known to work even for dehydration reactions with negative volume change (Jung et al., 2004). One consequence of reaction with the negative volume change is the formation of a paired stress field as a result of strain compatibility across the reaction front (Hacker, 1996; Kirby et al., 1996). Here we analyze waveforms of a tiny seismic cluster in the lower crust of the downgoing Pacific plate at a depth of 155 km and propose new evidence in favor of this mechanism: tensional earthquakes lying 1 km above compressional earthquakes, and earthquakes with highly similar waveforms lying on well-defined planes with complementary rupture areas. The tensional stress is interpreted to be caused by the dimensional mismatch between crust transformed to eclogite and underlying untransformed crust, and the earthquakes are interpreted to be facilitated by fluid produced by eclogitization. These observations provide seismic evidence for the dual roles of volume-change related stresses and fluid-related embrittlement as viable processes for nucleating earthquakes in downgoing oceanic lithosphere.

Lithospheric strength in the active boundary between the Pacific Plate and Baja California microplate constrained from lower crustal and upper mantle xenoliths

Science.gov (United States)

Chatzaras, Vasileios; van der Werf, Thomas; Kriegsman, Leo M.; Kronenberg, Andreas; Tikoff, Basil; Drury, Martyn R.

2017-04-01

The lower crust is the most poorly understood of the lithospheric layers in terms of its rheology, particularly at active plate boundaries. We studied naturally deformed lower crustal xenoliths within an active plate boundary, in order to link their microstructures and rheological parameters to the well-defined active tectonic context. The Baja California shear zone (BCSZ), located at the western boundary of the Baja California microplate, comprises the active boundary accommodating the relative motion between the Pacific plate and Baja California microplate. The basalts of the Holocene San Quintin volcanic field carry lower crustal and upper mantle xenoliths, which sample the Baja California microplate lithosphere in the vicinity of the BCSZ. The lower crustal xenoliths range from undeformed gabbros to granoblastic two-pyroxene granulites. Two-pyroxene geothermometry shows that the granulites equilibrated at temperatures of 690-920 oC. Phase equilibria (P-T pseudosections using Perple_X) indicate that symplectites with intergrown pyroxenes, plagioclase, olivine and spinel formed at 3.6-5.4 kbar, following decompression from pressures exceeding 6 kbar. FTIR spectroscopy shows that the water content of plagioclase varies among the analyzed xenoliths; plagioclase is relatively dry in two xenoliths while one xenolith contains hydrated plagioclase grains. Microstructural observations and analysis of the crystallographic texture provide evidence for deformation of plagioclase by a combination of dislocation creep and grain boundary sliding. To constrain the strength of the lower crust and upper mantle near the BCSZ we estimated the differential stress using plagioclase and olivine grain size paleopiezomtery, respectively. Differential stress estimates for plagioclase range from 10 to 32 MPa and for olivine are 30 MPa. Thus the active microplate boundary records elevated crustal temperatures, heterogeneous levels of hydration, and low strength in both the lower crust and

The Lu-Hf isotope composition of cratonic lithosphere: disequilibrium between garnet and clinopyroxene in kimberlite xenoliths

NARCIS (Netherlands)

Simon, N.S.C.; Carlson, R.W.; Pearson, D.G.; Davies, G.R.

2002-01-01

12th Annual V.M. Goldschmidt Conference Davos Switzerland, The Lu-Hf isotope composition of cratonic lithosphere: disequilibrium between garnet and clinopyroxene in kimberlite xenoliths (DTM, Carnegie Institution of Washington), Pearson, D.G. (University of Durham)

Intracratonic asthenosphere upwelling and lithosphere rejuvenation beneath the Hoggar swell (Algeria): Evidence from HIMU metasomatised lherzolite mantle xenoliths

Science.gov (United States)

Beccaluva, L.; Azzouni-Sekkal, A.; Benhallou, A.; Bianchini, G.; Ellam, R. M.; Marzola, M.; Siena, F.; Stuart, F. M.

2007-08-01

The mantle xenoliths included in Quaternary alkaline volcanics from the Manzaz-district (Central Hoggar) are proto-granular, anhydrous spinel lherzolites. Major and trace element analyses on bulk rocks and constituent mineral phases show that the primary compositions are widely overprinted by metasomatic processes. Trace element modelling of the metasomatised clinopyroxenes allows the inference that the metasomatic agents that enriched the lithospheric mantle were highly alkaline carbonate-rich melts such as nephelinites/melilitites (or as extreme silico-carbonatites). These metasomatic agents were characterized by a clear HIMU Sr-Nd-Pb isotopic signature, whereas there is no evidence of EM1 components recorded by the Hoggar Oligocene tholeiitic basalts. This can be interpreted as being due to replacement of the older cratonic lithospheric mantle, from which tholeiites generated, by asthenospheric upwelling dominated by the presence of an HIMU signature. Accordingly, this rejuvenated lithosphere (accreted asthenosphere without any EM influence), may represent an appropriate mantle section from which deep alkaline basic melts could have been generated and shallower mantle xenoliths sampled, respectively. The available data on lherzolite xenoliths and alkaline lavas (including He isotopes, Ra Pan-African basement. This can be considered a far-field foreland reaction of the Africa-Europe collisional system since the Eocene.

Elemental and isotopic (C, O, Sr, Nd) compositions of Late Paleozoic carbonated eclogite and marble from the SW Tianshan UHP belt, NW China: Implications for deep carbon cycle

Science.gov (United States)

Zhu, Jianjiang; Zhang, Lifei; Lü, Zeng; Bader, Thomas

2018-03-01

Subduction zones are important for understanding of the global carbon cycle from the surface to deep part of the mantle. The processes involved the metamorphism of carbonate-bearing rocks largely control the fate of carbon and contribute to local carbon isotopic heterogeneities of the mantle. In this study, we present petrological and geochemical results for marbles and carbonated eclogites in the Southwestern Tianshan UHP belt, NW China. Marbles are interlayered with coesite-bearing pelitic schists, and have Sr-Nd isotopic values (εNd (T=320Ma) = -3.7 to -8.9, 87Sr/86Sr (i) = 0.7084-0.7089), typical of marine carbonates. The marbles have dispersed low δ18OVSMOW values (ranging from 14 to 29‰) and unaffected carbon isotope (δ13CVPDB = -0.2-3.6‰), possibly due to infiltration of external H2O-rich fluids. Recycling of these marbles into mantle may play a key role in the carbon budget and contributed to the mantle carbon isotope heterogeneity. The carbonated eclogites have high Sr isotopic compositions (87Sr/86Sr (i) = 0.7077-0.7082) and positive εNd (T = 320 Ma) values (from 7.6 to 8.2), indicative of strong seafloor alteration of their protolith. The carbonates in the carbonated eclogites are mainly dolomite (Fe# = 12-43, Fe# = Fe2+/(Fe2+ + Mg)) that were added into oceanic basalts during seafloor alteration and experienced calcite - dolomite - magnesite transformation during the subduction metamorphic process. The uniformly low δ18O values (∼11.44‰) of carbonates in the carbontaed eclogites can be explained by closed-system equilibrium between carbonate and silicate minerals. The low δ13C values (from -3.3 to -7.7‰) of the carbonated eclogites most likely reflect contribution from organic carbon. Recycling of these carbonated eclogites with C isotope similar to typical mantle reservoirs into mantle may have little effect on the mantle carbon isotope heterogeneity.

Geological characters and petrological characters of metamorphosed medium-acidic intrusive complexes in Ludong Orogenic Belt,China

Institute of Scientific and Technical Information of China (English)

凌贤长; 胡庆立; 王丽霞

2002-01-01

Ludong orogenic belt in China is an importantal continent collision orogenic belt in eastern Asia, between Sino-Korean landmass and Yangtze landmass. The host rock of the orogenic belt is metamorphosed medium-acidic intrusive complexes, which can be divided into four types, that's, quartz dioritz, granite dioritz, monzonitic granite and undertint monzonitic granite, principal minerals are plagioclases, potassium feldspars and quartzs, minor minerals are hornblendes, biotites, clinopyxenes and garnets, accessory mineral types and assemblages are very similar, specially, various rocks are mainly fine-grained textures. They have the history of regional amphibolite facies metamorphism and deep-middle-shallow structural layer deformation, and are changed into various gneiss and tectonic system. There are many xenolithes of middle Proterozoic eclogite-host rock extrahigh-high pressure metamorphic complexes, a small xenolithes of early Proterozoic layered metamorphite system and granulites, and ultrabasic-basic rocks of various epoches in the metamorphosed medium-acidic intrusive complexes.

H Diffusion in Olivine and Pyroxene from Peridotite Xenoliths and a Hawaiian Magma Speedometer

Science.gov (United States)

Peslier, A. H.; Bizimis, M.

2014-01-01

Hydrogen is present as a trace element in olivine and pyroxene and its content distribution in the mantle results from melting and metasomatic processes. Here we examine how these H contents can be disturbed during decompression. Hydrogen was analyzed by FTIR in olivine and pyroxene of spinel peridotite xenoliths from Salt Lake Crater (SLC) nephelinites which are part of the rejuvenated volcanism at Oahu (Hawaii) [1,2]. H mobility in pyroxene resulting from spinel exsolution during mantle upwelling Most pyroxenes in SLC peridotites exhibit exsolutions, characterized by spinel inclusions. Pyroxene edges where no exsolution are present have less H then their core near the spinel. Given that H does not enter spinel [3], subsolidus requilibration may have concentrated H in the pyroxene adjacent to the spinel exsolution during mantle upwelling. H diffusion in olivine during xenolith transport by its host magma and host magma ascent rates Olivines have lower water contents at the edge and near fractures compared to at their core, while the concentrations of all other chemical elements appear homogeneous. This suggests that some of the initial water has diffused out of the olivine. Water loss from the olivine is thought to occur during host-magma ascent and xenolith transport to the surface [4-6]. Diffusion modeling matches best the data when the initial water content used is that measured at the core of the olivines, implying that mantle water contents are preserved at the core of the olivines. The 3225 cm(sup -1) OH band at times varies independantly of other OH bands, suggesting uneven H distribution in olivine defects likely acquired during mantle metasomatism just prior to eruption and unequilibrated. Diffusion times (1-48 hrs) combined with depths of peridotite equilibration or of magma start of degassing allow to calculate ascent rates for the host nephelinite of 0.1 to 27 m/s.

Three steps of serpentinization in an eclogitized oceanic serpentinization front (Lanzo Massif - Western Alps)

OpenAIRE

DEBRET, Baptiste; NICOLLET, christian; ANDREANI, Muriel; SCHWARTZ, Stéphane; GODARD, Marguerite

2013-01-01

The Lanzo peridotite massif is a fragment of oceanic lithosphere generated in an ocean-continent transition context and eclogitized during alpine collision. Despite the subduction history, the massif has preserved its sedimentary oceanic cover, suggesting that it may have preserved its oceanic structure. It is an exceptional case for studying the evolution of a fragment of the lithosphere from its oceanization to its subduction and then exhumation. We present a field and petrological study re...

UHP metamorphism recorded by kyanite-bearing eclogite in the Seve Nappe Complex of northern Jämtland, Swedish Caledonides

NARCIS (Netherlands)

Janák, M.; Van Roermund, H.; Majka, J.; Gee, D.

The first evidence for ultrahigh-pressure (UHP) metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides is recorded by kyanite-bearing eclogite, found in a basic dyke within a garnet peridotite body exposed close to the lake Friningen in northern Jämtland (central Sweden). UHP

Textures in spinel peridotite mantle xenoliths using micro-CT scanning: Examples from Canary Islands and France

Science.gov (United States)

Bhanot, K. K.; Downes, H.; Petrone, C. M.; Humphreys-Williams, E.

2017-04-01

Spinel pyroxene-clusters, which are intergrowths of spinel, orthopyroxene and clinopyroxene in mantle xenoliths, have been investigated through the use of micro-CT (μ-CT) in this study. Samples have been studied from two different tectonic settings: (1) the northern Massif Central, France, an uplifted and rifted plateau on continental lithosphere and (2) Lanzarote in the Canary Islands, an intraplate volcanic island on old oceanic lithosphere. μ-CT analysis of samples from both locations has revealed a range of spinel textures from small Lanzarote are regions that have experienced significant lithospheric thinning. This process provides a mechanism where the sub-solidus reaction of olivine + garnet = orthopyroxene + clinopyroxene + spinel is satisfied by providing a pathway from garnet peridotite to spinel peridotite. We predict that such textures would only occur in the mantle beneath regions that show evidence of thinning of the lithospheric mantle. Metasomatic reactions are seen around spinel-pyroxene clusters in some Lanzarote xenoliths, so metasomatism post-dated cluster formation.

Subducted slab-plume interaction traced by magnesium isotopes in the northern margin of the Tarim Large Igneous Province

Science.gov (United States)

Cheng, Zhiguo; Zhang, Zhaochong; Xie, Qiuhong; Hou, Tong; Ke, Shan

2018-05-01

Incorporation of subducted slabs may account for the geochemical and isotopic variations of large igneous provinces (LIPs). However, the mechanism and process by which subducted slabs are involved into magmas is still highly debated. Here, we report a set of high resolution Mg isotopes for a suite of alkaline and Fe-rich rocks (including basalts, mafic-ultramafic layered intrusions, diabase dykes and mantle xenoliths in the kimberlitic rocks) from Tarim Large Igneous Province (TLIP). We observed that δ26 Mg values of basalts range from -0.29 to - 0.45 ‰, -0.31 to - 0.42 ‰ for mafic-ultramafic layered intrusions, -0.28 to - 0.31 ‰ for diabase dykes and -0.29 to - 0.44 ‰ for pyroxenite xenoliths from the kimberlitic rocks, typically lighter than the normal mantle source (- 0.25 ‰ ± 0.04, 2 SD). After carefully precluding other possibilities, we propose that the light Mg isotopic compositions and high FeO contents should be ascribed to the involvement of recycled sedimentary carbonate rocks and pyroxenite/eclogite. Moreover, from basalts, through layered intrusions to diabase dykes, (87Sr/86Sr)i values and δ18OV-SMOW declined, whereas ε (Nd) t and δ26 Mg values increased with progressive partial melting of mantle, indicating that components of carbonate rock and pyroxenite/eclogite in the mantle sources were waning over time. In combination with the previous reported Mg isotopes for carbonatite, nephelinite and kimberlitic rocks in TLIP, two distinct mantle domains are recognized for this province: 1) a lithospheric mantle source for basalts and mafic-ultramafic layered intrusions which were modified by calcite/dolomite and eclogite-derived high-Si melts, as evidenced by enriched Sr-Nd-O and light Mg isotopic compositions; 2) a plume source for carbonatite, nephelinite and kimberlitic rocks which were related to magnesite or periclase/perovskite involvement as reflected by depleted Sr-Nd-O and extremely light Mg isotopes. Ultimately, our study suggests

Preliminary data on mantle xenoliths from the Feldstein basalt (Thuringia, Germany)

Science.gov (United States)

Kukuła, Anna; Puziewicz, Jacek; Ntaflos, Theodoros; Matusiak-Małek, Magdalena; Milke, Ralf

2014-05-01

Feldstein is an isolated outcrop of columnar basaltic rock nearby Themar, located 60 km south-west of Erfurt (Thuringia, Germany). The Feldstein alkali basalt (ca. 16.3 Ma) belongs to the Heldburger Gangschar subset of the Central European Volcanic Province (Abratis et al. 2007). The Feldstein alkali basalt contains peridotitic xenoliths, which were the subject of our study. Two groups of spinel peridotite xenoliths occur in the Feldstein basalt. Group A spinel peridotite (2 xenoliths) is characterized by protogranular texture with typical grain size of 2 - 3 mm (max 8 mm). It consists of olivine (90.28 - 91.36 % Fo, 0.35 - 0.45 wt. % NiO), orthopyroxene (mg# 0.91 - 0.92, Al 0.09 - 0.18 a pfu), clinopyroxene (mg# 0.93 - 0.95, Al 0.06 - 0.21 a pfu) and spinel (cr# 0.20 - 0.41, mg# 0.66 - 0.78). The mg# and Al content in clinopyroxene are negatively correlated following the depletion trend after variable degrees of partial melting of the same source. One of the studied samples contains clinopyroxene that does not plot on the general depletion trend but has significantly higher Al (0.15 - 0.21 a pfu) for similar mg # 0.93 - 0.94 with clinopyroxenes from this trend. However the primitive mantle normalized clinopyroxene REE patterns (concave upwards with LaN/YbN=0.11) indicate that they are the residues after elevated degrees of partial melting. The most magnesian clinopyroxene that is Ca-rich and Al-poor has REE abundances, typical for strongly depleted spinel peridotites. It has concave upwards primitive mantle normalized pattern and LaN/YbN=0.61. A slight increase of LaN and CeN with inflection point at PrN has been observed as well. The group B spinel peridotites have protogranular texture (3-4 mm, max 7 mm grains) and some of them contain several melt pockets of basaltic composition. It consists of olivine (88.95 - 91.32 % Fo, 0.34 - 0.47 wt.% NiO), orthopyroxene (mg# 0.90 - 0.93, Al 0.04 - 0.16 apfu) and clinopyroxene (mg# 0.90 - 0.93, Al 0.10 - 0.20 a pfu). The

Compositional trends among Kaapvaal Craton garnet peridotite xenoliths and their effects on seismic velocity and density

DEFF Research Database (Denmark)

Schutt, Derek; Lesher, Charles

2010-01-01

and clinopyroxene enrichment possibly as a consequence of melt infiltration. More than half of the mineral mode variance among Kaapvaal Craton xenoliths can be accounted for by opx enrichment. Melt depletion effects can account for as much as 30% of the variance, while less than 20% of the variance is associated...

Formation and temporal evolution of the Kalahari sub-cratonic lithospheric mantle: Constraints from Venetia xenoliths, South Africa

NARCIS (Netherlands)

Hin, R.C.; Morel, M.L.A.; Nebel, O.; Mason, P.R.D.; van Westrenen, W.; Davies, G.R.

2009-01-01

The ~533 Ma Venetia Diamond Mine is located between the Kaapvaal and Zimbabwe Cratons and the study of selected xenoliths provides the opportunity to investigate the temporal evolution of the sub-continental lithospheric mantle (SCLM) underneath southern Africa, as well as the extent and potentially

Combined Lu-Hf and Sm-Nd geochronology of the Mariánské Lázně Complex: New constraints on the timing of eclogite- and granulite-facies metamorphism

Science.gov (United States)

Collett, Stephen; Štípská, Pavla; Schulmann, Karel; Peřestý, Vít; Soldner, Jeremie; Anczkiewicz, Robert; Lexa, Ondrej; Kylander-Clark, Andrew

2018-04-01

Lu-Hf and Sm-Nd garnet-whole rock geochronology combined with petrographic observations, minero-chemical variations, thermodynamic modelling and structural data was used to constrain the P-T-t-d evolution of eclogites from the Mariánské Lázně Complex (Bohemian Massif). Boudins of mostly isotropic eclogite with relict steep eclogite-facies fabric are affected by steep migmatitic foliation, which is followed on a regional scale by the development of almost pervasive, predominantly SE-dipping, extensional foliation. The structural succession shows continuous transition from eclogite to garnetiferous migmatitic amphibolite and to amphibolite migmatite. A least retrogressed sample of eclogite shows clusters of fine-grained inclusion-poor garnet, omphacite relicts surrounded by a fine-grained clinopyroxene-plagioclase symplectite with minor amphibole, biotite-plagioclase intergrowths after white mica, kyanite with plagioclase-spinel coronas and accessory rutile. Rare potassic white mica occurs as inclusions in omphacite. A more retrogressed eclogite, with no omphacite or kyanite relicts, contains inclusion-poor garnet surrounded by amphibole-plagioclase corona in a matrix dominated by plagioclase-amphibole symplectite with minor clinopyroxene. In places, the symplectite is overgrown by coarse-grained amphibole. Peak P-T conditions, inferred from combined conventional thermobarometry and phase-equilibria modelling and based on inclusions of white mica (up to 3.33 Si p.f.u.), matrix omphacite (Jd33-36) and garnet core (Alm33-38Prp38-42Grs22-25Sps1) compositions are 25 kbar at 650-750 °C. A HT overprint occurred at 14-18 kbar and >800 °C based on coexisting clinopyroxene (Jd18-24), plagioclase (An18-35), and amphibole (Na(B) modelling of garnet mantle compositions. Lu-Hf and Sm-Nd garnet geochronology has been applied to both samples, an older age (c. 390 Ma) obtained by the Lu-Hf method is interpreted as the timing of HP metamorphism, while c. 15 Ma younger ages were

Isotope geochemistry of xenoliths from East Africa. Implications for development of mantle reservoirs and their interaction

Energy Technology Data Exchange (ETDEWEB)

Cohen, R S; O' Nions, R K [Cambridge Univ. (UK). Dept. of Earth Sciences; Dawson, J B [Sheffield Univ. (UK). Dept. of Geology

1984-05-01

Pd, Nd and Sr isotope analyses together with U, Pb, Sm, Nd, Rb and Sr concentrations have been obtained for separated phases of lherzolite and bulk rock mafic granulite xenoliths in Recent volcanics from Tanzania. A garnet lherzolite from the Lashaine vent has yielded the least radiogenic Pb (/sup 206/Pb//sup 204/PB=15.55) and Nd (/sup 143/Nd//sup 144/Nd=0.51127; epsilon/sup 0/sub(Nd)=-26.7) isotope compositions recorded so far for an ultramafic xenolith, and /sup 87/Sr//sup 86/Sr=0.83604. The Pb isotope compositions of the mafic granulites are variable 15.77xenoliths do not conform with those of MORB, particularly in terms of their Pb-Sr, and Nd-Pb relationship. In this regard they are similar to some ocean islands and could be a viable source material for some ocean island basalts at least. The mantle part of the continental lithosphere is as likely to contain recycled components derived from the continental crust as are other regions of mantle. If the mantle part of continental lithosphere is invoked as a source for ocean islands, it does not negate the possibility that substantial recycled components are involved.

The helium flux from the continents and ubiquity of low-3He/4He recycled crust and lithosphere

Science.gov (United States)

Day, James M. D.; Barry, Peter H.; Hilton, David R.; Burgess, Ray; Pearson, D. Graham; Taylor, Lawrence A.

2015-03-01

New helium isotope and trace-element abundance data are reported for pyroxenites and eclogites from South Africa, Siberia, and the Beni Bousera Massif, Morocco that are widely interpreted to form from recycled oceanic crustal protoliths. The first He isotope data are also presented for Archaean peridotites from the Kaapvaal (South Africa), Slave (Canada), and Siberian cratons, along with recently emplaced off-craton peridotite xenoliths from Kilbourne Hole, San Carlos (USA) and Vitim (Siberia), to complement existing 3He/4He values obtained for continental and oceanic peridotites. Helium isotope compositions of peridotite xenoliths vary from 7.3 to 9.6 RA in recently (volcanics that contain a contribution from asthenospheric sources. Using the new He isotope data for cratonic peridotites and assuming that significant portions (>50%) of the Archaean and Proterozoic continental lithospheric mantle are stable and unaffected by melt or fluid infiltration on geological timescales (>0.1 Ga), and that U and Th contents vary between cratonic lithosphere and non-cratonic lithosphere, calculations yield a 3He flux of 0.25-2.2 atoms/s/cm2 for the continental lithospheric mantle. These estimates differ by a factor of ten from non-cratonic lithospheric mantle and are closer to the observed 3He flux from the continents (<1 atoms/s/cm2). Pyroxenites and eclogites from the continental regions are all characterized by 3He/4He (0.03-5.6 RA) less than the depleted upper mantle, and relatively high U and Th contents. Together with oceanic and continental lithospheric peridotites, these materials represent reservoirs with low time-integrated 3He/(U + Th) in the mantle. Pyroxenites and eclogites are also characterized by higher Fe/Mg, more radiogenic Os-Pb isotope compositions, and more variable δ18O values (∼3‰ to 7‰), compared with peridotitic mantle. These xenoliths are widely interpreted to be the metamorphic/metasomatic equivalents of recycled oceanic crustal protoliths. The

Boron Isotopic Composition of Metasomatized Mantle Xenoliths from the Western Rift, East Africa

Science.gov (United States)

Hudgins, T.; Nelson, W. R.

2017-12-01

The Western Branch of the East African Rift System is known to have a thick lithosphere and sparse, alkaline volcanism associated with a metasomatized mantle source. Recent work investigating the relationship between Western Branch metasomatized mantle xenoliths and associated lavas has suggested that these metasomes are a significant factor in the evolution of the rift. Hydrous/carbonated fluids or silicate melts are potent metasomatic agents, however gaining insight into the source of a metasomatic agent proves challenging. Here we investigate the potential metasomatic fluid sources using B isotope analysis of mineral separates from Western Branch xenoliths. Preliminary SIMS analyses of phlogopite from Katwe Kikorongo and Bufumbira have and average B isotopic composition of -28.2‰ ± 5.1 and -16.4‰ ± 3.6, respectively. These values are are dissimilar to MORB (-7.5‰ ± 0.7; Marschall and Monteleone, 2015), primitive mantle (-10‰ ± 2; Chaussidon and Marty, 1995), and bulk continental crust (-9.1‰ ± 2.4; Marschall et al., 2017) and display significant heterogeneity across a relatively short ( 150km) portion of the Western Branch. Though displaying large variability, these B isotopic compositions are indicative of a metasomatic agent with a more negative B isotopic composition than MORB, PM, or BCC. These results are consistent with fluids that released from a subducting slab and may be related to 700 Ma Pan-African subduction.

Nanodiamond finding in the hyblean shallow mantle xenoliths.

Science.gov (United States)

Simakov, S K; Kouchi, A; Mel'nik, N N; Scribano, V; Kimura, Y; Hama, T; Suzuki, N; Saito, H; Yoshizawa, T

2015-06-01

Most of Earth's diamonds are connected with deep-seated mantle rocks; however, in recent years, μm-sized diamonds have been found in shallower metamorphic rocks, and the process of shallow-seated diamond formation has become a hotly debated topic. Nanodiamonds occur mainly in chondrite meteorites associated with organic matter and water. They can be synthesized in the stability field of graphite from organic compounds under hydrothermal conditions. Similar physicochemical conditions occur in serpentinite-hosted hydrothermal systems. Herein, we report the first finding of nanodiamonds, primarily of 6 and 10 nm, in Hyblean asphaltene-bearing serpentinite xenoliths (Sicily, Italy). The discovery was made by electron microscopy observations coupled with Raman spectroscopy analyses. The finding reveals new aspects of carbon speciation and diamond formation in shallow crustal settings. Nanodiamonds can grow during the hydrothermal alteration of ultramafic rocks, as well as during the lithogenesis of sediments bearing organic matter.

Depleted subcontinental lithospheric mantle and its tholeiitic melt metasomatism beneath NE termination of the Eger Rift (Europe): the case study of the Steinberg (Upper Lusatia, SE Germany) xenoliths

Science.gov (United States)

Kukuła, Anna; Puziewicz, Jacek; Matusiak-Małek, Magdalena; Ntaflos, Theodoros; Büchner, Jörg; Tietz, Olaf

2015-12-01

The ca. 30 Ma Steinberg basanite occurs at the NE termination of the Eger (Ohře) Rift in the NW Bohemian Massif, Central Europe, and belongs to the Cenozoic alkaline Central European Volcanic Province. The basanite hosts a suite of mantle xenoliths, most of which are harzburgites containing relatively magnesian olivine (Fo 90.5-91.6) and Al-poor (0.04-0.13 a pfu) orthopyroxene (mg# 0.90-0.92). Some of these harzburgites also contain volumetrically minor clinopyroxene (mg# 0.92-0.95, Al 0.03-0.13 a pfu) and have U-shaped LREE-enriched REE patterns. The Steinberg harzburgites are typical for the Lower Silesian - Upper Lusatian domain of the European subcontinental lithospheric mantle. They represent residual mantle that has undergone extensive partial melting and was subsequently affected by mantle metasomatism by mixed carbonatite-silicate melts. The Steinberg xenolith suite comprises also dunitic xenoliths affected by metasomatism by melt similar to the host basanite, which lowered the Fo content in olivine to 87.6 %. This metasomatism happened shortly before xenolith entrainment in the erupting lava. One of the xenoliths is a wehrlite (olivine Fo 73 %, clinopyroxene mg# 0.83-0.85, subordinate orthopyroxene mg# 0.76-0.77). Its clinopyroxene REE pattern is flat and slightly LREE-depleted. This wehrlite is considered to be a tholeiitic cumulate. One of the studied harzburgites contains clinopyroxene with similar trace element contents to those in wehrlite. This type of clinopyroxene records percolation of tholeiitic melt through harzburgite. The tholeiitic melt might be similar to Cenozoic continental tholeiites occurring in the Central European Volcanic Province (e.g., Vogelsberg, Germany).

The Paleozoic metamorphic history of the Central Orogenic Belt of China from 40Ar/39Ar geochronology of eclogite garnet fluid inclusions

NARCIS (Netherlands)

Qiu, H.N.; Wijbrans, J.R.

2008-01-01

The pressure-temperature-time evolution of the UHP eclogites of Dabie-Sulu, in the eastern sector of the Central Orogenic Belt of China shows a complex pattern of predominantly Triassic, and to a lesser extent Early Paleozoic ages.

Trace element distribution in peridotite xenoliths from Tok, SE Siberian craton: A record of pervasive, multi-stage metasomatism in shallow refractory mantle

Science.gov (United States)

Ionov, Dmitri A.; Chazot, Gilles; Chauvel, Catherine; Merlet, Claude; Bodinier, Jean-Louis

2006-03-01

Spinel peridotite xenoliths in alkali basalts at Tok, SE Siberian craton range from fertile lherzolites to harzburgites and wehrlites; olivine-rich (70-84%) rocks are dominant. REE patterns in the lherzolites range from nearly flat for fertile rocks (14-17% cpx) to LREE-enriched; the enrichments are positively correlated with modal olivine, consistent with high-permeability of olivine-rich rocks during melt percolation. Clinopyroxene in olivine-rich Tok peridotites typically has convex-upward trace element patterns (La/Nd PM 1); the LREE-enrichments are positively correlated with phosphorus abundances and are mainly hosted by accessory phosphates and P-rich cryptocrystalline materials. In addition to apatite, some Tok xenoliths contain whitlockite (an anhydrous, halogen-poor and Na-Mg-rich phosphate), which is common in meteorites and lunar rocks, but has not been reported from any terrestrial mantle samples. Some olivine-rich peridotites have generations of clinopyroxene with distinct abundances of Na, LREE, Sr and Zr. The mineralogical and trace element data indicate that the lithospheric mantle section represented by the xenoliths experienced a large-scale metasomatic event produced by upward migration of mafic silicate melts followed by percolation of low- T, alkali-rich melts and fluids. Chromatographic fractionation and fractional crystallisation of the melts close to the percolation front produced strong LREE-enrichments, which are most common in the uppermost mantle and are related to carbonate- and P 2O 5-rich derivatives of the initial melt. Reversal and gradual retreat of the percolation front during thermal relaxation to ambient geotherm ("retrograde" metasomatism) caused local migration and entrapment of small-volume residual fluids and precipitation of volatile-rich accessory minerals. A distinct metasomatic episode, which mainly produced "anhydrous" late-stage interstitial materials was concomitant with the alkali basaltic magmatism, which brought

Lithospheric mantle evolution in the Afro-Arabian domain: Insights from Bir Ali mantle xenoliths (Yemen)

Science.gov (United States)

Sgualdo, P.; Aviado, K.; Beccaluva, L.; Bianchini, G.; Blichert-Toft, J.; Bryce, J. G.; Graham, D. W.; Natali, C.; Siena, F.

2015-05-01

Detailed petrological and geochemical investigations of an extensive sampling of mantle xenoliths from the Neogene-Quaternary Bir Ali diatreme (southern Yemen) indicate that the underlying lithospheric mantle consists predominantly of medium- to fine-grained (often foliated) spinel-peridotites (85-90%) and spinel-pyroxenites (10-15%) showing thermobarometric estimates in the P-T range of 0.9-2.0 GPa and 900-1150 °C. Peridotites, including lherzolites, harzburgites and dunites delineate continuous chemical, modal and mineralogical variations compatible with large extractions of basic melts occurring since the late Proterozoic (~ 2 Ga, according to Lu-Hf model ages). Pyroxenites may represent intrusions of subalkaline basic melts interacting and equilibrated with the host peridotite. Subsequent metasomatism has led to modal changes, with evidence of reaction patches and clinopyroxene and spinel destabilization, as well as formation of new phases (glass, amphibole and feldspar). These changes are accompanied by enrichment of the most incompatible elements and isotopic compositions. 143Nd/144Nd ranges from 0.51419 to 0.51209 (εNd from + 30.3 to - 10.5), 176Hf/177Hf from 0.28459 to 0.28239 (εHf from + 64.4 to - 13.6), and 208Pb/204Pb from 36.85 to 41.56, thus extending from the depleted mantle (DM) towards the enriched OIB mantle (EM and HIMU) components. 3He/4He (R/RA) ratios vary from 7.2 to 7.9 with He concentrations co-varying with the most incompatible element enrichment, in parallel with metasomatic effects. These metasomatic events, particularly effective in harzburgites and dunites, are attributable to the variable interaction with alkaline basic melts related to the general extensional and rifting regime affecting the East Africa-Arabian domain during the Cenozoic. In this respect, Bir Ali mantle xenoliths resemble those occurring along the Arabian margins and the East Africa Rift system, similarly affected by alkaline metasomatism, whereas they are

Eclogite facies relics and a multistage breakdown in metabasites of the KTB pilot hole, NE Bavaria: implications for the Variscan tectonometamorphic evolution of the NW Bohemian Massif

Science.gov (United States)

O'Brien, Patrick J.; Röhr, Christian; Okrusch, Martin; Patzak, Margarete

1992-11-01

Complex reaction textures in coronitic metagabbros and retrograded eclogites of the KTB pilot and an adjacent drilling provide evidence for a multistage metamorphic history in the Variscan basement of the NW Bohemian Massif. The eclogites show complete metamorphic recrystallization leaving no textural or mineral relics of their igneous precursors. In contrast, textural relics of the igneous protolith are still preserved in the metagabbros where the metamorphic overprint under high pressure conditions achieved only partial replacement of the initial assemblage plagioclase + augite + amphibole (+olivine or orthopyroxene?) + ilmenite to form the eclogite facies assemblage garnet + omphacite + kyanite + zoisite + quartz+rutile. The garnets in the metagabbros occur in the typical ‘necklace’ fashion at the borders between the original plagioclase and mafic phase domains. In the same rocks, omphacite formed by a topotactic reaction mechanism replacing igneous augite as well as in smaller grains at the margins of the texturally igneous clinopyroxene where it occurs without fixed orientation with respect to the relict phase. Both eclogites and metagabbros show a partial breakdown under high pressure granulite (transitional to high pressure amphibolite) facies conditions during which omphacite broke down to vermicular symplectites of diopside + plagioclase. A later pervasive medium pressure metamorphism under amphibolite facies conditions led to the development of assemblages dominated by hornblende + plagioclase+titanite: phases prevailing in the overwhelming majority of the surrounding metabasites. Subsequent vein-associated retrogression produced minerals typical of the greenschist to zeolite facies. All metamorphic stages may be represented in a single thin section but although the overall reaction sequence is apparent, the obvious disequilibrium in the rocks makes the use of conventional geothermobarometry difficult. However, calculations made by assuming an

Rheological structure of a lithosphere-asthenosphere boundary zone, decoded from EBSD analysis of mantle xenoliths from Ichinomegata, NE Japan

Science.gov (United States)

Sato, Y.; Ozawa, K.

2017-12-01

Mantle xenoliths are fragments of mantle materials entrapped in alkali basalts or kimberlites and transported to the surface (Nixon, 1987). They provide information on rheological, thermal, chemical, petrological structures of the upper mantle (e.g. Green et al., 2010; McKenzie and Bickle, 1988; O'Reilly and Griffin, 1996). They potentially represent materials from a boundary zone of lithosphere and asthenosphere (LABZ), where the heat transportation mechanism changes from convection to conduction (Sleep, 2005, 2006). However, difficulties in geobarometry for spinel peridotite (e.g. O'Reilly et al., 1997) have hampered our understanding of shallow LABZ. Ichinomegata located in the back-arc side of NE Japan is a latest Pleistocene andesitic-dacitic volcano yielding spinel peridotite xenoliths (Katsui et al., 1979). Through our works (Sato and Ozawa, 2016, 2017a, 2017b), we have overcome difficulties in geobarometry of spinel peridotites and gained accurate thermal structure (0.74-1.60 GPa, 832-1084 °C) from eight of the nine examined xenoliths. The rheological and chemical features suggest drastic changes: undeformed (granular), depleted, subsolidus mantle representing lithospheric mantle (ca. 28-35 km) and deformed (porphyroclastic), fertile, hydrous supersolidus mantle representing rheological LABZ (ca. 35-54 km). We investigate depth dependent variation of crystallographic preferred orientation (CPO) of constituent minerals of the xenoliths by electron back-scattered diffraction analysis (using JSM-7000F with a CCD detector and the CHANNEL5 software at the University of Tokyo). A shallower (ca. 32 km) sample with tabulargranular texture and coarse olivine size (0.92 mm) has A-type olivine CPO with [100] maximum as reported by Satsukawa and Michibayashi (2014) (hereafter SM14), whereas a deep (ca. 51 km) sample with porphyroclastic texture and finer olivine size (0.46 mm) has CPO with weaker fabric intensity characterized by a [100] girdle similar to AG-type and

Evidence for partial melting of eclogite from the Moldanubian Zone of the Bohemian Massif, Czech Republic

Czech Academy of Sciences Publication Activity Database

Miyazaki, T.; Nakamura, D.; Tamura, A.; Svojtka, Martin; Arai, S.; Hirajima, T.

2016-01-01

Roč. 111, č. 6 (2016), s. 405-419 ISSN 1345-6296 Grant - others:Rada Programu interní podpory projektů mezinárodní spolupráce AV ČR(CZ) M100131203 Program:Program interní podpory projektů mezinárodní spolupráce AV ČR Institutional support: RVO:67985831 Keywords : partial melting * eclogite * leucocratic pockets * Bohemian Massif * Moldanubian Zone Subject RIV: DB - Geology ; Mineralogy Impact factor: 0.409, year: 2016

Mantle metasomatism in the Kaapvaal Craton lithosphere: constraints on the composition of the metasomatic agent from fluid inclusions in MARID-type xenoliths

Science.gov (United States)

Konzett, J.; Krenn, K.; Hauzenberger, Ch.

2012-04-01

The emplacement of both group I and group II kimberlites in the Kaapvaal Craton of the Kimberley region in South Africa is associated with an intense metasomatic alteration of the country rocks as evidenced by a diverse suite of xenoliths sampled by the kimberlites mainly comprising metasomatized peridotites and minor MARID-type xenoliths. These are characterized by hydrous potassic silicates and LILE-HFSE-rich titanates. Because the metasomatic agent is not preserved in these rocks its composition has to be inferred from that of the metasomatic assemblages. Here we present for the first time data on fluid inclusions from two MARID-xenoliths sampled by group-I kimberlites of the Kimberley cluster. They provide direct evidence for the nature of the metasomatic fluids involved in kimberlite-related metsomatism. The xenoliths contain phlogopite+K-richterite+diopside+ilmenite±rutile±apatite±zircon. Fluid inclusions with 4-10 µm in size were found in diopside, K-richterite and zircon and contain L+V+one-to-several daughter phases. Investigations with the freezing and heating stage indicate two different chemical systems for the fluids: (1) H2O-NaCl dominant fluids found as L+V+S inclusions in zircon together with abundant needle-like apatite, rutile and phlogopite solid inclusions. The fluid inclusions in part occur along zircon host-rutile/apatite inclusion grain boundaries which indicates that the fluids were trapped during zircon growth. They contain 30-32 mass% NaCl and show a density of 0.87-0.94 g/cm3. Halos of tiny fluid inclusions, however, indicate that most if not all zircon inclusions are decrepitated during ascent from depth and/or superheating during entrainment of the xenoliths into the kimberlite. Using EMPA, enstatite and a SiO2 polymorph were identified in opened fluid inclusions exposed at the surface of polished thin sections. Because these phases were exclusively found in the fluid inclusions, they are considered daughter crystals. The enstatite

The petrology, geochronology and significance of Granite Harbour Intrusive Complex xenoliths and outcrop sampled in western McMurdo Sound, Southern Victoria Land, Antarctica

International Nuclear Information System (INIS)

Martin, A.P.; Cooper, A.F.; Price, R.C.; Turnbull, R.E.; Roberts, N.M.W.

2015-01-01

Granite Harbour Intrusive Complex xenoliths in McMurdo Volcanic Group rocks and in situ outcrops have been studied from Mount Morning, western McMurdo Sound, Antarctica. Calc-alkalic samples have whole rock signatures and normative compositions similar to the Dry Valleys 1b suite, and zircon grains in one specimen yield a 545.2 ± 4.4 Ma crystallisation age. This supports subduction-related magmatism initiating in Southern Victoria Land by 545 Ma. A second group of xenoliths is alkalic, with titanite grains in one xenolith from this group dated at 538 ± 8 Ma. Whole rock chemistry, normative compositions and geochronology of the alkalic group are comparable to the Koettlitz Glacier Alkaline Suite (KGAS). The position of a proposed lower crustal discontinuity that may form a significant basement suture in the McMurdo Sound region is newly constrained to the east of Mount Morning, perhaps along the trace of the Discovery Glacier. The boundary between East and West Antarctica may also pass along the trace of the Discovery Glacier if, as previously hypothesised, its location is controlled by the basement suture. A significant basement suture may also have provided the necessary egress for the (regionally) early and sustained magmatic activity observed at Mount Morning over the last 24 million years. (author).

Mg isotope systematics during magmatic processes: Inter-mineral fractionation in mafic to ultramafic Hawaiian xenoliths

Science.gov (United States)

Stracke, A.; Tipper, E. T.; Klemme, S.; Bizimis, M.

2018-04-01

Observed differences in Mg isotope ratios between bulk magmatic rocks are small, often on a sub per mill level. Inter-mineral differences in the 26Mg/24Mg ratio (expressed as δ26Mg) in plutonic rocks are on a similar scale, and have mostly been attributed to equilibrium isotope fractionation at magmatic temperatures. Here we report Mg isotope data on minerals in spinel peridotite and garnet pyroxenite xenoliths from the rejuvenated stage of volcanism on Oahu and Kauai, Hawaii. The new data are compared to literature data and to theoretical predictions to investigate the processes responsible for inter-mineral Mg isotope fractionation at magmatic temperatures. Theory predicts up to per mill level differences in δ26Mg between olivine and spinel at magmatic temperatures and a general decrease in Δ26Mgolivine-spinel (=δ26Mgolivine - δ26Mgspinel) with increasing temperature, but also with increasing Cr# in spinel. For peridotites with a simple petrogenetic history by melt depletion, where increasing depletion relates to increasing melting temperatures, Δ26Mgolivine-spinel should thus systematically decrease with increasing Cr# in spinel. However, most natural peridotites, including the Hawaiian spinel peridotites investigated in this study, are overprinted by variable extents of melt-rock reaction, which disturb the systematic primary temperature and compositionally related olivine-spinel Mg isotope systematics. Diffusion, subsolidus re-equilibration, or surface alteration may further affect the observed olivine-spinel Mg isotope fractionation in peridotites, making Δ26Mgolivine-spinel in peridotites a difficult-to-apply geothermometer. The available Mg isotope data on clinopyroxene and garnet suggest that this mineral pair is a more promising geothermometer, but its application is restricted to garnet-bearing igneous (garnet pyroxenites) and metamorphic rocks (eclogites). Although the observed δ26Mg variation is on a sub per mill range in bulk magmatic rocks

Chromium-rich lawsonite in high-Cr eclogites from the Făgăras Massif (South Carpathians)

Science.gov (United States)

Negulescu, E.; Săbău, G.

2012-12-01

Lawsonite is a relatively rare phase in natural rocks, because of its thermal decomposition during exhumation, and Cr-bearing lawsonite being restricted to only a few occurrences worldwide. Here we report Cr-lawsonite in eclogites hosted in a medium-grade metamorphic complex. Several high-Cr eclogite lenses occur in the Topolog Complex (Făgăras Massif) of dominantly gneissic-amphibolitic composition. High Cr contents are the result of emerald-green mm-sized nodules containing Cr-rich minerals, embedded in a gray-green matrix of kyanite, clinopyroxene, garnet, amphibole, zoisite, and rutile. Garnets occur as porphyroblasts or in coronas around clinopyroxene aggregates probably replacing former magmatic pyroxene. Relict gabbroic textures (sometimes pegmatoid) and whole rock geochemistry indicate a gabbroic cumulate origin. The REE pattern, displaying a slight positive Eu anomaly and a tea spoon-shaped LREE depletion is also indicative of a cumulate origin, as also noted by Pe Piper & Piper (2002) for the Othrys gabbro (as well as others in the Vourinos and Pindos ophiolitic suites) with the same unusual REE-pattern. The emerald-green Cr-rich nodules are unevenly distributed in the rock, and always enclosed in Cr-rich clinopyroxenes (up to 5.46% Cr2O3) which may exhibit Cr-diffusion haloes towards normal Cr-free matrix pyroxene. The nodules consist of diablastic chromite, rutile and Cr-rich kyanite of up to 15.67 wt% Cr2O3, Cr-bearing epidote, to which Cr-rich staurolite (up to 10.45% Cr2O3; XMg up to 0.68) and Cr-rich lawsonite (up to 9.17% Cr2O3) may exceptionally associate. Cr concentrations in kyanite and lawsonite are, to our knowledge, the highest reported so far. Cr-lawsonite was identified in a single sample, as small single phase inclusions armoured in Cr-kyanite. Equilibrium PT-conditions of 2.6 GPa and 610o C were derived from the garnet-mantled clinopyroxene aggregates using multi-equilibria calculation with the PTGIBBS routine of Brandelik & Massonne

Eclogite nappe-stack in the Grivola-Urtier Ophiolites (Southern Aosta Valley, Western Alps)

Science.gov (United States)

Tartarotti, Paola

2013-04-01

In the Western Alpine chain, ophiolites represent a section of the Mesozoic Tethys oceanic lithosphere, involved in subduction during the convergence between the paleo-Africa and paelo-Europe continents during the Cretaceous - Eocene. The Western Alpine ophiolites consist of several tectonic units, the most famous being the Zermatt-Saas and Combin nappes, and other major ophiolite bodies as the Voltri, Monviso, and Rocciavrè that show different rock assemblages and contrasting metamorphic imprints. The Grivola-Urtier (GU) unit is exposed in the southern Aosta Valley, covering an area of about 100 km2; it is tectonically sandwiched between the continentally-derived Pennidic Gran Paradiso Nappe below, and the Austroalpine Mount Emilius klippe above. This unit has been so far considered as part of the Zermatt-Saas nappe extending from the Saas-Fee area (Switzerland) to the Aosta Valley (Italy). The GU unit consists of serpentinized peridotites that include pods and boudinaged layers of eclogitic Fe-metagabbro and trondhjemite, rodingites and chloriteschists transposed in the main foliation together with calcschists and micaschists. All rocks preserve particularly fresh eclogitic mineral assemblages. The contact between the serpentinites and calcshists is marked by a tectonic mélange consisting of mylonitic marble and calcschist with stretched and boudinaged serpentinite blocks. Continentally-derived allochthonous blocks ranging in size from100 meters to meters are also included within the ophiolites. New field, petrographic and geochemical data reveal the complex nature of the fossil Tethyan oceanic lithosphere exposed in the southern Aosta Valley, as well as the extent and size of the continental-oceanic tectonic mélange. The geological setting of the GU unit is here inferred as a key tool for understanding the complex architecture of the ophiolites in the Western Alps.

Deformation, static recrystallization, and reactive melt transport in shallow subcontinental mantle xenoliths (Tok Cenozoic volcanic field, SE Siberia)

Science.gov (United States)

Tommasi, Andréa; Vauchez, Alain; Ionov, Dmitri A.

2008-07-01

Partial melting and reactive melt transport may change the composition, microstructures, and physical properties of mantle rocks. Here we explore the relations between deformation and reactive melt transport through detailed microstructural analysis and crystallographic orientation measurements in spinel peridotite xenoliths that sample the shallow lithospheric mantle beneath the southeastern rim of the Siberian craton. These xenoliths have coarse-grained, annealed microstructures and show petrographic and chemical evidence for variable degrees of reaction with silicate melts and fluids, notably Fe-enrichment and crystallization of metasomatic clinopyroxene (cpx). Olivine crystal preferred orientations (CPO) range from strong to weak. [010]-fiber patterns, characterized by a point concentration of [010] normal to the foliation and by dispersion of [100] in the foliation plane with a weak maximum parallel to the lineation, predominate relative to the [100]-fiber patterns usually observed in lithospheric mantle xenoliths and peridotite massifs. Variations in olivine CPO patterns or intensity are not correlated with modal and chemical compositions. This, together with the analysis of microstructures, suggests that reactive melt percolation postdated both deformation and static recrystallization. Preferential crystallization of metasomatic cpx along (010) olivine grain boundaries points to an influence of the preexisting deformation fabrics on melt transport, with higher permeability along the foliation. Similarity between orthopyroxene (opx) and cpx CPO suggests that cpx orientations may be inherited from those of opx during melt-rock reaction. As observed in previous studies, reactive melt transport does not weaken olivine CPO and seismic anisotropy in the upper mantle, except in melt accumulation domains. In contrast, recovery and selective grain growth during static recrystallization may lead to development of [010]-fiber olivine CPO and, if foliations are

The peculiar case of Marosticano xenoliths: a cratonic mantle fragment affected by carbonatite metasomatism in the Veneto Volcanic Province (Northern Italy)

Science.gov (United States)

Brombin, Valentina; Bonadiman, Costanza; Coltorti, Massimo; Florencia Fahnestock, M.; Bryce, Julia G.; Marzoli, Andrea

2017-04-01

The Tertiary Magmatic Province of Veneto, known as Veneto Volcanic Province (VVP), in the Northern Italy, represents one of the most important volcanic provinces of the Adria Plate. It is composed by five volcanic districts: Val d'Adige, Marosticano, Mts. Lessini, Berici Hills and Euganean Hills. Most of the volcanic products are relatively undifferentiated lavas, from nephelinites to tholeiites in composition. Commonly VVP nephelinites and basanites carry mantle xenoliths. This study presents a petrological characterization of the new xenolith occurrence of Marosticano and comparison with previously studied VVP xenolith populations (i.e. from the Lessinean and Val d'Adige areas), which represent off-craton lithospheric mantle fragment affected by Na-alkaline silicate metasomatism (Siena & Coltorti 1989; Beccaluva et al., 2001; Gasperini et al., 2006). Marosticano (MA) peridotites are anhydrous spinel-bearing lherzolites and harzburgites, which are geochemically well distinguishible from the other VVP mantle xenoliths. Primary minerals record the "most restitic" composition of the VVP sampled mantle, even calling the geochemical features of a sub-cratonic mantle. Olivines in both lherzolites and harzburgites show high Ni contents compared with the Fo values (Ni→ lherzolite: 2600-3620 ppm; harzburgite: 2600-3540 ppm; Fo → lh: 91-92; hz: 90-93) that follow the trend of olivine from a cratonic area (Kelemen, 1998). Orthopyroxenes have mg# values with 1:1 ratio with coexisting olivines and Al2O3 contents always 0.5 wt%) contents are also the chemical characteristics of the clinopyroxenes. On the whole both MA pyroxenes show major element contents that recall the characteristics of those from cratonic (sp-bearing) peridotites (e.g. from Greenland, South Africa and Tanzania; Downes et al., 2004). In addition, the relationship between the high Fo content of olivine and the high chromium contents (cr#=(Cr/(Cr+Al)X100); lh: 30-53; hz: 38-67) in coexisting spinel, out of

The uniquely high-temperature character of Cullinan diamonds: A signature of the Bushveld mantle plume?

Science.gov (United States)

Korolev, N. M.; Kopylova, M.; Bussweiler, Y.; Pearson, D. G.; Gurney, J.; Davidson, J.

2018-04-01

The mantle beneath the Cullinan kimberlite (formerly known as "Premier") is a unique occurrence of diamondiferous cratonic mantle where diamonds were generated contemporaneously and shortly following a mantle upwelling that led to the formation of a Large Igneous Province that produced the world's largest igneous intrusion - the 2056 Ma Bushveld Igneous Complex (BIC). We studied 332 diamond inclusions from 202 Cullinan diamonds to investigate mantle thermal effects imposed by the formation of the BIC. The overwhelming majority of diamonds come from three parageneses: (1) lithospheric eclogitic (69%), (2) lithospheric peridotitic (21%), and (3) sublithospheric mafic (9%). The lithospheric eclogitic paragenesis is represented by clinopyroxene, garnet, coesite and kyanite. Main minerals of the lithospheric peridotitic paragenesis are forsterite, enstatite, Cr-pyrope, Cr-augite and spinel; the sublithospheric mafic association includes majorite, CaSiO3 phases and omphacite. Diamond formation conditions were calculated using an Al-in-olivine thermometer, a garnet-clinopyroxene thermometer, as well as majorite and Raman barometers. The Cullinan diamonds may be unique on the global stage in recording a cold geotherm of 40 mW/m2 in cratonic lithosphere that was in contact with underlying convecting mantle at temperatures of 1450-1550 °C. The studied Cullinan diamonds contain a high proportion of inclusions equilibrated at temperatures exceeding the ambient 1327 °C adiabat, i.e. 54% of eclogitic diamonds and 41% of peridotitic diamonds. By contrast, ≤ 1% of peridotitic diamond inclusions globally yield equally high temperatures. We propose that the Cullinan diamond inclusions recorded transient, slow-dissipating thermal perturbations associated with the plume-related formation of the 2 Ga Bushveld igneous province. The presence of inclusions in diamond from the mantle transition zone at 300-650 km supports this view. Cullinan xenoliths indicative of the thermal state of

The ongoing search for the oldest rock on the Danish island of Bornholm: new U-Pb zircon ages for a quartz-rich xenolith and country rock from the Svaneke Granite

DEFF Research Database (Denmark)

Waight, Tod Earle; Serre, Simon H.; Næsby, Sebastian H.

2017-01-01

Previous geochronological studies on the Danish island of Bornholm have not identified any rocks older than c. 1.46 Ga. New LA-ICP-MS U-Pb zircon ages are presented for a xenolith within, and the country rock gneiss adjacent to, the Svaneke Granite on Bornholm. The xenolith is fine......-grained and quartz-rich and was likely derived from either a quartz-rich sedimentary protolith or a hydrothermally altered felsic volcanic rock. The relatively fine-grained felsic nature of the country rock gneiss and the presence of large zoned feldspars that may represent phenocrysts suggest its protolith may have...... been a felsic volcanic or shallow intrusive rock. A skarn-like inclusion from a nearby locality likely represents an originally carbonate sediment and is consistent with supracrustal rocks being present at least locally. Zircon data from the xenolith define an upper intercept age of 1483 ± 12 Ma (2σ...

Comparison of Conventional, Trace Element, and Pseudosection Thermobarometry in UHP Eclogite, North Qaidam Terrane, Western China

Science.gov (United States)

Mattinson, C. G.; Regel, M. E.; Zhang, J.

2014-12-01

In the southeastern North Qaidam terrane, near Dulan, felsic host gneisses enclose minor eclogite lenses. A small (~3°C/km) increase in temperatures to the west based on Zr-in-Rt thermometry is supported by a textural trend of zoned garnet with prograde mineral inclusions in the west to unzoned garnet with only peak-stage inclusions in the east. A western eclogite sample contains minor Ep and trace Phe in addition to Grt-Omp-Qtz-Rt; a strong foliation is defined by banding of Grt and Omp. Garnet Ca falls significantly from core (Alm44Prp23Grs32) to rim (Alm48Prp27Grs23). Phengite contains 3.34-3.36 Si pfu. Conventional Grt-Omp-Phe thermobarometry yields 23-27 kbar, 660-730°C, and Zr-in-Rt thermometry yields 671 ± 9°C (n = 37). An isochemical phase diagram (pseudosection) for the system NCKFMASHO calculated with PerpleX indicates that garnet Xgrs decreases with increasing P and T, suggesting that observed garnet zoning reflects prograde growth. Garnet rim compositional isopleths intersect at 23-27 kbar, 580-620°C, depending on choice of effective bulk composition and assumed Fe3+/Fetotal. Garnet rims help define the foliation, suggesting that these P-T conditions apply to this deformation. An eastern eclogite sample contains minor Ep and abundant, coarse-grained Phe in addition to Grt-Omp-Qtz-Rt; foliation is weak. Garnet zoning is weak; compositions are Alm41-43Prp26-28Grs29-32. Phengite cores contain 3.48-3.56 Si pfu. Conventional Grt-Omp-Phe thermobarometry yields 32-36 kbar, 700-750°C, and Zr-in-Rt thermometry yields 691 ± 12°C (n = 34). Conventional thermobarometry and pseudosections yield similar peak P-T conditions, but from different mineral compositions: thermobarometry yields peak P from high-Ca Grt, but the pseudosection yields peak P from intermediate-Ca Grt; maximum Ca in Grt is predicted at lower P and T. Pseudosection-based peak Ts are significantly lower than those from Zr-in-Rt and conventional thermometry. Wide isopleth spacing in the

Fabrics produced mimetically during static metamorphism in retrogressed eclogites from the Zermatt-Saas zone, Western Italian Alps

Science.gov (United States)

McNamara, D. D.; Wheeler, J.; Pearce, M.; Prior, D. J.

2012-11-01

Lattice preferred orientations (LPOs) are commonly interpreted to form by dislocation creep. Consequently they are used to infer deformation at the metamorphic grade at which the minerals were stable, especially if those minerals show a shape fabric. Here we show that LPOs can occur through mimicry of a pre-existing LPO, so they formed statically, not during deformation. Omphacite and glaucophane LPOs occur in eclogite facies rocks from the Zermatt-Saas Unit of the Northwest Italian Alps. Barroisite grew during greenschist facies retrogression and has an LPO controlled significantly by the eclogite facies omphacite and glaucophane LPOs, rather than directly by deformation. Using spatially resolved lattice orientation data from the three key minerals, collected using electron backscatter diffraction, we deploy a new technique of interphase misorientation distribution analysis to prove this. Barroisite LPO develops by mimicry of omphacite (via a particular lattice orientation relationship) and by direct topotactic and epitactic replacement of glaucophane. LPO in turn influenced anisotropic grain growth, resulting in a barroisite grain shape fabric. Thus regional retrogression during exhumation of the Zermatt-Saas high-pressure rocks was, in large part, static, rather than dynamic as previously interpreted. In general the possibility of mimetic fabrics forming during metamorphic reactions must be borne in mind when interpreting direct structural observations and seismic anisotropy data in terms of deformation, in both crust and mantle.

Os-Hf-Sr-Nd isotope and PGE systematics of spinel peridotite xenoliths from Tok, SE Siberian craton: Effects of pervasive metasomatism in shallow refractory mantle

Science.gov (United States)

Ionov, Dmitri A.; Shirey, Steven B.; Weis, Dominique; Brügmann, Gerhard

2006-01-01

Os-Hf-Sr-Nd isotopes and PGE were determined in peridotite xenoliths carried to the surface by Quaternary alkali basaltic magmas in the Tokinsky Stanovik Range on the Aldan shield. These data constrain the timing and nature of partial melting and metasomatism in the lithospheric mantle beneath SE Siberian craton. The xenoliths range from the rare fertile spinel lherzolites to the more abundant, strongly metasomatised olivine-rich (70-84%) rocks. Hf-Sr-Nd isotope compositions of the xenoliths are mainly within the fields of oceanic basalts. Most metasomatised xenoliths have lower 143Nd / 144Nd and 176Hf / 177Hf and higher 87Sr / 86Sr than the host basalts indicating that the metasomatism is older and has distinct sources. A few xenoliths have elevated 176Hf / 177Hf (up to 0.2838) and plot above the Hf-Nd mantle array defined by oceanic basalts. 187Os / 188Os in the poorly metasomatised, fertile to moderately refractory (Al2O3 ≥ 1.6%) Tok peridotites range from 0.1156 to 0.1282, with oldest rhenium depletion ages being about 2 Ga. The 187Os / 188Os in these rocks show good correlations with partial melting indices (e.g. Al2O3, modal cpx); the intercept of the Al-187Os / 188Os correlation with lowest Al2O3 estimates for melting residues (∼0.3-0.5%) has a 187Os / 188Os of ∼0.109 suggesting that these peridotites may have experienced melt extraction as early as 2.8 Gy ago. 187Os / 188Os in the strongly metasomatised, olivine-rich xenoliths (0.6-1.3% Al2O3) ranges from 0.1164 to 0.1275 and shows no apparent links to modal or chemical compositions. Convex-upward REE patterns and high abundances of heavy to middle REE in these refractory rocks indicate equilibration with evolved silicate melts at high melt / rock ratios, which may have also variably elevated their 187Os / 188Os. This inference is supported by enrichments in Pd and Pt on chondrite-normalised PGE abundance patterns in some of the rocks. The melt extraction ages for the Tok suite of 2.0 to 2.8 Ga are

High-pressure metamorphic age and significance of eclogite-facies continental fragments associated with oceanic lithosphere in the Western Alps (Etirol-Levaz Slice, Valtournenche, Italy)

Science.gov (United States)

Fassmer, Kathrin; Obermüller, Gerrit; Nagel, Thorsten J.; Kirst, Frederik; Froitzheim, Nikolaus; Sandmann, Sascha; Miladinova, Irena; Fonseca, Raúl O. C.; Münker, Carsten

2016-05-01

The Etirol-Levaz Slice in the Penninic Alps (Valtournenche, Italy) is a piece of eclogite-facies continental basement sandwiched between two oceanic units, the blueschist-facies Combin Zone in the hanging wall and the eclogite-facies Zermatt-Saas Zone in the footwall. It has been interpreted as an extensional allochthon from the continental margin of Adria, emplaced onto ultramafic and mafic basement of the future Zermatt-Saas Zone by Jurassic, rifting-related detachment faulting, and later subducted together with the future Zermatt-Saas Zone. Alternatively, the Etirol-Levaz Slice could be derived from a different paleogeographic domain and be separated from the Zermatt-Saas Zone by an Alpine shear zone. We present Lu-Hf whole rock-garnet ages of two eclogite samples, one from the center of the unit and one from the border to the Zermatt-Saas Zone below. These data are accompanied by a new geological map of the Etirol-Levaz Slice and the surrounding area, as well as detailed petrology of these two samples. Assemblages, mineral compositions and garnet zoning in both samples indicate a clockwise PT-path and peak-metamorphic conditions of about 550-600 °C/20-25 kbar, similar to conditions proposed for the underlying Zermatt-Saas Zone. Prograde garnet ages of the two samples are 61.8 ± 1.8 Ma and 52.4 ± 2.1 Ma and reflect different timing of subduction. One of these is significantly older than published ages of eclogite-facies metamorphism in the Zermatt-Saas Zone and thus contradicts the hypothesis of Mesozoic emplacement. The occurrence of serpentinite and metagabbro bodies possibly derived from the Zermatt-Saas Zone inside the Etirol-Levaz Slice suggests that the latter is a tectonic composite. The basement slivers forming the Etirol-Levaz Slice and other continental fragments were subducted earlier than the Zermatt-Saas Zone, but nonetheless experienced similar pressure-temperature histories. Our results support the hypothesis that the Zermatt-Saas Zone and the

Restitic or not? Insights from trace element content and crystal - Structure of spinels in African mantle xenoliths

Science.gov (United States)

Lenaz, Davide; Musco, Maria Elena; Petrelli, Maurizio; Caldeira, Rita; De Min, Angelo; Marzoli, Andrea; Mata, Joao; Perugini, Diego; Princivalle, Francesco; Boumehdi, Moulay Ahmed; Bensaid, Idris Ali Ahmadi; Youbi, Nasrrddine

2017-05-01

The lithospheric architecture of Africa consists of several Archean cratons and smaller cratonic fragments, stitched together and flanked by polycyclic fold belts. Here we investigate the structure and chemistry of spinels from lithospheric mantle xenoliths from distinct tectonic settings, i.e. from the Saharan metacraton in Libya (Waw-En-Namus) which could show archaic chemical features, Cameroon (Barombi Koto and Nyos Lakes) where the Sub Continental Lithospheric Mantle was modified during the Pan-African event and fluxed by asthenospheric melts of the Tertiary Cameroon Volcanic Line and Morocco (Tafraoute, Bou-Ibalrhatene maars) in the Middle Atlas where different metasomatic events have been recorded. From a structural point of view it is to notice that the Libyan spinels can be divided into two groups having different oxygen positional parameter (u > 0.2632 and u Morocco spinels show a Tc in the range 630-760 °C. About 150 different spinels have been studied for their trace element content and it can be seen that many of them are related to Cr content, while Zn and Co are not and clearly distinguish the occurrences. Differences in the trace element chemistry, in the structural parameters and in the intracrystalline closure temperatures suggest that a different history should be considered for Cameroon, Morocco and LB I and LB II spinels. Even if it was not considered for this purpose, we tentatively used the Fe2 +/Fe3 + vs. TiO2 diagram that discriminate between peridotitic and the so-called "magmatic" spinels, i.e. spinel crystallized from melts. LB I and LB II spinels plot in the peridotitic field while Cameroon and Morocco spinels fall in the magmatic one. Consequently, the xenoliths sampled from a probably juvenile SCLM at the edge of the most important lithospheric roots (i.e. Cameroon and Morocco) apparently have spinels possibly fractionated in situ from percolating melts and do not represent a real spinel-peridotite facies. On the contrary mantle

Li Isotope Studies of Olivine in Mantle Xenoliths by SIMS

Science.gov (United States)

Bell, D. R.; Hervig, R. L.; Buseck, P. R.

2005-01-01

Variations in the ratio of the stable isotopes of Li are a potentially powerful tracer of processes in planetary and nebular environments [1]. Large differences in the 7Li/6Li ratio between the terrestrial upper mantle and various crustal materials make Li isotope composition a potentially powerful tracer of crustal recycling processes on Earth [2]. Recent SIMS studies of terrestrial mantle and Martian meteorite samples report intra-mineral Li isotope zoning [3-5]. Substantial Li isotope heterogeneity also exists within and between the components of chondritic meteorites [6,7]. Experimental studies of Li diffusion suggest the potential for rapid isotope exchange at elevated temperatures [8]. Large variations in 7Li, exceeding the range of unaltered basalts, occur in terrestrial mantle-derived xenoliths from individual localities [9]. The origins of these variations are not fully understood.

Formation of atoll garnets in the UHP eclogites of the Tso Morari Complex, Ladakh, Himalaya

Science.gov (United States)

Jonnalagadda, Mallika K.; Karmalkar, Nitin R.; Duraiswami, Raymond A.; Harshe, Shivani; Gain, Sarah; Griffin, William L.

2017-12-01

The eclogites of the Tso Morari Complex, Ladakh, NW Himalayas preserve both garnets with spectacular atoll textures, as well as whole porphyroblastic garnets. Whole garnets are euhedral, idiomorphic and enclose inclusions of amphibole, phengite and zoisite within the cores, and omphacite and quartz/coesite towards the rims. Detailed electron microprobe analyses and back-scattered electron images show well-preserved prograde zoning in the whole garnets with an increase in Mg and decrease in Ca and Mn contents from the core to the rim. The atoll garnets commonly consist of euhedral ring over island/peninsular core containing inclusions of phengite, omphacite and rarely amphibole between the core and ring. Compositional profiles across the studied atoll grains show elemental variations with higher concentrations of Ca and Mn with low Mg at the peninsula/island cores; contrary to this low Ca, Mn and high Mg is observed at the outer rings. Temperature estimates yield higher values at the Mg-rich atoll garnet outer rings compared to the atoll cores. Atoll garnet formation was favoured by infiltration of fluid formed due to breakdown of hydrous phases, and/or the release of structurally bounded OH from nominally anhydrous minerals at the onset of exhumation. Infiltration of fluids along pre-existing fracture pathways and along mineral inclusion boundaries triggered breakdown of the original garnet cores and released elements which were subsequently incorporated into the newly-grown garnet rings. This breakdown of garnet cores and inward re-growth at the outer ring produced the atoll structure. Calibrated geo-thermobarometers and mineral equilibria reflect that the Tso Morari eclogites attain peak pressures prior to peak temperatures representing a clockwise path of evolution.

Rhenium-osmium isotopes in pervasively metasomatized mantle xenoliths from the Bohemian Massif and implications for the reliability of Os model ages

Czech Academy of Sciences Publication Activity Database

Kochergina, Y. V.; Ackerman, Lukáš; Erban, V.; Matusiak-Malek, M.; Puziewicz, J.; Halodová, P.; Špaček, P.; Trubač, J.; Magna, T.

2016-01-01

Roč. 430, July 15 (2016), s. 90-107 ISSN 0009-2541 Institutional support: RVO:67985831 Keywords : Bohemian Massif * depletion age * Ohře/Eger rift * Os isotopes * peridotite xenolith * Re-Os * sub-continental lithospheric mantle Subject RIV: DB - Geology ; Mineralogy Impact factor: 3.347, year: 2016

Reworking of Archean mantle in the NE Siberian craton by carbonatite and silicate melt metasomatism: Evidence from a carbonate-bearing, dunite-to-websterite xenolith suite from the Obnazhennaya kimberlite

Science.gov (United States)

Ionov, Dmitri A.; Doucet, Luc S.; Xu, Yigang; Golovin, Alexander V.; Oleinikov, Oleg B.

2018-03-01

The Obnazhennaya kimberlite in the NE Siberian craton hosts a most unusual cratonic xenolith suite, with common rocks rich in pyroxenes and garnet, and no sheared peridotites. We report petrographic and chemical data for whole rocks (WR) and minerals of 20 spinel and garnet peridotites from Obnazhennaya with Re-depletion Os isotope ages of 1.8-2.9 Ga (Ionov et al., 2015a) as well as 2 pyroxenites. The garnet-bearing rocks equilibrated at 1.6-2.8 GPa and 710-1050 °C. Some xenoliths contain vermicular spinel-pyroxene aggregates with REE patterns in clinopyroxene mimicking those of garnet. The peridotites show significant scatter of Mg# (0.888-0.924), Cr2O3 (0.2-1.4 wt.%) and high NiO (0.3-0.4 wt.%). None are pristine melting residues. Low-CaO-Al2O3 (≤0.9 wt.%) dunites and harzburgites are melt-channel materials. Peridotites with low to moderate Al2O3 (0.4-1.8 wt.%) usually have CaO > Al2O3, and some have pockets of calcite texturally equilibrated with olivine and garnet. Such carbonates, exceptional in mantle xenoliths and reported here for the first time for the Siberian mantle, provide direct evidence for modal makeover and Ca and LREE enrichments by ephemeral carbonate-rich melts. Peridotites rich in CaO and Al2O3 (2.7-8.0 wt.%) formed by reaction with silicate melts. We infer that the mantle lithosphere beneath Obnazhennaya, initially formed in the Mesoarchean, has been profoundly modified. Pervasive inter-granular percolation of highly mobile and reactive carbonate-rich liquids may have reduced the strength of the mantle lithosphere leading the way for reworking by silicate melts. The latest events before the kimberlite eruption were the formation of the carbonate-phlogopite pockets, fine-grained pyroxenite veins and spinel-pyroxene symplectites. The reworked lithospheric sections are preserved at Obnazhennaya, but similar processes could erode lithospheric roots in the SE Siberian craton (Tok) and the North China craton, where ancient melting residues and

Precambrian U-Pb zircon ages in eclogites and garnet pyroxenites from South Brittany (France): an old oceanic crust in the West European Hercynian belt?

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Peucat, J. J.; Vidal, Ph.; Godard, G.; Postaire, B.

1982-08-01

U-Pb zircon ages have been determined for two eclogites from the Vendée and for two garnet pyroxenites from the Baie d'Audierne. In an episodic Pb loss model, the two discordia would give upper intercept ages around 1300-1250 Ma and lower intercepts ages of 436-384 Ma. Two interpretations are proposed: (1) The 1250-1300 Ma ages may reflect an unspecified upper mantle event or process; the Paleozoic ages correspond to the tectonic emplacement of an eclogitic mantle fragment into the continental crust. (2) The protolith may have been extracted from the upper mantle 1250-1300 Ma ago and stored in a crustal environment until it was metamorphosed under high-pressure conditions around 400 Ma ago. This latter model is favoured by available geologic and isotopic data. Consequently, we propose that a 1300 Ma old oceanic crust was tectonicly incorporated into a sialic basement during the Proterozoic. This mixture was subsequently subducted during the Paleozoic.

Immiscible melt droplets in garnet, as represented by ilmenite-magnetite-spinel spheroids in an eclogite-garnet peridotite association, Blanský les Granulite Massif, Czech Republic

Czech Academy of Sciences Publication Activity Database

Vrána, S.; Ackerman, Lukáš; Erban, V.; Halodová, P.

2016-01-01

Roč. 101, č. 1 (2016), s. 82-92 ISSN 0003-004X Institutional support: RVO:67985831 Keywords : eclogite * Fe-Ti-rich melt * garnet peridotite * garnetite * Ilmenite-magnetite-spinel * Invited Centennial article * Moldanubian Zone * UHP crystallization Subject RIV: DD - Geochemistry Impact factor: 2.021, year: 2016

Characterization of Organic Materials in the Xenolithic Clasts in Sharps (H3.4) Meteorite Using Microraman Spectroscopy

Science.gov (United States)

Chan, Q. H. S.; Zolensky, M. E.; Bodnar, R. J.; Kebukawa, Y.

2015-01-01

Graphitization of carbon is an irreversible process which alters the structure of graphitic materials in response to the increase in metamorphic grade (temperature and/or pressure). Carbonaceous materials offer a reliable geothermometer as their Raman spectra change systematically with increasing metamorphic grade [1-3]. In this study, we identified carbonaceous materials in the xenolithic clasts in Sharps and interpreted their metamorphic history by revealing the structural organization (order) of the polyaromatic organic phases using µ-Raman spectroscopy.

Characterization of Organic Materials in the Xenolithic Clasts in Sharps (H3.4) Meteorite Using Micro-Raman Spectroscopy

Science.gov (United States)

Chan, Q. H. S.; Zolensky, M. E.; Bodnar, R. J.; Kebukawa, Y.

2015-01-01

Graphitization of carbon is an irreversible process which alters the structure of graphitic materials in response to the increase in metamorphic grade (temperature and/or pressure). Carbonaceous materials offer a reliable geothermometer as their Raman spectra change systematically with increasing metamorphic grade. In this study, we identified carbonaceous materials in the xenolithic clasts in Sharps and interpreted their metamorphic history by revealing the structural organization (order) of the polyaromatic organic phases using micro-Raman spectroscopy.

Upper mantle xenoliths from the Pliocene Kozákov volcano (NE Bohemia): P–T–fO2 and geochemical constraints

Czech Academy of Sciences Publication Activity Database

Konečný, P.; Ulrych, Jaromír; Schovánek, P.; Huraiová, M.; Řanda, Zdeněk

2006-01-01

Roč. 57, č. 5 (2006), s. 379-396 ISSN 1335-0552 R&D Projects: GA AV ČR IAA3013403 Institutional research plan: CEZ:AV0Z30130516; CEZ:AV0Z10480505 Keywords : Bohemian Massif * Cenozoic volcanism * ultramafic xenoliths * spinel lherzolite * harzburgite Subject RIV: DB - Geology ; Mineralogy Impact factor: 0.364, year: 2006 http://www.geologicacarpathica.sk/src/main.php

Origin of eclogite and garnet pyroxenite from the Moldanubian Zone of the Bohemian Massif, Czech Republic and its implication to other mafic layers embedded in orogenic peridotites

Czech Academy of Sciences Publication Activity Database

Obata, M.; Hirajima, T.; Svojtka, Martin

2006-01-01

Roč. 88, 1-2 (2006), s. 321-340 ISSN 0930-0708 R&D Projects: GA AV ČR IAA3013006 Institutional research plan: CEZ:AV0Z30130516 Keywords : eclogite * pyroxenite * Bohemian Massif Subject RIV: DB - Geology ; Mineralogy Impact factor: 1.038, year: 2006

Chemical compositions and Sr, Nd isotope ratios of gabbroic xenoliths in calc-alkali andesites of Naeba and Torikabuto volcanoes, North Fossa Magna, central Japan

International Nuclear Information System (INIS)

Shimazu, Mitsuo; Kawano, Yoshinobu; Kaji, Kiyoshi; Igarashi, Satoshi.

1991-01-01

Gabbroic, doleritic and basaltic xenoliths found in calc-alkali andesites of Naeba and Torikabuto volcanoes are geochemically divided into three groups. Gabbro A of the group 1 from Naeba is rich in MgO and Ni, poor in alkalis, and shows depleted REE pattern resembling those of Ichinomegata (Sp. No.2232) and Hakone (HKG1, HKG2) volcanoes. On the basis the REE pattern and high Al IV contents in clinopyroxenes, gabbro A is interpreted to have been cumulate from a primary magma generated by partial melting of upper mantle. From REE pattern, gabbros of the group 2 from Ichinomegata (Sp. No.2218) may have derived from low alkali tholeiite magma which have been formed by removal of material such as the group 1 gabbro at shallow depth. Doleritic and basaltic xenoliths of the group 3 from Naeba, gabbroic xenoliths from Torikabuto and Umikawa are poorer in MgO and richer in alkalis than those of the group 1 and show enriched pattern in REE resembling that of high alkali tholeiite and contain clinopyroxenes having low Al IV . Therefore, these rocks are considered to be differentiates of high alkali tholeiite magma at shallow depth. On εNd- 87 Sr/ 86 Sr diagram, isotope data of gabbro A of the group 1 plot near those of andesites from Asama and Myoko volcanoes of the North Fossa Magna. It is interpreted that these rocks have derived from the same mantle source as the Asama and Myoko volcanoes which are richer in incompatible elements than those of MORB. (author)

Intra-slab COH fluid fluxes evidenced by fluid-mediated decarbonation of lawsonite eclogite-facies altered oceanic metabasalts

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Vitale Brovarone, Alberto; Chu, Xu; Martin, Laure; Ague, Jay J.; Monié, Patrick; Groppo, Chiara; Martinez, Isabelle; Chaduteau, Carine

2018-04-01

The interplay between the processes controlling the mobility of H2O and C-bearing species during subduction zone metamorphism exerts a critical control on plate tectonics and global volatile recycling. Here we present the first study on fresh, carbonate-bearing, lawsonite eclogite-facies metabasalts from Alpine Corsica, France, which reached the critical depths at which important devolatilization reactions occur in subducting slabs. The studied samples indicate that the evolution of oceanic crustal sequences subducted under present-day thermal regimes is dominated by localized fluid-rock interactions that are strongly controlled by the nature and extent of inherited (sub)seafloor hydrothermal processes, and by the possibility of deep fluids to be channelized along inherited or newly-formed discontinuities. Fluid channelization along inherited discontinuities controlled local rehydration and dehydration/decarbonation reactions and the stability of carbonate and silicate minerals at the blueschist-eclogite transition. Fluid-mediated decarbonation was driven by upward, up-temperature fluid flow in the inverted geothermal gradient of a subducting oceanic slab, a process that has not been documented in natural samples to date. We estimate that the observed fluid-rock reactions released 20-60 kg CO2 per m3 of rock (i.e. 0.7-2.1 wt% CO2), which is in line with the values predicted from decarbonation of metabasalts in open systems at these depths. Conversely, the estimated time-integrated fluid fluxes (20-50 t/m2) indicate that the amount of carbon transported by channelized fluid flow within the volcanic part of subducting oceanic plates is potentially much higher than previous numerical estimates, testifying to the percolation of C-bearing fluids resulting from devolatilization/dissolution processes operative in large reservoirs.

Partial reactive crystallization of variable CO2-bearing siliceous MORB-eclogite-derived melt in fertile peridotite and genesis of alkalic basalts with signatures of crustal recycling

Science.gov (United States)

Mallik, A.; Dasgupta, R.

2013-12-01

The presence of heterogeneity in the form of recycled altered oceanic crust (MORB-eclogite) has been proposed in the source of HIMU ocean island basalts (OIBs) [1]. Partial melts of recycled oceanic crust, however, are siliceous and Mg-poor and thus do not resemble the major element compositions of alkalic OIBs that are silica-poor and Mg-rich. In an upwelling heterogenous mantle, MORB-eclogite undergoes melting deeper than volatile-free peridotite, hence, andesitic partial melt derived from eclogite will react with subsolidus peridotite. We have examined the effect of such a melt-rock reaction under volatile-free conditions at 1375 °C, 3 GPa by varying the melt-rock ratio from 8 to 50 wt.% [2]. We concluded that the reacted melts reproduce certain major element characteristics of oceanic basanites, but not nephelinites. Also, the melt-rock reaction produces olivine and garnet-bearing websteritic residue. Because presence of CO2 has been invoked in the source of many HIMU ocean islands, the effect of CO2 on such a melt-rock reaction needs to be evaluated. Accordingly, we performed reaction experiments on mixtures of 25% and 33% CO2-bearing andesitic partial melt and peridotite at 1375 °C, 3 GPa by varying the dissolved CO2 content of the reacting melts from 1 to 5 wt.% (bulk CO2 from 0.25 to 1.6 wt.%) [3, this study]. Owing to melt-rock reaction, with increasing CO2 in the bulk mixture, (a) modes of olivine and cpx decrease while melt, opx and garnet increase, (b) reacted melts evolve to greater degree of Si-undersaturation (from andesite through basanite to nephelinite), (c) enhanced crystallization of garnet take place with higher CO2 in the melt, reducing alumina content of the reacted melts, and (d) CaO and MgO content of the reacted melts increase, without affecting FeO* and Na2O contents (indicating greater propensity of Ca2+ and Mg2+ over Fe2+ and Na+ to enter silicate melt as carbonate). For a given melt-MgO, the CO2-bearing reacted melts are a better

Mantle xenoliths from Nemby, Eastern Paraguay: O-Sr-Nd isotopes and trace elements of hosted clinopyroxenes

International Nuclear Information System (INIS)

Comin-Chiaramonti, P; Antonini, P; Girardi, V.A.V; Gomes, C.B; Laurora, S; Zanetti, A

2001-01-01

The Nemby spinel peridotites (Asuncion- Sapucai-Villarrica Graben, Central Eastern Paraguay) are variable in major element compositions, ranging from relatively 'fertile' to very depleted in basaltic component. Some of the xenoliths have exceeding high K 2 O (HK suite, distinct from the low K 2 O, LK-suite) and incompatible element (IE) contents compared with the composition of lherzolites which underwent partial melting during 'basalt-extraction'. The IE contents of clinopyroxenes encompass world-wide occurrences. This suggests that processes, other than depletion, occurred. Demarchi et al. (1988) have shown that K is mostly partitioned into glassy patches (blebs) in the xenoliths and glassy drops in clinopyroxenes. The blebs have been interpreted as derived from the breakdown of volatile-bearing wet phases, such as amphibole and/or phlogopite, which melted during ascent to the surface; the glassy drops in clinopyroxenes are generally interpreted as products of incongruent partial melting induced by decompression (Comin- Chiaramonti et al., 1986). Both probably represent the remnants of hydrous phases such as micas and/or amphiboles, and/or products induced by the influx of small-volume, volatile-rich melts (Petrini et al., 1994). In summary, most of the major element chemistry of the Nemby xenoliths (except for K 2 O and to a lesser extent for Na 2 O) are consistent with residual compositions after variable degrees of partial melting and metasomatic effects shown by alkali and IE enrichments in both whole rock and in clinopyroxene. Clinopyroxenes display variable REE enrichments, more evident in those crystals characterized by spongy texture and abundance of glassy drops. A possible explanation for the progressive enrichment of samples characterized by similar HREE and different LREE abundances is different ion-exchange processes (cf. 'simple mixing metasomatism model' of Song and Frey, 1989; Sen et al., 1993) due to the passage of LREE-rich chemical front on

Eclogite-facies metamorphism in impure marble from north Qaidam orogenic belt: Geodynamic implications for early Paleozoic continental-arc collision

Science.gov (United States)

Chen, Xin; Xu, Rongke; Schertl, Hans-Peter; Zheng, Youye

2018-06-01

In the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, impure marble and interbedded eclogite represent a particular sedimentary provenance and tectonic setting, which have important implications for a controversial problem - the dynamic evolution of early Paleozoic subduction-collision complexes. In this contribution, detailed field work, mineral chemistry, and whole-rock geochemistry are presented for impure marble to provide the first direct evidence for the recycling of carbonate sediments under ultrahigh-pressures during subduction and collision in the Yuka terrane, in the North Qaidam UHP metamorphic belt. According to conventional geothermobarometry, pre-peak subduction to 0.8-1.3 GPa/485-569 °C was followed by peak UHP metamorphism at 2.5-3.3 GPa/567-754 °C and cooling to amphibolite facies conditions at 0.6-0.7 GPa/571-589 °C. U-Pb dating of zircons from impure marble reveals a large group with ages ranging from 441 to 458 Ma (peak at 450 Ma), a smaller group ranging from 770 to 1000 Ma (peak at 780 Ma), and minor >1.8 Ga zircon aged ca. 430 Ma UHP metamorphism. The youngest detrital zircons suggest a maximum depositional age of ca. 442 Ma and a burial rate of ca. 1.0-1.1 cm/yr when combined with P-T conditions and UHP metamorphic age. The REE and trace element patterns of impure marble with positive Sr and U anomalies, negative high field strength elements (Nb, Ta, Zr, Hf, and Ti), and Ce anomalies imply that the marble had a marine limestone precursor. Impure marble intercalated with micaschist and eclogite was similar to limestone and siltstone protoliths deposited in continental fore-arc or arc setting with basic volcanic activity. Therefore, the Yuka terrane most likely evolved in a continental island arc setting during the Paleozoic. These data suggest that metasediments were derived from a mixture of Proterozoic continental crust and juvenile early Paleozoic oceanic and/or island arc crust. In addition, their protoliths were likely

Depletion, cryptic metasomatism, and modal metasomatism of central European lithospheric mantle: evidence from elemental and Li isotope compositions of spinel peridotite xenoliths, Kozákov volcano, Czech Republic

Czech Academy of Sciences Publication Activity Database

Medaris Jr., L. G.; Ackerman, Lukáš; Jelínek, E.; Magna, T.

2015-01-01

Roč. 104, č. 8 (2015), s. 1925-1956 ISSN 1437-3254 Institutional support: RVO:67985831 Keywords : Central European lithospheric mantle * geochemistry * geothermometry * Li isotopes * spinel peridotite xenoliths Subject RIV: DD - Geochemistry Impact factor: 2.133, year: 2015

Geology and geochemistry of the Redrock Granite and anorthosite xenoliths (Proterozoic in the northern Burro Mountains, Grant County, New Mexico, USA

Directory of Open Access Journals (Sweden)

Virginia T. McLemore

2002-01-01

Full Text Available Mineral ages from the A-type granites and anorthosite xenoliths in the Redrock area in the northwestern Burro Mountains in southwestern New Mexico cluster around ~1220–1225 Ma and provide yet another example of bimodal igneous activity during this time period in the southwestern United States. The metaluminous to peraluminous, marginally alkaline to subalkaline Redrock Granite exhibits the textural, mineralogical, and geochemical features of A-type granitethat was emplaced at a relatively high crustal level. Field relationships, whole rock and mineral geochemical and isotopic trends suggest that the four phases of the Redrock Granite are genetically related, with the miarolitic biotite/alkali feldspar granite being the youngest phase. Spatial relationships and geochemical data suggest that the anorthosite xenoliths were coeval with the RedrockGranite, which is consistent with the anorthosite being derived from the upper mantle, possibly due to deep mantle upwellings, and the Redrock Granite from the lower crust. The process involved melting in the upper mantle, emplacement of anorthosite in the crust resulting in partial crustal melting and thinning, and, finally, intrusion of shallow silicic plutons, the Redrock Granite. The Redrock Granite and anorthosite were presumably derived from sources characterized by subtle, long-term LREE depletion, with εNd (at 1220 Ma values on theorder of +1 to +2.

Petrogenesis of granitoids and associated xenoliths in the early Paleozoic Baoxu and Enping plutons, South China: Implications for the evolution of the Wuyi-Yunkai intracontinental orogen

Science.gov (United States)

Yu, Yang; Huang, Xiao-Long; Sun, Min; He, Peng-Li

2018-05-01

The early Paleozoic Wuyi-Yunkai orogen was associated with extensive felsic magmatic activities and the orogenic core was mainly distributed in the Yunkai and Wugong domains located in the western Cathaysia block and in the Wuyi domain located in the central part of the Cathaysia block. In order to investigate the evolution of the Wuyi-Yunkai orogen, elemental and Sr-Nd isotopic analyses were performed for granites from the Baoxu pluton in the Yunkai domain and from the Enping pluton in the central part of the Cathaysia block. The Baoxu pluton consists of biotite granite with abundant xenoliths of gneissic granite, granodiorite and diorite, and the Enping pluton is mainly composed of massive granodiorite. Biotite granites (441 ± 5 Ma) and gneissic granite xenolith (443 ± 4 Ma) of the Baoxu pluton are all weakly peraluminous (A/CNK = 1.05-1.10). They show high Sr/Y and La/Yb ratios and have negative bulk-rock εNd(t) values (-7.0 to -4.4), which are similar to coeval gneissic S-type granites in the Yunkai domain and were probably derived from dehydration melting of a sedimentary source with garnet residue in the source. Granodiorites (429 ± 3 Ma) from Enping and granodiorite xenolith (442 ± 4 Ma) from Baoxu are metaluminous and have REE patterns with enriched light REE and flat middle to heavy REE, possibly generated by the dehydration melting of an igneous basement at middle to lower crustal level. Diorite xenolith from Baoxu is ultrapotassic (K2O = 4.9 wt%), has high contents of MgO (7.0 wt%), Cr (379 ppm) and Ni (171 ppm) and shows pronounced negative Nb, Ta and Ti anomalies. This xenolith also has negative εNd(t) value (-3.6) and low Rb/Ba and high Ba/Sr ratios, and is thus interpreted to be derived from an enriched lithospheric mantle with the breakdown of phlogopite. Early Paleozoic I- and S-type granites in the Wuyi-Yunkai orogen mostly have negative εNd(t) values and do not have juvenile components, consistent with genesis by an intracontinental

Coronitic metagabbro and eclogite from the Grenville Province of western Quebec: interpretation of U-Pb geochronology and metamorphism

International Nuclear Information System (INIS)

Indares, A.; Dunning, G.

1997-01-01

We present new U-Pb and metamorphic data on high-pressure coronitic metagabbros from three distinct structural settings in the Parautochthonous belt of the Grenville Province in western Quebec. Intrusive ages are (i) 1217 -10 +15 Ma, for metagabbro close to the Grenville Front, correlative with the Sudbury dykes, defined in Ontario; (ii) 1403 -11 +14 Ma for an eclogitized lens at the base of the highest structural level (SL4), a new age for mafic magmatism in the western Grenville; and (iii) 1218 -34 +53 Ma for metagabbro from SL4, interpreted as correlative with metagabbros from the Algonquin and Shawanaga domains in Ontario. Metamorphism in all cases is Grenvillian, with the best constrained age of 1069 ± 3 Ma for the metagabbro of SL4. Metamorphic grade increases from the Grenville Front to the south. The mafic rocks preserve relict igneous textures overprinted by garnet + clinopyroxene that developed as coronas and (or) pseudomorphs after igneous phases. The highest grade metagabbros contain omphacite and some lack primary plagioclase, therefore being eclogites. However, interpretation of textures and mineral chemistry indicates that they were equilibrated during decompression (at 1350 MPa and 720 o C, sample 51: and at 1200 MPa and 740 o C, sample 29), so maximum depths of burial remain unconstrained. Their evolution is interpreted as follows: (i) high-pressure metamorphism by burial of the Laurentian margin under accreted terranes thrust toward the northwest between 1080 and 1060 Ma; (ii) residence at intermediate crustal levels, for a few tens of millions of years; and (iii) rapid exhumation by renewed thrusting that led to the emplacement of the high-pressure units over the northerly adjacent structural units of the Parautochthonous Belt. (author)

Trace element behavior during serpentinization/de-serpentinization of an eclogitized oceanic lithosphere: A LA-ICPMS study of the Lanzo ultramafic massif (Western Alps)

OpenAIRE

DEBRET, Baptiste; ANDREANI, Muriel; GODARD, Marguerite; NICOLLET, Christian; SCHWARTZ, Stéphane; LAFAY, Romain

2013-01-01

Serpentinites are one of the major components of the oceanic lithosphere and are stable in the slab and the mantle wedge up to 100-150 km depth in subduction zones. During oceanic mantle hydration and alteration, they trap trace and fluid mobile (FME: B, Li, As, Sb, Rb, Ba, Cs, Sr, U and Pb) elements that participate to elemental transfer occurring between the dehydrating slab and the mantle wedge in subduction context. The Lanzo massif is an eclogitized oceanic lithosphere that preserved its...

Combined iron and magnesium isotope geochemistry of pyroxenite xenoliths from Hannuoba, North China Craton: implications for mantle metasomatism

Science.gov (United States)

Zhao, Xin Miao; Cao, Hui Hui; Mi, Xue; Evans, Noreen J.; Qi, Yu Han; Huang, Fang; Zhang, Hong Fu

2017-06-01

We present high-precision iron and magnesium isotopic data for diverse mantle pyroxenite xenoliths collected from Hannuoba, North China Craton and provide the first combined iron and magnesium isotopic study of such rocks. Compositionally, these xenoliths range from Cr-diopside pyroxenites and Al-augite pyroxenites to garnet-bearing pyroxenites and are taken as physical evidence for different episodes of melt injection. Our results show that both Cr-diopside pyroxenites and Al-augite pyroxenites of cumulate origin display narrow ranges in iron and magnesium isotopic compositions (δ57Fe = -0.01 to 0.09 with an average of 0.03 ± 0.08 (2SD, n = 6); δ26Mg = - 0.28 to -0.25 with an average of -0.26 ± 0.03 (2SD, n = 3), respectively). These values are identical to those in the normal upper mantle and show equilibrium inter-mineral iron and magnesium isotope fractionation between coexisting mantle minerals. In contrast, the garnet-bearing pyroxenites, which are products of reactions between peridotites and silicate melts from an ancient subducted oceanic slab, exhibit larger iron isotopic variations, with δ57Fe ranging from 0.12 to 0.30. The δ57Fe values of minerals in these garnet-bearing pyroxenites also vary widely (-0.25 to 0.08 in olivines, -0.04 to 0.25 in orthopyroxenes, -0.07 to 0.31 in clinopyroxenes, 0.07 to 0.48 in spinels and 0.31-0.42 in garnets). In addition, the garnet-bearing pyroxenite shows light δ26Mg (-0.43) relative to the mantle. The δ26Mg of minerals in the garnet-bearing pyroxenite range from -0.35 for olivine and orthopyroxene, to -0.34 for clinopyroxene, 0.04 for spinel and -0.68 for garnet. These measured values stand in marked contrast to calculated equilibrium iron and magnesium isotope fractionation between coexisting mantle minerals at mantle temperatures derived from theory, indicating disequilibrium isotope fractionation. Notably, one phlogopite clinopyroxenite with an apparent later metasomatic overprint has the heaviest δ57Fe

The Effect of CO2 on Partial Reactive Crystallization of MORB-Eclogite-derived Basaltic Andesite in Peridotite and Generation of Silica-Undersaturated Basalts

Science.gov (United States)

Mallik, A.; Dasgupta, R.

2012-12-01

Recycled oceanic crust (MORB-eclogite) is considered to be the dominant heterogeneity in Earth's mantle. Because MORB-eclogite is more fusible than peridotite, siliceous partial melt derived from it must react with peridotite while the latter is still in the subsolidus state. Thus, studying such reactive process is important in understanding melting dynamics of the Earth's mantle. Reaction of MORB-eclogite-derived andesitic partial melt with peridotite can produce alkalic melts by partial reactive crystallization but these melts are not as silica-undersaturated as many natural basanites, nephelinites or melititites [1]. In this study, we constrain how dissolved CO2 in a siliceous MORB-eclogite-derived partial melt affects the reaction phase equilibria involving peridotite and can produce nephelinitic melts. Here we compare experiments on CO2-free [1] and 2.6 wt.% CO2 bearing andesitic melt+lherzolite mixtures conducted at 1375 °C and 3 GPa with added melt fraction of 8-50 wt.%. In both CO2-free and CO2-bearing experiments, melt and olivine are consumed and opx and garnet are produced, with the extent of modal change for a given melt-rock ratio being greater for the CO2-bearing experiments. While the residue evolves to a garnet websterite by adding 40% of CO2-bearing melt, the residue becomes olivine-free by adding 50% of the CO2-free melt. Opx mode increases from 12 to ~55 wt.% for 0 to 40% melt addition in CO2-bearing system and 12 to ~43 wt.% for 0 to 50% melt addition in CO2-free system. Garnet mode, for a similar range of melt-rock ratio, increases from ~10 to ~15 wt.% for CO2 bearing system and to ~11 wt.% for CO2-free system. Reacted melts from 25-33% of CO2-bearing melt-added runs contain ~39 wt.% SiO2 , ~11-13 wt.% TiO2, ~9 wt.% Al2O3, ~11 wt.% FeO*, 16 wt.% MgO, 10-11 wt.% CaO, and 3 wt.% Na2O whereas experiments with a similar melt-rock ratio in a CO2-free system yield melts with 44-45 wt.% SiO2, 6-7 wt.% TiO2, 13-14 wt.% Al2O3, 10-11 wt.% FeO*, 12-13 wt

A micro-scale investigation of melt production and extraction in the upper mantle based on silicate melt pockets in ultramafic xenoliths from the Bakony-Balaton Highland Volcanic Field (Western Hungary)

DEFF Research Database (Denmark)

Bali, Eniko; Zanetti, A.; Szabo, C.

2008-01-01

Mantle xenoliths in Neogene alkali basalts of the Bakony-Balaton Highland Volcanic Field (Western Hungary) frequently have melt pockets that contain silicate minerals, glass, and often carbonate globules. Textural, geochemical and thermobarometric data indicate that the melt pockets formed at rel...

Paleozoic and Paleoproterozoic Zircon in Igneous Xenoliths Assimilated at Redoubt Volcano, Alaska

Science.gov (United States)

Bacon, C. R.; Vazquez, J. A.; Wooden, J. L.

2010-12-01

Historically active Redoubt Volcano is a basalt-to-dacite cone constructed upon the Jurassic-early Tertiary Alaska-Aleutian Range batholith. New SHRIMP-RG U-Pb age and trace-element concentration results for zircons from gabbroic xenoliths and crystal-rich andesitic mush from a late Pleistocene pyroclastic deposit indicate that ~310 Ma and ~1865 Ma igneous rocks underlie Redoubt at depth. Two gabbros have sharply terminated prismatic zircons that yield ages of ~310 Ma. Zircons from a crystal mush sample are overwhelmingly ~1865 Ma and appear rounded due to incomplete dissolution. Binary plots of element concentrations or ratios show clustering of data for ~310-Ma grains and markedly coherent trends for ~1865-Ma grains; e.g., ~310-Ma grains have higher Eu/Eu* than most of the ~1865-Ma grains, the majority of which form a narrow band of decreasing Eu/Eu* with increasing Hf content which suggests that ~1865-Ma zircons come from igneous source rocks. It is very unlikely that detrital zircons from a metasedimentary rock would have this level of homogeneity in age and composition. One gabbro contains abundant ~1865 Ma igneous zircons, ~300-310 Ma fluid-precipitated zircons characterized by very low U and Th concentrations and Th/U ratios, and uncommon ~100 Ma zircons. We propose that (1) ~310 Ma gabbro xenoliths from Redoubt Volcano belong to the same family of plutons dated by Aleinikoff et al. (USGS Circular 1016, 1988) and Gardner et al. (Geology, 1988) located ≥500 km to the northeast in basement rocks of the Wrangellia and Alexander terranes and (2) ~1865 Ma zircons are inherited from igneous rock, potentially from a continental fragment that possibly correlates with the Fort Simpson terrane or Great Bear magmatic zone of the Wopmay Orogen of northwestern Laurentia. Possibly, elements of these Paleoproterozoic terranes intersected the Paleozoic North American continental margin where they may have formed a component of the basement to the Wrangellia

REE, Sm-Nd and U-Pb zircon study of eclogites from the Alpine External Massifs (Western Alps): Evidence for crustal contamination

International Nuclear Information System (INIS)

Paquette, J.L.; Eidgenoessische Technische Hochschule, Zurich; Menot, R.P.; Peucat, J.J.

1989-01-01

A geochemical and geochronological study of the Alpine External Crystalline Massifs (AECM) of Aiguilles Rouges, Belledonne and Argentera was undertaken in order to constrain the geodynamic evolution of this segment of the Variscan foldbelt. Another aim of the study is to characterize the behaviour of isotopic markers, in particular the U-Pb zircon system, under high-grade metamorphic conditions. The whole-rock geochemistry of eclogites and amphibolites was investigated using major and trace element (including the REE) analytical techniques; isotopic studies were performed by application of the Sm-Nd whole-rock and U-Pb zircon methods. In terms of regional geological history, the early development of metamorphic and magmatic activity in the AECM is typical of the extensional tectonic regime observed throughout the Variscan foldbelt during the Cambro-Ordovician (i.e. basic magmatism dated at 475-450 Ma). The composition of the metabasic rocks is closely similar to tholeiites emplaced into thinned continental crust which are generally associated with the initial stages of oceanic rifting. The source regions for these metabasics are characterized by initial ε Nd values between +6 and +8, suggesting depleted mantle sources influenced by a weak crustal component and/or the existence of a metasomatised lithosphere. The multi-stage eclogite-facies metamorphism is dated at 425-395 Ma (i.e. Silurian). An application of the U-Pb method, associated with the artificial abrasion of zircon grains, has led to the recognition of a weak crustal contamination in the metabasic protoliths. This is implied by the Archaean and Lower Proterozoic upper intercepts on Concordia - devoid of geological significance - which reflect the presence of a pre-existing basement to the AECM. (orig./WL)

Processes accompanying of mantle plume emplacement into continental lithosphere: Evidence from NW Arabian plate, Western Syria

Science.gov (United States)

Sharkov, E. V.

2015-12-01

Lower crustal xenoliths occurred in the Middle Cretaceous lamprophyre diatremes in Jabel Ansaria (Western Syria) (Sharkov et al., 1992). They are represented mainly garnet granulites and eclogite-like rocks, which underwent by deformations and retrograde metamorphism, and younger fresh pegmatoid garnet-kaersutite-clinopyroxene (Al-Ti augite) rocks; mantle peridotites are absent in these populations. According to mineralogical geothermobarometers, forming of garnet-granulite suite rocks occurred under pressure 13.5-15.4 kbar (depths 45-54 kn) and temperature 965-1115oC. At the same time, among populations of mantle xenoliths in the Late Cenozoic platobasalts of the region, quite the contrary, lower crustal xenoliths are absent, however, predominated spinel lherzolites (fragments of upper cooled rim of a plume head), derived from the close depths (30-40 km: Sharkov, Bogatikov, 2015). From this follows that ancient continental crust was existed here even in the Middle Cretaceous, but in the Late Cenozoic was removed by extended mantle plume head; at that upper sialic crust was not involved in geomechanic processes, because Precambrian metamorphic rocks survived as a basement for Cambrian to Cenozoic sedimentary cover of Arabian platform. In other words, though cardinal rebuilding of deep-seated structure of the region occurred in the Late Cenozoic but it did not affect on the upper shell of the ancient lithosphere. Because composition of mantle xenolithis in basalts is practically similar worldwide, we suggest that deep-seated processes are analogous also. As emplacement of the mantle plume heads accompanied by powerful basaltic magmatism, very likely that range of lower (mafic) continental crust existence is very convenient for extension of plume heads and their adiabatic melting. If such level, because of whatever reasons, was not reached, melting was limited but appeared excess of volatile matters which led to forming of lamprophyre or even kimberlite.

The geological record of base metal sulfides in the cratonic mantle: A microscale 187Os/188Os study of peridotite xenoliths from Somerset Island, Rae Craton (Canada)

Science.gov (United States)

Bragagni, A.; Luguet, A.; Fonseca, R. O. C.; Pearson, D. G.; Lorand, J.-P.; Nowell, G. M.; Kjarsgaard, B. A.

2017-11-01

We report detailed petrographic investigations along with 187Os/188Os data in Base Metal Sulfide (BMS) on four cratonic mantle xenoliths from Somerset Island (Rae Craton, Canada). The results shed light on the processes affecting the Re-Os systematics and provide time constraints on the formation and evolution of the cratonic lithospheric mantle beneath the Rae craton. When devoid of alteration, BMS grains mainly consist of pentlandite + pyrrhotite ± chalcopyrite. The relatively high BMS modal abundance of the four investigated xenoliths cannot be reconciled with the residual nature of these peridotites, but requires addition of metasomatic BMS. This is especially evident in the two peridotites with the highest bulk Pd/Ir and Pd/Pt. Metasomatic BMS likely formed during melt/fluid percolation in the Sub Continental Lithospheric Mantle (SCLM) as well as during infiltration of the host kimberlite magma, when djerfisherite crystallized around older Fe-Ni-sulfides. On the whole-rock scale, kimberlite metasomatism is visible in a subset of bulk xenoliths, which defines a Re-Os errorchron that dates the host magma emplacement. The 187Os/188Os measured in the twenty analysed BMS grains vary from 0.1084 to >0.17 and it shows no systematic variation depending on the sulfide mineralogical assemblage. The largest range in 187Os/188Os is observed in BMS grains from the two xenoliths with the highest Pd/Ir, Pd/Pt, and sulfide modal abundance. The whole-rock TRD ages of these two samples underestimate the melting age obtained from BMS, demonstrating that bulk Re-Os model ages from peridotites with clear evidence of metasomatism should be treated with caution. The TRD ages determined in BMS grains are clustered around 2.8-2.7, ∼2.2 and ∼1.9 Ga. The 2.8-2.7 Ga TRD ages document the main SCLM building event in the Rae craton, which is likely related to the formation of the local greenstone belts in a continental rift setting. The Paleoproterozoic TRD ages can be explained by

Rheological properties of the lower crust and upper mantle beneath Baja California: a microstructural study of xenoliths from San Quintin

Science.gov (United States)

Van der Werf, Thomas F.; Chatzaras, Vasileios; Tikoff, Basil; Drury, Martyn R.

2016-04-01

Baja California is an active transtensional rift zone, which links the San Andreas Fault with the East Pacific Rise. The erupted basalts of the Holocene San Quintin volcanic field contain xenoliths, which sample the lower crust and upper mantle beneath Baja California. The aim of this research is to gain insight in the rheology of the lower crust and the upper mantle by investigating the xenolith microstructure. Microstructural observations have been used to determine the dominant deformation mechanisms. Differential stresses were estimated from recrystallized grain size piezometry of plagioclase and clinopyroxene for the lower crust and olivine for the upper mantle. The degree of deformation can be inferred from macroscopic foliations and the deformation microstructures. Preliminary results show that both the lower crust and the upper mantle have been affected by multiple stages of deformation and recrystallization. In addition the dominant deformation mechanism in both the lower crust and the upper mantle is dislocation creep based on the existence of strong crystallographic preferred orientations. The differential stress estimates for the lower crust are 10-29 MPa using plagioclase piezometry and 12-35 MPa using clinopyroxene piezometry. For the upper mantle, differential stress estimates are 10-20 MPa. These results indicate that the strength of the lower crust and the upper mantle are very similar. Our data do not fit with the general models of lithospheric strength and may have important implications for the rheological structure of the lithosphere in transtensional plate margins and for geodynamic models of the region.

Extreme Hf-Os Isotope Compositions in Hawaiian Peridotite Xenoliths: Evidence for an Ancient Recycled Lithosphere

Science.gov (United States)

Bizimis, M.; Lassiter, J. C.; Salters, V. J.; Sen, G.; Griselin, M.

2004-12-01

We report on the first combined Hf-Os isotope systematics of spinel peridotite xenoliths from the Salt Lake Crater (SLC), Pali and Kaau (PK) vents from the island of Oahu, Hawaii. These peridotites are thought to represent the Pacific oceanic lithosphere beneath Oahu, as residues of MORB-type melting at a paleo-ridge some 80-100Ma ago. Clinopyroxene mineral separates in these peridotites have very similar Nd and Sr isotope compositions with the post erosional Honolulu Volcanics (HV) lavas that bring these xenoliths to the surface. This and their relatively elevated Na and LREE contents suggest that these peridotites are not simple residues of MORB-type melting but have experience some metasomatic enrichment by the host HV lavas. However, the SLC and PK xenoliths show an extreme range in Hf isotope compositions towards highly radiogenic values (ɛ Hf= 7-80), at nearly constant Nd isotope compositions (ɛ Nd= 7-10), unlike any OIB or MORB basalt. Furthermore, these Oahu peridotites show a bimodal distribution in their bulk rock 187Os/186Os ratios: the PK peridotites have similar ratios to the abyssal peridotites (0.130-0.1238), while the SLC peridotites have highly subchondritic ratios (0.1237-0.1134) that yield 500Ma to 2Ga Re-depletion ages. Hf-Os isotopes show a broad negative correlation whereby the samples with the most radiogenic 176Hf/177Hf have the most unradiogenic 187Os/186Os ratios. Based on their combined Hf-Os-Nd isotope and major element compositions, the PK peridotites can be interpreted as fragments of the Hawaiian lithosphere, residue of MORB melting 80-100Ma ago, that have been variably metasomatized by the host HV lavas. In contrast, the extreme Hf-Os isotope compositions of the SLC peridotites suggest that they cannot be the source nor residue of any kind of Hawaiian lavas, and that Hf and Os isotopes survived the metasomatism or melt-rock reaction that has overprinted the Nd and Sr isotope compositions of these peridotites. The ancient (>1Ga

Melt inclusion: methods, applications and problem: Silica-rich melts in quartz xenoliths from Vulcano islands and their bearing on processes of crustal melting and crust-magma interaction in the Aeolian Arc, Italy

NARCIS (Netherlands)

Frezzotti, M.L.; Zavon, V.; Peccerillo, A.; Nikogosian, I.

2002-01-01

Silica-rich melts in quartz xenoliths from Vulcano islands and their bearing on processes of crustal melting and crust-magma interaction in the Aeolian Arc, Italy Proceedings of workshop Melt inclusion: methods, applications and problem. Napoli, Italy, September 2002, p. 71-73

Lower-crustal xenoliths from Jurassic kimberlite diatremes, upper Michigan (USA): Evidence for Proterozoic orogenesis and plume magmatism in the lower crust of the southern Superior Province

Science.gov (United States)

Zartman, Robert E.; Kempton, Pamela D.; Paces, James B.; Downes, Hilary; Williams, Ian S.; Dobosi, Gábor; Futa, Kiyoto

2013-01-01

Jurassic kimberlites in the southern Superior Province in northern Michigan contain a variety of possible lower-crustal xenoliths, including mafic garnet granulites, rare garnet-free granulites, amphibolites and eclogites. Whole-rock major-element data for the granulites suggest affinities with tholeiitic basalts. P–T estimates for granulites indicate peak temperatures of 690–730°C and pressures of 9–12 kbar, consistent with seismic estimates of crustal thickness in the region. The granulites can be divided into two groups based on trace-element characteristics. Group 1 granulites have trace-element signatures similar to average Archean lower crust; they are light rare earth element (LREE)-enriched, with high La/Nb ratios and positive Pb anomalies. Most plot to the left of the geochron on a 206Pb/€204Pb vs 207Pb/€204Pb diagram, and there was probably widespread incorporation of Proterozoic to Archean components into the magmatic protoliths of these rocks. Although the age of the Group 1 granulites is not well constrained, their protoliths appear to be have been emplaced during the Mesoproterozoic and to be older than those for Group 2 granulites. Group 2 granulites are also LREE-enriched, but have strong positive Nb and Ta anomalies and low La/Nb ratios, suggesting intraplate magmatic affinities. They have trace-element characteristics similar to those of some Mid-Continent Rift (Keweenawan) basalts. They yield a Sm–Nd whole-rock errorchron age of 1046 ± 140 Ma, similar to that of Mid-Continent Rift plume magmatism. These granulites have unusually radiogenic Pb isotope compositions that plot above the 207Pb/€204Pb vs 206Pb/€204Pb growth curve and to the right of the 4·55 Ga geochron, and closely resemble the Pb isotope array defined by Mid-Continent Rift basalts. These Pb isotope data indicate that ancient continental lower crust is not uniformly depleted in U (and Th) relative to Pb. One granulite xenolith, S69-5, contains quartz, and has a

Metasomatized granulites of the Mozambique belt: consequences for lithospheric U, Th, REE fertilisation and metallogenesis in the ancient Gondwanaland supercontinent

International Nuclear Information System (INIS)

Andreoli, M.A.G.; Hart, R.J.

1988-02-01

The 1,0 Ga old Lurio belt extends for ca. 1 000 km from Nsanje (S Malawi) through NE Mozambique to the Indian ocean. Lower crustal levels are locally exposed along its southern tectonic front. In this article mineralogical and geochemical data for an andesinite-mafic-ultramafic suite from Nsanje were reported.The results indicate that this complex terrane equilibrated first at P ∼ 13 kbr and T ∼ 900 0 C and subsequently cooled under an eclogite-garnet granulite georem. During a later event (P ∼ 7-10 kbar, T ∼ 650 - 800 0 C) distinctive metasomatic mica, amphibole, scapolite, apatite, diopsidic pyroxene (MASAD)-bearing assemblages and pegmatoids were formed by CO 2 , Cl, H 2 O, S and F rich fluids with higher REE, U, Th and Zr concentrations than the high-grade precursors. MASAD and older high-pressure granulite parageneses underwent subsequent dehydratation and reequilibration under medium-pressure granulite facies conditions perhaps during the Lurio orogenic event. MASAD-like assemblages are relatively common within the late Proterozoic medium- and high-pressure granulite terranes of Central Gondwana, especially in the newly defined Lurio-Zambezi Eclogite Province. The data provide indicate that the metasomatizing, MASAD-forming fluids had crypto-carbonatitic affinities and were introduced into the crust from the upper mantle during protorifting episodes between ca. 1,1 and 0,5 Ga ago. The MASAD assemblages may therefore represent the crustal equivalent of the metasomatic and MARID suites discovered in mantle xenoliths

Geochemical and petrological constraints on mantle composition of the Ohře(Eger) rift, Bohemian Massif: peridotite xenoliths from the České Středohoří Volcanic complex and northern Bohemia

Czech Academy of Sciences Publication Activity Database

Ackerman, Lukáš; Medaris Jr., G.; Špaček, P.; Ulrych, Jaromír

2015-01-01

Roč. 104, č. 8 (2015), s. 1957-1979 ISSN 1437-3254 R&D Projects: GA ČR GA205/09/1170 Institutional support: RVO:67985831 Keywords : Bohemian Massif * mantle * metasomatism * Ohře(Eger) rift * xenolith Subject RIV: DB - Geology ; Mineralogy Impact factor: 2.133, year: 2015

Quartz exsolution topotaxy in clinopyroxene from the UHP eclogite of Weihai, China

Science.gov (United States)

Xu, Haijun; Zhang, Junfeng; Zong, Keqing; Liu, Liang

2015-06-01

Abundant oriented silica precipitates of α-quartz (4.0 ± 1.0 vol.%), in part coexisting with calcic amphiboles (topotactic relationships with their host clinopyroxenes. Three types of crystallographic topotactic relationship have been identified between quartz and host clinopyroxene: (1) 52% quartz precipitates share the same orientation for the c-axes with [0001]qz//[001]cpx; (2) 34% quartz precipitates share the same orientation for the a-axes with [11 2 bar0]qz//[001]cpx; and (3) 11% quartz precipitates share the same orientation for the s-planes with (11 2 bar1)qz//(100)cpx. Other quartz axes and planes disperse in large or small girdles around the shared axes or planes. Many quartz rods/needles are elongated parallel to the [001]cpx with the long axes of quartz being either [0001]qz or [11 2 bar0]qz. Amphibole precipitates have also a strong crystallographic relationship with host clinopyroxene, i.e., (100)amp//(100)cpx, [010]amp//[010]cpx, and [001]amp//[001]cpx. These results provide quantitative microstructural evidence supporting an exsolution origin for oriented quartz needles/rods in clinopyroxene and demonstrate that the exsolution of quartz from clinopyroxene occurred within the stability field of α-quartz rather than coesite. The oriented precipitates of α-quartz, in part coexisting with calcic amphiboles, in host clinopyroxene are probably promoted by supercritical fluid or partial melting during the early exhumation of eclogites. Our results suggest that oriented quartz precipitates in clinopyroxene cannot be used as an indisputable UHP-indicator.

Crystal preferred orientations of minerals from mantle xenoliths in alkali basaltic rocks form the Catalan Volcanic Zone (NE Spain)

Science.gov (United States)

Fernández-Roig, Mercè; Galán, Gumer; Mariani, Elisabetta

2015-04-01

Mantle xenoliths in alkali basaltic rocks from the Catalan Volcanic Zone, associated with the Neogene-Quaternary rift system in NE Spain, are formed of anhydrous spinel lherzolites and harzburgites with minor olivine websterites. Both peridotites are considered residues of variable degrees of partial melting, later affected by metasomatism, especially the harzburgites. These and the websterites display protogranular microstructures, whereas lherzolites show continuous variation between protogranular, porphyroclastic and equigranular forms. Thermometric data of new xenoliths indicate that protogranular harzburgites, lherzolites and websterites were equilibrated at higher temperatures than porphyroclastic and equigranular lherzolites. Mineral chemistry also indicates lower equilibrium pressure for porphyroclastic and equigranular lherzolites than for the protogranular ones. Crystal preferred orientations (CPOs) of olivine and pyroxenes from these new xenoliths were determined with the EBSD-SEM technique to identify the deformation stages affecting the lithospheric mantle in this zone and to assess the relationships between the deformation fabrics, processes and microstructures. Olivine CPOs in protogranular harzburgites, lherzolites and a pyroxenite display [010]-fiber patterns characterized by a strong point concentration of the [010] axis normal to the foliation and girdle distribution of [100] and [001] axes within the foliation plane. Olivine CPO symmetry in porphyroclastic and equigranular lherzolites varies continuously from [010]-fiber to orthorhombic and [100]-fiber types. The orthorhombic patterns are characterized by scattered maxima of the three axes, which are normal between them. The rare [100]-fiber patterns display strong point concentration of [100] axis, with normal girdle distribution of the other two axes, which are aligned with each other. The patterns of pyroxene CPOs are more dispersed than those of olivine, especially for clinopyroxene, but

Carbonatitic liquids and COH fluids from epidote-dolomite eclogites at 3.7 - 4.6 GPa: new perspectives on carbon transfer at subduction zones

Science.gov (United States)

Poli, S.

2013-12-01

Current knowledge on the solidus temperature for carbonate-bearing rocks suggests that carbonatitic liquids should not form in a subducted oceanic lithosphere, unless anomalous thermal relaxation occurs. For a mildly warm subduction path, COH-bearing basaltic eclogites are expected to loose all H2O component at epidote breakdown, located at approx. 2.8-3.0 GPa. Above this pressure limit, the solidus is that of a carbonated basaltic eclogite which shows a minimum temperature of 1020 °C at 4.0-4.5 GPa (Dasgupta et al. 2004). However, the oceanic crust includes a range of gabbroic rocks, altered on rifts and transforms, with large amounts of An-rich plagioclase. It has been shown that epidote disappearance with pressure depend on the normative anorthite content of the bulk composition considered (Poli et al. 2009); we therefore expect that altered gabbros might display a much wider pressure range where epidote persists, potentially affecting the solidus relationships. Notably, this applies to epidosite rocks formed in hydrothermal environments at oceanic settings, then recovered in high-pressure and ultra-high pressure terrains. New experimental data from 3.7 to 4.6 GPa, 750°C to 1000 °C are intended to unravel the effect of variable bulk and volatile compositions in model eclogites, enriched in the normative anorthite component (An37 and An45). Experiments are performed in piston cylinder and multianvil machines apparatus, using both single and, buffered, double capsule techniques. Garnet, clinopyroxene and coesite form in all syntheses. Lawsonite was found to persist at 3.7 GPa, 750 °C, with both dolomite and magnesite; at 3.8 GPa, 775-800 °C, fluid saturated conditions, epidote coexists with kyanite, dolomite and magnesite. The anhydrous assemblage garnet, omphacite, aragonite, kyanite is found at 4.2 GPa, 850 °C. At 900 °C, fluid-rich conditions, a silicate fluid/melt of granitoid composition, a carbonatitic melt and Na-carbonate are observed. Close to

U-Pb dating on zircons for the la Borie eclogite (Haut-Allier, France) and consequences for the pre-Variscan evolution of the Western Europe

Energy Technology Data Exchange (ETDEWEB)

Ducrot, J.; Lancelot, J.R. (Montpellier-2 Univ., 44 (France). Lab. de Geochimie-Isotopique); Marchand, J. (Nantes Univ., 34 (France). Lab. de Petrologie et Minerologie)

1983-03-01

A crystallization age of 432sub(-10)/sup +20/Ma has been determined for the 'La Borie' eclogite (Haut-Allier, France), by U-Pb dating on zircons. This age is yielded by the upper intercept with the Concordia curve. In agreement with recent dates obtained in the southern part of the Massif Central, in Brittany and in Vendee, these data indicate that the high-pressure/high-temperature-metamorphic phase representative of the pre-Variscan evolution of the western Europe, took place at the Silurian/Ordovician boundary. This high-pressure/high-temperature metamorphism could be significant of subduction processes more or less synchronous, preceeding the main continental collision responsible of the Variscan belt.

U-Pb dating on zircons for the la Borie eclogite (Haut-Allier, France) and consequences for the pre-Variscan evolution of the Western Europe

International Nuclear Information System (INIS)

Ducrot, J.; Lancelot, J.R.; Marchand, J.

1983-01-01

A crystallization age of 432sub(-10) +20 Ma has been determined for the 'La Borie' eclogite (Haut-Allier, France), by U-Pb dating on zircons. This age is yielded by the upper intercept with the Concordia curve. In agreement with recent dates obtained in the southern part of the Massif Central, in Brittany and in Vendee, these data indicate that the high-pressure/high-temperature-metamorphic phase representative of the pre-Variscan evolution of the western Europe, took place at the Silurian/Ordovician boundary. This high-pressure/high-temperature metamorphism could be significant of subduction processes more or less synchronous, preceeding the main continental collision responsible of the Variscan belt. (s.s.). (orig.)

Composition of uppermost mantle beneath the Northern Fennoscandia - numerical modeling and petrological interpretation

Science.gov (United States)

Virshylo, Ivan; Kozlovskaya, Elena; Prodaivoda, George; Silvennoinen, Hanna

2013-04-01

restricted volume in the multi-dimensional parameter space. In order to constrain concentration of minerals we used equilibrium of mineral associations for selected P-T condition obtained by free Gibbs energy minimization (c.f. Stixrude & Lithgow-Bertelloni, 2005). We also considered the mineralogical composition of upper mantle xenoliths, although the representativeness of xenoliths in Precambrian rocks could be treated with care, if one tries to describe the modern mantle. As a first step, we estimated 1D model of mineralogical composition in the depth range of 35-350 km using the IASP91 reference model (Kennett & Engdahl, 1991). Both the P- and S- wave velocities were used for inversion, in order to improve the reliability of the model. More comprehensive result could be obtained if density distribution is involved. In our study we used the 1D PEMC density model (Dziewonski, Hales & Lapwood, 1975) as it is the most adequate for the continental lithosphere. The 1D modeling showed that the garnet lherzolite model (forsterite, fayalite, enstatite, ferrosilite, diopside, jadeite, pyrope) can be considered as a basic one. The end-members of olivine and orthopyroxene solutions were included with the aim of Fe/Mg ratio estimation. Testing with modified models including hedenbergite, harzburgite spinel, etc. showed that these minerals have no significant influence on bulk elastic properties. Selected set of minerals allows modelling the most species of peridote-pyroxenite associations known from xenoliths investigations (Kukkonen, Kuusisto, Lehtonen, & Peltonen, 2008; Lehtonen, O'Brien, Peltonen, Johanson, & Pakkanen, 2004). However, there exist also a number of evidences for mantle eclogite xenoliths from the region under study and its surrounding (Lehtonen et al., 2004; Peltonen, Kinnunen, & Huhma, 2002). That is why we also made modelling for garnet-clinopyroxene model of eclogite. The volumetric mineral compositions obtained were transformed into weight concentration of rock

3D Numerical Examination of Continental Mantle Lithosphere Response to Lower Crust Eclogitization and Nearby Slab Subduction

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Janbakhsh, P.; Pysklywec, R.

2017-12-01

2D numerical modeling techniques have made great contribution to understanding geodynamic processes involved in crustal and lithospheric scale deformations for the past 20 years. The aim of this presentation is to expand the scope covered by previous researchers to 3 dimensions to address out-of-plane intrusion and extrusion of mantle material in and out of model space, and toroidal mantle wedge flows. In addition, 3D velocity boundary conditions can create more realistic models to replicate real case scenarios. 3D numerical experiments that will be presented are designed to investigate the density and viscosity effects of lower crustal eclogitization on the decoupling process of continental mantle lithosphere from the crust and its delamination. In addition, these models examine near-field effects of a subducting ocean lithosphere and a lithospheric scale fault zone on the evolution of the processes. The model solutions and predictions will also be compared against the Anatolian geology where subduction of Aegean and Arabian slabs, and the northern boundary with the North Anatolian Fault Zone are considered as two main contributing factors to anomalous crustal uplift, missing mantle lithosphere, and anomalous surface heat flux.

Structural and mineralogical studies of the Tso Morari Dome: Insight into the deformation kinematics of the eclogitic gneiss, Ladakh Himalaya, India

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Dutta, D.; Mukherjee, S.

2017-12-01

Coesite-bearing eclogites from the Tso Morari Dome (TMD) and the Kaghan valley (Pakistan) are two examples from the Himalayan orogen that attained UHP conditions within 5 Ma, by subducting the frontal part of the advancing Indian plate through a subduction channel, and subsequently extruded rapidly ( 17 mm yr-1). This study focuses on the deformation of the gneissic rock that hosts the UHP eclogites. 25 rock samples were collected from two transects viz. (A) Sumdo-Karzok and (B) Sumdo-Debring. Preliminary thin-section studies reveal differences in microstructural characters between the rocks of A and B. Although dynamically recrystallised quartz grains are present in all these samples, grain boundary migration recyrstallisation ( 530-650 °C) are better preserved in the rocks of A. Similarly, intra-granular fractures in both quartz and feldspars, the latter being dominant, are more prominent in the samples along A. Chessboard extinction patterns (> 700 °C) in quartz, micro-faults in plagioclase grains and undulatory extinction in micas are also present. Samples close to the Zildat shear zone (ZSZ; N margin of the TMD) exhibit medium-sized, lenticular quartzo-feldspathic grains. Their abundance wanes away from the fault possibly due to decreasing deformation intensity. XRD studies reveal a decline in the ratio of modal percentage K-feldspar to that of muscovite towards the N margin: the fall being more gradual along B. Biotites are less abundant ( 14 % towards the ZSZ. Previous workers reported similar increase in micaceous minerals in ductile- and brittle shear zones from other terrains, and suggested higher fluid activity as the key factor. Subduction of the Indian continental crust and subsequent exhumation, along the subduction channel, followed Coutte- and Poiseuille flows, respectively. Hence, rocks near the ZSZ should exhibit opposing shear senses, which we encounter both at micro- and meso-scales. Besides, Google Earth images show geomorphologic features

Deformation and seismic anisotropy of the subcontinental lithospheric mantle in NE Spain: EBSD data on xenoliths from the Catalan Volcanic Zone

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Fernández-Roig, Mercè; Galán, Gumer; Mariani, Elisabetta

2017-02-01

Mantle xenoliths in Neogene-Quaternary basaltic rocks related to the European Cenozoic Rift System serve to assess the evolution of the subcontinental lithospheric mantle beneath the Catalan Volcanic Zone in NE Spain. Crystallographic preferred orientations, major element composition of minerals, and temperature and pressure estimates have been used to this end. The mantle consists of spinel lherzolites, harzburgites and subordinate websterites. Protogranular microstructures are found in all peridotites and websterites, but lherzolites also display finer-grained porphyroclastic and equigranular microstructures. The dominant olivine deformation fabric is [010] fiber, but subordinate orthorhombic and [100]-fiber types are also present, especially in porphyroclastic and equigranular lherzolites. The fabric strength (J index = 10.12-1.91), equilibrium temperature and pressure are higher in xenoliths with [010]-fiber fabric and decrease in those with orthorhombic and [100]-fiber type. Incoherence between olivine and pyroxene deformation fabric is mostly found in porphyroclastic and equigranular lherzolites. Seismic anisotropy, estimated from the crystal preferred orientations, also decreases (AVp = 10.2-2.60%; AVs max = 7.95-2.19%) in porphyroclastic and equigranular lherzolites. The olivine [010]-fiber fabric points to deformation by simple shear or transpression which is likely to have occured during the development of late-Hercynian strike-slip shear zones, and to subsequent annealing during late Hercynian decompression, Permian and Cretaceous rifting. Also, it cannot be excluded that the percolation of mafic magmas during these extensional events provoked the refertilization of the lithospheric mantle. However, no clear relationship has been observed between fabric strength and mineral mode and composition. Later transtensional deformation during late Alpine orogenesis, at higher stress and decreasing temperature and pressure, transformed the earlier fabric into

Grain-scale Sr isotope heterogeneity in amphibolite (retrograded UHP eclogite, Dabie terrane): Implications for the origin and flow behavior of retrograde fluids during slab exhumation

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Guo, Shun; Yang, Yueheng; Chen, Yi; Su, Bin; Gao, Yijie; Zhang, Lingmin; Liu, Jingbo; Mao, Qian

2016-12-01

To constrain the origin and flow behavior of amphibolite-facies retrograde fluids during slab exhumation, we investigate the textures, trace element contents, and in situ strontium (Sr) isotopic compositions (using LA-MC-ICP-MS) of multiple types of epidote and apatite in the UHP eclogite and amphibolites from the Hualiangting area (Dabie terrane, China). The UHP epidote porphyroblasts in the eclogite (Ep-E), which formed at 28-30 kbar and 660-720 °C, contain high amounts of Sr, Pb, Th, Ba, and light rare earth elements (LREEs) and have a narrow range of initial 87Sr/86Sr ratios (0.70431 ± 0.00012 to 0.70454 ± 0.00010). Two types of amphibolite-facies epidote were recognized in the amphibolites. The first type of epidote (Ep-AI) developed in all the amphibolites and has slightly lower trace element contents than Ep-E. The Ep-AI has a same initial 87Sr/86Sr ratio range as the Ep-E and represents the primary amphibolite-facies retrograde product that is associated with an internally buffered fluid at 8.0-10.3 kbar and 646-674 °C. The other type of epidote (Ep-AII) occurs as irregular fragments, veins/veinlets, or reaction rims on the Ep-AI in certain amphibolites. Elemental X-ray maps reveal the presence of Ep-AI relics in the Ep-AII domains (appearing as a patchy texture), which indicates that Ep-AII most likely formed by the partial replacement of the Ep-AI in the presence of an infiltrating fluid. The distinctly lower trace element contents of Ep-AII are ascribed to element scavenging by a mechanism of dissolution-transport-precipitation during replacement. The Ep-AII in an individual amphibolite exhibits large intra- and inter-grain variations in the initial 87Sr/86Sr ratios (0.70493 ± 0.00030 to 0.70907 ± 0.00022), which are between those of the Ep-AI and granitic gneisses (wall rock of the amphibolites, 0.7097-0.7108). These results verify that the infiltrating fluid was externally derived from granitic gneisses. The matrix apatite in the amphibolites has

Bauxite to eclogite: Evidence for late Permian supracontinental subduction at the Red River shear zone, northern Vietnam

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Nakano, Nobuhiko; Osanai, Yasuhito; Nam, Nguyen Van; Tri, Tran Van

2018-03-01

We have investigated the geological processes recorded in aluminous granulites from the Red River shear zone in northern Vietnam using mineral and whole-rock chemistries, fluid inclusions, metamorphic pressure-temperature paths, and geochronology. The granulites are extremely rich in Al2O3 (36.3-50.9 wt%), TiO2, and total Fe2O3, and poor in SiO2 (7.9-24.1 wt%), MgO, CaO, Na2O, and K2O. The granulites are enriched in high-field-strength elements and rare earth elements, and severely depleted in large-ion lithophile elements. These features strongly suggest the protolith was lateritic bauxite. Moreover, the other elemental concentrations and the Zr/Ti ratios point to basaltic rock as the precursor of the bauxite. Some of the aluminous granulites contain high-pressure mineral inclusions of kyanite, staurolite, siderite, and rutile, none of which are observed in the matrix. Abundant primary carbonic fluid inclusions are observed in garnet, corundum, and staurolite, but are rare in quartz and zircon. The average densities of fluid inclusions in garnet, corundum, staurolite, quartz, and zircon are 1.00 ± 0.06, 1.07 ± 0.04, 1.09 ± 0.03, 0.29 ± 0.07, and 1.15 ± 0.05 g/cm3, respectively. The mineral features not only in the matrix and but also in garnet from all rock types, isochemical phase diagrams obtained for each bulk rock composition, and Zr-in-rutile thermometry indicate an early eclogite-facies metamorphism ( 2.5 GPa at 650 °C) and a subsequent nearly isothermal decompression. Zircons yield a wide range of U-Pb ages from 265 to 36 Ma, whereas the dark luminescent cores of the zircons, which contain high-density CO2 inclusions, yield a concordia age of 257 ± 8 Ma. These observations suggest that the dark luminescent zircon cores were formed at the same time as the garnet, corundum, and staurolite that contain high-density CO2 fluid inclusions. Based on the carbonic fluid inclusion isochore and the densities as well as calculated phase diagram, the concordia

A D- and N-15-Rich Micrometer-Sized Aggregate of Organic Matter in a Xenolithic Clast from the Zag Ordinary Chondrite

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Kebukawa, Yoko; Ito, Motoo; Zolensky, Michael E.; Rahman, Zia; Suga, Hiroki; Nakato, Aiko; Chan, Queenie H. S.; Fries, Marc; Takeichi, Yasuo; Takahashi, Yoshio;

Water in the Cratonic Mantle: Insights from FTIR Data on Lac De Gras Xenoliths (Slave Craton, Canada)

Science.gov (United States)

Peslier, Anne H.; Brandon, Alan D.; Schaffer, Lillian Aurora; O'Reilly, Suzanne Yvette; Griffin, William L.; Morris, Richard V.; Graff, Trevor G.; Agresti, David G.

2014-01-01

The mantle lithosphere beneath the cratonic part of continents is the deepest (> 200 km) and oldest (>2-3 Ga) on Earth, remaining a conundrum as to how these cratonic roots could have resisted delamination by asthenospheric convection over time. Water, or trace H incorporated in mineral defects, could be a key player in the evolution of continental lithosphere because it influences melting and rheology of the mantle. Mantle xenoliths from the Lac de Gras kimberlite in the Slave craton were analyzed by FTIR. The cratonic mantle beneath Lac de Gras is stratified with shallow (water contents extending to higher values than those from the shallow ones. The FTIR spectra of olivines from the shallow samples have more prominent Group II OH bands compared to the olivines from the deep samples, consistent with a more oxidized mantle environment. The range of olivine water content is similar to that observed in Kaapvaal craton peridotites at the same depths (129-184 km) but does not extend to as high values as those from Udachnaya (Siberian craton). The Slave, Kaapvaal and Siberian cratons will be compared in terms of water content distribution, controls and role in cratonic root longevity.

Floating stones off El Hierro, Canary Islands: xenoliths of pre-island sedimentary origin in the early products of the October 2011 eruption

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Troll, V. R.; Klügel, A.; Longpré, M.-A.; Burchardt, S.; Deegan, F. M.; Carracedo, J. C.; Wiesmaier, S.; Kueppers, U.; Dahren, B.; Blythe, L. S.; Hansteen, T. H.; Freda, C.; Budd, D. A.; Jolis, E. M.; Jonsson, E.; Meade, F. C.; Harris, C.; Berg, S. E.; Mancini, L.; Polacci, M.; Pedroza, K.

2012-03-01

A submarine eruption started off the south coast of El Hierro, Canary Islands, on 10 October 2011 and continues at the time of this writing (February 2012). In the first days of the event, peculiar eruption products were found floating on the sea surface, drifting for long distances from the eruption site. These specimens, which have in the meantime been termed "restingolites" (after the close-by village of La Restinga), appeared as black volcanic "bombs" that exhibit cores of white and porous pumice-like material. Since their brief appearance, the nature and origin of these "floating stones" has been vigorously debated among researchers, with important implications for the interpretation of the hazard potential of the ongoing eruption. The "restingolites" have been proposed to be either (i) juvenile high-silica magma (e.g. rhyolite), (ii) remelted magmatic material (trachyte), (iii) altered volcanic rock, or (iv) reheated hyaloclastites or zeolite from the submarine slopes of El Hierro. Here, we provide evidence that supports yet a different conclusion. We have analysed the textures and compositions of representative "restingolites" and compared the results to previous work on similar rocks found in the Canary Islands. Based on their high-silica content, the lack of igneous trace element signatures, the presence of remnant quartz crystals, jasper fragments and carbonate as well as wollastonite (derived from thermal overprint of carbonate) and their relatively high oxygen isotope values, we conclude that "restingolites" are in fact xenoliths from pre-island sedimentary layers that were picked up and heated by the ascending magma, causing them to partially melt and vesiculate. As they are closely resembling pumice in appearance, but are xenolithic in origin, we refer to these rocks as "xeno-pumice". The El Hierro xeno-pumices hence represent messengers from depth that help us to understand the interaction between ascending magma and crustal lithologies beneath the

Floating stones off El Hierro, Canary Islands: xenoliths of pre-island sedimentary origin in the early products of the October 2011 eruption

Directory of Open Access Journals (Sweden)

V. R. Troll

2012-03-01

Full Text Available A submarine eruption started off the south coast of El Hierro, Canary Islands, on 10 October 2011 and continues at the time of this writing (February 2012. In the first days of the event, peculiar eruption products were found floating on the sea surface, drifting for long distances from the eruption site. These specimens, which have in the meantime been termed "restingolites" (after the close-by village of La Restinga, appeared as black volcanic "bombs" that exhibit cores of white and porous pumice-like material. Since their brief appearance, the nature and origin of these "floating stones" has been vigorously debated among researchers, with important implications for the interpretation of the hazard potential of the ongoing eruption. The "restingolites" have been proposed to be either (i juvenile high-silica magma (e.g. rhyolite, (ii remelted magmatic material (trachyte, (iii altered volcanic rock, or (iv reheated hyaloclastites or zeolite from the submarine slopes of El Hierro. Here, we provide evidence that supports yet a different conclusion. We have analysed the textures and compositions of representative "restingolites" and compared the results to previous work on similar rocks found in the Canary Islands. Based on their high-silica content, the lack of igneous trace element signatures, the presence of remnant quartz crystals, jasper fragments and carbonate as well as wollastonite (derived from thermal overprint of carbonate and their relatively high oxygen isotope values, we conclude that "restingolites" are in fact xenoliths from pre-island sedimentary layers that were picked up and heated by the ascending magma, causing them to partially melt and vesiculate. As they are closely resembling pumice in appearance, but are xenolithic in origin, we refer to these rocks as "xeno-pumice". The El Hierro xeno-pumices hence represent messengers from depth that help us to understand the interaction between ascending magma and crustal lithologies

Theory of Earth

Science.gov (United States)

Anderson, D. L.

2014-12-01

Earth is an isolated, cooling planet that obeys the 2nd law. Interior dynamics is driven from the top, by cold sinking slabs. High-resolution broad-band seismology and geodesy has confirmed that mantle flow is characterized by narrow downwellings and ~20 broad slowly rising updrafts. The low-velocity zone (LVZ) consists of a hot melange of sheared peridotite intruded with aligned melt-rich lamellae that are tapped by intraplate volcanoes. The high temperature is a simple consequence of the thermal overshoot common in large bodies of convecting fluids. The transition zone consists of ancient eclogite layers that are displaced upwards by slabs to become broad passive, and cool, ridge feeding updrafts of ambient mantle. The physics that is overlooked in canonical models of mantle dynamics and geochemistry includes; the 2nd law, convective overshoots, subadiabaticity, wave-melt interactions, Archimedes' principle, and kinetics (rapid transitions allow stress-waves to interact with melting and phase changes, creating LVZs; sluggish transitions in cold slabs keep eclogite in the TZ where it warms up by extracting heat from mantle below 650 km, creating the appearance of slab penetration). Canonical chemical geodynamic models are the exact opposite of physics and thermodynamic based models and of the real Earth. A model that results from inverting the assumptions regarding initial and boundary conditions (hot origin, secular cooling, no external power sources, cooling internal boundaries, broad passive upwellings, adiabaticity and whole-mantle convection not imposed, layering and self-organization allowed) results in a thick refractory-yet-fertile surface layer, with ancient xenoliths and cratons at the top and a hot overshoot at the base, and a thin mobile D" layer that is an unlikely plume generation zone. Accounting for the physics that is overlooked, or violated (2nd law), in canonical models, plus modern seismology, undermines the assumptions and conclusions of these

Slab-derived components in the subcontinental lithospheric mantle beneath Chilean Patagonia: Geochemistry and Sr-Nd-Pb isotopes of mantle xenoliths and host basalt

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Jalowitzki, Tiago; Gervasoni, Fernanda; Conceição, Rommulo V.; Orihashi, Yuji; Bertotto, Gustavo W.; Sumino, Hirochika; Schilling, Manuel E.; Nagao, Keisuke; Morata, Diego; Sylvester, Paul

2017-11-01

In subduction zones, ultramafic xenoliths hosted in alkaline basalts can yield significant information about the role of potential slab-derived components in the subcontinental lithospheric mantle (SCLM). Chemical and isotopic heterogeneities in such xenoliths are usually interpreted to reflect melt extraction followed by metasomatic re-enrichment. Here we report new whole-rock major, trace element and isotopic (Sr-Nd-Pb) data for a Proterozoic suite of 17 anhydrous spinel-lherzolites and Eocene (new K-Ar data) host alkaline basalt found near Coyhaique ( 46°S), Aysén Region, Chile. These Patagonian nodules are located in a current back-arc position, 100 km east of the present day volcanic arc and 320 km from the Chile Trench. The mantle xenoliths consist of coarse- to medium-grained spinel-lherzolites with trace element compositions characteristic of a subduction zone setting, such as pronounced negative Nb, Ta and Ti anomalies coupled with significant enrichment of LILEs (e.g., U) and chalcophile elements (W, Pb and Sn). Most of them are characterized by flat to depleted light-rare earth element (LREE) patterns (Ce/YbN = 0.6-1.1) coupled with less radiogenic Sr-Pb (87Sr/86Sr = 0.702422-0.703479; 206Pb/204Pb = 18.212-18.539) and more radiogenic Nd isotopic compositions (143Nd/144Nd = 0.512994-0.513242), similar to the depleted mantle component (DMM or PREMA). In contrast, samples with slight LREE enrichment (Ce/YbN = 1.3-1.8) show more radiogenic Sr-Pb (87Sr/86Sr = 0.703791-0.704239; 206Pb/204Pb = 18.572-18.703) and less radiogenic Nd isotopic compositions (143Nd/144Nd = 0.512859-0.512934), similar to the EM-2 reservoir. These new geochemical and isotope data suggest that the Coyhaique spinel-lherzolites are derived from a heterogeneous SCLM resulting from mixing between a depleted mantle component and up to 10% of slab-derived components. The enriched component added to the SCLM represents variable extents of melts of both subducted Chile Trench sediments and

Conodont geothermometry in pyroclastic kimberlite: constraints on emplacement temperatures and cooling histories

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Pell, Jennifer; Russell, James K.; Zhang, Shunxin

2018-03-01

Kimberlite pipes from Chidliak, Baffin Island, Nunavut, Canada host surface-derived Paleozoic carbonate xenoliths containing conodonts. Conodonts are phosphatic marine microfossils that experience progressive, cumulative and irreversible colour changes upon heating that are experimentally calibrated as a conodont colour alteration index (CAI). CAI values permit us to estimate the temperatures to which conodont-bearing rocks have been heated. Conodonts have been recovered from 118 samples from 89 carbonate xenoliths collected from 12 of the pipes and CAI values within individual carbonate xenoliths show four types of CAI distributions: (1) CAI values that are uniform throughout the xenolith; (2) lower CAIs in core of a xenolith than the rim; (3) CAIs that increase from one side of the xenolith to the other; and, (4) in one xenolith, higher CAIs in the xenolith core than at the rim. We have used thermal models for post-emplacement conductive cooling of kimberlite pipes and synchronous heating of conodont-bearing xenoliths to establish the temperature-time history of individual xenoliths within the kimberlite bodies. Model results suggest that the time-spans for xenoliths to reach the peak temperatures recorded by CAIs varies from hours for the smallest xenoliths to 2 or 3 years for the largest xenoliths. The thermal modelling shows the first three CAI patterns to be consistent with in situ conductive heating of the xenoliths coupled to the cooling host kimberlite. The fourth pattern remains an anomaly.

Presumed Multiple Metasomatism underneath the Colorado Plateau; Decoding from Chemistry and Inclusion/Lamella Mineralogy of Diverse Garnets from the Garnet Ridge, Northern Arizona

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Sato, Y.; Ogasawara, Y.

2015-12-01

Various garnets containing the information on mantle petrology and related metasomatism occur at the Garnet Ridge, Colorado Plateau. The origins of garnets range from deep mantle to shallow continental crust. These garnets were delivered by kimberlitic diatreme of 30 Ma (Smith et al. 2004). We have classified the garnets into 10 groups (A to J, see figure) by naked eye observation, major chemistry, minor Na-Ti-P, inclusion/lamella mineralogy. Among them, groups A to D are of mantle origin, E to G of subducted oceanic crust origin, and H to J of continental crust origin. We summarized results as in the followings. A: Cr and pyrope-rich garnet has Cr2O3(0.8-6.3 wt.%) and inclusions of Ol, Cpx, Opx, Ti-Chu/Chn and carbonates, indicating carbonated garnet lherzolites as host. Cr contents negatively correlates with Na-Ti-P contents and occurrence of exsolved Rt, Ilm and crichtonite. This indicates Cr-rich end-member is the most "primitive" mantle garnet before metasomatism. B: Pyrope-rich reddish brown garnet of peridotitic origins was subdivided into 4 subgroups (B1 to B4, see figure). Compositional range in Ca-Mg-Fe triangle expands to Fe-rich side from group A. Exsolved Na-bearing amphibole and inclusions of Ap, carbonates and fluid were identified. These indicate metasomatism of group A. C: Garnet megacryst is coarse-grained garnet (2-10 cm across) with crystal faces. This garnet has wide chemical variation plotted in the center area of Ca-Mg-Fe triangle. D: Garnet aggregate has similar chemistry of group C and is composed of several grains. Grain boundaries of garnet were recognized by Rt, Ilm and other minerals and oscillatory zonings of Ca, Mg, Fe and Na-Ti-P. Fluid inclusions of groups C and D suggest these garnets might crystalized from fluid. E: Garnet in eclogite and F: Garnet in metasomatized eclogite are xenolith samples (the Fallaron Plate origin?). Aggregate of Zo+Ab contained in group E indicates decomposed precursor lawsonite inclusion. G: Quartz

Diamond xenolith and matrix organic matter in the Sutter's Mill meteorite measured by C-XANES

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Kebukawa, Yoko; Zolensky, Michael E.; Kilcoyne, A. L. David; Rahman, Zia; Jenniskens, Peter; Cody, George D.

2014-11-01

The Sutter's Mill (SM) meteorite fell in El Dorado County, California, on April 22, 2012. This meteorite is a regolith breccia composed of CM chondrite material and at least one xenolithic phase: oldhamite. The meteorite studied here, SM2 (subsample 5), was one of three meteorites collected before it rained extensively on the debris site, thus preserving the original asteroid regolith mineralogy. Two relatively large (10 μm sized) possible diamond grains were observed in SM2-5 surrounded by fine-grained matrix. In the present work, we analyzed a focused ion beam (FIB) milled thin section that transected a region containing these two potential diamond grains as well as the surrounding fine-grained matrix employing carbon and nitrogen X-ray absorption near-edge structure (C-XANES and N-XANES) spectroscopy using a scanning transmission X-ray microscope (STXM) (Beamline 5.3.2 at the Advanced Light Source, Lawrence Berkeley National Laboratory). The STXM analysis revealed that the matrix of SM2-5 contains C-rich grains, possibly organic nanoglobules. A single carbonate grain was also detected. The C-XANES spectrum of the matrix is similar to that of insoluble organic matter (IOM) found in other CM chondrites. However, no significant nitrogen-bearing functional groups were observed with N-XANES. One of the possible diamond grains contains a Ca-bearing inclusion that is not carbonate. C-XANES features of the diamond-edges suggest that the diamond might have formed by the CVD process, or in a high-temperature and -pressure environment in the interior of a much larger parent body.

Diversity of minor elements in olivines from mantle xenoliths (Wołek Hill, SW Poland) - PIXE measurements

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Nowak, Monika; Munnik, Frans; Michalak, Przemysław P.; Renno, Axel

2016-04-01

Wołek Hill is one of the best examined exposures of Cenozoic volcanic rocks from SW Poland (Nowak, 2012). This is related with two facts: a great amount of mantle xenoliths were collected from that outcrop and this is one of two occurrences in Poland were modal metasomatism (related with amphibole crystals presence) was recognized. Wołek Hill is a relatively small exposure and belongs to the Złotoryja Volcanic Field, which is one of the volcanic concentrations in the Polish part of the Central European Volcanic Province (Ladenberger et al. 2006). Based on previous observations olivine crystals from the inside part of xenoliths occasionally display internal inhomogeneity visible on a BSE image. Such inhomogeneity has been related to olivine "sub-grains" with slightly shifted crystal axis. Those "sub-grains" are visible in optical microscope as transitional lamellae (Nowak, Stawikowski 2009). Besides the mentioned visible inhomogeneity olivine crystals also show diversity in Ca content inside single crystals (sometimes even more than 200 ppm). EPMA standard measurements (15 kV, 20 nA, time: 40 seconds) were limited to beam size and detection limits of the microprobe (most of the obtained results oscillate close to the detection limits - or even below it). Special conditions EPMA analyses (15 kV, 100nA, time: 100s) confirmed the differences in Ca content in the studied olivines, but did not provide any idea on how to interpret the results (Nowak, 2012). In this short summary we present preliminary data of olivine minor element composition (Ca, Zn, Cr, Ti, Co, K and also Mn, Ni, Fe) obtained with Particle Induced X-ray Emission(PIXE) measurements performed at the HZDR in Dresden using a 3 MeV proton beam, 1-1.5 nA current and an acquisition time of 3 hrs for each scan. The size of individual scans varied from ca. 30 μm up to 60-65 μm, with 8 x 8 measurement points - min. 4 μm in diameter. The measurements have been analysed with the GeoPIXE software (Ryan, 2001

The internal structure of eclogite-facies ophiolite complexes: Implications from the Austroalpine outliers within the Zermatt-Saas Zone, Western Alps

Science.gov (United States)

Weber, Sebastian; Martinez, Raul

2016-04-01

The Western Alpine Penninic domain is a classical accretionary prism that formed after the closure of the Penninic oceans in the Paleogene. Continental and oceanic nappes were telescoped into the Western Alpine stack associated with continent-continent collision. Within the Western Alpine geologic framework, the ophiolite nappes of the Zermatt-Saas Zone and the Tsate Unit are the remnants of the southern branch of the Piemonte-Liguria ocean basin. In addition, a series of continental basement slices reported as lower Austroalpine outliers have preserved an eclogitic high-pressure imprint, and are tectonically sandwiched between these oceanic nappes. Since the outliers occur at an unusual intra-ophiolitic setting and show a polymetamorphic character, this group of continental slices is of special importance for understanding the tectono-metamorphic evolution of Western Alps. Recently, more geochronological data from the Austroalpine outliers have become available that make it possible to establish a more complete picture of their complex geological history. The Lu-Hf garnet-whole rock ages for prograde growth of garnet fall into the time interval of 52 to 62 Ma (Weber et al., 2015, Fassmer et al. 2015), but are consistently higher than the Lu-Hf garnet-whole rock ages from several other locations throughout the Zermatt-Saas zone that range from 52 to 38 Ma (Skora et al., 2015). This discrepancy suggests that the Austroalpine outliers may have been subducted earlier than the ophiolites of the Zermatt-Saas Zone and therefore have been tectonically emplaced into their present intra-ophiolite position. This points to the possibility that the Zermatt-Saas Zone consists of tectonic subunits, which reached their respective pressure peaks over a prolonged time period, approximately 10-20 Ma. The pressure-temperature estimates from several members of the Austroalpine outliers indicate a complex distribution of metamorphic peak conditions, without ultrahigh

Associated chemical and carbon isotopic composition variations in diamonds from Finsch and Premier kimberlite, South Africa

International Nuclear Information System (INIS)

Deines, P.

1984-01-01

The carbon isotopic composition of 66 inclusion-containing diamonds from the Premier kimberlite, South Africa, 93 inclusion-containing diamonds and four diamonds of two diamond-bearing peridotite xenoliths from the Finsch kimberlite, South Africa was measured. The data suggest a relationship between the carbon isotopic composition of the diamonds and the chemical composition of the associated silicates. For both kimberlites similar trends are noted for diamonds containing peridotite-suite inclusions (P-type) and for diamonds containing eclogite-suite inclusions (E-type): Higher delta 13 C P-type diamonds tend to have inclusions lower in SiO 2 , Al 2 O 3 , Cr 2 O 3 , MgO, Mg/(Mg + Fe) and higher in FeO and CaO. Higher delta 13 C E-type diamonds tend to have inclusions lower in SiO 2 , Al 2 O 3 , MgO, Mg/(Mg + Fe), Na 2 O, K 2 O, TiO 2 and higher in CaO, Ca/(Ca + Mg). Consideration of a number of different models that have been proposed for the genesis of kimberlites, their zenoliths and diamonds shows that they are all consistent with the conclusion that in the mantle, regions exist that are characterized by different mean carbon isotopic compositions. (author)

Formation of cratonic lithosphere: An integrated thermal and petrological model

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Herzberg, Claude; Rudnick, Roberta

2012-09-01

The formation of cratonic mantle peridotite of Archean age is examined within the time frame of Earth's thermal history, and how it was expressed by temporal variations in magma and residue petrology. Peridotite residues that occupy the lithospheric mantle are rare owing to the effects of melt-rock reaction, metasomatism, and refertilization. Where they are identified, they are very similar to the predicted harzburgite residues of primary magmas of the dominant basalts in greenstone belts, which formed in a non-arc setting (referred to here as "non-arc basalts"). The compositions of these basalts indicate high temperatures of formation that are well-described by the thermal history model of Korenaga. In this model, peridotite residues of extensive ambient mantle melting had the highest Mg-numbers, lowest FeO contents, and lowest densities at ~ 2.5-3.5 Ga. These results are in good agreement with Re-Os ages of kimberlite-hosted cratonic mantle xenoliths and enclosed sulfides, and provide support for the hypothesis of Jordan that low densities of cratonic mantle are a measure of their high preservation potential. Cratonization of the Earth reached its zenith at ~ 2.5-3.5 Ga when ambient mantle was hot and extensive melting produced oceanic crust 30-45 km thick. However, there is a mass imbalance exhibited by the craton-wide distribution of harzburgite residues and the paucity of their complementary magmas that had compositions like the non-arc basalts. We suggest that the problem of the missing basaltic oceanic crust can be resolved by its hydration, cooling and partial transformation to eclogite, which caused foundering of the entire lithosphere. Some of the oceanic crust partially melted during foundering to produce continental crust composed of tonalite-trondhjemite-granodiorite (TTG). The remaining lithosphere gravitationally separated into 1) residual eclogite that continued its descent, and 2) buoyant harzburgite diapirs that rose to underplate cratonic nuclei

The Last Word

Science.gov (United States)

Anderson, D. L.

2014-12-01

Earth is an isolated, cooling planet, that obeys the 2nd law of thermodynamics. Interior dynamics is driven from the top, by cold sinking slabs. High-resolution broad-band seismology and geodesy have confirmed that mantle flow is characterized by narrow downwellings and ~20 broad slowly rising updrafts. The low-velocity zone (LVZ) consists of a hot melange of sheared peridotite intruded with aligned melt-rich lamellae that are tapped by intraplate volcanoes. The high temperature is a simple consequence of the thermal overshoot common in large bodies of convecting fluids. The transition zone consists of ancient eclogite layers that are displaced upwards by slabs to become broad, passive, cool ridge-feeding updrafts of ambient mantle. The physics that is overlooked in canonical models of mantle dynamics and geochemistry includes; the 2nd law of thermodynamics, convective overshoots, subadiabaticity, wave-melt interactions, Archimedes principle, and kinetics. Rapid transitions allow stress-waves to interact with melting and phase changes, creating LVZs; sluggish transitions in cold slabs keep eclogite in the transition zone where it warms up by extracting heat from mantle below 650 km, creating the appearance of slab penetration. Canonical chemical geodynamic models are the exact opposite of physics- and thermodynamic-based models and of the real Earth. A model that results from inverting the assumptions regarding initial and boundary conditions (hot origin, secular cooling, no external power sources, cooling internal boundaries, broad passive upwellings, adiabaticity and whole-mantle convection not imposed, layering and self-organization allowed) results in a thick refractory-yet-fertile surface layer, with ancient xenoliths and cratons at the top and a hot overshoot at the base. A thin mobile D" layer results, that is an unlikely plume-generation zone. Accounting for the physics that is overlooked or violated (the 2nd law of thermodynamics) in canonical models, plus

Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

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van der Werf, Thomas; Chatzaras, Vasileios; Marcel Kriegsman, Leo; Kronenberg, Andreas; Tikoff, Basil; Drury, Martyn R.

2017-12-01

The rheology of lower crust and its transient behavior in active strike-slip plate boundaries remain poorly understood. To address this issue, we analyzed a suite of granulite and lherzolite xenoliths from the upper Pleistocene-Holocene San Quintín volcanic field of northern Baja California, Mexico. The San Quintín volcanic field is located 20 km east of the Baja California shear zone, which accommodates the relative movement between the Pacific plate and Baja California microplate. The development of a strong foliation in both the mafic granulites and lherzolites, suggests that a lithospheric-scale shear zone exists beneath the San Quintín volcanic field. Combining microstructural observations, geothermometry, and phase equilibria modeling, we estimated that crystal-plastic deformation took place at temperatures of 750-890 °C and pressures of 400-560 MPa, corresponding to 15-22 km depth. A hot crustal geotherm of 40 ° C km-1 is required to explain the estimated deformation conditions. Infrared spectroscopy shows that plagioclase in the mafic granulites is relatively dry. Microstructures are interpreted to show that deformation in both the uppermost lower crust and upper mantle was accommodated by a combination of dislocation creep and grain-size-sensitive creep. Recrystallized grain size paleopiezometry yields low differential stresses of 12-33 and 17 MPa for plagioclase and olivine, respectively. The lower range of stresses (12-17 MPa) in the mafic granulite and lherzolite xenoliths is interpreted to be associated with transient deformation under decreasing stress conditions, following an event of stress increase. Using flow laws for dry plagioclase, we estimated a low viscosity of 1.1-1.3×1020 Pa ṡ s for the high temperature conditions (890 °C) in the lower crust. Significantly lower viscosities in the range of 1016-1019 Pa ṡ s, were estimated using flow laws for wet plagioclase. The shallow upper mantle has a low viscosity of 5.7×1019 Pa ṡ s

Detecting of the processes of the diamond formation using the monomineral thermobarometry .

Science.gov (United States)

Ashchepkov, Igor; Afanasiev, Valentin; Pokhilenko, Lyudmila; Logvinova, Alla; Vladykin, Nikolai

2010-05-01

mvm-2 is related to the Diam-eclogites (To according to Krogh, 1988), Fe- Opx and chromites. And hottest (45mvm-2) refers to the magmatic type eclogites (cumulates) and some HT chromites (restites). In mantle columns beneath the Proterozoic pipes like Roberts Victor (Souter, Harte, 1988; Jacob ea, 2004), Premier (Gurney ea., 1985; Viljoin ea, 2009) the eclogitic DI trace mostly 40 mvm-2 geotherm but large amount of PT point drops onto advective hottest branch. Mostly eclogites are separated to several branches according to Mg#. The Fe- rich (ancient trondjemitic cumulates) are commonly low-To (LT). Conditions for the mantle beneath Tanzania (Stachel ea, 1998) and Ghana (Stachel ea, 1997) are close to 40mvm-2 geotherm from the deepest~80 kbar level to 35 kbar. The PT estimates for DI from Guinea (Denies, Harris, 2004), those from Angola kimberlites refer to colder geotherm branch. Similar but hotter conditions are detected for the V-Cm kimberlites from Botswana - Venetia (Viljoen ea 2009; Hin et al., 2009) which in mantle columns reveals subadiabatic branch from 1450oC (45mvm-2) but the Late Mesozoic pipes in this region like Letlhakane (Achtenbergh ea, 2001; Stienfenhofer ea, 1997; Deines, Harris, 2004), Orapa (Denies ea., 2009; Stachel ea., 2004) reveal conditions close to 42 mvm-2. PT for mantle xenoliths from the "of craton" Namibian pipes (Louwrensia, Hanous) (Mitchel, 1984; Boyd ea., 2004) trace the G-D boundary or are plotted above it and only relic associations are correspondent to the levels ~65-70 kbar. The PT for OPx and some Cpx in diamonds (Harris ea, 2004) from placers are close to this boundary but those for hot (1400oC) eclogites (magmatic type) are correspondent to the lower levels of mantle columns or coincides with the hottest PT conditions for peridotites. The location of the PT estimate for DI above the Diam -Graph boundary is probably due to metastable crystallization or mantle diapirism. In North America kimberlites the conditions for the DI are

A geochemical study of lithospheric mantle beneath Northern Victoria Land (Antarctica): main evidences from volatile content in ultramafic xenoliths

Science.gov (United States)

Correale, Alessandra; Pelorosso, Beatrice; Rizzo, Andrea Luca; Coltorti, Massimo; Italiano, Francesco; Bonadiman, Costanza

2017-04-01

A geochemical study of ultramafic xenoliths from Northern Victoria Land (Green Point, GP and Handler Ridge, HR), is carried out in order to investigate the features of the lithosphere mantle beneath the Western Antarctic Ridge System (WARS). The majority of samples is spinel anhydrous lherzolite with rare presence of secondary phases (secondary cpx and glass). Geothermobarometric calculations, based on the Fe/Mg distribution among the peridotite minerals reveal that Sub Continental Lithospheric Mantle (SCLM) beneath Handler Ridge records temperatures and redox conditions higher then Greene Point (P fixed at 15 Kbar). Moreover, geochemical models evidence that, GP mantle domain represents a residuum after ˜7 to 21 % of partial melting in the spinel stability field, which was variably affected by interaction with infiltrating melts, acting in different times, from at least Jurassic to Cenozoic (Pelorosso et al., 2016). Fluid inclusions (FI) entrapped in olivine and pyroxene crystals were investigated for elemental and isotopic contents of both, noble gases (He, Ne, Ar) and CO2. He, Ar and Ne concentrations range from 1.52×10-14 to 1.07×10-12, from 4.09×10-13 to 3.47×10-11and from 2.84×10-16 to 7.57×10-14 mol/g, respectively, while the CO2amounts are between 7.08×10-10 and 8.12×10-7 mol/g. The 3He/4He varies between 5.95 and 20.18 Ra (where Ra is the 3He/4He ratio of air), being the lowest and the highest values measured in the He-poorer samples. Post-eruptive input of cosmogenic 3He and radiogenic 4He seems to influence mainly the samples associated to a lower He concentrations, increasing and decreasing respectively their primordial 3He/4He values, that for all the other samples range between 6.76 and 7.45 Ra. This range reasonably reflects the isotope signature of mantle beneath the investigated areas. The 4He/40Ar* ratio corrected for atmospheric-derived contamination ranges between 0.004 and 0.39. The lowest 4He/40Ar* values (4He/40Ar*correspondence of

Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

Directory of Open Access Journals (Sweden)

T. van der Werf

2017-12-01

Full Text Available The rheology of lower crust and its transient behavior in active strike-slip plate boundaries remain poorly understood. To address this issue, we analyzed a suite of granulite and lherzolite xenoliths from the upper Pleistocene–Holocene San Quintín volcanic field of northern Baja California, Mexico. The San Quintín volcanic field is located 20 km east of the Baja California shear zone, which accommodates the relative movement between the Pacific plate and Baja California microplate. The development of a strong foliation in both the mafic granulites and lherzolites, suggests that a lithospheric-scale shear zone exists beneath the San Quintín volcanic field. Combining microstructural observations, geothermometry, and phase equilibria modeling, we estimated that crystal-plastic deformation took place at temperatures of 750–890 °C and pressures of 400–560 MPa, corresponding to 15–22 km depth. A hot crustal geotherm of 40 ° C km−1 is required to explain the estimated deformation conditions. Infrared spectroscopy shows that plagioclase in the mafic granulites is relatively dry. Microstructures are interpreted to show that deformation in both the uppermost lower crust and upper mantle was accommodated by a combination of dislocation creep and grain-size-sensitive creep. Recrystallized grain size paleopiezometry yields low differential stresses of 12–33 and 17 MPa for plagioclase and olivine, respectively. The lower range of stresses (12–17 MPa in the mafic granulite and lherzolite xenoliths is interpreted to be associated with transient deformation under decreasing stress conditions, following an event of stress increase. Using flow laws for dry plagioclase, we estimated a low viscosity of 1.1–1.3×1020 Pa ⋅ s for the high temperature conditions (890 °C in the lower crust. Significantly lower viscosities in the range of 1016–1019 Pa ⋅ s, were estimated using flow laws for wet plagioclase. The

Timing of metamorphism and exhumation in the Nordøyane ultra-high-pressure domain, Western Gneiss Region, Norway: New constraints from complementary CA-ID-TIMS and LA-MC-ICP-MS geochronology

Science.gov (United States)

Butler, J. P.; Jamieson, R. A.; Dunning, G. R.; Pecha, M. E.; Robinson, P.; Steenkamp, H. M.

2018-06-01

We present the results of a combined CA-ID-TIMS and LA-MC-ICP-MS U-Pb geochronology study of zircon and associated rutile and titanite from the Nordøyane ultra-high-pressure (UHP) domain in the Western Gneiss Region (WGR) of Norway. The dated samples include 4 eclogite bodies, 2 host-rock migmatites, and 2 cross-cutting pegmatites and leucosomes, all from the island of Harøya. Zircon from a coesite eclogite yielded an age of ca. 413 Ma, interpreted as the time of UHP metamorphism in this sample. Zircon data from the other eclogite bodies yielded metamorphic ages of ca. 413 Ma, 407 Ma, and 406 Ma; zircon trace-element data associated with 413 Ma and 407 Ma ages are consistent with eclogite-facies crystallization. In all of the eclogites, U-Pb dates from zircon cores, interpreted as the times of protolith crystallization, range from ca. 1680-1586 Ma, consistent with Gothian ages from orthogneisses in Nordøyane and elsewhere in the WGR. A zircon core age of ca. 943 Ma from one sample agrees with Sveconorwegian ages of felsic gneisses and pegmatites in the western part of the area. Migmatites hosting the eclogite bodies yielded zircon core ages of ca. 1657-1591 Ma and rim ages of ca. 395-392 Ma, interpreted as the times of Gothian protolith formation and Scandian partial melt crystallization, respectively. Pegmatite in an eclogite boudin neck yielded a crystallization age of ca. 388 Ma, interpreted as the time of melt crystallization. Rutile and titanite from 3 samples (an eclogite and two migmatites) yielded concordant ID-TIMS ages of 378-376 Ma. The results are similar to existing U-Pb data from other Nordøyane eclogites (415-405 Ma). In combination with previous pressure-temperature data from the coesite eclogite, these ages indicate that peak metamorphic conditions of 3 GPa/760 °C were reached ca. 413 Ma, followed by decompression to 1 GPa/810 °C by ca. 397 Ma and cooling below ca. 600 °C by ca. 375 Ma. The results are compatible with protracted UHP

Tracing subduction zone fluid-rock interactions using trace element and Mg-Sr-Nd isotopes

Science.gov (United States)

Wang, Shui-Jiong; Teng, Fang-Zhen; Li, Shu-Guang; Zhang, Li-Fei; Du, Jin-Xue; He, Yong-Sheng; Niu, Yaoling

2017-10-01

Slab-derived fluids play a key role in mass transfer and elemental/isotopic exchanges in subduction zones. The exhumation of deeply subducted crust is achieved via a subduction channel where fluids from various sources are abundant, and thus the chemical/isotopic compositions of these rocks could have been modified by subduction-zone fluid-rock interactions. Here, we investigate the Mg isotopic systematics of eclogites from southwestern Tianshan, in conjunction with major/trace element and Sr-Nd isotopes, to characterize the source and nature of fluids and to decipher how fluid-rock interactions in subduction channel might influence the Mg isotopic systematics of exhumed eclogites. The eclogites have high LILEs (especially Ba) and Pb, high initial 87Sr/86Sr (up to 0.7117; higher than that of coeval seawater), and varying Ni and Co (mostly lower than those of oceanic basalts), suggesting that these eclogites have interacted with metamorphic fluids mainly released from subducted sediments, with minor contributions from altered oceanic crust or altered abyssal peridotites. The positive correlation between 87Sr/86Sr and Pb* (an index of Pb enrichment; Pb* = 2*PbN/[CeN + PrN]), and the decoupling relationships and bidirectional patterns in 87Sr/86Sr-Rb/Sr, Pb*-Rb/Sr and Pb*-Ba/Pb spaces imply the presence of two compositionally different components for the fluids: one enriched in LILEs, and the other enriched in Pb and 87Sr/86Sr. The systematically heavier Mg isotopic compositions (δ26Mg = - 0.37 to + 0.26) relative to oceanic basalts (- 0.25 ± 0.07) and the roughly negative correlation of δ26Mg with MgO for the southwestern Tianshan eclogites, cannot be explained by inheritance of Mg isotopic signatures from ancient seafloor alteration or prograde metamorphism. Instead, the signatures are most likely produced by fluid-rock interactions during the exhumation of eclogites. The high Rb/Sr and Ba/Pb but low Pb* eclogites generally have high bulk-rock δ26Mg values

Density Sorting During the Evolution of Continental Crust

Science.gov (United States)

Kelemen, P. B.; Behn, M. D.; Hacker, B. R.

2015-12-01

We consider two settings - in addition to "delamination" of arc lower crust - in which dense, mafic eclogites founder into the convecting mantle while buoyant, felsic lithologies accumulate at the base of evolving continental crust. Arc processes play a central role in generating continental crust, but it remains uncertain how basaltic arc crust is transformed to andesitic continental crust. Dense, SiO2-poor products of fractionation may founder from the base of arc crust by "delamination", but lower arc crust after delamination has significantly different trace elements compared to lower continental crust (LCC). In an alternative model, buoyant magmatic rocks generated at arcs are first subducted, mainly via subduction erosion. Upon heating, these buoyant lithologies ascend through the mantle wedge or along a subduction channel, and are "relaminated" at
the base of overlying crust (e.g., Hacker et al EPSL 11, AREPS 15). Average buoyant lavas and plutons
for the Aleutians, Izu-Bonin-Marianas, Kohistan and Talkeetna arcs fall within the range of estimated LCC major and trace elements. Relamination is more efficient in generating continental crust than delamination. Himalayan cross-sections show Indian crust thrust beneath Tibetan crust, with no intervening mantle. There is a horizontal Moho at ca 80 km depth, extending from thickened Indian crust, across the region where Tibetan crust overlies Indian crust, into thickened Tibetan crust. About half the subducted Indian crust is present, whereas the other half is missing. Data (Vp/Vs; Miocene lavas formed by interaction of continental crust with mantle; xenolith thermometry) indicate 1000°C or more from ca 50 km depth to the Moho since the Miocene. We build on earlier studies (LePichon et al Tectonics 92, T'phys 97; Schulte-Pelkum et al Nature 05; Monsalve et al JGR 08) to advance the hypothesis that rapid growth of garnet occurs at 70-80 km and 1000°C within subducting Indian crust. Dense eclogites founder

Experimental Melting Study of Basalt-Peridotite Hybrid Source: Melting model of Hawaiian plume

Science.gov (United States)

Takahashi, E.; Gao, S.

2015-12-01

Eclogite component entrained in ascending plume is considered to be essentially important in producing flood basalts (e.g., Columbia River basalt, Takahashi et al., 1998 EPSL), alkalic OIBs (e.g., Kogiso et al.,2003), ferro-picrites (Tuff et al.,2005) and Hawaiian shield lavas (e.g., Hauri, 1996; Takahashi & Nakajima, 2002, Sobolev et al.,2005). Size of the entrained eclogite, which controls the reaction rates with ambient peridotite, however, is very difficult to constrain using geophysical observation. Among Hawaiian shield volcanoes, Koolau is the most enriched end-member in eclogite component (Frey et al, 1994). Reconstruction of Koolau volcano based on submarine study on Nuuanu landslide (AGU Monograph vol.128, 2002, Takahashi Garcia Lipman eds.) revealed that silica-rich tholeiite appeared only at the last stage (Makapuu stage) of Koolau volcano. Chemical compositions of lavas as well as isotopes change abruptly and coherently across a horizon (Shinozaki et al. and Tanaka et al. ibid.). Based on these observation, Takahashi & Nakajima (2002 ibid) proposed that the Makapuu stage lava in Koolau volcano was supplied from a single large eclogite block. In order to study melting process in Hawaiian plume, high-pressure melting experiments were carried out under dry and hydrous conditions with layered eclogite/peridotite starting materials. Detail of our experiments will be given by Gao et al (2015 AGU). Combined previous field observation with new set of experiments, we propose that variation in SiO2 among Hawaiian tholeiites represent varying degree of wall-rock interaction between eclogite and ambient peridotite. Makapuu stage lavas in Koolau volcano represents eclogite partial melts formed at ~3 GPa with various amount of xenocrystic olivines derived from Pacific plate. In other words, we propose that "primary magma" in the melting column of Hawaiian plume ranges from basaltic andesite to ferro-picrite depending on the lithology of the source. Solidus of

The composition and distribution of the rejuvenated component across the Hawaiian plume: Hf-Nd-Sr-Pb isotope systematics of Kaula lavas and pyroxenite xenoliths

Science.gov (United States)

Bizimis, Michael; Salters, Vincent J. M.; Garcia, Michael O.; Norman, Marc D.

2013-10-01

Rejuvenated volcanism refers to the reemergence of volcanism after a hiatus of 0.5-2 Ma following the voluminous shield building stage of Hawaiian volcanoes. The composition of the rejuvenated source and its distribution relative to the center of the plume provide important constraints on the origin of rejuvenated volcanism. Near-contemporaneous lavas from the Kaula-Niihau-Kauai ridge and the North Arch volcanic field that are aligned approximately orthogonally to the plume track can constrain the lateral geochemical heterogeneity and distribution of the rejuvenated source across the volcanic chain. Nephelinites, phonolites and pyroxenite xenoliths from Kaula Island have radiogenic Hf, Nd and unradiogenic Sr isotope compositions consistent with a time-integrated depleted mantle source. The pyroxenites and nephelinites extend to the lowest 208Pb/204Pb reported in Hawaiian rocks. These data, along with new Pb isotope data from pyroxenites from the Salt Lake Crater (Oahu) redefine the composition of the depleted end-member of the Hawaiian rejuvenated source at 208Pb/204Pb=37.35±0.05, 206Pb/204Pb = 17.75±0.03, ɛNd = 9-10, ɛHf ˜16-17 and 87Sr/88Sr Niihau-Kauai-North Arch transect are consistent with a larger proportion of the rejuvenated depleted component in the periphery of the plume track rather than along its axis.

Timing and duration of Variscan high-pressure metamorphism in the French Massif Central: A multimethod geochronological study from the Najac Massif

Science.gov (United States)

Lotout, Caroline; Pitra, Pavel; Poujol, Marc; Anczkiewicz, Robert; Van Den Driessche, Jean

2018-05-01

Accurate dating of eclogite-facies metamorphism is of paramount importance in order to understand the tectonic evolution of an orogen. An eclogite sample from the Najac Massif (French Massif Central, Variscan belt) displays a zircon-bearing garnet-omphacite-amphibole-rutile-quartz peak assemblage. Pseudosection modeling suggests peak pressure conditions of 15-20 kbar, 560-630 °C. Eclogite-facies garnet displays Lu-enriched cores and Sm-rich rims and yields a Lu-Hf age of 382.8 ± 1.0 Ma and a Sm-Nd age of 376.7 ± 3.3 Ma. The ages are interpreted as marking the beginning of the prograde garnet growth during the initial stages of the eclogite-facies metamorphism, and the high-pressure (and temperature) peak reached by the rock, respectively. Zircon grains display chondrite-normalized REE spectra with variably negative, positive or no Eu anomalies and are characterized by either enriched or flat HREE patterns. However, they yield a well constrained in situ LA-ICP-MS U-Pb age of 385.5 ± 2.3 Ma, despite this REE pattern variability. Zr zonation in garnet, Y content in zircon and the diversity of zircon HREE spectra may suggest that zircon crystallized prior to and during incipient garnet growth on the prograde P-T path, recording the initial stages of the eclogite-facies conditions. Consequently, the zircon age of 385.5 ± 2.3 Ma, comparable within error with the Lu-Hf age obtained on garnet, is interpreted as dating the beginning of the eclogite-facies metamorphism. Accordingly, the duration of the prograde part of the eclogite-facies event is estimated at 6.1 ± 4.3 Myr. Subsequent exhumation is constrained by an apatite U-Pb age at 369 ± 13 Ma.

Crustal thickness controlled by plate tectonics

DEFF Research Database (Denmark)

Artemieva, Irina M.; Meissner, Rolf

2012-01-01

/gabbro–eclogite phase transition in crustal evolution and the links between lithosphere recycling, mafic magmatism, and crustal underplating. We advocate that plate tectonics processes, togetherwith basalt/gabbro–eclogite transition, limit crustal thickness worldwide by providing effective mechanisms of crustal...

Journal of Earth System Science | Indian Academy of Sciences

Indian Academy of Sciences (India)

Keywords. Atoll garnet; eclogite; fluid infiltration; retrograde metamorphism; Ladakh Himalayas; Tso Morari Complex. Abstract. The eclogites of the Tso Morari Complex, Ladakh, NW Himalayas preserve both garnets with spectacular atoll textures, as well as whole porphyroblastic garnets. Whole garnets are euhedral, ...

Widespread melt/rock interaction and seismic properties of the lithosphere above mantle plumes: Evidence from mantle xenoliths from French Polynesia

Science.gov (United States)

Tommasi, A.; Godard, M.; Coromina, G.; Dautria, J. M.; Barczus, H.

2003-04-01

In addition to thermal erosion, plume/lithosphere interaction may induce significant changes in the lithosphere chemical composition. To constrain the extent of this process in an oceanic environment and its consequences on the lithosphere seismic properties, we studied the relationship between petrological processes and microstructure in mantle xenoliths from the Austral-Cook, Society and Marquesas islands. Olivine forsterite contents in our sp-peridotites vary continuously from Fo91 to Fo83, the lowest Fo being observed in dunites and wehrlites. Yet, their high Ni content (up to 2500 ppm) precludes a cumulate origin. These rocks are rather interpreted as resulting from melt/rock reactions involving olivine precipitation and pyroxene dissolution, the dunites indicating high melt-rock ratios. Moreover, wehrlites display poikiloblastic diopside enclosing corroded olivines. Late crystallization of clinopyroxene, also observed in lherzolites, may result from a near-solidus melt-freezing reaction occurring at the boundary of a partial melting domain developed at the expenses of lithospheric mantle. These data suggest that the lithosphere above a mantle plume undergoes a complex sequence of magmatic processes that significantly change its composition. Yet, crystal preferred orientations and thus seismic anisotropy are little affected by these processes. Lherzolites and harzburgites, independent from composition, show high-temperature porphyroclastic microstructures and strong olivine CPO. Although dunites and wehrlites display annealing microstructures to which is associated a progressive dispersion of the olivine CPO, very weak CPO are limited to a few dunites and wehrlites, suggesting that CPO destruction is restricted to domains of intense magma-rock interaction due to localized flow or accumulation of magmas. Conversely, the compositional changes result in lower seismic velocities for P- and S-waves. Relative to normal mantle, seismic anomalies may attain -2.5 (2

Diamond carbon sources: a comparison of carbon isotope models

International Nuclear Information System (INIS)

Kirkley, M.B.; Otter, M.L.; Gurney, J.J.; Hill, S.J.

1990-01-01

The carbon isotope compositions of approximately 500 inclusion-bearing diamonds have been determined in the past decade. 98 percent of these diamonds readily fall into two broad categories on the basis of their inclusion mineralogies and compositions. These categories are peridotitic diamonds and eclogitic diamonds. Most peridotitic diamonds have δ 13 C values between -10 and -1 permil, whereas eclogitic diamonds have δ 13 C values between -28 and +2 permil. Peridotitic diamonds may represent primordial carbon, however, it is proposed that initially inhomogeneous δ 13 C values were subsequently homogenized, e.g. during melting and convection that is postulated to have occurred during the first billion years of the earth's existence. If this is the case, then the wider range of δ 13 C values exhibited by eclogitic diamonds requires a different explanation. Both the fractionation model and the subduction model can account for the range of observed δ 13 C values in eclogitic diamonds. 16 refs., 2 figs

From opening to subduction of an oceanic domain constrained by LA-ICP-MS U-Pb zircon dating (Variscan belt, Southern Armorican Massif, France)

Science.gov (United States)

Paquette, J.-L.; Ballèvre, M.; Peucat, J.-J.; Cornen, G.

2017-12-01

In the Variscan belt of Western Europe, the lifetime and evolution of the oceanic domain is poorly constrained by sparse, outdated and unreliable multigrain ID-TIMS U-Pb zircon dating. In this article, we present a complete in situ LA-ICP-MS dataset of about 300 U-Pb zircon analyses obtained on most of the ophiolitic and eclogitic outcrops of Southern Brittany, comprising new dating of previously published zircon populations and newly discovered rock samples. In situ dating and cathodo-luminescence imaging of each zircon grain yields new absolute time-constraints on the evolution of the Galicia-Moldanubian Ocean. The new results confirm that the opening of this oceanic domain is well defined at about 490 Ma. In contrast, the generally-quoted 400-410 Ma-age for the high-pressure event related to the subduction of the oceanic crust is definitely not recorded in the zircons of the eclogites. In light of these new data, we propose that the obduction of oceanic rocks occurred at about 370-380 Ma while the high-pressure event is recorded at 355 Ma in only a few zircon grains of some eclogite samples. Additionally, this large scale dating project demonstrates that the zircons from eclogites do not systematically recrystallise during the high pressure event and consequently their U-Pb systems do not record that metamorphism systematically. These zircons rather preserve the isotopic memory of the magmatic crystallization of their igneous protolith. Another example of an eclogite sample from the French Massif Central illustrates the frequent mistake in the interpretation of the ages of the early hydrothermal alteration of zircons in the oceanic crust versus partial or complete recrystallization during eclogite facies metamorphism.

1.8 billion years of fluid-crust interaction: A zircon oxygen isotope record for the lower crust, western Churchill Province, Canadian Shield

Science.gov (United States)

Petts, Duane C.; Moser, Desmond E.; Longstaffe, Frederick J.; Davis, William J.; Stern, Richard A.

2014-04-01

The western Churchill Province of the Canadian Shield experienced a prolonged and complex formation history (ca. 4.04 to 1.70 Ga), with evidence for multiple episodes of orogenesis and regional magmatic activity. Here we report on the oxygen isotopic compositions of garnet and zircon recovered from lower crustal xenoliths, which have U-Pb ages between ca. 3.5 and 1.7 Ga. Overall, zircon from four metabasite xenoliths from the Rankin Inlet sample suite have δ18O values ranging from + 5.5 to + 8.6‰. Zircon from three metatonalite/anorthosite xenoliths and five metabasite xenoliths from the Repulse Bay sample suite have δ18O values of + 5.6 to + 8.3‰. High δ18O values (> + 6.0‰) for the oldest igneous zircon cores (ca. 3.5 Ga and 3.0-2.6 Ga) indicate that their metatonalite/anorthosite protolith magmas were generated from, or had assimilated, supracrustal rocks that interacted previously with surface-derived fluids. Igneous zircon cores (ca. 2.9-2.6 Ga) from one metabasite xenolith have δ18O values of + 5.6 to + 6.4‰, which suggests a formation from a mantle-derived basaltic/gabbroic magma. Metamorphic zircon cores (ca. 2.0-1.9 Ga) from one metabasite xenolith commonly have δ18O values between + 6.0 and + 6.3‰, which is indicative of a basalt/gabbro protolith and localized reworking of the lower crust caused by regional-scale plate convergence. The wide range of δ18O values (+ 5.5 to + 8.3‰) for ca. 1.75-1.70 Ga metamorphic zircon rims (identified in all xenoliths) indicates regional transient heating and reworking of mantle- and supracrustal-derived crust, induced by magmatic underplating along the crust-mantle boundary.

Xenoliths in Eocene lavas from Central Tibet record carbonated metasomatism of the lithosphere

Science.gov (United States)

Goussin, Fanny; Cordier, Carole; Boulvais, Philippe; Guillot, Stéphane; Roperch, Pierrick; Replumaz, Anne

2017-04-01

Cenozoic post-collisional volcanism of the Tibetan Plateau, emplaced on an accreted continental margin under compression, could bring important information regarding the edification of the Plateau. In this study, we combined petrography, whole rock geochemistry, stable isotopes and in situ mineral analysis to decipher the genesis of Eocene-Oligocene magmatic rocks from the Nangqian basin (35-38 Ma, [Spurlin et al., 2005; Xu et al., 2016]), located at the hinge between Central Tibet and the Eastern Indo-Asia Collision Zone. Our dataset includes potassic trachyandesites; amphibole-bearing potassic trachytes; and rare ultrapotassic (K2O/Na2O ≥ 4) mafic syenites. All samples have high REE abundances (La = 100 - 500 x primitive mantle). Fractionation of heavy REE (Gd/YbN > 3) indicates melting in the garnet stability field, and relative depletion in high-field strength elements (Nb, Ta) indicates a selective enrichment of the source by metasomatic fluids. This metasomatism event is also evidenced by the occurrence of re-equilibrated mantle xenocrysts of phlogopite (Mg# = 88 - 90 and Cr2O3 content = 0.9 - 1.82 wt%) in mafic syenites. Potassic trachyandesites have specific composition, with negative Zr-Hf anomaly and low Hf/Sm (0.2 - 0.4). Indeed, they include xenocrystic aggregates, composed of magmatic clinopyroxene, apatite and subordinate biotite and feldspar, with interstitial calcite and dolomite. δ18OV -SMOW (9.2 - 11.0 ) and δ13CV -PDB (-6.1 - -4.0 ) of these rocks indicate the presence of primary, mantle-derived carbonates. In situ analysis of the major and trace element compositions of the carbonates, clinopyroxenes and apatites further suggest that these aggregates represent cumulates of a carbonate-bearing magma. These xenoliths thus show that the lithospheric mantle was also metasomatized by CO2-rich fluids. Cenozoic carbonatites in China have been identified in Maoniuping in Western Sichuan (31.7 Ma), Lixian in the Western Qinlin (22-23 Ma), and

Deformation and hydration state of the lithospheric mantle beneath the Styrian Basin (Pannonian Basin, Eastern Austria)

Science.gov (United States)

Aradi, L. E.; Hidas, K.; Kovács, I. J.; Klébesz, R.; Szabo, C.

2016-12-01

In the Carpathian-Pannonian Region, Neogene alkali basaltic volcanism occurred in six volcanic fields, from which the Styrian Basin Volcanic Field (SBVF) is the westernmost one. In this study, we present new petrographic and crystal preferred orientation (CPO) data, and structural hydroxyl ("water") contents of upper mantle xenoliths from 12 volcanic outcrops across the SBVF. The studied xenoliths are mostly coarse granular lherzolites, amphiboles are present in almost every sample and often replace pyroxenes and spinels. The peridotites are highly annealed, olivines and pyroxenes do not show significant amount of intragranular deformation. Despite the annealed texture of the peridotites, olivine CPO is unambiguous, and varies between [010]-fiber, orthogonal and [100]-fiber symmetry. The CPO of pyroxenes is coherent with coeval deformation with olivine, showing [100]OL distributed subparallel to [001]OPX. The CPO of amphiboles suggest postkinematic epitaxial overgrowth on the precursor pyroxenes. The "water" content of the studied xenoliths exhibit rather high values, up to 10, 290 and 675 ppm in olivine, ortho- and clinopyroxene, respectively. Ortho- and clinopyroxene pairs show equilibrium in all samples, however "water" loss in olivines is observed in several xenoliths. The xenoliths show equilibrium temperatures from 850 to 1100 °C, which corresponds to lithospheric mantle depths between 30 and 60 km. Equilibrium temperatures show correlation with the varying CPO symmetries and grain size: coarser grained xenoliths with [100]-fiber and orthorhombic symmetry appear in the high temperature (>1000 °C) xenoliths, which is characteristic for asthenospheric origin. Most of the samples display transitional CPO symmetry between [010]-fiber and orthogonal, which indicate extensive lithospheric deformation under varying stress field from transtensional to transpressional settings. Based on the estimated seismic properties of the studied samples, a significant part of

Deformation and fluid-enhanced annealing in subcontinental lithospheric mantle beneath the Pannonian Basin (Styrian Basin, Eastern Austria)

Science.gov (United States)

Aradi, Laszlo; Hidas, Károly; János Kovács, István; Tommasi, Andrea; Garrido, Carlos; Szabó, Csaba

2017-04-01

In the Carpathian-Pannonian region, xenolith-bearing Neogene alkali basaltic volcanism occurred in five volcanic fields [1], from which the Styrian Basin Volcanic Field (SBVF) is the westernmost one. In this study, we present new petrographic and crystal preferred orientation (CPO) data, and structural hydroxyl ("water") contents of upper mantle xenoliths from 12 volcanic outcrops across the SBVF. The studied xenoliths are mostly coarse granular hydrous spinel lherzolites. Amphiboles, replacing pyroxenes and spinels, are present in almost every sample. The peridotites are highly annealed, olivines and pyroxenes show no significant amount of intragranular deformation. Despite the annealed texture of the peridotites, olivine CPO is unambiguous and varies between [010]-fiber, orthogonal and [100]-fiber symmetry. The CPO of pyroxenes is coherent with coeval deformation with olivine. The fabric and CPO of amphiboles suggest postkinematic epitaxial overgrowth on the precursor pyroxenes. The structural hydroxyl content of the studied xenoliths exhibits rather high, equilibrium values, up to 10, 290 and 675 ppm in olivine, ortho- and clinopyroxene, respectively. The olivines contain more structural hydroxyl in the annealed xenoliths than in the more deformed ones. The xenoliths show equilibrium temperatures from 850 to 1100 °C, which corresponds to lithospheric mantle depths between 30 and 60 km. Equilibrium temperatures show correlation with the varying CPO symmetries and grain size: coarser grained xenoliths with [100]-fiber and orthorhombic symmetry appear in the high temperature (>1000 °C) xenoliths, which is characteristic for asthenospheric environments [2]. Most of the samples display transitional CPO symmetry between [010]-fiber and orthogonal, which indicate lithospheric deformation under varying stress field from transtensional to transpressional settings [3], probably related to the Miocene evolution of the Pannonian Basin, during which varying compressive and

Within-plate Cenozoic Volcanism and Mantle Sources Within The Western-central Mediterranean Area

Science.gov (United States)

Beccaluva, L.; Bianchini, G.; Bonadiman, C.; Coltorti, M.; Siena, F.

An integrated study of anorogenic basic magmas and entrained mantle xenoliths rep- resents a promising approach for a comprehension of the magmatogenic events occur- ring within the lithospheric mantle in the western-central Mediterranean area. In this contribution we review the geochemical characteristics of mafic lavas and associated peridotite xenoliths from three anorogenic volcanic districts: Pliocene-Quaternary vol- canism of Sardinia; Pliocene-Quaternary volcanism of the Iblean area (eastern Sicily); Paleocene-Oligocene Veneto Volcanic Province. Investigations have been focused on 1) petrological features of parental magmas, which may contribute to infer the com- positional characteristics of mantle sources and to constrain the modes of partial melt- ing; 2) modelling the depletion events and metasomatic enrichments in mantle xeno- liths of the three volcanic districts, as well as the nature of their causative agents. Petrological features and Sr-Nd-Pb isotopic data, both of lava and xenoliths, indicate that DM+HIMU components distinguish the lithospheric mantle sections of Iblean and Veneto Volcanic Provinces. On the other hand, lavas and xenoliths from Sardinia display a significant different isotopic signature characterised by DM+EM1. Similar geochemical fingerprints, i.e. the significant presence of EM components are gener- ally recorded by mafic lavas and mantle xenoliths from the European Plate, whereas they are not observed in the stable African lithospheric domain.

New evidence for an old idea: Geochronological constraints for a paired metamorphic belt in the central European Variscides

Science.gov (United States)

Will, T. M.; Schmädicke, E.; Ling, X.-X.; Li, X.-H.; Li, Q.-L.

2018-03-01

New geochronological data reveal a prolonged tectonothermal evolution of the Variscan Odenwald-Spessart basement, being part of the Mid-German Crystalline Zone in central Europe. We report the results from (i) secondary ion mass spectrometry (SIMS) U-Pb dating of zircon, rutile and monazite, (ii) SIMS zircon oxygen isotope analyses, (iii) laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) zircon Lu-Hf isotope analyses and, (iv) LA-ICPMS zircon and rutile trace element data for a suite of metamorphic rocks (five amphibolite- and eclogite-facies mafic meta-igneous rocks and one granulite-facies paragneiss). The protoliths of the mafic rocks formed from juvenile as well as depleted mantle sources in distinct tectonic environments at different times. Magmatism took place at a divergent oceanic margin (possibly in a back-arc setting) at 460 Ma, in an intraoceanic basin at ca. 445 Ma and at a continental margin at 329 Ma. Regardless of lithology, zircon in eclogite, amphibolite and high-temperature paragneiss provide almost identical Carboniferous ages of 333.7 ± 4.1 Ma (eclogite), 329.1 ± 1.8 to 328.4 ± 8.9 Ma (amphibolite), and 334.0 ± 2.0 Ma (paragneiss), respectively. Rutile yielded ages of 328.6 ± 4.7 and 321.4 ± 7.0 Ma in eclogite and amphibolite, and monazite in high-temperature paragneiss grew at 330.1 ± 2.4 Ma (all ages are quoted at the 2σ level). The data constrain coeval high-pressure eclogite- and high-temperature granulite-facies metamorphism of the Odenwald-Spessart basement at ca. 330 Ma. Amphibolite-facies conditions were attained shortly afterwards. The lower plate eclogite formed in a fossil subduction zone and the upper plate high-temperature, low-pressure rocks are the remains of an eroded Carboniferous magmatic arc. The close proximity of tectonically juxtaposed units of such radically different metamorphic conditions and thermal gradients is characteristic for a paired metamorphic belt sensu Miyashiro

Peridotitic lithosphere metasomatised by volatile-bearing melts, and its association with intraplate alkaline HIMU-like magmatism

DEFF Research Database (Denmark)

Scott, James; Brenna, Marco; Crase, Jordan

2016-01-01

The role of lithospheric mantle metasomatized by CO2-bearing melts in the genesis of HIMU-like alkaline intraplate basalts is investigated using a suite of peridotite xenoliths from New Zealand. The xenoliths have Sr–Nd–Pb–Hf isotope compositions (87Sr/86Sr =0.7029, eNd = +5 to +6, 206Pb/204Pb = ...

Seismic anisotropy and compositionally induced velocity anomalies in the lithosphere above mantle plumes: a petrological and microstructural study of mantle xenoliths from French Polynesia

Science.gov (United States)

Tommasi, Andréa; Godard, Marguerite; Coromina, Guilhem; Dautria, Jean-Marie; Barsczus, Hans

2004-11-01

In addition to thermal erosion, plume/lithosphere interaction may induce significant changes in the lithosphere chemical composition. To constrain the extent of this process in an oceanic environment and its consequences on the lithosphere seismic properties, we investigated the relationship between petrological processes and microstructure in mantle xenoliths from different hotspots tracks in South Pacific Superswell region: the Austral-Cook, Society, and Marquesas islands in French Polynesia. Olivine forsterite contents in the studied spinel peridotites vary continuously from Fo91 to Fo83. Dunites and wehrlites display the lowest forsterite contents. Their microstructure and high Ni contents preclude a cumulate origin, suggesting that these rocks result from melt/rock reactions involving olivine precipitation and pyroxene dissolution. In addition, lherzolites and wehrlites display evidence of late crystallization of clinopyroxene, which may result from a near-solidus melt-freezing reaction. These data suggest that the lithosphere above a mantle plume undergoes a complex sequence of magmatic processes that significantly change its composition. These compositional changes, particularly iron enrichment in olivine, result in lower P- and S-waves velocities. Relative to normal lithospheric mantle, compositionally induced seismic anomalies may attain -2.2% for S-waves and -1% for P-waves. Smaller negative anomalies for P-waves are due to a higher sensitivity to modal composition. Conversely, crystal-preferred orientations (CPO) and seismic anisotropy are little affected by these processes. Lherzolites and harzburgites, independent from composition, show high-temperature porphyroclastic microstructures and strong olivine CPO. Dunites and wehrlites display annealing microstructures to which is associated a progressive dispersion of the olivine CPO. Very weak, almost random olivine CPO is nevertheless rare, suggesting that CPO destruction is restricted to domains of

Widespread melt/rock interaction and seismic properties of the lithosphere above mantle plumes: A petrological and microstructural study of mantle xenoliths from French Polynesia

Science.gov (United States)

Tommasi, A.; Godard, M.

2002-12-01

In addition to thermal erosion, plume/lithosphere interaction may induce significant changes in the lithosphere chemical composition. To constrain the extent of this process in an oceanic environment and its consequences on the lithosphere seismic properties, we studied the relationship between petrological processes and microstructure in mantle xenoliths from the Austral-Cook, Society and Marquesas islands. Olivine forsterite contents in our sp-peridotites vary continuously from Fo91 to Fo83, the lowest Fo being observed in dunites and wehrlites. Yet, their high Ni content (up to 2500 ppm) precludes a cumulate origin. These rocks are rather interpreted as resulting from melt/rock reactions involving olivine precipitation and pyroxene dissolution, the dunites indicating high melt-rock ratios. Moreover, wehrlites display poikiloblastic diopside enclosing corroded olivines. Late crystallization of clinopyroxene, also observed in lherzolites, may result from a near-solidus melt-freezing reaction occurring at the boundary of a partial melting domain developed at the expenses of lithospheric mantle. These data suggest that the lithosphere above a mantle plume undergoes a complex sequence of magmatic processes that significantly change its composition. Yet, crystal preferred orientations and thus seismic anisotropy are little affected by these processes. Lherzolites and harzburgites, independent from composition, show high-temperature porphyroclastic microstructures and strong olivine CPO. Although dunites and wehrlites display annealing microstructures to which is associated a progressive dispersion of the olivine CPO, very weak CPO are limited to a few dunites and wehrlites, suggesting that CPO destruction is restricted to domains of intense magma-rock interaction due to localized flow or accumulation of magmas. Conversely, the compositional changes result in lower seismic velocities for P- and S-waves. Relative to normal mantle, seismic anomalies may attain -2

Hydrogen storage in Earth's mantle and core

Science.gov (United States)

Prewitt, Charles T.

1994-01-01

Two different approaches to explaining how hydrogen might be stored in the mantle are illustrated by a number of papers published over the past 25-30 years, but there has been little attempt to provide objective comparisons of the two. One approach invokes the presence in the mantle of dense hydrous magnesium silicates (DHMS) stable at elevated pressures and temperatures. The other involves nominally anhydrous minerals (NAM) that contain hydrogen as a minor constituent on the ppm level. Experimental studies on DHMS indicate these phases may be stable to pressures and temperatures as high at 16 GPa and 1200 C. This temperature is lower than that indicated by a mantle geotherm at 16 GPa, but may be reasonable for a subducting slab. It is possible that other DHMS could be stable to even higher pressures, but little is known about maximum temperature limits. For NAM, small amounts of hydrogen (up to several hundred ppm) have been detected in olivine, orthopyroxene, clinopyroxene, and garnet recovered from xenoliths in kimberlites, eclogites, and alkali basalts; it has been demonstrated that synthetic wadsleyite and perovskite can accommodate significant amounts of hydrogen. A number of problems are associated with each possibility. For NAM originating in the mantle, one would like to assume that the hydrogen measured in samples recovered on Earth's surface was incorporated when the phase-crystallized at high temperatures and pressures, but it could have been introduced during transport to the surface. Major problems for the DHMS proponents are that none of these phases have been found as minerals and little is yet known about their stabilities in systems containing other cations such as Fe, Al, and Ca.

P-T Equilibrium Conditions of Xenoliths from the Udachnaya Kimberlite Pipe: Thermal Perturbations in the Lithospheric Mantle

Science.gov (United States)

Tychkov, Nikolay; Agashev, Alexey; Malygina, Elena; Pokhilenko, Nikolay

2014-05-01

Integrated study of 250 peridotite xenoliths from Udachnaya -East pipe show difference in mineral paragenesises and textural-structural peculiarities in the different level of cratonic lithosphere mantle (CLM). The compositions of minerals were determined using EPMA. Thermobarometric parameters (Brey, Kohller, 1990) were determined for all rocks occupying different fields on geothermal curve. The deepest layer (the pressure interval of 5.0-7.0 GPa) contains mostly pophyroclastic lherzolites. Anyway, some rocks of this layer have an idiomorphic texture being also enriched in incompatible components. Higher in the CLM sequence, the interval (4.2-6.3 GPa) is composed of the most depleted rocks: megacristalline ultradepleted harzburgite-dunites and depleted granular harzburgite-dunites, as well as lherzolites in a subordinate amount. They correspond strate to 35 mW/m2 and partly overlap the deeper layer in dapth. It is likely that rocks of this layer are in equilibrium and were not subject to significant secondary changes due to kimberlite magma intrusion. Thus, this interval of the CLM sequence reflects the true (relic) geotherm for the area of the Udachnaya kimberlite pipe. Moreover, it is obvious that this interval was a major supplier of diamonds into kimberlites of the Udachnaya pipe. The interval of 4.2-2.0 GPa in the CLM sequence is also composed of coarse depleted lherzolites and harzburgites. Rocks of this interval are slightly more enriched than those of the underlying interval. This is confirmed by the distinct predominance of lherzolites over harzburgite-dunites. The heat flow in this layer varies in the range of 38-45 mW/m2 and shows a general tendency to increase with decreasing depth. According to occurrence of nonequilibrium mineral assemblages and increased heat flow relative to the major heat flow of 35 mW/m2, this interval is similar to the deepest interval of secondary enriched rocks. Interval of less than 2.0 GPa composed of spinel lherzolites and

Implications of Zn/Fe ratios for the sources of Colorado Plateau basalts

Science.gov (United States)

Rudzitis, S.; Reid, M. R.

2011-12-01

Early Miocene to recent mafic magmatism migrated across the Arizona Transition Zone towards the center of the stable Colorado Plateau at a rate of ~ 3-6 km/Myr (Roy et al., 2009). Present-day volcanic centers are close to a stepwise change in the thickness of the lithosphere between the Colorado Plateau and Basin and Range. Accordingly, volcanic migration might track progressive thinning of the lithosphere towards the center of the Colorado Plateau. This project aims to determine the conditions of melt generation across the transition zone in order to investigate the temporal/spatial correlation between volcanism and thinning of the Colorado Plateau lithosphere. Pressure and temperature estimates for Colorado Plateau basalts can be obtained from the Mg and Si contents of melts (Lee et al, 2009) but require melting of a peridotitic source. Eclogite and pyroxenite xenoliths reported in Colorado Plateau basalts show that melt sources could be olivine-poor. Zn/Fe ratios in melts can help to distinguish contributions from olivine-poor sources because they are sensitive to differences in bulk chemistry and to mineralogy (Le Roux et al., 2010). Specifically, Zn/Fe is not fractionated between melt, olivine, and orthopyroxene, but is highly fractionated when clinopyroxene and garnet are present. Our work to date has focused on laser ablation-IC-PMS analysis of individual olivine grains from high-Mg basalts (>8.0 wt. %) from the San Francisco and Mormon Mountain volcanic fields. Preliminary values of Zn/Fe ratios that represent the averages of multiple analyses of several grains in individual samples range from 7.9 to 9.3 (x10000). Variations of up to 1.7 (x10000) in the ratios exist between individual grains within samples and could be the result of co-crystallization of clinopyroxene with olivine. The lowest values in each sample should approach the Zn/Fe ratios of parental melts, and are, in turn, similar to MORB values and predicted peridotite melts. The results suggest

The Mantle and Basalt-Crust Interaction Below the Mount Taylor Volcanic Field, New Mexico

Science.gov (United States)

Schrader, Christian M.; Crumpler, Larry S.; Schmidt, Marick E.

2010-01-01

The Mount Taylor Volcanic Field (MTVF) lies on the Jemez Lineament on the southeastern margin of the Colorado Plateau. The field is centered on the Mt. Taylor composite volcano and includes Mesa Chivato to the NE and Grants Ridge to the WSW. MTVF magmatism spans approximately 3.8-1.5 Ma (K-Ar). Magmas are dominantly alkaline with mafic compositions ranging from basanite to hy-basalt and felsic compositions ranging from ne-trachyte to rhyolite. We are investigating the state of the mantle and the spatial and temporal variation in basalt-crustal interaction below the MTVF by examining mantle xenoliths and basalts in the context of new mapping and future Ar-Ar dating. The earliest dated magmatism in the field is a basanite flow south of Mt. Taylor. Mantle xenolith-bearing alkali basalts and basanites occur on Mesa Chivato and in the region of Mt. Taylor, though most basalts are peripheral to the main cone. Xenolith-bearing magmatism persists at least into the early stages of conebuilding. Preliminary examination of the mantle xenolith suite suggests it is dominantly lherzolitic but contains likely examples of both melt-depleted (harzburgitic) and melt-enriched (clinopyroxenitic) mantle. There are aphyric and crystal-poor hawaiites, some of which are hy-normative, on and near Mt. Taylor, but many of the more evolved MTVF basalts show evidence of complex histories. Mt. Taylor basalts higher in the cone-building sequence contain >40% zoned plagioclase pheno- and megacrysts. Other basalts peripheral to Mt. Taylor and at Grants Ridge contain clinopyroxene and plagioclase megacrysts and cumulate-textured xenoliths, suggesting they interacted with lower crustal cumulates. Among the questions we are addressing: What was the chemical and thermal state of the mantle recorded by the basaltic suites and xenoliths and how did it change with time? Are multiple parental basalts (Si-saturated vs. undersaturated) represented and, if so, what changes in the mantle or in the tectonic

Water Distribution in the Continental and Oceanic Upper Mantle

Science.gov (United States)

Peslier, Anne H.

2015-01-01

Nominally anhydrous minerals such as olivine, pyroxene and garnet can accommodate tens to hundreds of ppm H2O in the form of hydrogen bonded to structural oxygen in lattice defects. Although in seemingly small amounts, this water can significantly alter chemical and physical properties of the minerals and rocks. Water in particular can modify their rheological properties and its distribution in the mantle derives from melting and metasomatic processes and lithology repartition (pyroxenite vs peridotite). These effects will be examined here using Fourier transform infrared spectrometry (FTIR) water analyses on minerals from mantle xenoliths from cratons, plume-influenced cratons and oceanic settings. In particular, our results on xenoliths from three different cratons will be compared. Each craton has a different water distribution and only the mantle root of Kaapvaal has evidence for dry olivine at its base. This challenges the link between olivine water content and survival of Archean cratonic mantle, and questions whether xenoliths are representative of the whole cratonic mantle. We will also present our latest data on Hawaii and Tanzanian craton xenoliths which both suggest the intriguing result that mantle lithosphere is not enriched in water when it interacts with melts from deep mantle upwellings (plumes).

New evidence for the asthenospheric origin of the Cameroon Volcanic Line from 1D shear wave velocities

CSIR Research Space (South Africa)

Tokam, AP

2013-10-01

Full Text Available the mantle composition beneath Ethiopia and southern Brazil (Keranen et al., 2009; Julia et al., 2008). Many petrological studies of ultramafic orogenic massifs and ultramafic xenoliths along the CVL (mainly around Mount Cameroon and the Adamawa....N. and Oya, M. 2010. Petrological and chemical variability of peridotite xenoliths from the Cameroon Volcanic Line, West Africa: an evidence from Plume emplacement. Journal of Mineralogical and Petrological Sciences, 107, 57-69. McKenzie, D...

MARID-type Glimmerites from Kimberley, South Africa: Metasomes or high-pressure cumulates?

Science.gov (United States)

Förster, Michael W.; Prelevic, Dejan; Buhre, Stephan; Jacob, Dorrit E.

2015-04-01

Mica- amphibole- rutile- ilmenite- diopside (MARID) xenoliths are alkali-rich, coarse-grained ultramafic rocks, typical for heavily metasomatized subcontinental lithospheric mantle (Dawson & Smith, 1977). They are produced either by interaction of mantle wall rock with lamproitic melts that percolate through the mantle (Dawson and Smith 1977; Sweeney 1993), or as direct crystallization products of those melts (Waters 1987). Two rock samples of mica-rich (>90% phlogopite) xenoliths from the Boshof Road Dump of the Bultfontein kimberlite diamond mine in Kimberley, South Africa were analyzed for major and trace elements of minerals. Millimeter sized phlogopite is the dominant mineral, making up more than 90% of the rock. Other phases are in descending order: diopside, K-richterite, rutile and ilmenite. Phlogopite is homogenous in composition and appears without zonation. They are perpotassic with K/Al between 1.1 and 1.2 at an Mg#-value of 84.5-86.5. Clinopyroxene is low in Al2O3 with values 90% phlogopite. Perpotassic phlogopites with K/Al >1 values are typical for MARID-type xenoliths by comprising low Mg# of 82-88 (Dawson 1987). We performed thermobarometric calculations on the clinopyroxenes, by using the equations of Putirka (2008). With a proposed lamproitic melt, like Waters (1987) suggested for a MARID parental magma, a pressure of 13 kbar (39 km) and a temperature of 1300 C was calculated. This depth coincides with the crustal thickness of the Kaapvaal craton (Nguuri et al. 2001). However, the pressure calculations depend on the fractionation of Al between melt and mineral and are not realistic for low-Al diopsides. Calculations by Konzett et al. (2014) yielded 4.2 GPa (155 km) by using a Ca-in-opx thermometer and a cratonic geotherm of 40 mW/m² and seem to be more realistic. By applying a sandwich experimental approach, mixing glimmerite samples with harzburgitic peridotites, we hope to achieve deeper insights into the origin of MARID-type glimmerites

El Hierro's floating stones as messengers of crust-magma interaction at depth

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Burchardt, S.; Troll, V. R.; Schmeling, H.; Koyi, H.; Blythe, L. S.; Longpré, M. A.; Deegan, F. M.

2012-04-01

During the early stages of the submarine eruption that started on October 10 2011 south of El Hierro, Canary Islands, Spain, peculiar eruption products were found floating on the sea surface. These centimetre- to decimetre-sized "bombs" have been termed "restingolites" after the nearby village La Restinga and consist of a basaltic rind and a white to light grey core that resembles pumice in texture. According to Troll et al. (2011; see also Troll et al. EGU 2012 Abstracts), this material consists of a glassy matrix hosting extensive vesicle networks, which results in extremely low densities allowing these rocks to float on sea water. Mineralogical and geochemical analyses reveal that the "restingolites" originate from the sedimentary rocks (sand-, silt-, and mudstones) that form layer 1 of the oceanic crust beneath El Hierro. During the onset and early stages of the eruption, magma ponded at the base of this sedimentary sequence, breaking its way through the sedimentary rocks to the ocean floor. The textures of the "restingolites" reveal that crust-magma interaction during fragmentation and transport of the xenoliths involved rapid partial melting and volatile exsolution. Xenoliths strikingly similar to those from El Hierro are known from eruptions on other Canary Islands (e.g. La Palma, Gran Canaria, and Lanzarote). In fact, they resemble in texture xenoliths of various protoliths from volcanic areas worldwide (e.g. Krakatao, Indonesia, Cerro Quemado, Guatemala, Laacher See, Germany). This indicates that the process of partial melting and volatile exsolution, which the "restingolites" bear witness of, is probably occurring frequently during shallow crustal magma emplacement. Thermomechanical numerical models of the effect of the density decrease associated with the formation of vesicle networks in partially molten xenoliths show that xenoliths of crustal rocks initially sink in a magma chamber, but may start to float to the chamber roof once they start to heat up

Titanates of the lindsleyite-mathiasite (LIMA) group reveal isotope disequilibrium associated with metasomatism in the mantle beneath Kimberley (South Africa)

Science.gov (United States)

Giuliani, Andrea; Woodhead, Jon D.; Phillips, David; Maas, Roland; Davies, Gareth R.; Griffin, William L.

2018-01-01

Radiogenic isotope variations unrelated to radiogenic ingrowth are common between minerals found in metasomatised mantle xenoliths entrained in kimberlite, basalts and related magmas. As the metasomatic minerals are assumed to have been in isotopic equilibrium originally, such variations are typically attributed to contamination by the magma host and/or interaction with mantle fluids during or before xenolith transport to surface. However, the increasing evidence of metasomatism by multiple, compositionally distinct fluids permeating the lithospheric mantle, coeval with specific magmatic events, suggests that isotopic disequilibrium might be a consequence of discrete, though complex, metasomatic events. Here we provide clear evidence of elemental and Sr isotope heterogeneity between coeval Ti-rich LIMA (lindsleyite-mathiasite) minerals at the time of their formation in the mantle. LIMA minerals occur in close textural association with clinopyroxene and phlogopite in low-temperature (∼800-900 °C), strongly metasomatised mantle xenoliths from the ∼84 Ma Bultfontein kimberlite (South Africa). Previous U/Pb dating of the LIMA phases was used to argue that each xenolith recorded a single event of LIMA crystallisation at ∼180-190 Ma, coeval with the emplacement of Karoo magmas. SEM imaging reveals that up to four types of LIMA phases coexist in each xenolith, and occasionally in a single LIMA grain. Major element and in situ Sr isotope analyses of the different LIMA types show that each phase has a distinct elemental composition and initial 87Sr/86Sr ratio (e.g., 0.7068-0.7086 and 0.7115-0.7129 for two LIMA types in a single xenolith; 0.7053-0.7131 across the entire sample suite). These combined age and isotopic constraints require that multiple fluids metasomatised these rocks at broadly the same time (i.e. within a few thousands to millions of years), and produced similar mineralogical features. Elemental and isotopic variations between different LIMA types

New constraints on the textural and geochemical evolution of the upper mantle beneath the Styrian basin

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Aradi, Laszlo; Hidas, Károly; Zanetti, Alberto; János Kovács, István; Patkó, Levente; Szabó, Csaba

2016-04-01

Plio-Pleistocene alkali basaltic volcanism sampled sporadically the upper mantle beneath the Carpathian-Pannonian Region (CPR, e.g. [1]). Lavas and pyroclasts often contain mantle derived xenoliths, and the majority of them have been extensively studied [1], except the westernmost Styrian Basin Volcanic Field (SBVF, Eastern Austria and Slovenia). In the SBVF only a few volcanic centers have been studied in details (e.g. Kapfenstein & Tobaj). Based on these studies, the upper mantle beneath the SBVF is consists of dominantly high temperature, texturally and geochemically homogeneous protogranular spinel lherzolite. New major and trace element data from rock-forming minerals of ultramafic xenoliths, coupled with texture and deformation analysis from 12 volcanic outcrops across the SBVF, suggest that the lithospheric roots of the region are more heterogeneous than described previously. The studied xenoliths are predominantly lherzolite, amphibole is a common phase that replaces pyroxenes and spinels and proves modal metasomatism. Phlogopite coupled with apatite is also present in amphibole-rich samples. The texture of the xenoliths is usually coarse-grained and annealed with low abundance of subgrain boundaries in both olivine and pyroxenes. Olivine crystal preferred orientation (CPO) varies between the three most abundant one: [010]-fiber, orthogonal and [100]-fiber symmetry [2]. The CPO of pyroxenes is usually coherent with coeval deformation with olivine, however the CPO of amphibole is suggesting postkinematic epitaxial overgrowth on the precursor pyroxenes. According to equilibrium temperatures, the studied xenolith suite samples a broader temperature range (850-1100 °C) than the literature data, corresponding to mantle depths between 30 and 60 km, which indicates that the xenolith suite only represents the shallower part of the recent 100 km thick lithospheric mantle beneath the SBVF. The equilibrium temperatures show correlation with the varying CPO symmetries

Characterization of carbonaceous matter in xenolithic clasts from the Sharps (H3.4) meteorite: Constraints on the origin and thermal processing

Energy Technology Data Exchange (ETDEWEB)

Kebukawa, Yoko [Yokohama National Univ. (Japan); Zolensky, Michael E. [NASA Johnson Space Center, Houston, TX (United States); Chan, Queenie H. S. [NASA Johnson Space Center, Houston, TX (United States); Nagao, Keisuke [Univ. of Tokyo (Japan); Kilcoyne, A. L. David [Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); Bodnar, Robert J. [Virginia Polytechnic Inst. and State Univ. (Virginia Tech), Blacksburg, VA (United States); Farley, Charles [Virginia Polytechnic Inst. and State Univ. (Virginia Tech), Blacksburg, VA (United States); Rahman, Zia [NASA Johnson Space Center, Houston, TX (United States); Le, Loan [NASA Johnson Space Center, Houston, TX (United States); Cody, George D. [Carnegie Inst. of Washington, Washington, DC (United States)

2016-09-25

Primitive xenolithic clasts, often referred to as “dark clasts”, are well known in many regolith breccias. The Sharps H3.4 ordinary chondrite contains unusually large dark clasts up to ~1 cm across. Poorly-graphitized carbon (PGC), with Fe, Ni metal and described as “carbon-rich aggregates”, has been found in these clasts (Brearley, 1990). We report detailed analyses of carbonaceous matter in several identical Sharps clasts using FTIR, Raman, C-XANES, and TEM that provide insight on the extent of thermal processing and possible origin of such clasts. We also prepared acid residues of the clasts using the HCl/HF method and conducted mass spectrometric analysis of the entrained noble gases. Carbonaceous matter is often used to infer thermal history due to its sensitivity to thermal processes. The FTIR spectra of the acid residue from the Sharps clast suggest that carbonaceous matter in the clast contains less hydrogen and oxygen compared to acid residues from typical type 3.4 ordinary chondrites. The metamorphic temperatures obtained by Raman spectroscopy ranges between ~380 °C and ~490 °C. TEM observations indicate that the clasts experienced a peak temperature of 300 °C to 400 °C, based on the carbon d 002 layer lattice spacing of C-rich aggregates. These estimates are consistent with an earlier estimate of 330 ± 50 °C, that is also estimated by the d 002 layer lattice spacing (Brearley, 1990). It should be noted that the lattice spacing thermometer is based on terrestrial metamorphose rocks, and thus temperature was probably underestimated. Meanwhile, the C-XANES spectra of the C-rich aggregates show high exciton intensities, indicative of graphene structures that developed at around 700–800 °C following an extensive period of time (millions of years), however, the surrounding matrix areas experienced lower temperatures of less than 300–500 °C. Noble gas analysis of the acid residue from the Sharps clasts shows that the residue is

Geology of the Northern part of the Strath Ossian Granite, Scotland

International Nuclear Information System (INIS)

Henderson, W.G.

1982-12-01

The Strath Ossian Granite is made up of granodiorite, dark, variable 'granodiorites' interpreted as mobilised diorite or basic material, appinite and porphyritic granodiorite. Huge rafts of psammitic metasediments occur within the mass and three fracture-zones and numerous dykes, dominantly of porphyrite, cut across it in a north-easterly direction. Granite emplacement may have occurred in stages, early batches being xenolith-rich and later ones xenolith-poor. New batches were intruded centrally, which created strong radial stresses, sufficiently strong to make room for the intrusion by forcing the metasedimentary country rocks downwards and aside. (author)

Evolution of supercritical fluid in deeply subducted continental crust: a case study of composite granite-quartz veins in the Sulu belt, China

Science.gov (United States)

Wang, S.; Wang, L.; Brown, M.

2016-12-01

Although fluid plays a key role in element transport and rock strength during subduction to and exhumation from ultrahigh pressure (UHP) metamorphic conditions, the source of supercritical fluid at P above the second critical endpoints (SCE) and the subsequent evolution are not well constrained. To provide insight into the evolution of supercritical fluid in continental subduction zones, we undertook an integrated study of composite granite-quartz veins in retrogressed and migmatitic UHP eclogite at General's Hill, N of Qingdao, in the central Sulu belt. The composite veins are irregularly distributed in the eclogite, which occurs as blocks within gneiss. The granite component is enriched in large ion lithophile elements and light rare earth elements but depleted in high field strength elements and heavy rare earth elements, indicating crystallization from a melt phase of crustal origin. Additionally, the granite contains high modal phengite (22-30 vol%) and clinozoisite/epidote (3-10 vol%), implying precipitation from a H2O-rich silicate melt. By contrast, the quartz component is dominated by SiO2 (99.10 wt%), and contains low total rare earth elements (ΣREE = 0.46 ppm), indicating precipitation from an aqueous fluid. The crystallization age of the composite veins is 221 ± 2 Ma, which is younger than the UHP metamorphism in the Sulu belt at ca 230 Ma, consistent with formation during exhumation. Initial 176Hf/177Hf ratios and δ18O values of metamorphic zircons from the composite veins, and Sr-Nd isotope compositions of the granites all lie between values for eclogite and gneiss, indicating a mixed source. Accordingly, we propose that a supercritical fluid generated from the gneiss and the included blocks of eclogite at P-T conditions above the SCE for both compositions became trapped in the eclogite during exhumation. At P below the SCE for the hydrous granite system, the mixed supercritical fluid separated into immiscible aqueous melt and aqueous fluid and

Transdomes sampling of lower and middle crust

Science.gov (United States)

Teyssier, C. P.; Whitney, D. L.; Roger, F.; Rey, P. F.

2015-12-01

Migmatite transdomes are formed by lateral and upward flow of partially molten crust in transtension zones (pull-apart structures). In order to understand the flow leading to this type of domes, 3D numerical models were set-up to simulate the general case of an extensional domain located between two strike-slip faults (pull-apart or dilational bridge). Results show that upper crust extension induces flow of the deep, low-viscosity crust, with rapid upward movement of transdome material when extension becomes localized. At this point a rolling hinge detachment allows rapid removal of upper crust. The internal structure of transdomes includes a subvertical high strain zone located beneath the zone of localized upper crust extension; this shear zone separates two elongate subdomes of foliation that show refolded/sheath folds. Lineation tends to be oriented dominantly subhorizontal when the amount of strike-slip motion is greater than the amount of upward flow of dome rocks. Models also predict nearly isothermal decompression of transdome material and rapid transfer of ~50 km deep rocks to the near surface. These model results are compared to the structural and metamorphic history of several transdomes, and in particular the Variscan Montagne Noire dome (French Massif Central) that consists of two domes separated by a complex high strain zone. The Montagne Noire dome contains ~315 Ma eclogite bodies (U-Pb zircon age) that record 1.4 GPa peak pressure. The eclogite bodies are wrapped in highly sheared migmatite that yield 314-310 Ma monazite ages interpreted as the metamorphism and deformation age. Based on these relations we conclude that the Montagne Noire transdome developed a channel of partially molten crust that likely entrained eclogite bodies from the deep crust (~50 km) before ascending to the near-surface. One implication of this work is that the flowing crust was deeply seated in the orogen although it remained a poor recorder of peak pressure of metamorphism

Unraveling the history of complex zoned garnets from the North Motagua Mélange (Guatemala)

Science.gov (United States)

Barickman, M. H.; Martin, C.; Flores, K. E.; Harlow, G. E.; Bonnet, G.

2016-12-01

The Guatemala Suture Zone (GSZ) is situated in central Guatemala, between the North American and Caribbean plates. Two serpentinite mélanges straddle the Motagua Fault system: the North Motagua Mélange (NMM) and the South Motagua Mélange (SMM). In this study, chemically zoned garnet grains from four eclogite blocks from the NMM were analyzed by EMPA for major elements and LA-ICP-MS for trace elements to unravel the geological history of the eclogites. These eclogites typically consist of euhedral to subhedral garnets, partly retrogressed omphacite grains, and accessory minerals such as phengite and epidote as inclusions in garnet. EBSD was employed to examine apparent garnet inclusions in garnet. The garnet grains in NMM eclogites display complex chemical zonations: all grains roughly show a spessartine-rich core, an almandine-rich core and/or intermediate zone, and a pyrope and grossular-rich rim. Additionally, crystal resorption can be observed between the different zones, and the pyrope-grossular rim can display oscillatory zoning. Finally, grossular-rich zones (crystallographically syntactic) within garnet are present in all studied samples. REE and spider diagrams do not show any significant difference in the patterns of the different zones within the garnet, or indicating that the chemical environment from which each garnet zone grew was broadly the same. The lack of significant variation in LILE content indicates that a fluid influx during garnet growth is unlikely. Consequently, we interpret that garnet grains grew in a largely closed system; however, the presence of the grossular-rich zones, argues for occasional excursions into conditions when either two garnets crystallized or Ca-rich overgrowths that were largely resorbed prior to subsequent continued garnet growth.

Petrology and deformation style of lithospheric mantle beneath the Heldburg Dike swarm (Central Germany) subset of Central European Volcanic Province

Science.gov (United States)

Kukuła, Anna; Puziewicz, Jacek; Hidas, Károly; Ntaflos, Theodoros; Matusiak-Małek, Magdalena; Milke, Ralf

2017-04-01

The Heldburg Dike swarm is a set of Cenozoic alkali basalt dikes occurring in the central part of Germany at the border between Thuringia and Bavaria. We studied xenoliths from Strauf, Feldstein, Bramberg and from the active quarry in Zeilberg. The peridotites from Strauf, Feldstein and Bramberg have the composition of spinel lherzolite (15), spinel harzburgite (9) and dunite (3). They vary in size from 1.5 cm (Strauf) up to 20 cm (Zeilberg). We distinguish groups (A, A- and B) of peridotites based on different forsterite content in olivine. Group A consists of olivine (89.6 - 91.8 Fo), orthopyroxene (Mg# 0.90-0.93, Al 0.05-0.18 a pfu), clinopyroxene (Mg# 0.87-0.95, Al 0.06-0.26 a pfu) and spinel (Cr# 0.13-0.65, Mg# 0.54-0.78). Clinopyroxene rare earth elements (REE) patterns are S-shaped (Feldstein, Bramberg) or U-shaped (Strauf); spoon-shaped patterns occur occasionally. Trace element (TE) patterns show negative Nb, Ta, Zr, Hf, Ti and positive Th, U anomalies. The most magnesian clinopyroxene (xenolith 3140, Feldstein) is strongly aluminous and LREE depletedwith weak anomalies in TE patterns. Group A- is contains olivine (88.9-89.5 Fo), orthopyroxene (Mg# 0.89-0.90, Al 0.10-0.13 a pfu) and clinopyroxene (Mg# 0.90-0.92, Al 0.10-0.17 a pfu). Clinopyroxene is increasingly enriched in REEs from Lu to La. TE patterns are similar to those of group A but with less pronounced anomalies. Group B (3 xenoliths only) consists of olivine Fo 86.7-88.9, orthopyroxene (Mg# 0.88-0.89, Al 0.07-0.19 a pfu), clinopyroxene (Mg# 0.88-0.90, Al 0.10-0.26 a pfu). Clinopyroxene is enriched in LREE, concave upward in Pr. TE patterns are similar to those in group A. One of group B harzburgites contains grains (up to 0.5 mm) of Ca-Mg carbonate located in interstices. The clinopyroxene chemical composition plots away from the melting trend in the MgO-Al2O3 diagram of Upton et al. (2011), suggesting a later addition of the clinopyroxene. The composition of orthopyroxene corresponds to ca. 15

Eocene deep crust at Ama Drime, Tibet

DEFF Research Database (Denmark)

Kellett, Dawn; Cottle, John; Smit, Matthijs Arjen

2014-01-01

Granulitized eclogite-facies rocks exposed in the Ama Drime Massif, south Tibet, were dated by Lu-Hf garnet geochronology. Garnet from the three samples analyzed yielded Lu-Hf ages of 37.5 ± 0.8 Ma, 36.0 ± 1.9 Ma, and 33.9 ± 0.8 Ma. Eclogitic garnet growth is estimated at ca. 38 Ma, the oldest age...... burial and exhumation of a cold subducted slab. The rocks instead resulted from crustal thickening during the early stages of continental collision, and resided in the lower-middle crust for >20 m.y. before they were exhumed and reheated. These new data provide solid evidence for the Indian crust having...

Domain structure and texture in fine grained symplectite from garnet breakdown in peridotite xenoliths (Zinst, Bavaria, Bohemian Massif)

Science.gov (United States)

Habler, G.; Špaček, P.; Abart, R.

2012-04-01

Lherzolite xenoliths entrained in Oligocene basanite at the locality of Zinst (Bavaria, western Bohemian Massif) contain rare fine-grained symplectites forming about 5 millimeter sized patches comprising several microstructurally and chemically distinct concentric zones. The symplectites reflect a complex reaction history of lherzolite during decompression and interaction with melt. Here we focus on ultra-fine grained symplectite with an integrated bulk composition expressed in terms of garnet end-member component percentages as Prp(69-71)Alm(11-13)Grs(2.5-5)And(7.5-10)Uvr(4). According to the composition and the microstructural occurrence in lherzolite the ultra-fine grained symplectite is interpreted as a product of isochemical garnet breakdown, although the precursor phase is not preserved. Under cross polarized light patches with similar extinction show a domain microstructure in symplectite. BSE images reveal an intimate intergrowth of orthopyroxene, spinel and plagioclase. All phases have a shape preferred orientation within distinct domains, whereas discontinuous SPO changes occur at microstructural domain boundaries. Three types of symplectite were microstructurally discerned: The most pristine type A occurs in a 10-30 micrometers wide zone along the symplectite margin. Spinel forms several tens of nanometers wide rods or lamellae within Opx, whereas Pl and Opx represent the symplectite matrix. All phases show a strong SPO with the maximum elongation perpendicular to the symplectite boundary. At edges of this interface, the SPO of the symplectite phases changes accordingly. Discontinuities in SPO may coincide with changes in crystallographic orientation. EBSD data showed that symplectite phases have strict crystallographic orientation relations with Opx(100)//Spl(111) and Opx(010)//Spl(110). Whereas the initial lattice orientation is controlled by adjacent phases at the symplectite boundary, the crystallographic orientation within symplectite domains

Separate zones of sulfate and sulfide release from subducted mafic oceanic crust

Science.gov (United States)

Tomkins, Andrew G.; Evans, Katy A.

2015-10-01

Liberation of fluids during subduction of oceanic crust is thought to transfer sulfur into the overlying sub-arc mantle. However, despite the importance of sulfur cycling through magmatic arcs to climate change, magma oxidation and ore formation, there has been little investigation of the metamorphic reactions responsible for sulfur release from subducting slabs. Here, we investigate the relative stability of anhydrite (CaSO4) and pyrite (FeS2) in subducted basaltic oceanic crust, the largest contributor to the subducted sulfur budget, to place constraints on the processes controlling sulfur release. Our analysis of anhydrite stability at high pressures suggests that this mineral should dominantly dissolve into metamorphic fluids released across the transition from blueschist to eclogite facies (∼450-650 °C), disappearing at lower temperatures on colder geothermal trajectories. In contrast, we suggest that sulfur release via conversion of pyrite to pyrrhotite occurs at temperatures above 750 °C. This higher temperature stability is indicated by the preservation of pyrite-bornite inclusions in coesite-bearing eclogites from the Sulu Belt in China, which reached temperatures of at least 750 °C. Thus, sulfur may be released from subducting slabs in two separate pulses; (1) varying proportions of SO2, HSO4- and H2S are released via anhydrite breakdown at the blueschist-eclogite transition, promoting oxidation of remaining silicates in some domains, and (2) H2S is released via pyrite breakdown well into the eclogite facies, which may in some circumstances coincide with slab melting or supercritical liquid generation driven by influx of serpentinite-derived fluids. These results imply that the metallogenic potential in the sub-arc mantle above the subducting slab varies as a function of subduction depth, having the greatest potential above the blueschist-eclogite transition given the association between oxidised magmas and porphyry Cu(-Au-Mo) deposits. We speculate

A mantle origin for Paleoarchean peridotitic diamonds from the Panda kimberlite, Slave Craton: Evidence from 13C-, 15N- and 33,34S-stable isotope systematics

Science.gov (United States)

Cartigny, Pierre; Farquhar, James; Thomassot, Emilie; Harris, Jeffrey W.; Wing, Bozwell; Masterson, Andy; McKeegan, Kevin; Stachel, Thomas

2009-11-01

In order to address diamond formation and origin in the lithospheric mantle underlying the Central Slave Craton, we report N- and C-stable isotopic compositions and N-contents and aggregation states for 85 diamonds of known paragenesis (73 peridotitic, 8 eclogitic and 4 from lower mantle) from the Panda kimberlite (Ekati Mine, Lac de Gras Area, Canada). For 12 peridotitic and two eclogitic sulfide inclusion-bearing diamonds from this sample set, we also report multiple-sulfur isotope ratios. The 73 peridotitic diamonds have a mean δ13C-value of - 5.2‰ and range from - 6.9 to - 3.0‰, with one extreme value at - 14.1‰. The associated δ15N-values range from - 17.0 to + 8.5‰ with a mean value of - 4.0‰. N-contents range from 0 to 1280 ppm. The 8 eclogitic diamonds have δ13C-values ranging from - 11.2 to - 4.4‰ with one extreme value at - 19.4‰. Their δ15N ranges from - 2.1 to + 7.9‰ and N-contents fall between 0 and 3452 ppm. Four diamonds with an inferred lower mantle origin are all Type II (i.e. nitrogen-free) and have a narrow range of δ13C values, between - 4.5 and - 3.5‰. The δ34S of the 14 analyzed peridotitic and eclogitic sulfide inclusions ranges from - 3.5 to +5.7‰. None of them provide evidence for anomalous δ33S-values; observed variations in δ33S are from +0.19 to - 0.33‰, i.e. within the 2 sigma uncertainties of mantle sulfur ( δ33S = 0‰). At Panda, the N contents and the δ13C of sulfide-bearing peridotitic diamonds show narrower ranges than silicate-bearing peridotitic diamonds. This evidence supports the earlier suggestion established from eclogitic diamonds from the Kaapvaal that sulfide-(±silicate) bearing diamonds sample a more restricted portion of sublithospheric mantle than silicate-(no sulfide) bearing diamonds. Our findings at Panda suggest that sulfide-bearing diamonds should be considered as a specific diamond population on a global-scale. Based on our study of δ34S, Δ 33S, δ15N and δ13C, we find no

The subcontinental versus suboceanic debate. Pt. 1

International Nuclear Information System (INIS)

Allegre, C.J.; Dupre, B.; Lambret, B.; Richard, P.; Paris-6 Univ., 75

1981-01-01

Pb, Nd and Sr isotopic compositions have been determined in lherzolite-xenolith-bearing alkali-basalts from the center of the African shield. The present data are very similar to those reported for ocean-island basalts and do not support the hypothesis of different mantle sources for alkali-basalts from continental and oceanic areas. From these observations and on the basis of data obtained for xenolith in kimberlite and for tholeiitic continental basalts one may infer the following terrestrial mantle structure: whereas oceanic tholeiites would originate in upper oceanic mantle, oceanic and continental alkali basalts would come from the lower mantle and tholeiitic continental basalts from the continental lithosphere. (orig.)

Clinopyroxenes still trapped in diamonds: high-energy synchrotron X-ray diffraction as a chemical probe

Science.gov (United States)

Casati, Nicola; Nestola, Fabrizio; Alvaro, Matteo; Wilhelm, Heribert; Kleppe, Annette; Nimis, Paolo; Harris, Jeffrey W.

2014-05-01

radiation at I15 extreme-conditions beamline, Diamond Light Source Ltd. Such approach makes the absorption from the large diamond host nearly negligible (i.e. with a decrease in the mass attenuation coefficient of more than 75% with respect to a conventional laboratory X-ray source for single crystal diffraction) and allows to collect extremely high quality data on the inclusions. The details will be discussed. We thank the Diamond Light Source for beamtime allocation (beamline I15, experiment number EE8754-1). The research was also funded by the ERC Starting Grant 2012 to FN. References Fedortchouk Y., Manghnani M.H., Hushur A., Shiryaev A., Nestola F. (2011) An atomic force microscopy study of diamond dissolution features: The effect of H2O and CO2 in the fluid on diamond morphology. American Mineralogist, 96, 1768-1775. Nestola F., Longo M., McCammon C., Boffa Ballaran T. (2007) Crystal-structure refinement of Na-bearing clinopyroxenes from mantle-derived eclogite xenoliths. American Mineralogist, 92, 1242-1245.

A petrological view of early Earth geodynamics

Science.gov (United States)

Herzberg, C.

2003-04-01

Xenoliths of low T Archean cratonic mantle consist mostly of harzburgite and lherzolite with geochemical depletions that are characterisitc of igneous residues. Many authors have identified the complementary magmas as komatiites. This model is re-examined in light of work presented in Herzberg & O'Hara (2002) and found to be problematic. Munro-type alumina-undepleted komatiites from Alexo, Pyke Hill, and other locations often contain olivine phenocrysts with maximum Mg# \\cong 94. Residues of fractional melting would consist of pure dunite having Mg# = 97-98, but these are not observed. Residues of equilibrium melting would also be pure dunite with Mg# = 94, but these are also not observed. Olivines with Mg# = 94 are found in rare harzburgites, indicating that residues of alumina-undepleted komatiite have either been overprinted by subsequent magmatism or they have been geodynamically eroded. Alumina-undepleted komatiites can be successfully modeled with a primary magma containing 30% MgO produced by 0.5 mass fractions of equilibrium melting of depleted peridotite. A hot plume interpretation is consistent with both the petrology and helium isotopic compositions of alumina-undepleted komatiites. But what about cratonic mantle? The FeO and MgO contents of residues of fertile mantle peridotite formed by both equilibrium and fractional melting can be predicted and applied to xenoliths of cratonic mantle in most cases. Application to xenoliths from the Kaapvaal and Slave cratons is not possible owing to a second stage of Opx enrichment, but results can be applied to most xenoliths from Siberia, Tanzania, Somerset Island, and east Greenland as they contain less than 45% SiO_2. These xenoliths are very similar to residues produced by fractional melting. Pressures of initial melting were mostly 3 to 5 GPa, but can be as high 7 GPa. Pressures of final melting were highly variable and can be as low as 1 GPa. Potential temperatures (T_P) were typically 1450 to 1600oC and

CO2-SO3-rich (carbonate-sulfate) melt/fluids in the lithosphere beneath El Hierro, Canary Islands.

Science.gov (United States)

Oglialoro, E.; Ferrando, S.; Malaspina, N.; Villa, I. M.; Frezzotti, M. L.

2015-12-01

Mantle xenoliths from the island of El Hierro, the youngest of the Canary Islands, have been studied to characterize fluxes of carbon in the lithosphere of an OIB volcanism region. Fifteen xenoliths (4-10 cm in diameter) were collected in a rift lava flow (15-41 ka) at a new xenolith locality in El Julan cliff (S-SW of the island). Peridotites consist of protogranular to porphyroblastic spinel harzburgites, lherzolites, and subordinate dunites. One spinel clinopyroxenite, and one olivine-websterite were also analyzed. Ultramafic xenoliths were classified as HEXO (harzburgite and dunite with exsolved orthopyroxene), HLCO (harzburgite and lherzolite containing orthopyroxene without visible exsolution lamellae), and HTR (transitional harzburgite with exsolved orthopyroxene porphyroclasts, and poikilitic orthopyroxene) following [1]. While HLCO and HTR peridotites contain mostly CO2 fluid inclusions, HEXO peridotites preserve an early association of melt/fluid inclusions containing dominantly carbonate/sulfate/silicate glass, evolving to carbonate/sulfate/phosphate/spinel aggregates, with exsolved CO2 (± carbonates, anhydrite and H2O). Chemical and Raman analyses identify dolomite, Mg-calcite, anhydrite, sulfohalite [Na6(SO4)2FCl] (± other anhydrous and hydrous alkali-sulfates), apatite, and Cr-spinel in the inclusions. Sulfides are noticeably absent. The microstructure and chemical composition of the metasomatic fluids indicate that the peridotites were infiltrated by a carbonate-sulfate-silicate melt/fluid enriched in CO2, H2O, and P. A mantle origin for this fluid is supported by high densities of CO2inclusions (> 1g/cm3), determined by Raman microspectroscopy and cross-checked by microthermometry. Consequently, El Julan peridotites provide the first evidence for liberating oxidized C and S fluxes from the Earth lithosphere in an OIB source region, and suggest that oxidation of sulfide to sulfate can occur during small-degree partial melting of the upper mantle

Multiple enrichment of the Carpathian-Pannonian mantle: Pb-Sr-Nd isotope and trace element constraints

Science.gov (United States)

Rosenbaum, Jeffrey M.; Wilson, Marjorie; Downes, Hilary

1997-07-01

Pb isotope compositions of acid-leached clinopyroxene and amphibole mineral separates from spinel peridotite mantle xenoliths entrained in Tertiary-Quaternary alkali basalts from the Carpathian-Pannonian Region of eastern Europe provide important constraints on the processes of metasomatic enrichment of the mantle lithosphere in an extensional tectonic setting associated with recent subduction. Principal component analysis of Pb-Sr-Nd isotope and rare earth element compositions of the pyroxenes is used to identify the geochemical characteristics of the original lithospheric mantle protolith and a spectrum of infiltrating metasomatic agents including subduction-related aqueous fluids and silicate melts derived from a subduction-modified mantle wedge which contains a St. Helena-type (HIMU) plume component. The mantle protolith is highly depleted relative to mid-ocean ridge basalt-source mantle with Pb-Nd-Sr isotope compositions consistent with an ancient depletion event. Silicate melt infiltration into the protolith accounts for the primary variance in the Pb-Sr-Nd isotope compositions of the xenoliths and has locally generated metasomatic amphibole. Infiltration of aqueous fluids has introduced radiogenic Pb and Sr without significantly perturbing the rare earth element signature of the protolith. The Pb isotope compositions of the fluid-modified xenoliths suggest that they reacted with aqueous fluids released from a subduction zone which had equilibrated with sediment derived from an ancient basement terrain. We propose a model for mantle lithosphere evolution consistent with available textural and geochemical data for the xenolith population. The Pb-Sr-Nd isotope compositions of both alkaline mafic magmas and rare, subduction-related, calc-alkaline basaltic andesites from the region provide important constraints for the nature of the asthenospheric mantle wedge and confirm the presence of a HIMU plume component. These silicate melts contribute to the metasomatism

Moho vs crust-mantle boundary: Evolution of an idea

Science.gov (United States)

O'Reilly, Suzanne Y.; Griffin, W. L.

2013-12-01

The concept that the Mohorovicic Discontinuity (Moho) does not necessarily coincide with the base of the continental crust as defined by rock-type compositions was introduced in the early 1980s. This had an important impact on understanding the nature of the crust-mantle boundary using information from seismology and from deep-seated samples brought to the surface as xenoliths in magmas, or as tectonic terranes. The use of empirically-constrained P-T estimates to plot the locus of temperature vs depth for xenoliths defined a variety of geotherms depending on tectonic environment. The xenolith geotherms provided a framework for constructing lithological sections through the deep lithosphere, and revealed that the crust-mantle boundary in off-craton regions commonly is transitional over a depth range of about 5-20 km. Early seismic-reflection data showed common layering near the Moho, correlating with the petrological observation of multiple episodes of basaltic intrusion around the crust-mantle boundary. Developments in seismology, petrophysics and experimental petrology have refined interpretation of lithospheric domains. The expansion of in situ geochronology (especially zircon U-Pb ages and Hf-isotopes; Os isotopes of mantle sulfides) has defined tectonic events that affected whole crust-mantle sections, and revealed that the crust-mantle boundary can change in depth through time. However, the nature of the crust-mantle boundary in cratonic regions remains enigmatic, mainly due to lack of key xenoliths or exposed sections. The observation that the Moho may lie significantly deeper than the crust-mantle boundary has important implications for modeling the volume of the crust. Mapping the crust using seismic techniques alone, without consideration of the petrological problems, may lead to an overestimation of crustal thickness by 15-30%. This will propagate to large uncertainties in the calculation of elemental mass balances relevant to crust-formation processes

40Ar/39Ar laser-probe dating of diamond inclusions from the Premier kimberlite

International Nuclear Information System (INIS)

Phillips, D.; Onstott, T.C.; Harris, J.W.; Strathclyde Univ., Glasgow

1989-01-01

Inclusions encapsulated by diamonds at the time of their formation provide a means for determining diamond crystallization ages and the chemistry of the surrounding upper mantle at that time. Sm-Nd studies of peridotitic inclusions, from Cretaceous-age kimberlites in southern Africa, suggest that the diamonds formed 3.3 Gyr ago. By contrast, eclogite-suite inclusions generally yield younger ages, sometimes approaching the time of kimberlite eruption. Here we report the results of 40 Ar/ 39 Ar laser-probe analyses of individual eclogitic clinopyroxene inclusions from Premier diamonds, which yield a mean age of 1,198±14 Myr. This age agrees well with Sm-Nd and 40 Ar/ 39 Ar analyses on similar Premier inclusions, and is indistinguishable from the inferred time of emplacement of the host kimberlite (1,150-1,230 Myr), which implies that diamond formation was essentially synchronous with kimberlite generation. The extrapolated non-radiogenic 40 Ar/ 36 Ar ratio of 334±102 is similar to the present-day atmospheric composition. This value is inconsistent with Sr and Nd isotopic signatures from Premier eclogite inclusions, which suggest a depleted mantle source ( 40 Ar/ 36 Ar>20,000). Pre-entrapment equilibration of the inclusions with an 36 Ar-rich fluid is the most probable explanation for the low non-radiogenic ( 40 Ar/ 36 Ar) composition. (author)

Kankan diamonds (Guinea) III: δ13C and nitrogen characteristics of deep diamonds

Science.gov (United States)

Stachel, T.; Harris, J. W.; Aulbach, S.; Deines, P.

Diamonds from the Kankan area in Guinea formed over a large depth profile beginning within the cratonic mantle lithosphere and extending through the asthenosphere and transition zone into the lower mantle. The carbon isotopic composition, the concentration of nitrogen impurities and the nitrogen aggregation level of diamonds representing this entire depth range have been determined. Peridotitic and eclogitic diamonds of lithospheric origin from Kankan have carbon isotopic compositions (δ13C: peridotitic -5.4 to -2.2‰ eclogitic -19.7 to -0.7‰) and nitrogen characteristics (N: peridotitic 17-648 atomic ppm; eclogitic 0-1,313 atomic ppm; aggregation from IaA to IaB) which are generally typical for diamonds of these two suites worldwide. Geothermobarometry of peridotitic and eclogitic inclusion parageneses (worldwide sources) indicates that both suites formed under very similar conditions within the cratonic lithosphere, which is not consistent with a derivation of diamonds with light carbon isotopic composition from subducted organic matter within subducting oceanic slabs. Diamonds containing majorite garnet inclusions fall to the isotopically heavy side (δ13C: -3.1‰ to +0.9‰) of the worldwide diamond population. Nitrogen contents are low (0-126 atomic ppm) and one of the two nitrogen-bearing diamonds shows such a low level of nitrogen aggregation (30% B-centre) that it cannot have been exposed to ambient temperatures of the transition zone (>=1,400 °C) for more than 0.2 Ma. This suggests rapid upward transport and formation of some Kankan diamonds pene-contemporaneous to Cretaceous kimberlite activity. Similar to these diamonds from the asthenosphere and the transition zone, lower mantle diamonds show a small shift towards isotopic heavy compositions (-6.6 to -0.5‰, mode at -3.5‰). As already observed for other mines, the nitrogen contents of lower mantle diamonds were below detection (using FTIRS). The mutual shift of sublithospheric diamonds towards

The role of amphibole in Merapi arc magma petrogenesis: insights from petrology and geochemistry of lava hosted xenoliths and xenocrysts

Science.gov (United States)

Chadwick, J. P.; Troll, V. R.; Schulz, B.; Dallai, L.; Freda, C.; Schwarzkopf, L. M.; Annersten, H.; Skogby, H.

2010-05-01

Recently, increasing attention has been paid to the role of amphibole in the differentiation of arc magmas. The geochemical composition of these magmas suggests that deep to mid crustal fractionation of amphibole has occurred. However, this phase is typically an infrequent modal phenocryst phase in subduction zone eruptive deposits(1). Nevertheless, erupted material only represents a portion of the magmatism produced in subduction zone settings, with many opportunities for melts to stall on route to the surface. This discrepancy between whole rock geochemistry and petrological interpretation of arc magmas has lead many scientists to postulate that, at mid to deep crustal levels, there may be significant volumes of amphibole bearing lithologies. Amphibole instability at shallow levels can also contribute to its scarcity in eruptive deposits. This argument is strengthened by field and petrological evidence, including the widespread occurrence of amphibole-rich intrusive rocks in exhumed orogenicbelts formed during subduction zone activity, e.g. the Adamello batholith (2),as well as the presence of amphibole-rich xenoliths and xenocrysts preserved in arc lavas worldwide, e.g. in Indonesia, Antilles, and Central America. Thus, amphibole appears to play an integral role in subduction zone magmatism and identifying and constraining this role is central to understanding arc magma petrogenisis. Amphibole-rich melts or bodies in the deep to mid crust could be a significant hydrous reservoir for intra-crustal melts and fluids (1). In this preliminary study, we have carried out petrological and geochemical analyses of recent basaltic andesite and amphibole bearing crystalline igneous inclusions and xenocrysts from Merapi volcano in Java, Indonesia. The basaltic andesite geochemistry is consistent with amphibole fractionation and the crystalline inclusions are cogenetic to the Merapi magmatic system. These inclusions are likely to represent fractionation residues reflecting

Water contents of clinopyroxenes from sub-arc mantle peridotites

Science.gov (United States)

Turner, Michael; Turner, Simon; Blatter, Dawnika; Maury, Rene; Perfit, Michael; Yogodzinski, Gene

2017-01-01

One poorly constrained reservoir of the Earth's water budget is that of clinopyroxene in metasomatised, mantle peridotites. This study presents reconnaissance Sensitive High-Resolution, Ion Microprobe–Stable Isotope (SHRIMP–SI) determinations of the H2O contents of (dominantly) clinopyroxenes in rare mantle xenoliths from four different subduction zones, i.e. Mexico, Kamchatka, Philippines, and New Britain (Tabar-Feni island chain) as well as one intra-plate setting (western Victoria). All of the sub-arc xenoliths have been metasomatised and carry strong arc trace element signatures. Average measured H2O contents of the pyroxenes range from 70 ppm to 510 ppm whereas calculated bulk H2O contents range from 88 ppm to 3 737 ppm if the variable presence of amphibole is taken into account. In contrast, the intra-plate, continental mantle xenolith from western Victoria has higher water contents (3 447 ppm) but was metasomatised by alkali and/or carbonatitic melts and does not carry a subduction-related signature. Material similar to the sub-arc peridotites can either be accreted to the base of the lithosphere or potentially be transported by convection deeper into the mantle where it will lose water due to amphibole breakdown.

Silica-enriched mantle sources of subalkaline picrite-boninite-andesite island arc magmas

Science.gov (United States)

Bénard, A.; Arculus, R. J.; Nebel, O.; Ionov, D. A.; McAlpine, S. R. B.

2017-02-01

Primary arc melts may form through fluxed or adiabatic decompression melting in the mantle wedge, or via a combination of both processes. Major limitations to our understanding of the formation of primary arc melts stem from the fact that most arc lavas are aggregated blends of individual magma batches, further modified by differentiation processes in the sub-arc mantle lithosphere and overlying crust. Primary melt generation is thus masked by these types of second-stage processes. Magma-hosted peridotites sampled as xenoliths in subduction zone magmas are possible remnants of sub-arc mantle and magma generation processes, but are rarely sampled in active arcs. Published studies have emphasised the predominantly harzburgitic lithologies with particularly high modal orthopyroxene in these xenoliths; the former characteristic reflects the refractory nature of these materials consequent to extensive melt depletion of a lherzolitic protolith whereas the latter feature requires additional explanation. Here we present major and minor element data for pristine, mantle-derived, lava-hosted spinel-bearing harzburgite and dunite xenoliths and associated primitive melts from the active Kamchatka and Bismarck arcs. We show that these peridotite suites, and other mantle xenoliths sampled in circum-Pacific arcs, are a distinctive peridotite type not found in other tectonic settings, and are melting residues from hydrous melting of silica-enriched mantle sources. We explore the ability of experimental studies allied with mantle melting parameterisations (pMELTS, Petrolog3) to reproduce the compositions of these arc peridotites, and present a protolith ('hybrid mantle wedge') composition that satisfies the available constraints. The composition of peridotite xenoliths recovered from erupted arc magmas plausibly requires their formation initially via interaction of slab-derived components with refractory mantle prior to or during the formation of primary arc melts. The liquid

Transition Metal Systematics of Opx-Enriched Harzburgites From the Cascades Arc With Implications for the Origin of Cratonic Peridotites

Science.gov (United States)

Turner, S. J.

2007-12-01

A number of peridotite xenoliths collected from the Simcoe volcanic field region of the Cascades arc exhibit notable enrichment of modal orthopyroxene. The process driving this enrichment is most likely metasomatism of the mantle wedge by Si-rich fluids derived ultimately from the underlying slab. By investigating the resultant elemental systematics associated with subduction zone metasomatism of this type, we hope to shed light on the origin of other opx-rich peridotites, such as those seen in many cratonic xenolith suites. The xenoliths found in the Simcoe volcanic field provide a rare opportunity to examine the composition of sub arc mantle, as it is unusual to find mantle xenoliths in volcanic arc lavas. The samples were analyzed using laser ablation ICPMS and their bulk compositions were reconstructed from point-counted mineral modes. Two-pyroxene mineral thermometry of the samples yield temperatures of approximately 1000 degrees C, corresponding to a depth of origin at uppermost mantle pressures if typical arc geotherms are assumed. Most of the peridotites are harzburgites or olivine-orthopyroxenites (Mg#s 0.88-0.9; opx mode 0.15-0.9), with small amounts of clinopyroxene (rare earths, consistent with a metasomatic origin for these opx-rich harzburgites. Of note is the counterintuitive systematics of Zn. Whole-rock Zn decreases with opx, but Zn in olivine also decreases with opx mode while Zn in opx increases with opx mode, hence the decrease in whole- rock Zn is not simply due to mechanical segregation of harzburgite into opx- and ol-rich zones. In summary, the REE signatures suggest the subducting slab as the most likely candidate for the source of the fluids that caused the opx enrichment. The opx-enrichment itself and the unusual trends in Zn suggest a reaction between a silicic fluid and normal harzburgite. Moreover, the concomitant decrease in olivine and whole-rock Zn with opx mode suggests significant leaching of Zn from the peridotite during this

Compositional Variation of Terrestrial Mantle Apatites and Implications for the Halogen and Water Budgets of the Terrestrial Mantle

Science.gov (United States)

Roden, M.; Patino Douce, A. E.; Chaumba, J. B.; Fleisher, C.; Yogodzinski, G. M.

2011-12-01

Apatite in ultramafic xenoliths from various tectonic enviroments including arc (Kamchatka), plume (Hawaii), and intraplate (Lunar Crater, Nunivak, Colorado Plateau) were analyzed by electron microprobe with the aim of characterizing the Cl and F contents, and from these measured compositions to infer the nature of fluids/melts that the apatites equilibrated with. The impetus for the study derived from the generalization of O'Reilly and Griffin (1) that mantle-derived metasomatic apatites tend to be Cl-rich and mantle-derived igneous apatites tend to be F-rich. Our work largely corroborates their generalization with Cl- and/or H2O-rich compositions characterizing the apatites from Nunivak and Kamchatka while apatites from igneous or Group II xenoliths tend to be Cl-poor and be either nearly pure fluorapatite or a mix of hydroxylapatite and fluorapatite. We attribute the Cl-rich nature of the Kamchatka apatites to formation from Cl-rich fluids generated from subducted lithosphere; however the Nunivak occurrence is far removed from subducted lithosphere and may reflect a deep seated source for Cl as also indicated by brine inclusions in diamonds, Cl-rich apatites in carbonate-bearing xenoliths and a Cl-rich signature in some plumes such as Iceland, Azores and Samoa. One curious aspect of mantle-derived apatite compositions is that xenoliths with evidence of carbonatitic metasomatism commonly have Cl-rich apatites while apatites from carbonatites are invariably Cl-poor - perhaps reflecting loss of Cl in fluids evolved from the carbonatitic magma. Apatites from Group II xenoliths at Hawaii are solid solutions between fluorapatite and hydroxylapatite and show no evidence for deep-seated Cl at Hawaii. Samples of the terrestrial mantle are almost uniformly characterized by mineral assemblages with a single Ca-rich phosphate phase but the mantles of Mars, Vesta and the Moon have two Ca-rich phosphates, apatite and volatile-poor merrillite - apatite compositions existing

Metamorphic P-T-t-d evolution of (U)HP metabasites from the South Tianshan accretionary complex (NW China) - Implications for rock deformation during exhumation in a subduction channel

Czech Academy of Sciences Publication Activity Database

Soldner, J.; Oliot, E.; Schulmann, K.; Štípská, P.; Kusbach, Vladimír; Anczkiewicz, R.

2017-01-01

Roč. 47, July (2017), s. 161-187 ISSN 1342-937X Institutional support: RVO:67985530 Keywords : eclogite * Tianshan massif * (U)HP metamorphic belt Subject RIV: DB - Geology ; Mineralogy OBOR OECD: Geology Impact factor: 6.959, year: 2016

Las rocas catazonales de la región de Cabo Ortegal

NARCIS (Netherlands)

Vogel, D.E.

1967-01-01

Paragneisses and associated mafic igneous rocks have been subjected twice to regional metamorphism and extensive folding. During the first, catazonal, metamorphism, the mafic rocks have been converted to eclogites and pyrigarnites (hornblende-granulite facies rocks); both characterized by the

Dynamics of Saxothuringian subduction channel/wedge constrained by phase equilibria modelling and micro-fabric analysis

Czech Academy of Sciences Publication Activity Database

Collett, S.; Štípská, P.; Kusbach, Vladimír; Schulmann, K.; Marciniak, G.

2017-01-01

Roč. 35, č. 3 (2017), s. 253-280 ISSN 0263-4929 Institutional support: RVO:67985530 Keywords : eclogite * Bohemian Massif * thermodynamic modelling * micro-fabric analysis * subduction and exhumation dynamics Subject RIV: DB - Geology ; Mineralogy OBOR OECD: Geology Impact factor: 3.594, year: 2016

the origin of late archaean granitoids in the sukumaland greenstone

African Journals Online (AJOL)

Mgina

melting at the base of a late Archaean thickened sub-arc basaltic crust. Melting to form the Suite 1 granitoids occurred in the eclogite stability field whereas Suite 2 formed by melting at shallower ... of TTG in terms of slab melting processes.

Late Archaean mantle metasomatism below eastern Indian craton ...

Indian Academy of Sciences (India)

R. Narasimhan (Krishtel eMaging) 1461 1996 Oct 15 13:05:22

crop NDD display mainly four distinct orientations viz., N-S, NNE-SSW, ...... also be trapped either along the grain boundary ... faster plate movements, high angle subduction would be ..... istry of oxide minerals in polymictic xenoliths from the.

Carbonate Cements from the Sverrefjell and Sigurdfjell Volcanoes, Svalbard Norway: Analogs for Martian Carbonates

Science.gov (United States)

Blake, D. F.; Treiman, A. H.; Morris, R.; Bish, D.; Amundsen, H.E.F.; Steele, A.

2011-01-01

The Sverrefjell and Sigurdfjell volcanic complexes erupted at 1Ma on Svalbard, Norway. Sverrefjell is a cone of cinders, pillow lavas and dikes; Sigurdfjell is elongate in outcrop and may represent a fissure eruption [1]. The lavas of both volcanos were volatile rich. The volcanos erupted under ice and were subsequently dissected by glaciation (glacial eratics are present on most of Sverrefjell, even on its summit). Eruption beneath an ice sheet is inferred, based on the presence of pillow lavas from near sea level to 1000 m above sea level. Sverrefjell contains the largest fraction of ultramafic xenoliths of any volcanic complex in the world, in places accounting for as much as 50% of the volume of the outcrop. The Sverrefjell and Sigurdfell volcanos contain carbonate cements of several varieties: (1) Amundsen [2] reported Mg-Fe-rich carbonate in sub-mm globules in basalts and ultramafic xenoliths from the volcanos. These globules are the best terrestrial analogs to the carbonate globules in the Mars meteorite ALH84001 [3]. (2) Thick (1-3 cm) coatings of carbonate cement drape the walls of vertical volcanic pipes or conduits on the flanks and near the present summit of Sverrefjell. Similar occurrences are found on Sigurdfjell. (3) Breccia-filled pipes or vents occur on Sverrefjell and Siggurdfjell in which the breccia fragments are cemented by carbonate. The fragments themselves commonly contain carbonate globules similar to those found in the basalts and ultramafic xenoliths.

Evidence for the presence of carbonate melt during the formation of cumulates in the Colli Albani Volcanic District, Italy

Science.gov (United States)

Shaw, Cliff S. J.

2018-06-01

Fergusite and syenite xenoliths and mafic lapilli from two locations in the Villa Senni ignimbrite of the Colli Albani Volcanic District show evidence for fractionation of a silicate magma that led to exsolution of an immiscible carbonate melt. The fergusite xenoliths are divided into two groups on the basis of their clinopyroxene compositions. Group 1 clinopyroxene records the crystallisation of a silicate melt and enrichment of the melt in Al, Ti and Mn and depletion in Si as well as enrichment in incompatible trace elements. The second group of clinopyroxene compositions (group 2) comes mainly from Ba-F-phlogopite- and Ti-andradite-bearing fergusites. They have significantly higher Si and lower Al and Ti and, like the coexisting phlogopite and garnet are strongly enriched in Mn. The minerals in the fergusites containing group 2 clinopyroxene are enriched in Ba, Sr, Cs, V and Li all of which are expected to partition strongly into a carbonate melt phase relative to the coexisting silicate melt. The compositional data suggest that the group 1 fergusites record sidewall crystallisation of CO2-rich silicate melt and that once the melt reached a critical degree of fractionation, carbonate melt exsolved. The group 2 fergusites record continued crystallisation in this heterogeneous silicate - carbonate melt system. Composite xenoliths of fergusite and thermometamorphic skarn record contact times of hundreds to a few thousand years indicating that fractionation and assimilation was relatively rapid.

Magma-carbonate interaction processes and associated CO2 release at Merapi volcano, Indonesia: insights from experimental petrology

NARCIS (Netherlands)

Deegan, F.M.; Troll, V.R.; Freda, C.; Misti, V.; Chadwick, J.P.

2010-01-01

There is considerable evidence for continuing, late-stage interaction between the magmatic system at Merapi volcano, Indonesia, and local crustal carbonate (limestone). Calc-silicate xenoliths within Merapi basaltic-andesite eruptive rocks display textures indicative of intense interaction between

Characterization of Spitsbergen Disks by Transmission Electron Microscopy and Raman Spectroscopy

Science.gov (United States)

Thomas-Keprta, K. L.; Clemett, S. J.; Le, L.; Ross, K.; McKay, David S.; Gibson, E. K., Jr.

2010-01-01

'Carbonate disks' found in the fractures and pores spaces of peridotite xenoliths and basalts from the island of Spitsbergen in the Norwegian Svalbard archipelago have been suggested to be "The best (and best documented) terrestrial analogs for the [Martian meteorite] ALH84001 carbonate globules ..." Previous studies have indicated that Spitsbergen carbonates show broadly comparable internal layering and mineral compositions to ALH84001 carbonate-magnetite disks. We report here for the first time, the detailed mineral characterization of Spitsbergen carbonates and their spatial relationship to the host mineral assemblages in the xenolith, using high resolution TEM (as used previously for ALH84001 carbonate disks). These studies were conducted in concert with complementary Raman and SEM analysis of the same samples. Our results indicate that there are significant chemical and physical differences between the disks in Spitsbergen and the carbonates present in ALH84001.

Hydrothermal flake graphite mineralisation in Paleoproterozoic rocks of south-east Greenland

DEFF Research Database (Denmark)

Rosing-Schow, Nanna; Bagas, Leon; Kolb, Jochen

2017-01-01

Flake graphite mineralisation is hosted in the Kuummiut Terrane of the Paleoproterozoic Nagssugtoqidian Orogen, south-east Greenland. Eclogite-facies peak-metamorphic assemblages record temperatures of 640–830 °C and pressures of 22–25 kbar, and are retrogressed in the high-pressure amphibolite-f...

Scandian Ultrahigh-Pressure Metamorphism of Proterozoic Basement Rocks on Fjortoft and Otroy, Western Gneiss Region, Norway

NARCIS (Netherlands)

Carswell, D.A.; van Roermund, H.L.M.; Wiggers de Vries, D.F.

2006-01-01

Electron microprobe mineral composition data are presented for samples of exsolved and recrystallized garnet websterites within the mantle-derived peridotite bodies and of external orthopyroxene eclogite lenses directly enclosed within the gneisses on the islands of Fjørtoft and Otrøy in the Western

Garnet Pyroxenites from Kaula, Hawaii: Implications for Plume-Lithosphere Interaction

Science.gov (United States)

Bizimis, M.; Garcia, M. O.; Norman, M. D.

2006-12-01

The presence of garnet pyroxenite xenoliths on Oahu and Kaula Islands, Hawaii, provides the rare opportunity to investigate the composition of the deeper oceanic mantle lithosphere and the nature of plume-lithosphere interaction in two dimensions, downstream from the center of the Hawaiian plume. Kaula (60 miles SW of Kauai) is on the same bathymetric shallow as Kauai and the Kaula-Niihau-Kauai islands form a cross-trend relationship to the Hawaiian Island ridge. Here, we present the first Sr-Nd isotope data on clinopyroxenes (cpx) from Kaula pyroxenites, and we compare them with the Salt Lake Crater (SLC) pyroxenites from Oahu. The Kaula cpx major element compositions overlap those of the (more variable) SLC pyroxenites (e.g. Mg# = 0.79-0.83), except for their higher Al2O3 contents (9% vs. 5-8%) than the SLC. The Kaula cpx are LREE enriched with elevated Dy/Yb ratios, similar to the SLC pyroxenites and characteristic of the presence of garnet that preferentially incorporates the HREE. In Sr-Nd isotope space, the Kaula pyroxenite compositions (87Sr/86Sr= 0.70312-0.70326, ɛNd= 7.2-8.6) overlap those of both the Oahu-Kauai post erosional lavas and the SLC pyroxenites, falling at the isotopically depleted end of the Hawaiian lava compositions. The depleted Sr-Nd isotope compositions of the Kaula pyroxenites suggest that they are not related to the isotopically enriched shield stage Hawaiian lavas, either as a source material (i.e. recycled eclogite) or as cumulates. Their elevated 87Sr/86Sr ratios relative to MORB also suggests that they are not likely MORB-related cumulates. The similarities between the Oahu and Kaula pyroxenites, some 200 km apart, suggest the widespread presence of pyroxenitic material in the deeper (>60km) Pacific lithosphere between Oahu and Kaula-Kauai, as high pressure cumulates from melts isotopically similar to the secondary Hawaiian volcanism. The presence of this material within the lower lithosphere is consistent with seismic observations

Axial‐type olivine crystallographic preferred orientations: the effect of strain geometry on mantle texture

NARCIS (Netherlands)

Chatzaras, V.; Kruckenberg, Seth C.; Cohen, Shaina M.; Medaris Jr., L. Gordon; Withers, Anthony C.; Bagley, Brian

The effect of finite strain geometry on crystallographic preferred orientation (CPO) is poorly constrained in the upper mantle. Specifically, the relationship between shape preferred orientation (SPO) and CPO in the mantle rocks remains unclear. We analyzed a suite of 40 spinel peridotite xenoliths

Mantle strength of the San Andreas fault system and the role of mantle-crust feedbacks

NARCIS (Netherlands)

Chatzaras, V.; Tikoff, B.; Newman, J.; Withers, A.C.; Drury, M.R.

2015-01-01

In lithospheric-scale strike-slip fault zones, upper crustal strength is well constrained from borehole observations and fault rock deformation experiments, but mantle strength is less well known. Using peridotite xenoliths, we show that the upper mantle below the San Andreas fault system

Water Content in the SW USA Mantle Lithosphere: FTIR Analysis of Dish Hill and Kilbourne Hole Pyroxenites

Science.gov (United States)

Gibler, Robert; Peslier, Anne H.; Schaffer, Lillian Aurora; Brandon, Alan D.

2014-01-01

Kilbourne Hole (NM, USA) and Dish Hill (CA, USA) mantle xenoliths sample continental mantle in two different tectonic settings. Kilbourne Hole (KH) is located in the Rio Grande rift. Dish Hill (DH) is located in the southern Mojave province, an area potentially affected by subduction of the Farallon plate beneath North America. FTIR analyses were obtained on well characterized pyroxenite, dunite and wehrlite xenoliths, thought to represent crystallized melts at mantle depths. PUM normalized REE patterns of the KH bulk-rocks are slightly LREE enriched and consistent with those of liquids generated by 6% melting of a spinel peridotite source. Pyroxenite pyroxenes have no detectable water but one DH wehrlite, which bulk-rock is LREE enriched, has 4 ppm H2O in orthopyroxene and 2 Ga. The Farallon subduction appears to have enriched in water the southwestern United States lithospheric mantle further east than DH, beneath the Colorado plateau.

Subduction of Proterozoic to Late Triassic continental basement in the Guatemala suture zone: A petrological and geochronological study of high-pressure metagranitoids from the Chuacús complex

Science.gov (United States)

Maldonado, Roberto; Ortega-Gutiérrez, Fernando; Ortíz-Joya, Guillermo A.

2018-05-01

Many continental subduction complexes contain abundant granitic rocks coexisting with minor volumes of eclogite-facies rocks. Characterization of granitic protoliths is crucial to decipher the origin of subducted continental crust, whereas knowledge of its metamorphic evolution is required to constrain the mechanisms of burial and exhumation. In this work we present geochronological and petrological evidence that demonstrate the occurrence of a subducted Proterozoic to Late Triassic granitic basement in the Chuacús complex of central Guatemala. Metagranitoids exposed in this area are interlayered with eclogite and other high-pressure rocks, and their structure is considerably variable due to strain partitioning during deformation. Laser ablation-inductively coupled plasma-mass spectrometry U-Pb zircon data from two ferroan metagranites yield protolith crystallization ages of ca. 1.1 Ga and their trace-element abundances suggest an origin related to intraplate magmatism, while a high-silica, peraluminous metagranite is dated at 1.0 Ga and was probably originated by partial melting of a high-grade continental crust. On the other hand, two megacrystic to augen metagranitoids yield protolith crystallization ages of ca. 224 Ma, which are identical within errors to the protolith age of hosted eclogitic metabasites. Their high incompatible trace element abundances together with the observed spatial-temporal relationships with mafic protoliths suggest that Late Triassic bimodal magmatism in the Chuacús complex was probably originated in a within-plate setting. Regardless of their age or structure, the studied metagranites preserve evidences for high-pressure metamorphic equilibration, such as the occurrence of Ca-rich garnet (XCa up to 0.52) in association with phengite (Si contents of up to 3.4 pfu) and rutile. The integration of Zr-in-rutile thermometry and phengite barometry allows the peak metamorphic conditions to be constrained at 640-680 °C and 13 kbar. This

The South Tibetan Tadpole Zone: Ongoing density sorting at the Moho beneath the Indus-Tsangpo suture zone (and beneath volcanic arcs?)

Science.gov (United States)

Kelemen, Peter; Hacker, Bradley

2016-04-01

Some Himalayan cross-sections show Indian crust thrust beneath Tibetan crust, with no intervening mantle wedge (e.g., Powell & Conaghan 73), others indicate thickening of both crustal sections, juxtaposed along a steep suture (e.g., Dewey & Burke 73), and many combine features of both end-members (e.g., Argand 24). To understand crustal scale structure and related phenomena, we focus on data from an area in southern Tibet at 28-30°N, 84-91°E. 21st century observations in this area show a horizontal Moho at ca 80 km depth, extending from thickened Indian crust, across a region where Tibetan crust is interpreted to overlie Indian crust, into thickened Tibetan crust (Zhao et al 01; Monsalve et al 08; Wittlinger et al 09; Nabelek et al 09; Kind et al 02; Schulte-Pelkum et al 05; Shi et al 15). About half the subducted Indian crustal volume is present, whereas the other half is missing (Replumaz et al 10). Vp/Vs indicates the alpha-beta quartz transition is at ca 50 km depth (Sheehan et al 13). Miocene lavas include primitive andesites probably formed by interaction of crustal material with mantle peridotite at > 1000°C (Turner et al 93; Williams et al 01, 04; Chung et al 05). Thermobarometry of xenoliths in a 12.7 Ma dike records ~ 1100°C at 2.2-2.6 GPa and 920°C at 1.7 GPa (Chan et al 09). Biotite-rich pyroxenites among the xenoliths, similar to those in central Tibet (Hacker et al 00) and the Pamirs (Hacker et al 05), may form via reaction of hot crustal lithologies and mantle peridotite (e.g., Sekine & Wyllie 82, 83). These data, taken together, indicate Miocene to present day temperatures exceeding 800°C from ca 50 km depth to the Moho, unlike thermal models with a hot mid-crust and cold Moho (McKenzie & Priestley 08, Craig et al 12, Wang et al 13; Nabelek & Nabelek 14), and despite the observation of numerous, near-Moho earthquakes (Chen & Molnar 83; Chen & Yang 04; Monsalve et al 06; Priestley et al 08; Craig et al 12) interpreted by many as brittle failure

Microstructural evolution and seismic anisotropy of upper mantle rocks in rift zones. Geologica Ultraiectina (300)

NARCIS (Netherlands)

Palasse, L.N.

2008-01-01

This thesis investigates field-scale fragments of subcontinental upper mantle rocks from the ancient Mesozoic North Pyrenean rift and Plio-Pleistocene xenoliths from the active Baja California rift, in order to constrain the deformation history of the uppermost mantle. The main focus of the study is

Heterogeneous hydrogen distribution in orthopyroxene from veined mantle peridotite (San Carlos, Arizona): Impact of melt-rock interactions

Science.gov (United States)

Denis, Carole M. M.; Demouchy, Sylvie; Alard, Olivier

2018-03-01

Experimental studies have shown that hydrogen embedded as a trace element in mantle mineral structures affects the physical properties of mantle minerals and rocks. Nevertheless, hydrogen concentrations in mantle minerals are much lower than predicted by hydrogen solubilities obtained experimentally at high pressure and temperature. Here, we report textural analyses and major and trace element concentrations (including hydrogen) in upper mantle minerals from a spinel-bearing composite xenolith (dunite and pyroxenite) transported by silica-undersaturated mafic alkaline lavas from the San Carlos volcanic field (Arizona, USA). Our results suggest that the composite xenolith results from the percolation-reaction of a basaltic liquid within dunite channels, and is equilibrated with respect to trace elements. Equilibrium temperatures range between 1011 and 1023 °C. Hydrogen concentrations (expressed in ppm H2O by weight) obtained from unpolarized and polarized Fourier transform infrared spectroscopy are low, with average values water stored in the Earth's upper mantle.

The continental lithosphere: a geochemical perspective

International Nuclear Information System (INIS)

Hawkesworth, C.J.; Person, G.; Turner, S.P.; Calsteren, P. Van; Gallagher, K.

1993-01-01

The lithosphere is the cool strong outler layer of the Earth that is effectively a boundary layer to the convecting interior. The evidence from mantle xenoliths and continental basalts is that the lower continental crust and uppermost mantle are different beneath Archaen and proterozoic areas. Mantle xenoliths from Archaen terrains, principally the Kaapvaal craton in southern Africa, are significantly depleted in Fe and other major elements which are concentrated in basalts. Nd and Os isotope data on inclusions in diamonds and peridoties respectively, indicate that such mantle is as old as the overlying Archaen crust. Since it appears to have been coupled to the overlying crust, and to have been isolated from the homogenising effects of convection for long periods of time, it is inferred to be within the continental lithosphere. The mantle lithosphere beneath Proterozoic and younger areas is less depleted in major elements, and so it is more fertile, less buoyant, and therefore thinner, than the Archaen mantle lithosphere. (author). 136 refs, 14 figs

The application of integrated geophysical methods composed of AMT and high-precision ground magnetic survey to the exploration of granite uranium deposits

International Nuclear Information System (INIS)

Qiao Yong; Shen Jingbang; Wu Yong; Wang Zexia

2014-01-01

Introduced two methods composed of AMT and high-precision ground magnetic survey were used to the exploration of granite uranium deposits in the Yin gongshan areas middle part of the Nei Monggol. Through experiment of methods and analysis of applicated results, think that AMT have good vertical resolution and could preferably survey thickness of rockmass, position of fracture and deep conditions, space distribution features of fracture zone ect, but it is not clear for rockmass, xenolith of reflection. And high-precision ground magnetic survey could delineate rockmass, xenolith of distribution range and identify the rock contact zone, fracture ect, but it generally measure position and it is not clear for occurrence, extension. That can resolve some geological structures by using the integrated methods and on the basis of sharing their complementary advantages. Effective technological measures are provided to the exploration of deep buried uranium bodies in the granite uranium deposits and outskirt extension of the deposit. (authors)

LATE TRIASSIC OBLIQUE EXTRUSION OF UHP/HP COMPLEXES IN THE ATBASHI ACCRETIONARY COMPLEX OF SOUTH TIANSHAN, KYRGYZSTAN

Directory of Open Access Journals (Sweden)

Wenjiao Xiao

2017-01-01

Full Text Available The exhumation and tectonic emplacement of eclogites and blueschists take place in forearc accretionary complexes by either forearc- or backarc-directed extrusion, but few examples have been well analysed in detail. Here we present an example of oblique wedge extrusion of UHP/HP rocks in the Atbashi accretionary complex of the Kyrgyz South Tianshan.

The lithospheric mantle below southern West Greenland

DEFF Research Database (Denmark)

Sand, Karina Krarup; Waight, Tod Earle; Pearson, D. Graham

2009-01-01

Geothermobarometry of primarily garnet lherzolitic xenoliths from several localities in southern West Greenland is applied to address the diamond potential, pressure and temperature distribution and the stratigraphy of the subcontinental lithospheric mantle ~600 Ma ago. The samples are from kimbe...... into the reworked Archean North of the Naqssugtoqidian deformation front....

Las rocas básicas de Monte Castelo (La Coruña)

NARCIS (Netherlands)

Warnaars, F.W.

1967-01-01

A gabbroic complex is found 30 km to the North of Santiago de Compostela, Galicia. It turns out to be a multiple intrusion of different sills with a North-South trend. The borders of the sills are indicated by the occurrence of biotite and garnet, and sometimes xenoliths were found.

Microstructural and seismic properties of the upper mantle underneath a rifted continental terrane (Baja California): An example of sub-crustal mechanical asthenosphere?

NARCIS (Netherlands)

Palasse, L.N.; Vissers, R.L.M.; Paulssen, H.; Basu, A.R.; Drury, M.R.

2012-01-01

The Gulf of California rift is a young and active plate boundary that links the San Andreas strike-slip fault system in California to the oceanic spreading system of the East Pacific Rise. The xenolith bearing lavas of the San Quintin volcanic area provide lower crust and upper mantle samples from

U-Pb GEOCHRONOLOGY OF METASEDIMENTARY SCHISTS IN ...

African Journals Online (AJOL)

PROF EKWUEME

The geology of Akwnaga area of northcentral Nigeria is dominated by schists, metaigneous gneisses and weakly to non- foliated granites. The Schist shows grades up to the sillimanite zone of the Barrovian type regional metamorphism and occurs as xenoliths in other rocks of the area. The zircons separated from a sample ...

Petrographic and geochemical comparisons between the lower crystalline basement-derived section and the granite megablock and amphibolite megablock of the Eyreville B core, Chesapeake Bay impact structure, USA

Science.gov (United States)

Townsend, G.N.; Gibson, R.L.; Horton, J. Wright; Reimold, W.U.; Schmitt, R.T.; Bartosova, K.

2009-01-01

The Eyreville B core from the Chesapeake Bay impact structure, Virginia, USA, contains a lower basement-derived section (1551.19 m to 1766.32 m deep) and two megablocks of dominantly (1) amphibolite (1376.38 m to 1389.35 m deep) and (2) granite (1095.74 m to 1371.11 m deep), which are separated by an impactite succession. Metasedimentary rocks (muscovite-quartz-plagioclase-biotite-graphite ?? fibrolite ?? garnet ?? tourmaline ?? pyrite ?? rutile ?? pyrrhotite mica schist, hornblende-plagioclase-epidote-biotite- K-feldspar-quartz-titanite-calcite amphibolite, and vesuvianite-plagioclase- quartz-epidote calc-silicate rock) are dominant in the upper part of the lower basement-derived section, and they are intruded by pegmatitic to coarse-grained granite (K-feldspar-plagioclase-quartz-muscovite ?? biotite ?? garnet) that increases in volume proportion downward. The granite megablock contains both gneissic and weakly or nonfoliated biotite granite varieties (K-feldspar-quartz-plagioclase-biotite ?? muscovite ?? pyrite), with small schist xenoliths consisting of biotite-plagioclase-quartz ?? epidote ?? amphibole. The lower basement-derived section and both megablocks exhibit similar middleto upper-amphibolite-facies metamorphic grades that suggest they might represent parts of a single terrane. However, the mica schists in the lower basement-derived sequence and in the megablock xenoliths show differences in both mineralogy and whole-rock chemistry that suggest a more mafi c source for the xenoliths. Similarly, the mineralogy of the amphibolite in the lower basement-derived section and its association with calc-silicate rock suggest a sedimentary protolith, whereas the bulk-rock and mineral chemistry of the megablock amphibolite indicate an igneous protolith. The lower basement-derived granite also shows bulk chemical and mineralogical differences from the megablock gneissic and biotite granites. ?? 2009 The Geological Society of America.

Petrographic and geochemical comparisons between the lower crystalline basement-derived section and the granite megablock and amphibolite megablock of the Eyreville-B core, Chesapeake Bay impact structure

Science.gov (United States)

Townsend, Gabrielle N.; Gibson, Roger L.; Horton, J. Wright; Reimold, Wolf Uwe; Schmitt, Ralf T.; Bartosova, Katerina

2009-01-01

The Eyreville B core from the Chesapeake Bay impact structure, Virginia, USA, contains a lower basement-derived section (1551.19 m to 1766.32 m deep) and two megablocks of dominantly (1) amphibolite (1376.38 m to 1389.35 m deep) and (2) granite (1095.74 m to 1371.11 m deep), which are separated by an impactite succession. Metasedimentary rocks (muscovite-quartz-plagioclase-biotite-graphite ± fibrolite ± garnet ± tourmaline ± pyrite ± rutile ± pyrrhotite mica schist, hornblende-plagioclase-epidote-biotite-K-feldspar-quartz-titanite-calcite amphibolite, and vesuvianite-plagioclase-quartz-epidote calc-silicate rock) are dominant in the upper part of the lower basement-derived section, and they are intruded by pegmatitic to coarse-grained granite (K-feldspar-plagioclase-quartz-muscovite ± biotite ± garnet) that increases in volume proportion downward. The granite megablock contains both gneissic and weakly or nonfoliated biotite granite varieties (K-feldspar-quartz-plagioclase-biotite ± muscovite ± pyrite), with small schist xenoliths consisting of biotite-plagioclase-quartz ± epidote ± amphibole. The lower basement-derived section and both megablocks exhibit similar middle- to upper-amphibolite-facies metamorphic grades that suggest they might represent parts of a single terrane. However, the mica schists in the lower basement-derived sequence and in the megablock xenoliths show differences in both mineralogy and whole-rock chemistry that suggest a more mafic source for the xenoliths. Similarly, the mineralogy of the amphibolite in the lower basement-derived section and its association with calc-silicate rock suggest a sedimentary protolith, whereas the bulk-rock and mineral chemistry of the megablock amphibolite indicate an igneous protolith. The lower basement-derived granite also shows bulk chemical and mineralogical differences from the megablock gneissic and biotite granites.

Isotopic characterisation of the sub-continental lithospheric mantle beneath Zealandia, a rifted fragment of Gondwana

DEFF Research Database (Denmark)

Waight, Tod Earle; Scott, James M.; van der Meer, Quinten Har Adriaan

2013-01-01

The greater New Zealand region, known as Zealandia, represents an amalgamation of crustal fragments accreted to the paleo-Pacific Gondwana margin and which underwent significant thinning during the subsequent split from Australia and Antarctica in the mid-Cretaceous following opening of the Tasma...... Sea and the Southern Ocean. We present Sr, Nd and Pb isotopes and laser ablation trace element data for a comprehensive suite of clinopyroxene separates from spinel peridotite xenoliths (lherzolite to harzburgite) from the sub-continental lithospheric mantle across southern New Zealand...... composition, age or geographical separation. These isotopic compositions indicate that the sub-continental lithospheric mantle under southern New Zealand has a regionally distinct and pervasive FOZO to HIMU – like signature. The isotopic signatures are also similar to those of the alkaline magmas...... that transported the xenoliths and suggest that most of the HIMU signature observed in the volcanics could be derived from a major source component in the sub-continental lithospheric mantle. Trace element abundances in clinopyroxene are highly heterogeneous and vary from LREE-enriched, relatively flat and MORB...

Fertile lithospheric mantle beneath the northwestern North China and its implication for the subduction of the Paleo-Asian Ocean

Science.gov (United States)

Dai, H. K.; Zheng, J.; Su, Y. P.; Xiong, Q.; Pan, S. K.

2017-12-01

The nature of the sub-continental lithospheric mantle (SCLM) beneath the western North China Craton (NCC) is poorly known, which hinders understanding the cratonic response to the southward subduction of the Paleo-Asian Ocean. Mineral chemical data of spinel lherzolite xenoliths from newly discovered Cenozoic Langshan basalts in the northwestern part of the craton have been integrated with data from other localities across the western NCC, to put constrains on the SCLM nature and to explore the reworking processes involved. Compositions of mineral cores (i.e., Mg# in olivine = 88 91) and P-T estimates ( 1.2 GPa, 950 oC) suggest the Langshan xenoliths/xenocrysts represent fragments of the uppermost SCLM and experienced ancient continental crust, and 2) the sharp decrease in lithospheric thickness from the inner part to the northern margin of the western NCC, the SCLM beneath the northwestern part should have been strongly rejuvenated or replaced by fertile and non-cratonic mantle. Combined with other geological evidence on the northwestern margin, the mantle replacement and metasomatism were likely triggered by southward subduction of the Paleo-Asian Ocean.

Long wavelength magnetic anomalies over continental rifts in cratonic region

Science.gov (United States)

Friedman, S. A.; Persaud, P.; Ferre, E. C.; Martín-Hernández, F.; Feinberg, J. M.

2017-12-01

New collections of unaltered mantle xenoliths shed light on potential upper mantle contributions to long wavelength magnetic anomalies (LWMA) in continental rifts in cratonic / shield areas. The new material originates from the East African Rift (Tanzania), the Rio Grande Rift (U.S.A.), the Rhine Rift (Germany), and the West Antarctic Rift (Antarctica). The xenoliths sample the uppermost ( 0.2 or Fe geotherms (>60ºC/km) that are characteristic of rifted regions preclude any contribution to LWMA at depths >10 km. Hence, only upper basalts and hypovolcanic mafic sills would constitute potential magnetic sources. In contrast, the margins of these rifted regions consist of refractory cratonic domains, often characterized by oxidized sublithospheric mantle that host significant concentrations of primary magnetite. The higher NRMs of these peridotites (up to 15 A/m, Qn > 2.5) combined with much lower geotherms (as low as 15ºC/km) allows for a 5 to 10 km layer of uppermost mantle to potentially contribute to LWMA. Assuming that Qn values in rift margins are also gradient across the rift would primarily reflect thermal equilibration over time.

Origin of Fe-rich lherzolites and wehrlites from Tok, SE Siberia by reactive melt percolation in refractory mantle peridotites

Science.gov (United States)

Ionov, Dmitri A.; Chanefo, Ingrid; Bodinier, Jean-Louis

2005-10-01

Lherzolite-wehrlite (LW) series xenoliths from the quaternary Tok volcanic field in the southeastern Siberian craton are distinguished from the more common lherzolite-harzburgite (LH) series by (a) low Mg numbers (0.84-0.89) at high modal olivine (66-84%) and (b) widespread replacement of orthopyroxene (0-12%) and spinel by clinopyroxene (7-22%). The LW series peridotites are typically enriched in Ca, Fe, Mn and Ti, and depleted in Si, Ni and Cr relative to refractory LH series rocks (Mg number ≥0.89), which are metasomatised partial melting residues. Numerical modelling of Fe-Mg solid/liquid exchange during melt percolation demonstrates that LW series rocks can form by reaction of host refractory peridotites with evolved (Mg numbers 0.6-0.7), silica-undersaturated silicate melts at high melt/rock ratios, which replace orthopyroxene with clinopyroxene and decrease Mg numbers. This process is most likely related to underplating and fractionation of basaltic magma in the shallow mantle, which also produced olivine-clinopyroxene cumulates found among the Tok xenoliths.

Journal of Earth System Science | Indian Academy of Sciences

Indian Academy of Sciences (India)

The common growth of `late' muscovite over sillimanite in migmatite may be the result of this process, and in ux of exogenous H2O may not be necessary. However, in general, metasomatism in the evolution of the crust remains a contentious issue. Processes in the lower-most crust may be inferred from studies of xenolith ...

Homogenisation of sulphide inclusions within diamonds: A new approach to diamond inclusion geochemistry

Science.gov (United States)

McDonald, Iain; Hughes, Hannah S. R.; Butler, Ian B.; Harris, Jeffrey W.; Muir, Duncan

2017-11-01

Base metal sulphide (BMS) inclusions in diamonds provide a unique insight into the chalcophile and highly siderophile element composition of the mantle. Entombed within their diamond hosts, these provide a more robust (closed system) sample, from which to determine the trace element, Re-Os and S-isotopic compositions of the mantle than mantle xenoliths or orogenic peridotites, as they are shielded from alteration during ascent to the Earth's crust and subsequent surface weathering. However, at temperatures below 1100 °C some BMS inclusions undergo subsolidus re-equilibration from an original monosulphide solid solution (Mss) and this causes fractionation of the major and trace elements within the inclusions. Thus to study the subjects noted above, current techniques require the entire BMS inclusion to be extracted for analyses. Unfortunately, 'flaking' of inclusions during break-out is a frequent occurrence and hence the risk of accidentally under-sampling a portion of the BMS inclusion is inherent in current practices. This loss may have significant implications for Re-Os isotope analyses where incomplete sampling of a Re-rich phase, such as chalcopyrite that typically occurs at the outer margins of BMS inclusions, may induce significant bias in the Re-Os and 187Os/188Os measurements and resulting model and isochron ages. We have developed a method for the homogenisation of BMS inclusions in diamond prior to their break-out from the host stone. Diamonds are heated to 1100 °C and then quenched to chemically homogenise any sulphide inclusions for both major and trace elements. Using X-ray Computed Microtomography (μCT) we determine the shape and spatial setting of multiple inclusions within a host stone and crucially show that the volume of a BMS inclusion is the same both before and after homogenisation. We show that the homogenisation process significantly reduces the inherent variability of in situ analysis when compared with unhomogenised BMS, thereby

Neutron activation analysis of the rare earth elements in rocks from the earth's upper mantle and deep crust

International Nuclear Information System (INIS)

Stosch, H.-G.; Koetz, J.; Herpers, U.

1986-01-01

Three techniques for analyzing rare earth elements (REE) in geological materials are described, i.e. instrumental neutron activation analysis (INAA), neutron activation analysis with pre-irradiation chemical REE separation (PCS-NAA) and radiochemical neutron activation analysis (RNAA). The knowledge of REE concentrationd in eclogites, peridotites and minerals from the earth's lower crust and upper mantle is very useful in constraining their petrogenetic history. (author)

Lawsonite Microstructures and Fabric Development at the Slab-Mantle Interface

Science.gov (United States)

Fornash, K.; Whitney, D.; Teyssier, C. P.; Seaton, N. C.

2016-12-01

Lawsonite is of critical importance to element and water cycling in subduction zones because it has a high H2O content, is stable at high pressures, and is a significant reservoir for trace elements in HP/LT rocks. In addition, the presence, abundance, and crystallographic orientation of lawsonite can affect the deformation and rheological behavior of subducted oceanic crust and sediments, and may influence the seismic properties of subducted slabs. The scarcity of unaltered lawsonite in HP/LT rocks exhumed to the Earth's surface, particularly in eclogite, however, has prevented an understanding of the deformation behavior of lawsonite and the factors controlling the development of fabrics in lawsonite, which are important for understanding the effects of lawsonite on the physical properties of subducted slabs. One of the few places in the world with unaltered lawsonite in eclogite and blueschist facies rocks is the Sivrihisar Massif, Turkey, which contains a coherent sequence of lawsonite-bearing metabasaltic and metasedimentary rocks that were metamorphosed and deformed at the slab-mantle interface (45 - 80 km), and therefore provide an opportunity to systematically compare lawsonite fabrics in rocks with different modal amounts of rheologically significant minerals (e.g., glaucophane, omphacite, quartz), integrated with information about lawsonite crystal size, compositional zoning patterns, shape, and twinning. Studies to date of lawsonite CPO from natural lawsonite-bearing rocks have resulted in two patterns: one is characterized by a concentration of [001] axes parallel to lineation and the other is characterized by a concentration of [001] axes perpendicular to foliation. We have documented the first type in metabasalt and the second in quartzite, but other researchers have documented both types in metabasalt. Regardless of variations in lawsonite CPO or rock type, omphacite and glaucophane CPO remain consistent. Quartz c-axis patterns vary in eclogite- and

The fate of carbonates along a subducting slab

Science.gov (United States)

Bouilhol, P.; Debret, B.; Inglis, E.

2017-12-01

Carbon long-term cycling is a subject of recent controversy as new mass balance calculations suggest that most carbon is transferred from the slab to the mantle wedge by fluids during subduction, limiting the efficiency of carbon recycling to the deep mantle. Here, we examine the mobility of carbon at large scale during subduction through field, petrographic and geochemical studies on exhumed portion of the alpine slab that have recorded different metamorphic conditions during subduction. We studied serpentinite samples, metasomatic horizon between serpentinites and sediments, as well as veins hosted in serpentinites. Samples are from the Western Alps (Queyras and Zermatt) and have recorded a prograde metamorphic history from low temperature blueshist to eclogite facies P-T conditions. We show that during subduction there are several stages of carbonate precipitation and dissolution at metasomatic interfaces between metasedimentary and ultramafic rocks in the slab, as well as within the serpentinites. The early stage of subduction sees carbonate precipitation from the sediment derived fluids into the serpentnites. At higher temperature, when the dehydration shift from sediment to serpentinite dominated, the carbonates are dissolved, inducing the release of CO2 rich fluids. This occurs before the eclogite facies is attained, providing strong evidence for the mobility of carbon in fluids during the early stages of subduction. These fluids are a potential metasomatic agent for the fore-arc mantle wedge, corroborating the observation of carbonate bearing veins in sub-arc mantle ultramafic rocks. In eclogite facies conditions, olivine and carbonate veins within the serpentinites witness the mobility of CO2 during serpentinite dehydration, and may provide clues about the large scale recycling of CO2 within the deep mantle, as well as secondary precipitation associated with exhumation. Trace elements, Fe and Zn isotopic composition of the different samples provides

Fast intraslab fluid-flow events linked to pulses of high pore fluid pressure at the subducted plate interface

Science.gov (United States)

Taetz, Stephan; John, Timm; Bröcker, Michael; Spandler, Carl; Stracke, Andreas

2018-01-01

A better understanding of the subduction zone fluid cycle and its chemical-mechanical feedback requires in-depth knowledge about how fluids flow within and out of descending slabs. Relicts of fluid-flow systems in exhumed rocks of fossil subduction zones allow for identification of the general relationships between dehydration reactions, fluid pathway formation, the dimensions and timescales of distinct fluid flow events; all of which are required for quantitative models for fluid-induced subduction zone processes. Two types of garnet-quartz-phengite veins can be distinguished in an eclogite-facies mélange block from the Pouébo Eclogite Mélange, New Caledonia. These veins record synmetamorphic internal fluid release by mineral breakdown reactions (type I veins), and infiltration of an external fluid (type II veins) with the associated formation of a reaction selvage. The dehydration and fluid migration documented by the type I veins likely occurred on a timescale of 105-106 years, based on average subduction rates and metamorphic conditions required for mineral dehydration and fluid flow. The timeframe of fluid-rock interaction between the external fluid and the wall-rock of the type II veins is quantified using a continuous bulk-rock Li-diffusion profile perpendicular to a vein and its metasomatic selvage. Differences in Li concentration between the internal and external fluid reservoirs resulted in a distinct diffusion profile (decreasing Li concentration and increasing δ7 Li) as the reaction front propagated into the host rock. Li-chronometric constraints indicate that the timescales of fluid-rock interaction associated with type II vein formation are on the order of 1 to 4 months (0.150-0.08+0.14 years). The short-lived, pulse-like character of this process is consistent with the notion that fluid flow caused by oceanic crust dehydration at the blueschist-to-eclogite transition contributes to or even dominates episodic pore fluid pressure increases at the

Deep structure of Pyrenees range (SW Europe) imaged by joint inversion of gravity and teleseismic delay time

Science.gov (United States)

Dufréchou, G.; Tiberi, C.; Martin, R.; Bonvalot, S.; Chevrot, S.; Seoane, L.

2018-04-01

We present a new model of the lithosphere and asthenosphere structure down to 300 km depth beneath the Pyrenees from the joint inversion of recent gravity and teleseismic data. Unlike previous studies, crustal correction were not applied on teleseismic data in order (i) to preserve the consistency between gravity data, which are mainly sensitive to the density structure of the crust.lithosphere, and travel time data, and (ii) to avoid the introduction of biases resulting from crustal reductions. The density model down to 100 km depth is preferentially used here to discuss the lithospheric structure of the Pyrenees, whereas the asthenospheric structure from 100 km to 300 km depth is discussed from our velocity model. The absence of a high density anomaly in our model between 30-100 km depth (except the Labourd density anomaly) in the northern part of the Pyrenees seems to preclude eclogitization of the subducted Iberian crust at the scale of the entire Pyrenean range. Local eclogitization of the deep Pyrenean crust beneath the western part of the Axial Zone (West of Andorra) associated with the positive Central density anomaly is proposed. The Pyrenean lithosphere in density and velocity models appears segmented from East to West. No clear relation between the along-strike segmentation and mapped major faults is visible in our models. The Pyrenees' lithosphere segments are associated to different seismicity pattern in the Pyrenees suggesting a possible relation between the deep structure of the Pyrenees and its seismicity in the upper crust. The concentration of earthquakes localized just straight up the Central density anomaly can result of the subsidence and/or delamination of an eclogitized Pyrenean deep root. The velocity model in the asthenosphere is similar to previous studies. The absence of a high-velocity anomaly in the upper mantle and transition zone (i.e. 125 to 225 km depth) seems to preclude the presence of a detached oceanic lithosphere beneath the

Petrologic and zircon U-Pb geochronological characteristics of the pelitic granulites from the Badu Complex of the Cathaysia Block, South China

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Zhao, Lei; Zhou, Xiwen; Zhai, Mingguo; Liu, Bo; Cui, Xiahong

2018-06-01

The recognition of the Indosinian Orogeny in the South China block has been controversial and difficult because of strong weathering and thick cover. High temperature (HT) and high pressure (HP) metamorphic rocks related to this orogeny were considered to be absent from this orogenic belt until the recent discovery of eclogite and granulite facies meta-igneous rocks, occurring as lenses within the meta-sedimentary rocks of the Badu Complex. However, metamorphic state of these meta-sedimentary rocks is still not clear. Besides, there have been no geochronological data of HT pelitic granulites previously reported from the Badu Complex. This paper presents petrographic characteristics and zircon geochronological results on the newly discovered kyanite garnet gneiss, pyroxene garnet gneiss and the HT pelitic granulites (sillimanite garnet gneiss). Mineral assemblages are garnet + sillimanite + ternary feldspar + plagioclase + quartz + biotite for the HT pelitic granulite, kyanite + ternary feldspar + garnet + sillimanite + plagioclase + quartz + biotite for the kyanite garnet gneiss, and garnet + biotite + pyroxene + plagioclase + ternary feldspar + quartz for the pyroxene garnet gneiss, respectively. Decompressional coronas around garnet grains can be observed in all these pelitic rocks. Typical granulite facies mineral assemblages and reaction textures suggest that these rocks experienced HP granulite facies metamorphism and overprinted decompression along a clockwise P-T loop. Results from integrated U-Pb dating and REE analysis indicate the growth of metamorphic zircons from depleted heavy REE sources (100-50 chondrite) compared with detrital zircons derived from granitic sources (typically > 1000 chondrite). Metamorphic zircons in HP granulite exhibit no or subdued negative Eu anomalies, which perhaps indicate zircon overgrowth under eclogite facies conditions. The zircon overgrowth ages range from 250 to 235 Ma, suggesting that HP granulite (eclogite) to

The strain-dependent spatial evolution of garnet in a high- P ductile shear zone from the Western Gneiss Region (Norway): a synchrotron X-ray microtomography study

Energy Technology Data Exchange (ETDEWEB)

Macente, A. [School of Geosciences, University of Edinburgh, The King' s Building James Hutton Road Edinburgh EH9 3FE UK; Fusseis, F. [School of Geosciences, University of Edinburgh, The King' s Building James Hutton Road Edinburgh EH9 3FE UK; Menegon, L. [School of Geography, Earth and Environmental Sciences, Faculty of Science and Engineering, Plymouth University, Fitzroy Drake Circus Plymouth Devon PL4 8AA UK; Xianghui, X. [Argonne National Laboratory, 9700 S. Cass Ave Building 431-B003 Argonne IL USA; John, T. [Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstr. 74-100 12249 Berlin Germany

2017-03-27

Reaction and deformation microfabrics provide key information to understand the thermodynamic and kinetic controls of tectono-metamorphic processes, however they are usually analysed in two dimensions, omitting important information regarding the third spatial dimension. We applied synchrotron-based X-ray microtomography to document the evolution of a pristine olivine gabbro into a deformed omphacite-garnet eclogite in four dimensions, where the 4th dimension is represented by the degree of strain. In the investigated samples, which cover a strain gradient into a shear zone from the Western Gneiss Region (Norway), we focused on the spatial transformation of garnet coronas into elongated clusters of garnets with increasing strain. Our microtomographic data allowed quantification of garnet volume, shape and spatial arrangement evolution with increasing strain. We combined microtomographic observations with light microscope- and backscatter electron images as well as electron microprobe- (EMPA) and electron backscatter diffraction (EBSD) analysis to correlate mineral composition and orientation data with the X-ray absorption signal of the same mineral grains. With increasing deformation, the garnet volume almost triples. In the low strain domain, garnets form a well interconnected large garnet aggregate that develops throughout the entire Page 1 of 52 sample. We also observed that garnet coronas in the gabbros never completely encapsulate olivine grains. In the most highly deformed eclogites, the oblate shapes of garnet clusters reflect a deformational origin of the microfabrics. We interpret the aligned garnet aggregates to direct synkinematic fluid flow and consequently influence the transport of dissolved chemical components. EBSD analyses reveal that garnet show a near-random crystal preferred orientation that testifies no evidence for crystal plasticity. There is, however evidence for minor fracturing, neo-nucleation and overgrowth. Microprobe chemical analysis

Silicate melt metasomatism in the lithospheric mantle beneath SW Poland

Science.gov (United States)

Puziewicz, Jacek; Matusiak-Małek, Magdalena; Ntaflos, Theodoros; Grégoire, Michel; Kukuła, Anna

2014-05-01

The xenoliths of peridotites representing the subcontinental lithospheric mantle (SCLM) beneath SW Poland and adjacent parts of Germany occur in the Cenozoic alkaline volcanic rocks. Our study is based on detailed characterization of xenoliths occurring in 7 locations (Steinberg in Upper Lusatia, Księginki, Pilchowice, Krzeniów, Wilcza Góra, Winna Góra and Lutynia in Lower Silesia). One of the two major lithologies occurring in the xenoliths, which we call the "B" lithology, comprises peridotites (typically harzburgites) with olivine containing from 90.5 to 84.0 mole % of forsterite. The harzburgites contain no clinopyroxene or are poor in that mineral (eg. in Krzeniów the group "B" harzburgites contain pfu in ortho-, and pfu in clinopyroxene). The exception are xenoliths from Księginki, which contain pyroxenes characterised by negative correlation between mg# and Al. The REE patterns of both ortho- and clinopyroxene in the group "B" peridotites suggest equilibration with silicate melt. The rocks of "B" lithology were formed due to alkaline silicate melt percolation in the depleted peridotitic protolith. The basaltic melts formed at high pressure are usually undersaturated in both ortho- and clinopyroxene at lower pressures (Kelemen et al. 1992). Because of cooling and dissolution of ortho- and clinopyroxene the melts change their composition and become saturated in one or both of those phases. Experimental results (e.g. Tursack & Liang 2012 and references therein) show that the same refers to alkaline basaltic silicate melts and that its reactive percolation in the peridotitic host leads to decrease of Mg/(Mg+Fe) ratios of olivine and pyroxenes. Thus, the variation of relative volumes of olivine and orthopyroxene as well as the decrease of mg# of rock-forming silicates is well explained by reactive melt percolation in the peridotitic protolith consisting of high mg# olivine and pyroxenes (in the area studied by us that protolith was characterised by olivine

COMPOSITIONAL AND THERMAL DIFFERENCES BETWEEN LITHOSPHERIC AND ASTHENOSPHERIC MANTLE AND THEIR INFLUENCE ON CONTINENTAL DELAMINATION

Directory of Open Access Journals (Sweden)

A. I. Kiselev

2015-01-01

Full Text Available The lower part of lithosphere in collisional orogens may delaminate due to density inversion between the asthenosphere and the cold thickened lithospheric mantle. Generally, standard delamination models have neglected density changes within the crust and the lithospheric mantle, which occur due to phase transitions and compositional variations upon changes of P-T parameters. Our attention is focused on effects of phase and density changes that may be very important and even dominant when compared with the effect of a simple change of the thermal mantle structure. The paper presents the results of numerical modeling for eclogitization of basalts of the lower crust as well as phase composition changes and density of underlying peridotite resulted from tectonic thickening of the lithosphere and its foundering into the asthenosphere. As the thickness of the lower crust increases, the mafic granulite (basalt passes into eclogite, and density inversion occurs at the accepted crust-mantle boundary (P=20 kbar because the newly formed eclogite is heavier than the underlying peridotite by 6 % (abyssal peridotite, according to [Boyd, 1989]. The density difference is a potential energy for delamination of the eclogitic portion of the crust. According to the model, P=70 kbar and T=1300 °C correspond to conditions at the lower boundary of the lithosphere. Assuming the temperature adiabatic distribution within the asthenosphere, its value at the given parameters ranges from 1350 °C to 1400 °C. Density inversion at dry conditions occurs with the identical lithospheric and asthenospheric compositions at the expense of the temperature difference at 100 °C with the density difference of only 0.0022 %. Differences of two other asthenospheric compositions (primitive mantle, and lherzolite KH as compared to the lithosphere (abyssal peridotite are not compensated for by a higher temperature. The asthenospheric density is higher than that of the lithospheric base

Formation and Evolution of the Continental Lithospheric Mantle: Perspectives From Radiogenic Isotopes of Silicate and Sulfide Inclusions in Macrodiamonds

Science.gov (United States)

Shirey, S. B.; Richardson, S. H.

2007-12-01

Silicate and sulfide inclusions that occur in diamonds comprise the oldest (>3 Ga), deepest (>140 km) samples of mantle-derived minerals available for study. Their relevance to the evolution of the continental lithosphere is clear because terrestrial macrodiamonds are confined to regions of the Earth with continental lithospheric mantle keels. The goals of analytical work on inclusions in diamond are to obtain paragenesis constraints, radiogenic ages, and initial isotopic compositions. The purpose is to place diamond formation episodes into the broader framework of the geological processes that create and modify the continental lithosphere and to relate the source of the C and N in diamond-forming fluids to understanding the Earth's C and N cycles in the Archean. Although sulfide and silicate inclusions rarely occur in the same diamond, they both can be grouped according to their geochemical similarity with the chief rock types that comprise the mantle keel: peridotite and eclogite. Silicate inclusions are classified as harzburgitic (depleted; olivine > Fo91, garnet Cr2O3 > 3 wt% and CaO from 0 to 5 wt%), lherzolitic (fertile), or eclogitic (basaltic; garnet Cr2O3 14 wt%; Os > 2 ppm) versus eclogitic (Ni bearing kimberlites, and the generosity of mining companies because of the extreme rarity of inclusions in suites of mostly gem-quality diamonds. Most isotopic work has been on the Kaapvaal-Zimbabwe craton with lesser work on the Slave, Siberian, and Australian cratons. Sm-Nd ages on silicate suites and Re-Os ages on sulfide suites confirm diamond formation from the Mesoarchean though the Neoproterozoic. Most important are the systematics across cratons in the context of crustal geology that lead to generalities about craton evolution. Inclusion suites date mantle keels as Mesoarchean and clearly point to subduction as the major process to form the earliest continental nuclei and to amalgamate the cratons in their present form. This is evident from the elevated

Seismic anisotropy from compositional banding in granulites from the deep magmatic arc of Fiordland, New Zealand

Science.gov (United States)

Cyprych, Daria; Piazolo, Sandra; Almqvist, Bjarne S. G.

2017-11-01

We present calculated seismic velocities and anisotropies of mafic granulites and eclogites from the Cretaceous deep lower crust (∼40-65 km) of Fiordland, New Zealand. Both rock types show a distinct foliation defined by cm-scale compositional banding. Seismic properties are estimated using the Asymptotic Expansion Homogenisation - Finite Element (AEH-FE) method that, unlike the commonly used Voigt-Reuss-Hill homogenisation, incorporates the phase boundary network into calculations. The predicted mean P- and S-wave velocities are consistent with previously published data for similar lithologies from other locations (e.g., Kohistan Arc), although we find higher than expected anisotropies (AVP ∼ 5.0-8.0%, AVS ∼ 3.0-6.5%) and substantial S-wave splitting along foliation planes in granulites. This seismic signature of granulites results from a density and elasticity contrast between cm-scale pyroxene ± garnet stringers and plagioclase matrix rather than from crystallographic orientations alone. Banded eclogites do not show elevated anisotropies as the contrast in density and elastic constants of garnet and pyroxene is too small. The origin of compositional banding in Fiordland granulites is primarily magmatic and structures described here are expected to be typical for the base of present day magmatic arcs. Hence, we identify a new potential source of anisotropy within this geotectonic setting.

Alkaline and carbonatite metasomatism of lithospheric mantle beneath SW Poland- Pilchowice case

Science.gov (United States)

Ćwiek, Mateusz; Matusiak-Małek, Magdalena; Puziewicz, Jacek; Ntaflos, Theodoros

2014-05-01

The Cenozoic basanites from Pilchowice (SW Poland) form volcanic plug located exactly at Intra- Sudetic Fault. These basanites belong to the Polish part of the Central European Volcanic Province and contain numerous, usually small (pfu and mg# from 0.915- 0.920 . One xenolith contains clinopyroxene with abundant spongy rims. Primary clinopyroxene is very rare and Al-enriched (mg# 0.92, 0.17 atoms of Al pfu). The spinel is Cr enriched (cr# 0.46-0.68) and is usually associated with clinopyroxene. Orthopyroxene is depleted in REE compared to primitive mantle. Orthopyroxene from majority of xenoliths are strongly LREE depleted ((La/Lu)N = 0.03-0.21). All studied peridotites contain clinopyroxene which is enriched (2 to 70 times) in REE compared to primitive mantle. Clinopyroxene patterns show relative low HREE concentration ((La/Lu)N = 4.75- 19.99), moreover patterns from three samples are convex- upward shaped with inflection point on Nd ((La/Nd)N = 0.36-0.96). Clinopyroxene- poor lithology, high cr# in spinel and LREE- depleted nature of orthopyroxene suggest that upper mantle sampled by Pilchowice basanite is a restite after partial melting. The LREE enriched composition of clinopyroxene suggest that peridotites were metasomatised. Clinopyroxene convex- upward shaped REE plots with inflection point on Nd is typical for metasomatism related with alkaline melt. On the other hand very low ratios of Ti/ Eu (24.8- 738.9) and high (La/ Yb)N (3.5- 17) ratio (Coltorti, 1999) suggest that the metasomatic agent was either a mixture of alkaline silicate melt with carbonatite or peridotite reaction with two independent agents is recorded. This study is a part of MSc thesis of the first author and was possible thanks to the project NCN 2011/03/B/ST10/06248 of Polish National Centre for Science. Coltorti, M., Bonadiman, C., Hinton, R. W., Siena, F. & Upton, B. G. J. (1999). Carbonatite metasomatism of the oceanic upper mantle: Evidence from clinopyroxenes and glasses in

Recycling of Oceanic Lithosphere: Water, fO2 and Fe-isotope Constraints

Science.gov (United States)

Bizmis, M.; Peslier, A. H.; McCammon, C. A.; Keshav, S.; Williams, H. M.

2014-01-01

Spinel peridotite and garnet pyroxenite xenoliths from Hawaii provide important clues about the composition of the oceanic lithosphere, and can be used to assess its contribution to mantle heterogeneity upon recycling. The peridotites have lower bulk H2O (approximately 70-114 ppm) than the MORB source, qualitatively consistent with melt depletion. The garnet pyroxenites (high pressure cumulates) have higher H2O (200-460 ppm, up to 550 ppm accounting for phlogopite) and low H2O/Ce ratios (less than 100). The peridotites have relatively light Fe-isotopes (delta Fe -57 = -0.34 to 0.13) that decrease with increasing depletion, while the pyroxenites are significantly heavier (delta Fe-57 up to 0.3). The observed xenolith, as well as MORB and OIB total Fe-isotope variability is larger that can be explained by existing melting models. The high H2O and low H2O/Ce ratios of pyroxenites are similar to estimates of EM-type OIB sources, while their heavy delta Fe-57 are similar to some Society and Cook-Austral basalts. Therefore, recycling of mineralogically enriched oceanic lithosphere (i.e. pyroxenites) may contribute to OIB sources and mantle heterogeneity. The Fe(3+)/Sigma? systematics of these xenoliths also suggest that there might be lateral redox gradients within the lithosphere, between juxtaposed oxidized spinel peridotites (deltaFMQ = -0.7 to 1.6, at 15 kb) and more reduced pyroxenites (deltaFMQ = -2 to -0.4, at 20-25kb). Such mineralogically and compositionally imposed fO2 gradients may generate local redox melting due to changes in fluid speciation (e.g. reduced fluids from pyroxenite encountering more oxidized peridotite). Formation of such incipient, small degree melts could further contribute to metasomatic features seen in peridotites, mantle heterogeneity, as well as the low velocity and high electrical conductivity structures near the base of the lithosphere and upper mantle.

Rapid Grain Size Reduction in the Upper Mantle at a Plate Boundary

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Kidder, S. B.; Scott, J.; Prior, D. J.; Lubicich, E. J.

2017-12-01

A few spinel peridotite xenoliths found near the Alpine Fault, New Zealand, exhibit a mylonitic texture and, locally, an extremely fine 30 micron grain size. The harzburgite xenoliths were emplaced in a 200 km-long elongate dike zone interpreted as a gigantic tension fracture or Reidel shear associated with Alpine Fault initiation 25 Ma. The presence of thin ( 1 mm) ultramylonite zones with px-ol phase mixing and fine grain sizes, minimal crustal-scale strain associated with the dike swarm, and the absence of mylonites at four of the five xenolith localities associated with the dike swarm indicate that upper mantle deformation was highly localized. Strings of small, recrystallized grains (planes in 3D) are found in the interiors of olivine porphyroclasts. In some cases, bands 1-2 grains thick are traced from the edges of olivine grains and terminate in their interiors. Thicker zones of recrystallized grains are also observed crossing olivine porphyroclasts without apparent offset of the unrecrystallized remnants of the porphyroclasts. We suggest a brittle-plastic origin for these features since the traditional recrystallization mechanisms associated with dislocation creep require much more strain than occurred within these porphyroclasts. Analogous microstructures in quartz and feldspar in mid-crust deformation zones are attributed to brittle-plastic processes. We hypothesize that such fine-grained zones were the precursors of the observed, higher-strain ultramylonite zones. Given the size of the new grains preserved in the porphyroclasts ( 100 micron) and a moho temperature > 650°C, grain growth calculations indicate that the observed brittle-plastic deformation occurred <10,000 yrs. prior to eruption. It is likely then that either brittle-plastic deformation was coeval with the ductile shearing occurring in the ultramylonite bands, or possibly, if deformation can be separated into brittle-plastic (early) and ductile (later) phases, that the entire localization

Geochemistry of ultramafic xenoliths in Cenozoic alkali basalts from ...

Indian Academy of Sciences (India)

enriched signatures, indicating the heterogeneous characteristics in the mantle source and the influence of different degrees of .... of physical integrity of the craton resulted from ... of old lithospheric relicts and newly accreted man- tle (Xu et al ...

Noble gas composition of subcontinental lithospheric mantle: An extensively degassed reservoir beneath Southern Patagonia

Science.gov (United States)

Jalowitzki, Tiago; Sumino, Hirochika; Conceição, Rommulo V.; Orihashi, Yuji; Nagao, Keisuke; Bertotto, Gustavo W.; Balbinot, Eduardo; Schilling, Manuel E.; Gervasoni, Fernanda

2016-09-01

Patagonia, in the Southern Andes, is one of the few locations where interactions between the oceanic and continental lithosphere can be studied due to subduction of an active spreading ridge beneath the continent. In order to characterize the noble gas composition of Patagonian subcontinental lithospheric mantle (SCLM), we present the first noble gas data alongside new lithophile (Sr-Nd-Pb) isotopic data for mantle xenoliths from Pali-Aike Volcanic Field and Gobernador Gregores, Southern Patagonia. Based on noble gas isotopic compositions, Pali-Aike mantle xenoliths represent intrinsic SCLM with higher (U + Th + K)/(3He, 22Ne, 36Ar) ratios than the mid-ocean ridge basalt (MORB) source. This reservoir shows slightly radiogenic helium (3He/4He = 6.84-6.90 RA), coupled with a strongly nucleogenic neon signature (mantle source 21Ne/22Ne = 0.085-0.094). The 40Ar/36Ar ratios vary from a near-atmospheric ratio of 510 up to 17700, with mantle source 40Ar/36Ar between 31100-6800+9400 and 54000-9600+14200. In addition, the 3He/22Ne ratios for the local SCLM endmember, at 12.03 ± 0.15 to 13.66 ± 0.37, are higher than depleted MORBs, at 3He/22Ne = 8.31-9.75. Although asthenospheric mantle upwelling through the Patagonian slab window would result in a MORB-like metasomatism after collision of the South Chile Ridge with the Chile trench ca. 14 Ma, this mantle reservoir could have remained unhomogenized after rapid passage and northward migration of the Chile Triple Junction. The mantle endmember xenon isotopic ratios of Pali-Aike mantle xenoliths, which is first defined for any SCLM-derived samples, show values indistinguishable from the MORB source (129Xe/132Xe =1.0833-0.0053+0.0216 and 136Xe/132Xe =0.3761-0.0034+0.0246). The noble gas component observed in Gobernador Gregores mantle xenoliths is characterized by isotopic compositions in the MORB range in terms of helium (3He/4He = 7.17-7.37 RA), but with slightly nucleogenic neon (mantle source 21Ne/22Ne = 0.065-0.079). We

Melt-peridotite reactions in upwelling EM1-type eclogite bodies

DEFF Research Database (Denmark)

Søager, Nina; Holm, Paul Martin

2013-01-01

or simple variations in degrees of mantle melting. The difference is also clear in major elements where the low Nb/U basalts have markedly higher alkali contents but lower FeO and Ni than the high Nb/U basalts. Four melt components have been identified based on olivine fractionation corrected compositions...

Deformation of the Songshugou ophiolite in the Qinling orogen

Science.gov (United States)

Sun, Shengsi; Dong, Yunpeng

2017-04-01

The Qinling orogen, middle part of the China Central Orogenic Belt, is well documented that was constructed by multiple convergences and subsequent collisions between the North China and South China Blocks mainly based on geochemistry and geochronology of ophiolites, magmatic rocks as well as sedimentary reconstruction. However, this model is lack of constraints from deformation of subduction/collision. The Songshugou ophiolite outcropped to the north of the Shangdan suture zone represents fragments of oceanic crust and upper mantle. Previous works have revealed that the ophiolite was formed at an ocean ridge and then emplaced in the northern Qinling belt. Hence, deformation of the ophiolite would provide constraints for the rifting and subduction processes. The ophiolite consists chiefly of metamorphosed mafic and ultramafic rocks. The ultramafic rocks contain coarse dunite, dunitic mylonite and harzburgite, with minor diopsidite veins. The mafic rocks are mainly amphibolite, garnet amphibolite and amphibole schist, which are considered to be eclogite facies and retrograde metamorphosed oceanic crust. Amphibole grains in the mafic rocks exhibit a strong shape-preferred orientation parallel to the foliation, which is also parallel to the lithologic contacts between mafic and ultramafic rocks. Electron backscattered diffraction (EBSD) analyses show strong olivine crystallographic preferred orientations (CPO) in dunite including A-, B-, and C-types formed by (010)[100], (010)[001] and (100)[001] dislocation slip systems, respectively. A-type CPO suggests high temperature plastic deformation in the upper mantle. In comparison, B-type may be restricted to regions with significantly high water content and high differential stress, and C-type may also be formed in wet condition with lower differential stress. Additionally, the dunite evolved into amphibolite facies metamorphism with mineral assemblages of olivine + talc + anthophyllite. Assuming a pressure of 1.5 GPa

A new subdivision of the central Sesia Zone (Aosta Valley, Italy)

Science.gov (United States)

Giuntoli, Francesco; Engi, Martin; Manzotti, Paola; Ballèvre, Michel

2015-04-01

The Sesia Zone in the Western Alps is a continental terrane probably derived from the NW-Adriatic margin and polydeformed at HP conditions during Alpine convergence. Subdivisions of the Sesia Zone classically have been based on the dominant lithotypes: Eclogitic Micaschist Complex, Seconda Zona Diorito-Kinzigitica, and Gneiss Minuti Complex. However, recent work (Regis et al., 2014) on what was considered a single internal unit has revealed that it comprises two or more tectonic slices that experienced substantially different PTDt-evolutions. Therefore, detailed regional petrographic and structural mapping (1:3k to 1:10k) was undertaken and combined with extensive sampling for petrochronological analysis. Results allow us to propose a first tectonic scheme for the Sesia Zone between the Aosta Valley and Val d'Ayas. A set of field criteria was developed and applied, aiming to recognize and delimit the first order tectonic units in this complex structural and metamorphic context. The approach rests on three criteria used in the field: (1) Discontinuously visible metasedimentary trails (mostly carbonates) considered to be monocyclic (Permo-Mesozoic protoliths); (2) mappable high-strain zones; and (3) visible differences in the metamorphic imprint. None of these key features used are sufficient by themselves, but in combination they allow us to propose a new map that delimits main units. We propose an Internal Complex with three eclogitic sheets, each 0.5-3 km thick. Dominant lithotypes include micaschists associated with mafic rocks and minor orthogneiss. The main foliation is of HP, dipping moderately NW. Each of these sheets is bounded by (most likely monometamorphic) sediments, <10-50 m thick. HP-relics (of eclogite facies) are widespread, but a greenschist facies overprint locally is strong close to the tectonic contact to neighbouring sheets. An Intermediate Complex lies NW of the Internal Complex and comprises two thinner, wedge-shaped units termed slices. These

The evaluation of the statistical monomineral thermobarometric methods for the reconstruction of the lithospheric mantle structure

Science.gov (United States)

Ashchepkov, I.; Vishnyakova, E.

2009-04-01

The modified versions of the thermobarometers for the mantle assemblages were revised sing statistical calibrations on the results of Opx thermobarometry. The modifications suggest the calculation of the Fe# of coexisting olivine Fe#Ol according to the statistical approximations by the regressions obtained from the xenoliths from kimberlite data base including >700 associations. They allow reproduces the Opx based TP estimates and to receive the complete set of the TP values for mantle xenoliths and xenocrysts. For GARNET Three variants of barometer give similar results. The first is published (Ashchepkov, 2006). The second is calculating the Al2O3 from Garnet for Orthopyroxene according to procedure: xCrOpx=Cr2O3/CaO)/FeO/MgO/500 xAlOpx=1/(3875*(exp(Cr2O3^0.2/CaO)-0.3)*CaO/989+16)-XcrOpx Al2O3=xAlOp*24.64/Cr2O3^0.2*CaO/2.+FeO*(ToK-501)/1002 And then it suppose using of the Al2O3 in Opx barometer (McGregor, 1974). The third variant is transformation of the G. Grutter (2006) method by introducing of the influence of temperature. P=40+(Cr2O3)-4.5)*10/3-20/7*CaO+(ToC)*0.0000751*MgO)*CaO+2.45*Cr2O3*(7-xv(5,8)) -Fe*0.5 with the correction for P>55: P=55+(P-55)*55/(1+0.9*P) Average from this three methods give appropriate values comparable with determined with (McGregor,1974) barometer. Temperature are estimating according to transformed Krogh thermometer Fe#Ol_Gar=Fe#Gar/2+(T(K)-1420)*0.000112+0.01 For the deep seated associations P>55 kbar T=T-(0.25/(0.4-0.004*(20-P))-0.38/Ca)*275+51*Ca*Cr2-378*CaO-0.51)-Cr/Ca2*5+Mg/(Fe+0.0001)*17.4 ILMENITE P= ((TiO2-23.)*2.15-(T0-973)/20*MgO*Cr2O3 and next P=(60-P)/6.1+P ToK is determined according to (Taylor et al , 1998) Fe#Ol_Chr =(Fe/(Fe+Mg)ilm -0.35)/2.252-0.0000351*(T(K)-973) CHROMITE The equations for PT estimates with chromite compositions P=Cr/(Cr+Al)*T(K)/14.+Ti*0.10 with the next iteration P=-0.0053*P^2+1.1292*P+5.8059 +0.00135*T(K)*Ti*410-8.2 For P> 57 P=P+(P-57)*2.75 Temperature estimates are according to the O

A petro-structural review of the Zermatt-Saas Fee zone

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Schenker, Filippo Luca; Markus Schmalholz, Stefan; Baumgartner, Lukas

2014-05-01

The Zermatt-Saas Fee zone (ZSZ) is an imbricate of fragments of blueschist- to eclogite-facies metabasalts and metagabbros, serpentinites and mélange zones containing blocks of the above mentioned rocks. The ZSZ is usually interpreted as a fragment of oceanic crust belonging to the Piemont-Ligurian (Tethyan) Ocean that was accreted into the Alpine nappe pile. In the last decades the discovery of several Ultra-High Pressure (UHP, >2.7 GPa at 550-600 °C from coesite bearing eclogites and diamond-bearing fluid inclusions in garnet) localities lead to the interpretation of deep subduction (> 100 km) of the ZSZ in the Eocene, and subsequent uplift from mantle depth with high exhumation rates (e.g. Amato et al., 1999). However, these high pressures are in apparent contrast to the regional metamorphic conditions that reflect pressures peaking at < 2 GPa for 550-600°C (blueschist and eclogite mineral assemblages in mafic rocks). These latter metamorphic conditions do not need anomalous high burial histories and exhumation velocities higher than the plate velocities. The magnitude and distribution of pressure in the tectonic units of the ZSZ are important for constraining dynamic models for the evolution of the ZSZ and the Western Alps. Before entering into dynamic models, we propose a petro-structural overview where the published petrological data on pressure and temperature are critically reviewed, and positioned on a geological map and cross section in order to integrate them into the proper structural and tectonic framework. The questions we seek to answer are: How is the pressure distributed within the main tectonic units and within the entire ZSZ? Do we observe sharp or gradual pressure gradients within the ZSZ? Can the UHP conditions be averaged/extended to the entire ZSZ? If not, do they correspond to conditions of observable subunits, or do they reflect anomalies in the pressure field? Answering these questions is fundamental to better understand the

A Missing Link in Understanding Mantle Wedge Melting, Higashi-akaishi Peridotite, Japan

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Till, C. B.; Carlson, R. W.; Grove, T. L.; Wallis, S.; Mizukami, T.

2009-12-01

The Sanbagawa subduction-type metamorphic belt in SW Japan represents the deepest exposed portion of a Mesozoic accretionary complex along the Japanese island arc. Located on the island of Shikoku, the Higashi-akaishi peridotite body is the largest ultramafic lens within the Sanbagawa belt and is dominantly composed of dunite, lherzolite and garnet clinopyroxenite, interfingered in one locality with quartz-rich eclogite. Previous work indicates the P-T history of the peridotite includes rapid prograde metamorphism with peak temperatures of 700-810°C and pressures of 2.9-3.8 GPa at approximately 110-120 Ma. Here we present major and trace element and isotopic data for samples within the Higashi-akaishi peridotite body that suggest it records subduction zone melting processes. Ultramafic samples range from 40-52 wt. % SiO2 and 21-45 wt. % MgO with olivine and clinopyroxene Mg#s as high as 0.93 and have trace element concentrations diagnostic of subduction zone processes. The quartz-rich eclogite contains 62 wt. % SiO2, 6 wt. % MgO and 13 wt. % Al2O3 and has trace element concentrations that are enriched relative to the ultramafic samples. 87Sr/86Sr (.703237-.704288), 143Nd/144Nd (ɛNd=+2-6) and Pb isotopic compositions are within the range of Japanese arc rocks. 187Os/188Os values range from typical mantle values (0.123-0.129), to slightly elevated (0.133) in one peridotite with an unusually low Os content, to a high of 0.145 in the quartz-rich eclogite. The presence of garnet porphyroblasts that enclose primary euhedral chlorite, together with the chemical evidence, suggest these samples are associated with mantle melting in the presence of H2O near their peak P-T conditions and may represent both residues and trapped melts within a paleo-mantle wedge. The peak P-T conditions of these rocks are also similar to the solidus conditions of H2O-saturated fertile mantle based on experimental determinations. Thus the Higashi-akaishi peridotite may be a real world analog

Migmatitic rocks southwest of the Barberton greenstone belt

International Nuclear Information System (INIS)

Robb, L.J.; Anhaeusser, C.R.

1981-01-01

A geologic survey was done on the migmatitic rocks southwest of the Barberton greenstone belt. A table is given on the chemical analyses of components from migmatic outcrops in this area, as well as on the chemical analyses of some selected rock types found in greenstone xenoliths, together with leuco-biotite tomalite/tronomjemite gneisses in the area surrounding the Boesmanskop syenite pluton. Isotope dating was also used in the survey

Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab.

Science.gov (United States)

Kawamoto, Tatsuhiko; Yoshikawa, Masako; Kumagai, Yoshitaka; Mirabueno, Ma Hannah T; Okuno, Mitsuru; Kobayashi, Tetsuo

2013-06-11

Slab-derived fluids play an important role in heat and material transfer in subduction zones. Dehydration and decarbonation reactions of minerals in the subducting slab have been investigated using phase equilibria and modeling of fluid flow. Nevertheless, direct observations of the fluid chemistry and pressure-temperature conditions of fluids are few. This report describes CO2-bearing saline fluid inclusions in spinel-harzburgite xenoliths collected from the 1991 Pinatubo pumice deposits. The fluid inclusions are filled with saline solutions with 5.1 ± 1.0% (wt) NaCl-equivalent magnesite crystals, CO2-bearing vapor bubbles, and a talc and/or chrysotile layer on the walls. The xenoliths contain tremolite amphibole, which is stable in temperatures lower than 830 °C at the uppermost mantle. The Pinatubo volcano is located at the volcanic front of the Luzon arc associated with subduction of warm oceanic plate. The present observation suggests hydration of forearc mantle and the uppermost mantle by slab-derived CO2-bearing saline fluids. Dehydration and decarbonation take place, and seawater-like saline fluids migrate from the subducting slab to the mantle wedge. The presence of saline fluids is important because they can dissolve more metals than pure H2O and affect the chemical evolution of the mantle wedge.

Evaluating the importance of metamorphism in the foundering of continental crust.

Science.gov (United States)

Chapman, Timothy; Clarke, Geoffrey L; Piazolo, Sandra; Daczko, Nathan R

2017-10-12

The metamorphic conditions and mechanisms required to induce foundering in deep arc crust are assessed using an example of representative lower crust in SW New Zealand. Composite plutons of Cretaceous monzodiorite and gabbro were emplaced at ~1.2 and 1.8 GPa are parts of the Western Fiordland Orthogneiss (WFO); examples of the plutons are tectonically juxtaposed along a structure that excised ~25 km of crust. The 1.8 GPa Breaksea Orthogneiss includes suitably dense minor components (e.g. eclogite) capable of foundering at peak conditions. As the eclogite facies boundary has a positive dP/dT, cooling from supra-solidus conditions (T > 950 ºC) at high-P should be accompanied by omphacite and garnet growth. However, a high monzodioritic proportion and inefficient metamorphism in the Breaksea Orthogneiss resulted in its positive buoyancy and preservation. Metamorphic inefficiency and compositional relationships in the 1.2 GPa Malaspina Pluton meant it was never likely to have developed densities sufficiently high to founder. These relationships suggest that the deep arc crust must have primarily involved significant igneous accumulation of garnet-clinopyroxene (in proportions >75%). Crustal dismemberment with or without the development of extensional shear zones is proposed to have induced foundering of excised cumulate material at P > 1.2 GPa.

A heat pipe mechanism for volcanism and tectonics on Venus

International Nuclear Information System (INIS)

Turcotte, D.L.

1989-01-01

A heat pipe mechanism is proposed for the transport of heat through the lithosphere on Venus. This mechanism allows the crust and lithosphere on Venus to be greater than 150 km thick. A thick crust and thick lithosphere can explain the high observed topography and large associated gravity anomalies. For a 150-km-thick lithosphere the required volcanic flux on Venus is 200 km 3 /yr; this is compared with a flux of 17 km 3 /yr associated with the formation of the oceanic crust on Earth. A thick basaltic crust on Venus is expected to transform to eclogite at a depth of 60 to 80 km; the dense eclogite would contribute the lithospheric delamination that returns the crust to the interior of the planet completing the heat pipe cycle. Topography and the associated gravity anomalies can be explained by Airy compensation of the thick crust. The principal observation that is contrary to this hypothesis is the mean age of the surface that is inferred from crater statistics; the minimum mean age is about 130 Ma, and this implies an upper limit of 2 km 3 /yr for the surface volcanic flux. If the heat pipe mechanism was applicable on Earth in the Archean, it would provide the thick lithosphere implied by isotopic data from diamonds

First Report on Hawaiian Carbonatites

OpenAIRE

A. Rocholl; K. P. Jochum; B. Plessen; D. Rhede; R. L. Romer; R. Wirth

2015-01-01

Carbonatites are common in continental settings but have, so far, only been identified at two oceanic localities, the Cape Verde and Canary Islands, both resting on > 130 Ma old, thick and cool oceanic crust. Here, we report on the first carbonatites observed in a hotter and younger mid-ocean setting, occuring as xenolithic fragments in nephelinitic tuffs at Salt Lake Crater (SLC), Oahu, Hawaii. The existence of Hawaiian carbonatites has been hypothesized before from ...

Molybdenum isotope fractionation in the mantle

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Liang, Yu-Hsuan; Halliday, Alex N.; Siebert, Chris; Fitton, J. Godfrey; Burton, Kevin W.; Wang, Kuo-Lung; Harvey, Jason

2017-02-01

We report double-spike molybdenum (Mo) isotope data for forty-two mafic and fifteen ultramafic rocks from diverse locations and compare these with results for five chondrites. The δ98/95Mo values (normalized to NIST SRM 3134) range from -0.59 ± 0.04 to +0.10 ± 0.08‰. The compositions of one carbonaceous (CI) and four ordinary chondrites are relatively uniform (-0.14 ± 0.01‰, 95% ci (confidence interval)) in excellent agreement with previous data. These values are just resolvable from the mean of 10 mid-ocean ridge basalts (MORBs) (0.00 ± 0.02‰, 95% ci). The compositions of 13 mantle-derived ultramafic xenoliths from Kilbourne Hole, Tariat and Vitim are more diverse (-0.39 to -0.07‰) with a mean of -0.22 ± 0.06‰ (95% ci). On this basis, the isotopic composition of the bulk silicate Earth (BSE or Primitive Mantle) is within error identical to chondrites. The mean Mo concentration of the ultramafic xenoliths (0.19 ± 0.07 ppm, 95% ci) is similar in magnitude to that of MORB (0.48 ± 0.13 ppm, 95% ci), providing evidence, either for a more compatible behaviour than previously thought or for selective Mo enrichment of the subcontinental lithospheric mantle. Intraplate and ocean island basalts (OIBs) display significant isotopic variability within a single locality from MORB-like to strongly negative (-0.59 ± 0.04‰). The most extreme values measured are for nephelinites from the Cameroon Line and Trinidade, which also have anomalously high Ce/Pb and low Mo/Ce relative to normal oceanic basalts. δ98/95Mo correlates negatively with Ce/Pb and U/Pb, and positively with Mo/Ce, explicable if a phase such as an oxide or a sulphide liquid selectively retains isotopically heavy Mo in the mantle and fractionates its isotopic composition in low degree partial melts. If residual phases retain Mo during partial melting, it is possible that the [Mo] for the BSE may be misrepresented by values estimated from basalts. This would be consistent with the high Mo

Inclusions in diamonds constrain thermo-chemical conditions during Mesozoic metasomatism of the Kaapvaal cratonic mantle

Science.gov (United States)

Weiss, Yaakov; Navon, Oded; Goldstein, Steven L.; Harris, Jeff W.

2018-06-01

Fluid/melt inclusions in diamonds, which were encapsulated during a metasomatic event and over a short period of time, are isolated from their surrounding mantle, offering the opportunity to constrain changes in the sub-continental lithospheric mantle (SCLM) that occurred during individual thermo-chemical events, as well as the composition of the fluids involved and their sources. We have analyzed a suite of 8 microinclusion-bearing diamonds from the Group I De Beers Pool kimberlites, South Africa, using FTIR, EPMA and LA-ICP-MS. Seven of the diamonds trapped incompatible-element-enriched saline high density fluids (HDFs), carry peridotitic mineral microinclusions, and substitutional nitrogen almost exclusively in A-centers. This low-aggregation state of nitrogen indicates a short mantle residence times and/or low mantle ambient temperature for these diamonds. A short residence time is favored because, elevated thermal conditions prevailed in the South African lithosphere during and following the Karoo flood basalt volcanism at ∼180 Ma, thus the saline metasomatism must have occurred close to the time of kimberlite eruptions at ∼85 Ma. Another diamond encapsulated incompatible-element-enriched silicic HDFs and has 25% of its nitrogen content residing in B-centers, implying formation during an earlier and different metasomatic event that likely relates to the Karoo magmatism at ca. 180 Ma. Thermometry of mineral microinclusions in the diamonds carrying saline HDFs, based on Mg-Fe exchange between garnet-orthopyroxene (Opx)/clinopyroxene (Cpx)/olivine and the Opx-Cpx thermometer, yield temperatures between 875-1080 °C at 5 GPa. These temperatures overlap with conditions recorded by touching inclusion pairs in diamonds from the De Beers Pool kimberlites, which represent the mantle ambient conditions just before eruption, and are altogether lower by 150-250 °C compared to P-T gradients recorded by peridotite xenoliths from the same locality. Oxygen fugacity (fO2

Detailed thermal fingerprinting of obduction-related processes: insights from Northern New Caledonia

Science.gov (United States)

Vitale Brovarone, A.; Agard, P.; Monié, P.; Chauvet, A.

2012-04-01

Northern New Caledonia comprises large and continuous units of high-pressure metamorphic rocks formed in response to the Eocene subduction of a continental margin, classically viewed as a northern extension of the Norfolk ridge, below an oceanic island-arc system (well-exposed in southern New Caledonia) [1, 2]. Metamorphic conditions increase gradually towards the east, providing a continuous window on prograde metamorphism from low-grade, lawsonite-bearing assemblages to epidote-bearing eclogite [3, 4, 5]. Unfortunately, available P-T constraints are mostly restricted to metamafics, but are almost completely lacking in metasediments, which represent the dominant lithology in these mainly continental-derived units. This is due both to the lack of diagnostic mineral assemblages in the metasedimentary lower grade units and to the intense late retrogression affecting the higher-grade metasediments. We herein present an extensive thermometric dataset (encompassing the area from Hienghene to the south and from Koumac to Pouébo) obtained via Raman Spectroscopy of Carbonaceous Material (RSCM), which provides quantitative estimates of the peak metamorphic temperature of CM-bearing metasediments in the range ~200-650°C [6, 7]. Metamorphic conditions vary from about 200 °C in the lower-grade units and progressively increase toward the east to about 550 °C in the eclogite facies unit. Sharp metamorphic gaps are nevertheless found across some major tectonic boundaries (such as the Gendarmerie fault zone). Importantly, the main metamorphic units defined by means of our dataset do not always match with previous studies. This dataset also provides useful insights on the architecture of the high-pressure belt in Northern New Caledonia, where structures are poorly exposed due to thick vegetation. [1] Cluzel, D., Aitchison, J.C., Picard, C., 2001. Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic forearc of New Caledonia (Southwest Pacific

Magnesium isotopic composition of the Earth and chondrites

Science.gov (United States)

Teng, Fang-Zhen; Li, Wang-Ye; Ke, Shan; Marty, Bernard; Dauphas, Nicolas; Huang, Shichun; Wu, Fu-Yuan; Pourmand, Ali

2010-07-01

To constrain further the Mg isotopic composition of the Earth and chondrites, and investigate the behavior of Mg isotopes during planetary formation and magmatic processes, we report high-precision (±0.06‰ on δ 25Mg and ±0.07‰ on δ 26Mg, 2SD) analyses of Mg isotopes for (1) 47 mid-ocean ridge basalts covering global major ridge segments and spanning a broad range in latitudes, geochemical and radiogenic isotopic compositions; (2) 63 ocean island basalts from Hawaii (Kilauea, Koolau and Loihi) and French Polynesia (Society Island and Cook-Austral chain); (3) 29 peridotite xenoliths from Australia, China, France, Tanzania and USA; and (4) 38 carbonaceous, ordinary and enstatite chondrites including 9 chondrite groups (CI, CM, CO, CV, L, LL, H, EH and EL). Oceanic basalts and peridotite xenoliths have similar Mg isotopic compositions, with average values of δ 25Mg = -0.13 ± 0.05 (2SD) and δ 26Mg = -0.26 ± 0.07 (2SD) for global oceanic basalts ( n = 110) and δ 25Mg = -0.13 ± 0.03 (2SD) and δ 26Mg = -0.25 ± 0.04 (2SD) for global peridotite xenoliths ( n = 29). The identical Mg isotopic compositions in oceanic basalts and peridotites suggest that equilibrium Mg isotope fractionation during partial melting of peridotite mantle and magmatic differentiation of basaltic magma is negligible. Thirty-eight chondrites have indistinguishable Mg isotopic compositions, with δ 25Mg = -0.15 ± 0.04 (2SD) and δ 26Mg = -0.28 ± 0.06 (2SD). The constancy of Mg isotopic compositions in all major types of chondrites suggest that primary and secondary processes that affected the chemical and oxygen isotopic compositions of chondrites did not significantly fractionate Mg isotopes. Collectively, the Mg isotopic composition of the Earth's mantle, based on oceanic basalts and peridotites, is estimated to be -0.13 ± 0.04 for δ 25Mg and -0.25 ± 0.07 for δ 26Mg (2SD, n = 139). The Mg isotopic composition of the Earth, as represented by the mantle, is similar to chondrites

Composition et évolution du manteau lithosphérique nord-africain : évidences pétrologiques et géochimiques à partir des enclaves de manteau échantillonnées par le volcanisme cénozoïque intraplaque du Moyen Atlas (Maroc)

OpenAIRE

Pezzali , Irene

2013-01-01

The Ph.D. study is aimed at characterising the composition of pyroxenite xenoliths brought to the surface by Cenozoic intraplate volcanism in the Azrou Timahdite district of Middle Atlas (Morocco) to unravel their origin and significance in the frame of the geodynamic evolution of the North Africa lithospheric mantle. The interpretations are based on a petrological approach and on reliable geochemical information at both bulk rock and mineral scale. The data are used to address a largely deba...

Deeply subducted continental fragments - Part 2: Insight from petrochronology in the central Sesia Zone (western Italian Alps)

Science.gov (United States)

Giuntoli, Francesco; Lanari, Pierre; Burn, Marco; Kunz, Barbara Eva; Engi, Martin

2018-02-01

Subducted continental terranes commonly comprise an assembly of subunits that reflect the different tectono-metamorphic histories they experienced in the subduction zone. Our challenge is to unravel how, when, and in which part of the subduction zone these subunits were juxtaposed. Petrochronology offers powerful tools to decipher pressure-temperature-time (P-T-t) histories of metamorphic rocks that preserve a record of several stages of transformation. A major issue is that the driving forces for re-equilibration at high pressure are not well understood. For example, continental granulite terrains subducted to mantle depths frequently show only partial and localized eclogitization. The Sesia Zone (NW Italy) is exceptional because it comprises several continental subunits in which eclogitic rocks predominate and high-pressure (HP) assemblages almost completely replaced the Permian granulite protoliths. This field-based study comprises both main complexes of the Sesia terrane, covering some of the recently recognized tectonic subunits involved in its assembly; hence our data constrain the HP tectonics that formed the Sesia Zone. We used a petrochronological approach consisting of petrographic and microstructural analysis linked with thermodynamic modelling and U-Th-Pb age dating to reconstruct the P-T-t trajectories of these tectonic subunits. Our study documents when and under what conditions re-equilibration took place. Results constrain the main stages of mineral growth and deformation, associated with fluid influx that occurred in the subduction channel. In the Internal Complex (IC), pulses of fluid percolated at eclogite facies conditions between 77 and 55 Ma with the HP conditions reaching ˜ 2 GPa and 600-670 °C. By contrast, the External Complex (EC) records a lower pressure peak of ˜ 0.8 GPa for 500 °C at ˜ 63 Ma. The juxtaposition of the two complexes occurred during exhumation, probably at ˜ 0.8 GPa and 350 °C; the timing is constrained between 46

Volatile (Li, B, F and Cl) mobility during amphibole breakdown in subduction zones

Science.gov (United States)

Debret, Baptiste; Koga, Kenneth T.; Cattani, Fanny; Nicollet, Christian; Van den Bleeken, Greg; Schwartz, Stephane

2016-02-01

Amphiboles are ubiquitous minerals in the altered oceanic crust. During subduction, their breakdown is governed by continuous reactions up to eclogitic facies conditions. Amphiboles thus contribute to slab-derived fluid throughout prograde metamorphism and continuously record information about volatile exchanges occurring between the slab and the mantle wedge. However, the fate of volatile elements and especially halogens, such as F and Cl, in amphibole during subduction is poorly constrained. We studied metagabbros from three different localities in the Western Alps: the Chenaillet ophiolite, the Queyras Schistes Lustrés and the Monviso meta-ophiolitic complexes. These samples record different metamorphic conditions, from greenschist to eclogite facies, and have interacted with different lithologies (e.g. sedimentary rocks, serpentinites) from their formation at mid-oceanic ridge, up to their devolatilization during subduction. In the oceanic crust, the initial halogen budget is mostly stored in magmatic amphibole (F = 300-7000 ppm; Cl = 20-1200 ppm) or in amphibole corona (F = 100-7000 ppm; Cl = 80-2000 ppm) and titanite (F = 200-1500 ppm; Cl govern the halogen fluxes between the crust and the overlying mantle and/or the plate interface in subduction zones. Li and B are poorly stored in the oceanic crust (< 5 ppm). In subduction zones, prograde metamorphism of metagabbros is first marked by the crystallization of glaucophane at the expense of magmatic and amphibole coronas. This episode is accompanied with a decrease of halogen concentrations in amphiboles (< 200 ppm of F and Cl) suggesting that these elements can be transferred to the mantle wedge by fluids. In the Queyras Schistes Lustrés complex, the intense deformation and the abundant devolatilization of metasedimentary rocks produce large fluid flows that promote rock chemical hybridization (metasomatic mixing with hybrid composition between metasedimentary rock and metagabbro) at the metasedimentary rock

The depth of sub-lithospheric diamond formation and the redistribution of carbon in the deep mantle

Science.gov (United States)

Beyer, Christopher; Frost, Daniel J.

2017-03-01

Most diamonds form in the Earth's lithosphere but a small proportion contain Si-rich majoritic garnet inclusions that indicate formation in the deeper mantle. The compositions of syngenetic garnet inclusions can potential yield information on both the depth and mantle lithology in which the diamonds formed. Pressure dependent changes in garnet compositions have been calibrated using the results of experiments conducted in a multi-anvil apparatus at pressures between 6 and 16 GPa and temperatures of 1000 to 1400 °C. Using the results of these experiments a barometer was formulated based on an empirical parameterisation of the two major majoritic substitutions, referred to as majorite (Maj; Al3+ =Mg2+ +Si4+), and Na-majorite (Na-Maj; Mg2+ +Al3+ =Na+ +Si4+). Moreover, previously published experimental garnet compositions from basaltic, kimberlite, komatiite and peridotite bulk compositions were included in the calibration, which consequently covers pressures from 6 to 20 GPa and temperatures from 900 to 2100 °C. Experimental pressures are reproduced over these conditions with a standard deviation of 0.86 GPa. The barometer is used to determine equilibration pressures of approximately 500 reported garnet inclusions in diamonds from a range of localities. As the majority of these inclusions are proposed to be syngenetic this allows a detailed picture of diamond formation depths and associated source rocks to be established using inclusion chemistry. Geographic differences in diamond source rocks are mapped within the sub-lithospheric mantle to over 500 km depth. Continuous diamond formation occurs over this depth range within lithologies with eclogitic affinities but also in lithologies that appear transitional between eclogitic and peridotitic bulk compositions, with an affinity to pyroxenites. The geographic differences between eclogitic and pyroxenitic diamond source rocks are rationalised in terms of diamond formation within downwelling and upwelling regimes

Thermal Evolution of Juvenile Subduction Zones ' New Constraints from Lu-Hf Geochronology on HP oceanic rocks (Halilbaǧi, Central Anatolia)

Science.gov (United States)

Pourteau, Amaury; Scherer, Erik; Schmidt, Alexander; Bast, Rebecca

2015-04-01

The thermal structure of subduction zones plays a key role on mechanical and chemical processes taking place along the slab-mantle interface. Until now, changes through time of this thermal structure have been explored mostly by the means of numerical simulations. However, both "warm" (i.e., epidote-bearing), and "cold" (i.e., lawsonite-bearing) HP oceanic rocks have been reported in some fossil subduction complexes exposed at the Earth's surface (e.g., Franciscan Complex, California; Rio San Juan Complex, Hispañola; Halilbağı Unit, Central Anatolia). These a-priori "incompatible" rocks witness different thermal stages of ancient subduction zones and their study might provide complementary constraints to numerical models. To decipher the meaning of these contrasting metamorphic rocks in the Halilbağı Unit, we are carrying out Lu-Hf geochronology on garnet (grt) and lws from a variety of HP oceanic rocks, as well as the metamorphic sole of the overlying ophiolite. We selected five samples that are representative of the variety of metamorphic evolutions (i.e. peak conditions and P-T paths) encountered in this area. Preliminary analyses yielded 110 Ma (grt-hbl isochron) for a sub-ophiolitic grt amphibolite; 92 Ma (grt-omp) for an eclogite with prograde and retrograde ep; 90 Ma (grt-omp) for an eclogitic metabasite with prograde ep and retrograde ep+lws; 87 Ma (grt-gln) for a lws eclogite with prograde ep; and 86 Ma (grt-gln) for a blueschist with prograde and retrograde lws. These ages are mainly two-point isochrons. Further-refined data will be presented at the EGU General Assembly 2015, in Vienna. The consistent younging trend from "warm" to "cold" metamorphic rocks revealed by these first-order results points to metamorphic-sole formation during the initiation of intra-oceanic subduction at ~110 Ma, and subsequent cooling of the slab-mantle interface between 92 and 86 Ma. Therefore, the contrasting metamorphic evolutions encountered in the Halilbağı Unit

Preservation of an Archaean whole rock Re-Os isochron for the Venetia lithospheric mantle: Evidence for rapid crustal recycling and lithosphere stabilisation at 3.3 Ga

Science.gov (United States)

van der Meer, Quinten H. A.; Klaver, Martijn; Reisberg, Laurie; Riches, Amy J. V.; Davies, Gareth R.

2017-11-01

Re-Os and platinum group element analyses are reported for peridotite xenoliths from the 533 Ma Venetia kimberlite cluster situated in the Limpopo Mobile Belt, the Neoarchaean collision zone between the Kaapvaal and Zimbabwe Cratons. The Venetian xenoliths provide a rare opportunity to examine the state of the cratonic lithosphere prior to major regional metasomatic disturbance of Re-Os systematics throughout the Phanerozoic. The 32 studied xenoliths record Si-enrichment that is characteristic of the Kaapvaal lithospheric mantle and can be subdivided into five groups based on Re-Os analyses. The most pristine group I samples (n = 13) display an approximately isochronous relationship and fall on a 3.28 ± 0.17 Ga (95 % conf. int.) reference line that is based on their mean TMA age. This age overlaps with the formation age of the Limpopo crust at 3.35-3.28 Ga. The group I samples derive from ∼50 to ∼170 km depth, suggesting coeval melt depletion of the majority of the Venetia lithospheric mantle column. Group II and III samples have elevated Re/Os due to Re addition during kimberlite magmatism. Group II has otherwise undergone a similar evolution as the group I samples with overlapping 187Os/188Os at eruption age: 187Os/188OsEA, while group III samples have low Os concentrations, unradiogenic 187Os/188OsEA and were effectively Re-free prior to kimberlite magmatism. The other sample groups (IV and V) have disturbed Re-Os systematics and provide no reliable age information. A strong positive correlation is recorded between Os and Re concentrations for group I samples, which is extended to groups II and III after correction for kimberlite addition. This positive correlation precludes a single stage melt depletion history and indicates coupled remobilisation of Re and Os. The combination of Re-Os mobility, preservation of the isochronous relationship, correlation of 187Os/188Os with degree of melt depletion and lack of radiogenic Os addition puts tight constraints on

High-temperature peridotites - lithospheric or asthenospheric?

International Nuclear Information System (INIS)

Hops, J.J.; Gurney, J.J.

1990-01-01

High-temperature peridotites by definition yield equilibration temperatures greater than 1100 degrees C. On the basis of temperature and pressure calculations, these high-temperature peridotites are amongst the deepest samples entrained by kimberlites on route to the surface. Conflicting models proposing either a lithospheric or asthenospheric origin for the high-temperature peridotites have been suggested. A detailed study of these xenoliths from a single locality, the Jagersfontein kimberlite in the Orange Free State, has been completed as a means of resolving this controversy. 10 refs., 2 figs

Lateral heterogeneity and vertical stratification of cratonic lithospheric keels: a case study of the Siberian craton

DEFF Research Database (Denmark)

Artemieva, Irina; Cherepanova, Yulia; Herceg, Matija

2014-01-01

by regional xenolith P-T arrays,lithosphere density heterogeneity as constrained by free-board and satellite gravity data, and the non-thermalpart of upper mantle seismic velocity heterogeneity based on joint analysis of thermal and seismic tomography data.Density structure of the cratonic lithosphere...... and strongly depleted lithospheric mantle of the Archean nuclei, particularly below the Anabar shield.Since we cannot identify the depth distribution of density anomalies, we complement the approach by seismicdata. An analysis of temperature-corrected seismic velocity structure indicates strong vertical...

Nd-Sr isotopic geochemistry and U-Pb geochronology of the Fe granitic gneiss and Lajedo Granodiorite: implications for paleoproterozoic evolution of the Mineiro Belt, southern Sao Francisco craton, Brazil

Energy Technology Data Exchange (ETDEWEB)

Teixeira, Wilson [Universidade de Sao Paulo (USP), SP (Brazil). Inst. de Geociencias. Centro de Pesquisas Geocronologicas]. E-mail: wteixeir@usp.br; Avila, Ciro Alexandre [Universidade Federal do Rio de Janeiro (UFRJ), RJ (Brazil). Museu Nacional. Dept. de Geologia e Paleontologia]. E-mail: avila@mn.ufrj.br; Nunes, Luciana Cabral [Universidade de Sao Paulo (USP), SP (Brazil). Inst. de Geociencias]. E-mail: luciana@igc.usp.br

2008-07-01

The Fe granitic gneiss and Lajedo granodiorite belong to a voluminous felsic-mafic plutonism, tectonically linked to Paleoproterozoic magmatic evolution of the Mineiro Belt, southern portion of the Sao Francisco Craton, central-eastern Brazil. The Fe pluton is located north of the Lenheiros shear zone and is intrusive with respect to the Rio das Mortes greenstone belt and pyroxenite - gabbroic bodies, as indicated by xenoliths of gneiss and amphibolite, in the first case, and pyroxenite in the latter. The Lajedo granodiorite is located south of the Lenheiros shear zone and cuts the metamafic rocks of the Forro peridotite - pyroxenite and mafic and intermediate rocks of the Nazareno greenstone belt, as evidenced by xenoliths from the latter unit. The modal composition of the Fe granitic gneiss lies within the ranges of monzogranite and syenogranite. It is peraluminous and shows a large variation in K{sub 2}O content, which implies a middle-K calc-alkaline to high-K calc-alkaline tendency. The Lajedo modal composition is consistent with granodioritic and tonalitic compositions. It indicates a predominantly peraluminous composition and calc-alkaline character. The U-Pb zircon crystallization age of the Fe granitic gneiss is 2191 {+-} 9 Ma, whereas the Lajedo granodiorite yields 2208 {+-} 26 Ma. The Nd/Sr characteristics of the Fe and Lajedo plutons are consistent with mixtures of enriched mantle (EMI-type), DMM and crustal components during magma genesis in a plutonic arc setting, while the low {sup 87}Sr/{sup 86}Sri ratios point to contribution of mafic rock protoliths during magma genesis. This is also in accordance with the characteristic xenoliths observed within the investigated plutons from the Nazareno and Rio das Mortes greenstone belts. The Fe granitic gneiss and Lajedo granodiorite show tectonic characteristics which are comparable to those of nearby coeval plutons: Brito quartz-diorite (2221 +- 2 Ma), Brumado de Cima granodiorite (2219 {+-} 2 Ma), Brumado

Magmatic systems of large continental igneous provinces

Directory of Open Access Journals (Sweden)

E. Sharkov

2017-07-01

Full Text Available Large igneous provinces (LIPs formed by mantle superplume events have irreversibly changed their composition in the geological evolution of the Earth from high-Mg melts (during Archean and early Paleoproterozoic to Phanerozoic-type geochemically enriched Fe-Ti basalts and picrites at 2.3 Ga. We propose that this upheaval could be related to the change in the source and nature of the mantle superplumes of different generations. The first generation plumes were derived from the depleted mantle, whereas the second generation (thermochemical originated from the core-mantle boundary (CMB. This study mainly focuses on the second (Phanerozoic type of LIPs, as exemplified by the mid-Paleoproterozoic Jatulian–Ludicovian LIP in the Fennoscandian Shield, the Permian–Triassic Siberian LIP, and the late Cenozoic flood basalts of Syria. The latter LIP contains mantle xenoliths represented by green and black series. These xenoliths are fragments of cooled upper margins of the mantle plume heads, above zones of adiabatic melting, and provide information about composition of the plume material and processes in the plume head. Based on the previous studies on the composition of the mantle xenoliths in within-plate basalts around the world, it is inferred that the heads of the mantle (thermochemical plumes are made up of moderately depleted spinel peridotites (mainly lherzolites and geochemically-enriched intergranular fluid/melt. Further, it is presumed that the plume heads intrude the mafic lower crust and reach up to the bottom of the upper crust at depths ∼20 km. The generation of two major types of mantle-derived magmas (alkali and tholeiitic basalts was previously attributed to the processes related to different PT-parameters in the adiabatic melting zone whereas this study relates to the fluid regime in the plume heads. It is also suggested that a newly-formed melt can occur on different sides of a critical plane of silica undersaturation and can

Nd-Sr isotopic geochemistry and U-Pb geochronology of the Fe granitic gneiss and Lajedo Granodiorite: implications for paleoproterozoic evolution of the Mineiro Belt, southern Sao Francisco craton, Brazil

International Nuclear Information System (INIS)

Teixeira, Wilson; Avila, Ciro Alexandre; Nunes, Luciana Cabral

2008-01-01

The Fe granitic gneiss and Lajedo granodiorite belong to a voluminous felsic-mafic plutonism, tectonically linked to Paleoproterozoic magmatic evolution of the Mineiro Belt, southern portion of the Sao Francisco Craton, central-eastern Brazil. The Fe pluton is located north of the Lenheiros shear zone and is intrusive with respect to the Rio das Mortes greenstone belt and pyroxenite - gabbroic bodies, as indicated by xenoliths of gneiss and amphibolite, in the first case, and pyroxenite in the latter. The Lajedo granodiorite is located south of the Lenheiros shear zone and cuts the metamafic rocks of the Forro peridotite - pyroxenite and mafic and intermediate rocks of the Nazareno greenstone belt, as evidenced by xenoliths from the latter unit. The modal composition of the Fe granitic gneiss lies within the ranges of monzogranite and syenogranite. It is peraluminous and shows a large variation in K 2 O content, which implies a middle-K calc-alkaline to high-K calc-alkaline tendency. The Lajedo modal composition is consistent with granodioritic and tonalitic compositions. It indicates a predominantly peraluminous composition and calc-alkaline character. The U-Pb zircon crystallization age of the Fe granitic gneiss is 2191 ± 9 Ma, whereas the Lajedo granodiorite yields 2208 ± 26 Ma. The Nd/Sr characteristics of the Fe and Lajedo plutons are consistent with mixtures of enriched mantle (EMI-type), DMM and crustal components during magma genesis in a plutonic arc setting, while the low 87 Sr/ 86 Sri ratios point to contribution of mafic rock protoliths during magma genesis. This is also in accordance with the characteristic xenoliths observed within the investigated plutons from the Nazareno and Rio das Mortes greenstone belts. The Fe granitic gneiss and Lajedo granodiorite show tectonic characteristics which are comparable to those of nearby coeval plutons: Brito quartz-diorite (2221 +- 2 Ma), Brumado de Cima granodiorite (2219 ± 2 Ma), Brumado de Baixo

Crustal and mantle velocity models of southern Tibet from finite frequency tomography

Science.gov (United States)

Liang, Xiaofeng; Shen, Yang; Chen, Yongshun John; Ren, Yong

2011-02-01

Using traveltimes of teleseismic body waves recorded by several temporary local seismic arrays, we carried out finite-frequency tomographic inversions to image the three-dimensional velocity structure beneath southern Tibet to examine the roles of the upper mantle in the formation of the Tibetan Plateau. The results reveal a region of relatively high P and S wave velocity anomalies extending from the uppermost mantle to at least 200 km depth beneath the Higher Himalaya. We interpret this high-velocity anomaly as the underthrusting Indian mantle lithosphere. There is a strong low P and S wave velocity anomaly that extends from the lower crust to at least 200 km depth beneath the Yadong-Gulu rift, suggesting that rifting in southern Tibet is probably a process that involves the entire lithosphere. Intermediate-depth earthquakes in southern Tibet are located at the top of an anomalous feature in the mantle with a low Vp, a high Vs, and a low Vp/Vs ratio. One possible explanation for this unusual velocity anomaly is the ongoing granulite-eclogite transformation. Together with the compressional stress from the collision, eclogitization and the associated negative buoyancy force offer a plausible mechanism that causes the subduction of the Indian mantle lithosphere beneath the Higher Himalaya. Our tomographic model and the observation of north-dipping lineations in the upper mantle suggest that the Indian mantle lithosphere has been broken laterally in the direction perpendicular to the convergence beneath the north-south trending rifts and subducted in a progressive, piecewise and subparallel fashion with the current one beneath the Higher Himalaya.

Hydration of an active shear zone: Interactions between deformation, metasomatism and magmatism - the spinel-lherzolites from the Montferrier (southern France) Oligocene basalts

International Nuclear Information System (INIS)

Cabanes, N.; Briqueu, L.

1987-01-01

Geochemical and textural investigations have been simultaneously performed on spinel-lherzolite xenoliths from the Oligo-Miocene alkali basalts of Montferrier (southern France). All the investigated samples have undergone a deformation very particular by intense shearing under high stresses (up to 1.75 kbar), low temperatures (≤900 0 C) and strain rates of about 10 -18 to 10 -15 s -1 . Mineral chemistry reveals that the Montferrier lherzolites are fragments of an undepleted relatively shallow upper mantle level located at a depth of 50 km (15 kbar). Moreover, Na and Ti enrichment in diopside would reflect a metasomatic event, also emphasized by the common occurrence of pargasite in 50-70% of the investigated samples. Crystallization of this amphibole is attributed to a hydrous infiltration which is related in time and space to the deformation. Indeed, amphibole is preferentially concentrated in strongly deformed zones and in kink-band boundaries of orthopyroxene porphyroclasts. Moreover, the grain boundaries were used by the pervasive agent to percolate into the lherzolite: significant chemical variations (increase in MgO: 15% and decrease in Al 2 O 3 : 55%) are observed within the range of 7-5 μm adjacent to the grain boundary. Finally, Sr isotopic data ( 87 Sr/ 86 Sr) demonstrate that the amphibole, i.e. the metasomatic agent, is genetically related to the host lava of the xenoliths. Thus, the hydrous silicate liquid from which the amphibole has crystallized may be an early percolation of the ascending alkali magma. (orig.)

Mantle compositions below petit-spot volcanoes of the NW Pacific Plate

Science.gov (United States)

Hirano, N.

2017-12-01

Monogenetic petit-spot volcanoes of a few kilometers in diameter and geothermal gradient than the conventional GDH1 model; Machida et al., 2015; Yamamoto et al., 2014). The fact that the majority of the petit-spot lava samples do not contain olivine phenocrysts and have differentiated compositions (45-52 wt% SiO2, Mg# values of 50-65) indicates that these magmas have undergone differentiation in a magma chamber. However, geobarometry indicates that the deepest-sourced associated peridotitic xenoliths were derived from a depth of 42 km (Yamamoto et al., 2014). This indicates that melt fractionation must have occurred at depths greater than the middle lithosphere, a situation where the depth of fractionation could correlate with the rotation of the σ3 stress axis from the extensionally lower to the compressional upper part of the lithosphere. This rotation is the result of concave flexure prior to the outer rise of the subduction zone (Valentine & Hirano, 2010). Pilet et al. (2016) and Yamamoto et al. (2009) reported that these xenoliths were derived from a metasomatized region of the mantle, with this region metasomatized by prior melts of petit-spot magmas in the province. The strategic analysis of xenocrystic olivines from several petit-spot volcanoes also indicates that more depleted compositions are located in areas more proximal to the trench. This indicates that the lithospheric mantle in this region must have been significantly metasomatized prior to the onset of trench subduction.

Formation of atoll garnets in the UHP eclogites of the Tso Morari ...

Indian Academy of Sciences (India)

Mallika K Jonnalagadda

2017-11-22

Nov 22, 2017 ... Shivani Harshe1, Sarah Gain2 and William L Griffin2. 1. Department of ..... M orari complex. L1. -. N ormal. G arnet. T rav erse. Mineral. G arnet. A mp. Garnet. Cp x. G ...... along with white mica occur as shear bands (0.5. GPa and 500 ..... erals in the crust; Treatise on Geochemistry, Amsterdam. Elsevier 3 ...

EUNAseis

DEFF Research Database (Denmark)

Artemieva, Irina; Thybo, Hans

2013-01-01

; (4) The thickness ratio between upper-middle (Vp b 6.8 km/s) and lower (Vp N 6.8 km/s) crystalline crust is indicative of crustal origin: oceanic, transitional, platform, or extended crust; (5) Continental rifting generally thins the upper-middle crust significantly without changing Vp. Lower crust...... experiences less thinning, also without changing Vp, suggesting a complex interplay of magmatic underplating, gabbro-eclogite phase transition and delamination; (6) Crustal structure of the Barents Sea shelf differs from rifted continental crust; and (7) Most of the North Atlantic Ocean north of 55°N has...

An overview of the lithological and geochemical characteristics of the Mesoarchean (ca. 3075) Ivisaartoq greenstone belt, southern West Greenland

DEFF Research Database (Denmark)

Polat, A.; Frei, Robert; Appel, P.W.U.

2008-01-01

Archean greenstone belts in the area. The Ivisaartoq greenstone belt is the largest Mesoarchean supracrustal lithotectonic assemblage in the Nuuk region. The belt contains well-preserved primary magmatic structures including pillow lavas, volcanic breccias, and cumulate (picrite) layers. It also includes...... depleted initial Nd isotopic signatures ( Nd = +4.2 to +5.0) than gabbros, diorites, and tholeiitic basalts ( Nd = +0.3 to +3.1), consistent with a strongly depleted mantle source. In some areas gabbros include up to 15 cm long white inclusions (xenoliths). These inclusions are composed primarily (>90...

Hf isotope evidence for variable slab input and crustal addition in basalts and andesites of the Taupo Volcanic Zone, New Zealand

DEFF Research Database (Denmark)

Waight, Tod Earle; Troll, Valentin R.; Gamble, J.A.

2017-01-01

-Nd isotopes. Here we present new Hf isotope data for a selection of volcanic rocks and crustal lithologies fromthe Taupo Volcanic Zone (TVZ), NewZealand and propose that the scatter in Hf-Nd isotopes indicates heterogeneity in the parental magmas prior to interactions with crustal lithologies. The observed......, whereas younger lavas have probably interacted more with mid- to shallow crustal meta-sedimentary greywacke-argillite lithologies of the Permian to Cretaceous composite Torlesse Terrane. Hf-Nd isotopic compositions of meta-igneous granulite xenoliths from Mt. Ruapehu are consistent with previous...

Origin and Distribution of Water Contents in Continental and Oceanic Lithospheric Mantle

Science.gov (United States)

Peslier, Anne H.

2013-01-01

The water content distribution of the upper mantle will be reviewed as based on the peridotite record. The amount of water in cratonic xenoliths appears controlled by metasomatism while that of the oceanic mantle retains in part the signature of melting events. In both cases, the water distribution is heterogeneous both with depth and laterally, depending on localized water re-enrichments next to melt/fluid channels. The consequence of the water distribution on the rheology of the upper mantle and the location of the lithosphere-asthenosphere boundary will also be discussed.

Shear wave velocity structure of the lower crust in southern Africa: evidence for compositional heterogeneity within Archaean and Proterozoic terrains

CSIR Research Space (South Africa)

Kgaswane, EM

2009-12-01

Full Text Available locations are shown with solid triangles, and event locations are shown with white circles. Most of the events are located outside of the area shown in the map. B12304 KGASWANE ET AL.: CRUSTAL STRUCTURE OF SOUTHERN AFRICA 6 of 19 B12304 velocity profiles... supergroup (white line) taken from van der Westhuizen et al. [2006] and the location of lower crust xenoliths obtained from Pretorius and Barton [2003] and Schmitz and Bowring [2003a]. The number labels 1–15 represent the names of the kimberlites: 1...

Lateral heterogeneity and vertical stratification of cratonic lithospheric keels: examples from Europe, Siberia, and North America

DEFF Research Database (Denmark)

Artemieva, Irina; Cherepanova, Yulia; Herceg, Matija

of the Precambrian lithosphere based on surface heat flow data, (ii) non-thermal part of upper mantle seismic velocity heterogeneity based on a joint analysis of thermal and seismic tomography data, and (iii) lithosphere density heterogeneity as constrained by free-board and satellite gravity data. The latter...... of the Gondwanaland does not presently exceed 250 km depth. An analysis of temperature-corrected seismic velocity structure indicates strong vertical and lateral heterogeneity of the cratonic lithospheric mantle, with a pronounced stratification in many Precambrian terranes; the latter is supported by xenolith data...

Evidence of Arc Magma Genesis in a Paleo-Mantle Wedge, the Higashi-Akaishi Peridotite, Japan

Science.gov (United States)

Till, C. B.; Guild, M. R.; Grove, T. L.; Carlson, R. W.

2014-12-01

Located in the Sanbagawa subduction-related high-pressure metamorphic belt in SW Japan on the island of Shikoku, the Higashi-akaishi peridotite body is composed of dunite, lherzolite and garnet clinopyroxenite, interfingered in one locality with quartz-rich eclogite. Previous work indicates the P-T history of the peridotite includes rapid prograde metamorphism with peak temperatures of 700-810°C and pressures of 2.9-3.8 GPa [1] at ~88-89 Ma followed by rapid exhumation at >2.5 cm/yr [2,3]. Major and trace element and isotopic data from samples within the Higashi-akaishi peridotite presented here and in another recent study [4] provide a record of subduction zone melting processes in a paleo-mantle wedge. Ultramafic samples range from 40-52 wt.% SiO2, 1-11 wt.% Al2O3 and 21-45 wt.% MgO with olivine and clinopyroxene Mg#'s as high as 0.93. The quartz-rich eclogite contains 62 wt.% SiO2, 6 wt.% MgO and 13 wt.% Al2O3 with trace element concentrations that are enriched relative to the ultramafic samples. 87Sr/86Sr (.703237-.704288), 143Nd/144Nd (ɛNd=+2 to +6) and Pb isotopic compositions are within the range of previously studied Japanese arc rocks. We interpret the pyroxenites as shallowly crystallized cumulates with varying amounts of trapped hydrous melt and the harzburgites as residues of melting. The peak P-T conditions of these rocks are similar to the solidus conditions of H2O-saturated fertile mantle near the base of the mantle wedge [5,6]. The presence of garnet porphyroblasts that enclose primary euhedral chlorite together with the chemical evidence, suggest these samples are associated with mantle melting in the presence of H2O. Major element modeling suggests the quartz-rich eclogite composition can be reproduced through mixing melts of subducted sediment with wet peridotite melts in the mantle wedge. Thus the Higashi-aikashi rock suite provides an in-situ record of the beginnings of hydrous melting and the mechanisms of metasomatism in the mantle wedge

Deeply subducted continental fragments - Part 1: Fracturing, dissolution-precipitation, and diffusion processes recorded by garnet textures of the central Sesia Zone (western Italian Alps)

Science.gov (United States)

Giuntoli, Francesco; Lanari, Pierre; Engi, Martin

2018-02-01

Contiguous continental high-pressure terranes in orogens offer insight into deep recycling and transformation processes that occur in subduction zones. These remain poorly understood, and currently debated ideas need testing. The approach we chose is to investigate, in detail, the record in suitable rock samples that preserve textures and robust mineral assemblages that withstood overprinting during exhumation. We document complex garnet zoning in eclogitic mica schists from the Sesia Zone (western Italian Alps). These retain evidence of two orogenic cycles and provide detailed insight into resorption, growth, and diffusion processes induced by fluid pulses in high-pressure conditions. We analysed local textures and garnet compositional patterns, which turned out remarkably complex. By combining these with thermodynamic modelling, we could unravel and quantify repeated fluid-rock interaction processes. Garnet shows low-Ca porphyroclastic cores that were stable under (Permian) granulite facies conditions. The series of rims that surround these cores provide insight into the subsequent evolution: the first garnet rim that surrounds the pre-Alpine granulite facies core in one sample indicates that pre-Alpine amphibolite facies metamorphism followed the granulite facies event. In all samples documented, cores show lobate edges and preserve inner fractures, which are sealed by high-Ca garnet that reflects high-pressure Alpine conditions. These observations suggest that during early stages of subduction, before hydration of the granulites, brittle failure of garnet occurred, indicating high strain rates that may be due to seismic failure. Several Alpine rims show conspicuous textures indicative of interaction with hydrous fluid: (a) resorption-dominated textures produced lobate edges, at the expense of the outer part of the granulite core; (b) peninsulas and atoll garnet are the result of replacement reactions; and (c) spatially limited resorption and enhanced transport

Deeply subducted continental fragments – Part 2: Insight from petrochronology in the central Sesia Zone (western Italian Alps

Directory of Open Access Journals (Sweden)

F. Giuntoli

2018-02-01

Full Text Available Subducted continental terranes commonly comprise an assembly of subunits that reflect the different tectono-metamorphic histories they experienced in the subduction zone. Our challenge is to unravel how, when, and in which part of the subduction zone these subunits were juxtaposed. Petrochronology offers powerful tools to decipher pressure–temperature–time (P–T–t histories of metamorphic rocks that preserve a record of several stages of transformation. A major issue is that the driving forces for re-equilibration at high pressure are not well understood. For example, continental granulite terrains subducted to mantle depths frequently show only partial and localized eclogitization. The Sesia Zone (NW Italy is exceptional because it comprises several continental subunits in which eclogitic rocks predominate and high-pressure (HP assemblages almost completely replaced the Permian granulite protoliths. This field-based study comprises both main complexes of the Sesia terrane, covering some of the recently recognized tectonic subunits involved in its assembly; hence our data constrain the HP tectonics that formed the Sesia Zone. We used a petrochronological approach consisting of petrographic and microstructural analysis linked with thermodynamic modelling and U–Th–Pb age dating to reconstruct the P–T–t trajectories of these tectonic subunits. Our study documents when and under what conditions re-equilibration took place. Results constrain the main stages of mineral growth and deformation, associated with fluid influx that occurred in the subduction channel. In the Internal Complex (IC, pulses of fluid percolated at eclogite facies conditions between 77 and 55 Ma with the HP conditions reaching  ∼  2 GPa and 600–670 °C. By contrast, the External Complex (EC records a lower pressure peak of  ∼  0.8 GPa for 500 °C at  ∼  63 Ma. The juxtaposition of the two complexes occurred during exhumation

A new garnet-orthopyroxene thermometer developed: method, results and applications

Science.gov (United States)

Olivotos, Spyros-Christos; Kostopoulos, Dimitrios

2014-05-01

The Fe-Mg exchange reaction between garnet and orthopyroxene is a robust geothermometer that has extensively been used to retrieve metamorphic temperatures from granulitic and peridotitic/pyroxenitic lithologies with important implications on the thermal state of the continental lithosphere. More than 800 experimental mineral pairs from both simple and complex systems were gleaned from the literature covering the P-T range 0.5-15 GPa / 800-1800°C. Grt was treated as a senary (Py, Alm, Grs, Sps, Kno and Uv), whereas Opx as a septenary (En, Fs, Di, Hd, FeTs, MgTs and MgCrTs) solid solution. For Opx, Al in the M1 site was calculated following Carswell (1991) and Fe/Mg equipartitioning between sites was assumed. A mixing on sites model was employed to calculate mole fractions of components for both minerals. With regard to the excess free energy of solution and activity coefficients the formalism of Mukhopadhyay et al. (1993) was adopted treating both minerals as symmetric regular solutions. Calibration was achieved in multiple steps; in each step ΔS was allowed to vary until the standard deviation of the differences between experimental and calculated temperature for all experiments was minimised. The experiment with the largest absolute relative deviation in temperature was then eliminated and the process was repeated. The new thermometer reproduces the experimental data to within 50°C and is independent of P-T-X variations within the bounds of the calibrant data set. Application of our new calibration to metamorphosed crustal and mantle rocks that occur both as massifs and xenoliths in volcanics suggested the following results. Granulite terranes have recorded differences in temperature between peak and re-equilibration conditions in the range 100-340°C, primarily depending on the mechanism and rate of exhumation. Several provinces retain memory of discrete cooling pulses (e.g. Palni Hills, South Harris, Adirondacks, E. Antarctic Belt, Aldan Shield) whereas

Chlorellestadite, Ca5(SiO4)1.5(SO4)1.5Cl, a new ellestadite- group mineral from the Shadil-Khokh volcano, South Ossetia

Science.gov (United States)

Środek, Dorota; Galuskina, Irina O.; Galuskin, Evgeny; Dulski, Mateusz; Książek, Maria; Kusz, Joachim; Gazeev, Viktor

2018-05-01

Chlorellestadite (IMA2017-013), ideally Ca5(SiO4)1.5(SO4)1.5Cl, the Cl-end member of the ellestadite group was discovered in a calcium-silicate xenolith in rhyodacite lava from the Shadil Khokh volcano, Greater Caucasus, South Ossetia. Chlorellestadite forms white, tinged with blue or green, elongate crystals up to 0.2-0.3 mm in length. Associated minerals include spurrite, larnite, chlormayenite, rondorfite, srebrodolskite, jasmundite and oldhamite. The empirical crystal chemical formula of the holotype specimen is Ca4.99Na0.01(SiO4)1.51(SO4)1.46(PO4)0.03(Cl0.61OH0.21F0.11)Σ0.93. Unit-cell parameters of chlorellestadite are: P63/m, a = 9.6002(2), c = 6.8692(2) Å, V = 548.27(3)Å3, Z = 2. Chlorellestadite has a Mohs hardness of 4-4.5 and a calculated density of 3.091 g/cm3. The cleavage is indistinct, and the mineral shows irregular fracture. The Raman spectrum of chlorellestadite is similar to the spectra of other ellestadite group minerals, with main bands located at 267 cm-1 (Ca-O vibrations), and between 471 and 630 cm-1 (SiO4 4- and SO4 2- bending vibrations) and 850-1150 cm-1 (SiO4 4- and SO4 2- stretching modes). Chlorellestadite forms in xenoliths of calcium-silicate composition when they are exposed to Cl-bearing volcanic exhalations at about 1000 °C under low pressure conditions.

Seismic properties of the upper mantle beneath Lanzarote (Canary Islands): Model predictions based on texture measurements by EBSD

Science.gov (United States)

Vonlanthen, Pierre; Kunze, Karsten; Burlini, Luigi; Grobety, Bernard

2006-12-01

We present a petrophysical analysis of upper mantle xenoliths, collected in the Quaternary alkali basalt fields (Series III and IV) from the island of Lanzarote. The samples consist of eight harzburgite and four dunite nodules, 5 to 15 cm in size, and exhibit a typical protogranular to porphyroclastic texture. An anomalous foliation resulting from strong recovery processes is observed in half of the specimens. The lattice preferred orientations (LPO) of olivine, orthopyroxene and clinopyroxene were measured using electron backscatter diffraction (EBSD). In most samples, olivine exhibits LPOs intermediate between the typical single crystal texture and the [100] fiber texture. Occasionally, the [010] fiber texture was also observed. Simultaneous occurrence of both types of fiber textures suggests the existence of more than one deformation regime, probably dominated by a simple shear component under low strain rate and moderate to high temperature. Orthopyroxene and clinopyroxene display a weaker but significant texture. The LPO data were used to calculate the seismic properties of the xenoliths at PT conditions obtained from geothermobarometry, and were compared to field geophysical data reported from the literature. The velocity of P-waves (7.9 km/s) obtained for a direction corresponding to the existing seismic transect is in good agreement with the most recent geophysical interpretation. Our results are consistent with a roughly W-E oriented fastest P-wave propagation direction in the uppermost mantle beneath the Canary Islands, and with the lithosphere structure proposed by previous authors involving a crust-mantle boundary at around 18 km in depth, overlaid by intermediate material between 11 and 18 km.

Timing and composition of continental volcanism at Harrat Hutaymah, western Saudi Arabia

Science.gov (United States)

Duncan, Robert A.; Kent, Adam J R; Thornber, Carl; Schliedler, Tyler D; Al-Amri, Abdullah M

2016-01-01

Harrat Hutaymah is an alkali basalt volcanic field in north-central Saudi Arabia, at the eastern margin of a large Neogene continental, intraplate magmatic province. Lava flow, tephra and spatter cone compositions in the field include alkali olivine basalts and basanites. These compositions contrast with the predominantly tholeiitic, fissure-fed basalts found along the eastern margin of the Red Sea. The Hutaymah lava flows were erupted through Proterozoic arc-associated plutonic and meta-sedimentary rocks of the Arabian shield, and commonly contain a range of sub-continental lithospheric xenoliths, although the lavas themselves show little indication of crustal contamination. Previous radiometric dating of this volcanic field (a single published K–Ar age; 1.8 Ma) is suspiciously old given the field measurement of normal magnetic polarity only (i.e. Brunhes interval, ≤ 780 Ka). We report new age determinations on 14 lava flows by the 40Ar–39Ar laser step heating method, all younger than ~ 850 Ka, to better constrain the time frame of volcanism, and major, trace and rare earth element compositions to describe the chemical variation of volcanic activity at Harrat Hutaymah. Crystal fractionation was dominated by olivine ± clinopyroxene at a range of upper mantle and crustal pressures. Rapid ascent and eruption of magma is indicated by the array of lower crustal and lithospheric xenoliths observed in lava flows and tephra. Modeling suggests 1–7% melting of an enriched asthenospheric mantle source occurred beneath Harrat Hutaymah under a relatively thick lithospheric cap (60–80 km).

Geologic map of Harrat Hutaymah, with petrologic classification and distribution of ultramafic inclusions, Saudi Arabia

Science.gov (United States)

Thornber, Carl R.

1990-01-01

This map shows detailed geology of the Quaternary and Tertiary volcanic deposits that comprise Harrat Hutaymah and an updated and generalized compilation of the underlying Proterozoic and Paleozoic basement rocks. Quaternary alluvial cover and details of basement geology (that is, faults, dikes, and other features) are not shown. Volcanic unit descriptions and contact relations are based upon field investigation by the author and on compilation and revision of mapping Kellogg (1984; northern half of area) and Pallister (1984; southern half of area). A single K-Ar date of 1.80 ± 0.05 Ma for an alkali olivine basalt flow transected by the Al Hutaymah tuff ring (Pallister, 1984) provides the basis for an estimated late Tertiary to Quaternary age range for all harrat volcanic units other than unit Qtr (tuff reworked during Quaternary age time). Contact relations and unit descriptions for the basement rocks were compiled from Pallister (1984), Kellogg (1984 and 1985), DuBray (1984), Johnson and Williams (1984), Vaslet and others (1987), Cole and Hedge (1986), and Richter and others (1984). All rock unit names in this report are informal and capitalization follows Saudi Arabian stratigraphic nomenclature (Fitch, 1980). Geographic information was compiled from Pallister (1984), Kellogg (1984), and Fuller (in Johnson and Williams, 1984) and from field investigation by the author in 1986. The pie diagrams on the map show the distribution and petrology of ultramafic xenoliths of Harrat Hutaymah. The pie diagrams are explained by a detailed classification of ultramafic xenoliths that is introduced in this report.

Preliminary evidence for a Hercynian age of the Versoyen complex, western Alps; Evidence preliminaire d'un age Hercynian pour le complexe du Versoyen, Alpes occidentales

Energy Technology Data Exchange (ETDEWEB)

Scharer, U. [Paris-7 Univ., Lab. de Geochronologie, UMR 7578, 75 (France); Cannic, S.; Lapierre, H. [Universite Joseph-Fourier, Lab. de Geologie des Chaines Alpines, Upres A5025, Institut Dolomieu, Grenoble I, 38 (France)

2000-03-01

To date the magmatic event that generated the Versoyen mafic complex, four fractions of zircon from a cross-cutting leuco-gabbro dike, has been analyzed by the U-Pb method, defining a regression line that intercepts the concordia curve at 309 {+-} 6 (2 {sigma}) Ma and 3 240 {+-} 34 Ma. These two ages can be interpreted to date, respectively, emplacement of the leuco-gabbro into the Versoyen complex, and the age of inherited Archean zircon cores, present in the newly formed crystals. The age of 309 Ma suggests that both Versoyen mafic magmatism and subsequent eclogite facies metamorphism belong to the Hercynian, and not the Alpine orogenic cycle. (authors)

A new model of crustal structure of Siberia

DEFF Research Database (Denmark)

Cherepanova, Yulia; Artemieva, Irina; Thybo, Hans

2010-01-01

to the Verkoyansk Ridge/Lena river in the east, and from the Arctic shelf in the north to the Tien Shan and Altay-Sayans mountains in the south. The new crustal model is based on our new ("from scratch") compilation of all available reliable seismic data and includes the results of seismic reflection, refraction...... orientation. Low surface heat flow (on average around 20-22 microW/m3) and the absence of the high-velocity (Vp>7.2 km/s) lowercrustal layer in the block with the thick crust suggest that eclogitization in the crustal root was subdued, thus allowing preservation of the ultra thick, seismically distinguishable...

An oxygen isotope study of quartz veins within eclogites from the Dabie terrane

Institute of Scientific and Technical Information of China (English)

LI; Yiliang

2001-01-01

［1］Liu, T. S., Loess and the Environment, Beijing: China Ocean Press, 1985, 1-251.［2］Chen, L. X., Zhu, Q. G., Luo, H. B. et al., East Asian Monsoon, Beijing: China Meteorology Press, 1991, 28-61.［3］An, Z. S., Liu, T. S., Lu, Y. C. et al., The long-term palaeomonsoon variation recorded by the loess-palaeosol sequence in central China, Quaternary International, 1990, (7/8): 91-95.［4］Guo, Z. T., Liu, T. S., Fedoroff, N. et al., Shift of the monsoon intensity on the Loess Plateau at ca. 0.85 MaBP, Chinese Science Bulletin, 1993, 38(2): 586-591.［5］Chen, J., An, Z. S., Wang, Y. J. et al., Distributions of Rb and Sr in the Luochuan loess-paleosol sequence of China during the last 800 ka: Implications for paleomonsoon variations, Science in China, Ser. D, 1999, 42(3): 225-232.［6］Chen, J., Wang, Y. J., Ji, J. F. et al., Rb/Sr variations and its climatic stratigraphical significance of a loess-paleosol profile from Luochuan, Shaanxi Province, Quaternary Sciences (in Chinese), 1999, 19(4): 350-356.［7］Guo, Z. T.,Liu, T. S., Fedoroff, N. et al., Climate extremes in loess of China coupled with the strength of deep-water for-mation in the North Atlantic, Global and Planetary Change, 1998, 18: 113-128.［8］Guo, Z. T., Liu, T. S., An, Z. S., Paleosols of the last 0.15 Ma in the Weinan loess section and their paleoclimate signifi-cance, Quaternary Sciences (in Chinese), 1994, 14(3): 256-269.［9］Guo, Z, T,, Fedoroff, N., Liu, T. S., Micromorphology of the loess-paleosol sequence of the last 130 ka in China and pa-leoclimatic event, Science in China (in Chinese), Ser. D, 1996, 26(3): 392-398.［10］Guo, Z., Liu, T., Guiot, J., et al., High frequency pulses of East Asian monsoon climate in the last two glaciations: Link with the North Atlantic, Climate Dynamics, 1996, 12: 701-709.［11］Guo, Z. T., Peng, S. Z., Wei, L. Y. et al., Weathering signals of Millennial-Scale oscillations of the East Asian Summer monsoon over the last 220 ka, Chinese Science Bulletin, 1999, 44 (supplement 1): 20-25.［12］Ding, Z. L., Yu, Z. W., Rutter, N. W. et al., Towards an orbital time scale for Chinese loess deposits, Quaternary Science Review, 1994,13: 39-70.［13］Duchaufour, Ph., Pedologie, Tome 1: Pedogenese et Classification, Paris-New York-Barcelone-Milan: Masson, 1983, 1-477.［14］Singer, M. J., Bowen, L. H., Verosub, K. L. et al., Mossbauer spectroscopic evidence for citrate-bicarbonate-dithionite ex-traction of maghemite from soils, Clays and Clay Minerals,1995, 43: 1-7.［15］Hunt, C. P., Singer, M. J., Kletetschka, G. et al., Effect of citrate-bicarbonate-dithionite treatment on fine-grained mag-netite and maghemite, Earth Planet. Sci. Lett., 1995, 130: 87-94.［16］Mehra, O., Jackson, M. L., Iron oxide removal from soil and clay by a dithionite-citrate system buffered with sodium bi-carbonate, Clay and Clay Minerals, 1960, 7: 317-327.［17］McKeague, J. A., Manual on soil sampling and methods of analysis, Toronto: Canadian Society of Soil Science, 1981, 1-212.［18］Kukla, G., An, Z. S., Melice, J. L. et al., Magnetic susceptibility record of Chinese loess, Transaction of Royal Society of Edinburgh, Earth Sciences, 1990, 81: 263-288.［19］Guo, Z. T., Wei, L. Y., Lu, H. Y. et al., Changes in the composition of Late Pleistocene aeolian dust and the environmental significance, Quaternary Sciences (in Chinese), 1999, 19(1): 41-48.［20］Heller, F., Liu, X. M., Liu, T. S. et al., Magnetic susceptibility of loess in China, Earth and Planetary Science Letters, 1991,103: 301-310.［21］Yang, J. D., Chen, J., An, Z. S. et al., Variations in 87Sr/86Sr Ratios of calcites in Chinese loess: A proxy for chemical weathering associated with the East Asian Summer monsoon, Palaeogeography Palaeoclimatology Palaeoecology, 2000,157: 151-159.［22］Verosub, K. L., Fine, P., Singer, M. J. et al., Pedogenesis and paleoclimate: Interpretation of the magnetic susceptibility record of Chinese loess-paleosol sequences, Geology, 1993, 21: 1011-1014.［23］Banerjee, S. K., Hunt, C. P., Liu, X., Separation of local signals from the regional paleomonsoon record of the Chinese loess plateau: A rock magnetic approach, Geophys. Res. Lett., 1993, 20: 843-846.［24］Vidic, N. J., TenPas, J. D., Verosub, K. L. et al., Separation of pedogenic and lithogenic components of magnetic suscepti-bility in the Chinese loess/paleosol sequence as determined by the CBD procedure and a mixing analysis, Geophys. J. Int., 2000, 142: 551-562.［25］Deng, C. L., Yuan, B. Y., Zhu, R. X. et al., Magnetic susceptibility of Holocene loess-black loam sequence from Jiaodao, Shanxi, before and after citrate-bicarbonate-dithionite extraction, Chinese Journal of Geophysics (in Chinese), 2000, 43(4): 514-520.［26］Hunt, C. P., Banerjee, S. K., Han, J. M. et al., Rock-magnetic proxies of climate change in the loess-palaeosol sequences of the western Loess Plateau of China, 1995, Geophys. J. Int., 1995, 123: 232-244.［27］Liu, X. M., Liu, T. S., Heller, F. et al., The grain size of magnetic minerals in Chinese loess and its implications for paleo-climate, Science in China (in Chinese), Ser. B, 1991, 21(6): 639-644.［28］Chen, F. H., Bloemendal, J., Feng, Z. D. et al., East Asian monsoon variations during oxygen isotope stage 5: Evidence from the northwestern margin of the Chinese Loess Plateau, Quaternary Science Reviews, 1999, 18: 1127-1135.［29］Pan, B. T., Wang, J. M., Loess record of Qinghai-Xizang Plateau monsoon variations in the eastern part of the plateau since the Last Interglacial, Quaternary Sciences (in Chinese), 1999, 19(4): 330-335.［30］Imbrie, J., Imbrie, J. Z., Modeling the climate response to orbital variation, Science, 1980, 207: 943-953.［31］Liu, T. S., Quaternary Environment (in Chinese), Beijing: Science Press, 1997, 189-239.

EBSD-based investigations of upper mantle xenoliths, snowball garnets and advanced ceramics

OpenAIRE

Vonlanthen, Pierre; Grobéty, Bernard

2007-01-01

L’automatisation au début des années ’90 de la technique de diffraction par électrons rétro-diffusés ou electron backscatter diffraction (EBSD) a complètement révolutionné les analyses de textures menées tant sur les roches que sur les matériaux industriels. Parmi plusieurs avancées majeures, cette méthode permet une détermination relativement rapide et simple de l’orientation cristallographique des grains contenus dans les solides polycristallins. L’intérêt est de taille non seulement dans l...

Orogenic, Ophiolitic, and Abyssal Peridotites

Science.gov (United States)

Bodinier, J.-L.; Godard, M.

2003-12-01

"Tectonically emplaced" mantle rocks include subcontinental, suboceanic, and subarc mantle rocks that were tectonically exhumed from the upper mantle and occur:(i) as dispersed ultramafic bodies, a few meters to kilometers in size, in suture zones and mountain belts (i.e., the "alpine," or "orogenic" peridotite massifs - De Roever (1957), Thayer (1960), Den Tex (1969));(ii) as the lower ultramafic section of large (tens of kilometers) ophiolite or island arc complexes, obducted on continental margins (e.g., the Oman Ophiolite and the Kohistan Arc Complex - Coleman (1971), Boudier and Coleman (1981), Burg et al. (1998));(iii) exhumed above the sea level in ocean basins (e.g., Zabargad Island in the Red Sea, St. Paul's islets in the Atlantic and Macquarie Island in the southwestern Pacific - Tilley (1947), Melson et al. (1967), Varne and Rubenach (1972), Bonatti et al. (1981)).The "abyssal peridotites" are samples from the oceanic mantle that were dredged on the ocean floor, or recovered from drill cores (e.g., Bonatti et al., 1974; Prinz et al., 1976; Hamlyn and Bonatti, 1980).Altogether, tectonically emplaced and abyssal mantle rocks provide insights into upper mantle compositions and processes that are complementary to the information conveyed by mantle xenoliths (See Chapter 2.05). They provide coverage to vast regions of the Earth's upper mantle that are sparsely sampled by mantle xenoliths, particularly in the ocean basins and beneath passive continental margins, back-arc basins, and oceanic island arcs.Compared with mantle xenoliths, a disadvantage of some tectonically emplaced mantle rocks for representing mantle compositions is that their original geodynamic setting is not exactly known and their significance is sometimes a subject of speculation. For instance, the provenance of orogenic lherzolite massifs (subcontinental lithosphere versus upwelling asthenosphere) is still debated (Menzies and Dupuy, 1991, and references herein), as is the original setting

Earthquakes, fluid pressures and rapid subduction zone metamorphism

Science.gov (United States)

Viete, D. R.

2013-12-01

High-pressure/low-temperature (HP/LT) metamorphism is commonly incomplete, meaning that large tracts of rock can remain metastable at blueschist- and eclogite-facies conditions for timescales up to millions of years [1]. When HP/LT metamorphism does take place, it can occur over extremely short durations (the role of fluids in providing heat for metamorphism [2] or catalyzing metamorphic reactions [1]. Earthquakes in subduction zone settings can occur to depths of 100s of km. Metamorphic dehydration and the associated development of elevated pore pressures in HP/LT metamorphic rocks has been identified as a cause of earthquake activity at such great depths [3-4]. The process of fracturing/faulting significantly increases rock permeability, causing channelized fluid flow and dissipation of pore pressures [3-4]. Thus, deep subduction zone earthquakes are thought to reflect an evolution in fluid pressure, involving: (1) an initial increase in pore pressure by heating-related dehydration of subduction zone rocks, and (2) rapid relief of pore pressures by faulting and channelized flow. Models for earthquakes at depth in subduction zones have focussed on the in situ effects of dehydration and then sudden escape of fluids from the rock mass following fracturing [3-4]. On the other hand, existing models for rapid and incomplete metamorphism in subduction zones have focussed only on the effects of heating and/or hydration with the arrival of external fluids [1-2]. Significant changes in pressure over very short timescales should result in rapid mineral growth and/or disequilibrium texture development in response to overstepping of mineral reaction boundaries. The repeated process of dehydration-pore pressure development-earthquake-pore pressure relief could conceivably produce a record of episodic HP/LT metamorphism driven by rapid pressure pulses. A new hypothesis is presented for the origins of HP/LT metamorphism: that HP/LT metamorphism is driven by effective pressure

Chemical and oxygen isotope zonings in garnet from subducted continental crust record mineral replacement and metasomatism

Science.gov (United States)

Vho, Alice; Rubatto, Daniela; Regis, Daniele; Baumgartner, Lukas; Bouvier, Anne-Sophie

2017-04-01

Garnet is a key mineral in metamorphic petrology for constraining pressure, temperature and time paths. Garnet can preserve multiple growth stages due to its wide P-T stability field and the relatively slow diffusivity for major and trace elements at sub-solidus temperatures. Pressure-temperature-time-fluid paths of the host rock may be reconstructed by combining metamorphic petrology with microscale trace element and oxygen isotope measurements in garnet. Subduction zones represent relevant geological settings for geochemical investigation of element exchanges during aqueous fluid-rock interactions. The Sesia Zone consists of a complex continental sequence containing a variety of mono-metamorphic and poly-metamorphic lithologies such as metagranitoids, sediments and mafic boudins. The precursor Varisican-Permian amphibolite-facies basement (6-9 kbar 650-850°C; Lardeaux and Spalla, 1991; Robyr et al., 2013) experienced high pressure metamorphism (15-22 kbar 500-550°C; Regis, et al. 2014; Robyr et al., 2013) during Alpine subduction. In different lithologies of the Internal Complex (Eclogitic Micaschist Complex), including metabasites from the Ivozio Complex, Ti-rich metasediments from Val Malone and pre-Alpine Mn-quartzites associated to metagabbros from Cima Bonze, garnet is abundant and shows a variety of complex textures that cannot be reconciled with typical growth zoning, but indicate resorption and replacement processes and possible metasomatism. In-situ, microscale oxygen isotopes analysis of garnet zones was performed by ion microprobe with the SwissSIMS Cameca IMS 1280-HR at University of Lausanne and SHRIMP-SI at the Australian National University. Each sample has a distinct δ18O composition, and the δ18O values show different degrees of variation between domains. Homogeneously low values of < 5‰ are measured in the garnets from the Ivozio Complex metagabbro. Intragrain variations of up to 3.5‰ in the porphyroblasts from Val Malone metasediments

Linking Serpentinite Geochemistry with Possible Alteration and Evolution of Supra-Subduction Wedge Mantle

Science.gov (United States)

Scambelluri, M.; Cannaò, E.; Agostini, S.; Gilio, M.

2016-12-01

Serpentinites are able to transport and release volatiles and fluid-mobile elements (FME) found in arc magmas. Constraining the trace element compositions of these rocks and of fluids released by de-serpentinization improves our knowledge of mass transfer from subduction zones to volcanic arcs, and of the role of slab and wedge mantle in this global process. Studies of high-pressure ultramafic rocks exhumed from plate interface settings reveal the fluid/rock interactions atop the slab and the processes that can affect the mantle wedge. Alpine eclogite-facies antigorite serpentinite (Voltri Massif) and fully de-serpentinized meta-peridotite (Cima di Gagnone) are enriched in sediment-derived As, Sb, U, Pb before peak dehydration. Their Sr, Pb and B isotopic compositions are reset during prograde (forearc) interaction with slab fluids. The eclogitic garnet and olivine from the Cima di Gagnone metaperidotite trap primary inclusions of the fluid released during breakdown of antigorite and chlorite. The inclusions display FME enrichments (high Cl, S; variable Cs, Rb, Ba, B, Pb, As, Sb) indicating element release from rocks to fluids during dehydration under subarc conditions. Our studies show that serpentinized mantle rocks from subduction zones sequester FME from slab fluids and convey these components and radiogenic isotopes into the mantle wedge upon dehydration. The geochemical processes revealed by such plate-interface rocks can apply to the supra-subduction mantle. Shallow element release from slabs to mantle wedge, downdrag of this altered mantle and its subsequent (subarc) dehydration transfers crust-derived FMEs to the arc magma sources without the need of concomitant subarc dehydration/melting of metasedimentary slab components. The slab signature detected in arc lavas can thus result from geochemical mixing of sediment, oceanic crust and ultramafic reservoirs into altered wedge-mantle rocks, rather than being attributed to multiple fluids.

Multiple tectonic mode switches indicate short-duration heat pulses in a Mio-Pliocene metamorphic core complex, West Papua, Indonesia

Science.gov (United States)

White, L. T.; Hall, R.; Gunawan, I.

2017-12-01

The Wandaman Peninsula is a narrow (2 km) promontory in remote western New Guinea. The peninsula is almost entirely composed of medium- to high-grade metamorphic rocks considered to be associated with a Mio-Pliocene metamorphic core complex. Previous work has shown that the uplift and exhumation of the core complex has potentially brought some extremely young eclogite to the surface. These might be comparable to the world's youngest (4.3 Ma) eclogites found in the D'Entrecasteaux Islands at the opposite end of New Guinea. We show that tectonic history of this region is complex. This is because the metamorphic sequences in the Wandaman Peninsula record multiple phases of deformation, all within the last few million years. This is demonstrated through methodical collation of cross-cutting relations from field and microstructural studies across the peninsula. The first phase of deformation and metamorphism is associated with crustal extension and partial melting that took place at 5-7 Ma according to new U-Pb data from metamorphic zircons. This extensional phase ceased after a tectonic mode switch and the region was shortened. This is demonstrated by two phases of folding (1. recumbent and 2. open) that overprint the earlier extensional fabrics. All previous structures were later overprinted by brittle extensional faults and uplift. This extensional phase is still taking place today, as is indicated by submerged forests exposed along the coastline associated with recent earthquakes and hot springs. The sequence of metamorphic rocks that are exposed in the Wandaman Peninsula show that stress and thermal conditions can change rapidly. If we consider that the present is a key to the past, then such results can identify the duration of deformation and metamorphic events more accurately than in much older orogenic systems.

Comment on ;Evolution of high-pressure mafic granulites and pelitic gneisses from NE Madagascar: Tectonic implications;. Tectonophysics, 662, 219-242 (2015) by Ishwar-Kumar et al.

Science.gov (United States)

Goncalves, Philippe; Brandt, Sönke; Nicollet, Christian; Tucker, Robert

2017-05-01

Determining the possible tectonic regimes active during the Neoproterozoic is crucial for the knowledge of the evolution of the super-continent Gondwana. In Madagascar, that occupies a key position in Gondwana, there is an on-going debate regarding the location of possible suture zones and the implications in terms of paleo-geography. Recognizing high-pressure to ultra-high pressure conditions in mafic rocks is commonly viewed as a strong argument for paleo-subduction zones. Ishwar-Kumar et al. (2015) report unusual high pressure conditions (24 kbar) in Neoproterozoic to Cambrian rocks from North-Central Madagascar (Andriamena Complex). They propose a geodynamic model in which exhumation of the high pressure terranes from up to 80 km to 40 km occurred via vertical extrusion during the collision of various crustal blocks after subduction and closure of an oceanic domain during the formation of Gondwana in the late Neoproterozoic to Cambrian. We question this model and in particular the (ultra-)high pressure conditions reported, because their estimation is based on a misinterpretation of the petrography and inaccurate thermodynamic modeling for the crucial metabasite sample. The authors suggest that garnet-quartz coronas around orthopyroxene and ilmenite coexist with clinopyroxene. The postulated garnet-clinopyroxene-quartz assemblage is interpreted to document an eclogite facies overprint. However, the presence of abundant plagioclase in the sample and the lack of high jadeite content in clinopyroxene clearly refute the postulated eclogite facies conditions. According to the presented photographs clinopyroxene is part of the rock matrix. We therefore suggest that the sample represents a common two-pyroxene granulite, formed at mid- to low-pressure granulite facies conditions of > 700 °C and Madagascar, this interpretation is not justified by the data presented by Ishwar-Kumar et al. (2015).

Early Palaeozoic orogenesis along the Indian margin of Gondwana: Tectonic response to Gondwana assembly

Science.gov (United States)

Cawood, Peter A.; Johnson, Michael R. W.; Nemchin, Alexander A.

2007-03-01

SHRIMP U-Pb dating of zircons from a peralkaline S-type Lesser Himalayan granite from the Kathmandu region, Nepal indicate an age of emplacement of 475 Ma. The granites along with metasedimentary xenoliths show a similar signature of inherited detrital zircon ages ranging from Archaean to early Palaeozoic with prominent late Mesoproterozoic (Grenvillian) and Neoproterozoic (Pan-African) age peaks and have a maximum age of 500 Ma based on the youngest detrital grains. Deformation structures in xenoliths are truncated by the granite and along with the granites are assigned to a Cambro-Ordovician orogenic event, herein termed the Bhimphedian Orogeny that can be traced across the Himalaya from Pakistan to the eastern Himalaya and possibly extends west into Afghanistan. We interpret the orogeny as being related to Andean-type orogenic activity on the northern margin of the Indian continent, following Gondwana assembly. The magmatic arc was associated with andesitic and basaltic volcanism and was active from c. 530 to 490 Ma. The arc activity overlaps with, and is succeeded by, regional deformation, crustal melting and S-type granite emplacement that extended to 470 Ma. Orogenic activity was driven by coupling across the plate margin either during on-going subduction or through accretion of microcontinental ribbons, possibly represented by the Lhasa and Qiangtang blocks. It represents the termination of an orogenic cycle, termed the North Indian Orogen that commenced with Neoproterozoic rifting and passive margin development and terminated with the Bhimphedian Orogeny. This was succeeded by a return to passive margin setting along the northern Indian margin of Gondwana which continued until the Cenozoic Himalayan Orogeny.

U-Pb thermochronology of the lower crust: producing a long-term record of craton thermal evolution

Science.gov (United States)

Blackburn, T.; Bowring, S. A.; Mahan, K. H.; Perron, T.; Schoene, B.; Dudas, F. O.

2010-12-01

The EarthScope initiative is focused on providing an enhanced view of the North American lithosphere and the present day stress field of the North American continent. Of key interest is the interaction between convecting asthenosphere and the conducting lithospheric mantle that underlie the continents, especially the cold ‘keels’ that underlie Archean domains. Cratonic regions are in general characterized by minimal erosion and or sediment accumulation. The Integration of seismic tomography, and mantle xenolith studies reveal a keel of seismically fast and relatively buoyant and viscous mantle; physical properties that are intimately linked with the long-term stability and topographic expression of the region. Missing from this model of the continental lithosphere is the 4th dimension--time--and along with it our understanding of the long-term evolution of these stable continental interiors. Here we present a thermal record from the North American craton using U-Pb thermochronology of lower crustal xenoliths. The use of temperature sensitive dates on lower crustal samples can produce a unique time-temperature record for a well-insulated and slowly cooling lithosphere. The base of the crust is insulated enough to remain unperturbed by any plausible changes to surface topography, yet unlike the subadjacent lithospheric mantle, contains accessory phases amenable to U-Pb dating (rutile, apatite, titanite). With near steady state temperatures in the lower crust between 400-600 °C, U-Pb thermochronometers with similar average closure temperatures for Pb are perfectly suited to record the long-term cooling of the lithosphere. Xenoliths from multiple depths, and across the craton yield time-temperature paths produced from U-Pb thermochronometers that record extremely slow cooling (<0.25 °C/Ma) over time scales of billions of years. Combining these data with numerical thermal modeling allow constraints to be placed on the dominant heat transfer mechanisms operating

Composition of the earth's upper mantle. II - Volatile trace elements in ultramafic xenoliths

Science.gov (United States)

Morgan, J. W.; Wandless, G. A.; Petrie, R. K.; Irving, A. J.

1980-01-01

Radiochemical neutron activation analysis was used to determine the nine volatile elements Ag, Bi, Cd, In, Sb, Se, Te, Tl, and Zn in 19 ultramafic rocks, consisting mainly of spinel and garnet lherzolites. A sheared garnet lherzolite, PHN 1611, may approximate undepleted mantle material and tends to have a higher volatile element content than the depleted mantle material represented by spinel lherzolites. Comparisons of continental basalts with PHN 1611 and of oceanic ridge basalts with spinel lherzolites show similar basalt: source material partition factors for eight of the nine volatile elements, Sb being the exception. The strong depletion of Te and Se in the mantle, relative to lithophile elements of similar volatility, suggests that 97% of the earth's S, Se and Te may be in the outer core.

What can blueschists tell about the Deep? High Pressure in the Anatolide - Taurid Belt

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Oberhaensli, R.

2011-12-01

High-pressure metamorphic terranes in the Anatolide - Taurid belt document the complex distribution of paleo-sutures in the Tethyan realm. Field based petrologic studies of metapelites in the Anatolide-Taurid realm allow to trace HP-LT metamorphism not only in the well known ophiolitic Tavsanli Zone (2.4 GPa/500 °C) but also in the Afyon Zone (0.9 GPa/350 °C), the Menderes Massif (1.2 Gpa/500 °C;) and in the Lycian Nappes (1.0 Gpa/400 °C) - all situated north of the so called Taurid Platform. While the HP metamorphism is dated to 90-80 Ma (Rb/Sr; Ar/Ar) in the Tavsanli Zone, it ranges from 60-70 Ma (Ar/Ar) in the Afyon Zone and its tectonic equivalent, the Lycian Nappes. The Afyon Zone s.l. is closely related to the glaucophane- lawsonite-bearing rocks of the Tavsanli Zone and its eastward extension. Blueschist-facies metamorphism is documented by Fe,Mg-carpholite in regionally distributed metapelites and glaucophane in sparse mafic rocks (Afyon, Menderes, Lycia). Since observations of HP are based on Fe,Mg-carpholite bearing metasediments and not on mafic blueschists new thermodynamic data and petrologic modelling was elaborated to match P-T data and field-based observations. Moreover, newly formed phengitic mica allows precise dating. Both, Tavsanli and Afyon Zones can be followed along strike over more than 600 km and around the southern edge of the Central Anatolian Crystalline Complex. The two zones are situated north of the Taurid Platform and correlate with the Amasia Zone in Armenia. To the extreme East the Bitlis Complex underwent a LT - HP metamorphic blueschist evolution (1,1 GPa/ 350 °C; glaucophane, Fe,Mg-carpholite) in its sedimentary cover while the basement is eclogitic. Depending on the structural position and mineral association of phengitic mica metamorphic ages of the Bitlis blueschists scatter around 70-80 Ma. Eclogites from the basement are slightly older. These LT-HP units cannot be correlated with the Tavsanli - Afyon blueschist belts

A hybrid composite dike suite from the northern Arabian Nubian Shield, southwest Jordan: Implications for magma mixing and partial melting of granite by mafic magma

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Jarrar, Ghaleb H.; Yaseen, Najel; Theye, Thomas

2013-03-01

The Arabian Nubian Shield is an exemplary juvenile continental crust of Neoproterozoic age (1000-542 Ma). The post-collisional rift-related stage (~ 610 to 542 Ma) of its formation is characterized among others by the intrusion of several generations of simple and composite dikes. This study documents a suite of hybrid composite dikes and a natural example of partial melting of granite by a mafic magma from the northernmost extremity of Arabian Nubian Shield in southwest Jordan. The petrogenesis of this suite is discussed on the basis of field, petrographic, geochemical, and Rb/Sr isotopic data. These dikes give spectacular examples of the interaction between basaltic magma and the granitic basement. This interaction ranges from brecciation, partial melting of the host alkali feldspar granite to complete assimilation of the granitic material. Field structures range from intrusive breccia (angular partially melted granitic fragments in a mafic groundmass) to the formation of hybrid composite dikes that are up to 14 m in thickness. The rims of these dikes are trachyandesite (latite) with alkali feldspar ovoids (up to 1 cm in diameter); while the central cores are trachydacite to dacite and again with alkali feldspar ovoids and xenoliths from the dike rims. The granitic xenoliths in the intrusive breccia have been subjected to at least 33% partial melting. A seven-point Rb/Sr isochron from one of these composite dikes yields an age of 561 ± 33 Ma and an initial 87Sr/86Sr ratio of 0.70326 ± 0.0003 (2σ) and MSWD of 0.62. Geochemical modeling using major, trace, rare earth elements and isotopes suggests the generation of the hybrid composite dike suite through the assimilation of 30% to 60% granitic crustal material by a basaltic magma, while the latter was undergoing fractional crystallization at different levels in the continental crust.

Hf-Nd isotope decoupling in the oceanic lithosphere: constraints from spinel peridotites from Oahu, Hawaii

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Bizimis, Michael; Sen, Gautam; Salters, Vincent J. M.

2004-01-01

We present a detailed geochemical investigation on the Hf, Nd and Sr isotope compositions and trace and major element contents of clinopyroxene mineral separates from spinel lherzolite xenoliths from the island of Oahu, Hawaii. These peridotites are believed to represent the depleted oceanic lithosphere beneath Oahu, which is a residue of a MORB-related melting event some 80-100 Ma ago at a mid-ocean ridge. Clinopyroxenes from peridotites from the Salt Lake Crater (SLC) show a large range of Hf isotopic compositions, from ɛHf=12.2 (similar to the Honolulu volcanics series) to extremely radiogenic, ɛHf=65, at nearly constant 143Nd/ 144Nd ratios ( ɛNd=7-8). None of these samples show any isotopic evidence for interaction with Koolau-type melts. A single xenolith from the Pali vent is the only sample with Hf and Nd isotopic compositions that falls within the MORB field. The Hf isotopes correlate positively with the degree of depletion in the clinopyroxene (e.g. increasing Mg#, Cr#, decreasing Ti and heavy REE contents), but also with increasing Zr and Hf depletions relative to the adjacent REE in a compatibility diagram. The Lu/Hf isotope systematics of the SLC clinopyroxenes define apparent ages of 500 Ma or older and these compositions cannot be explained by mixing between any type of Hawaiian melts and the depleted Pacific lithosphere. Metasomatism of an ancient (e.g. 1 Ga or older) depleted peridotite protolith can, in principle, explain these apparent ages and the Nd-Hf isotope decoupling, but requires that the most depleted samples were subject to the least amount of metasomatism. Alternatively, the combined isotope, trace and major element compositions of these clinopyroxenes are best described by metasomatism of the 80-100 Ma depleted oceanic lithosphere by melts products of extensive mantle-melt interaction between Honolulu Volcanics-type melts and the depleted lithosphere.

Orogenic inheritance and continental breakup: Wilson Cycle-control on rift and passive margin evolution

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Schiffer, C.; Petersen, K. D.

2016-12-01

Rifts often develop along suture zones between previously collided continents, as part of the Wilson cycle. The North Atlantic is such an example, formed where Pangaea broke apart along Caledonian and Variscan sutures. Dipping upper mantle structures in E. Greenland and Scotland, have been interpreted as fossil subduction zones and the seismic signature indicates the presence of eclogite and serpentinite. We speculate that this orogenic material may impose a rheological control upon post-orogenic extension and we use thermo-mechanical modelling to explore such effects. Our model includes the following features: 1) Crustal thickness anomalies, 2) Eclogitised mafic crust emplaced in the mantle lithosphere, and 3) Hydrated mantle peridotite (serpentinite) formed in a pre-rift subduction setting. Our models indicate that the inherited structures control the location and the structural and magmatic evolution of the rift. Rifting of thin initial crust allows for relatively large amounts of serpentinite to be preserved within the uppermost mantle. This facilitates rapid continental breakup and serpentinite exhumation. Magmatism does not occur before continental breakup. Rifts in thicker crust preserve little or no serpentinite and thinning is more focused in the mantle lithosphere, rather than in the crust. Continental breakup is therefore preceded by magmatism. This implies that pre-rift orogenic properties may determine whether magma-poor or magma-rich conjugate margins are formed. Our models show that inherited orogenic eclogite and serpentinite are deformed and partially emplaced either as dipping structures within the lithospheric mantle or at the base of the thinned continental crust. The former is consistent with dipping sub-Moho reflectors often observed in passive margins. The latter provides an alternative interpretation of `lower crustal bodies' which are often regarded as igneous bodies. An additional implication of our models is that serpentinite, often

Metamorphism and Shear Localization in the Oceanic and Continental Lithosphere: A Local or Lithospheric-Scale Effect?

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Montesi, L.

2017-12-01

Ductile rheologies are characterized by strain rate hardening, which favors deformation zones that are as wide as possible, thus minimizing strain rate and stress. By contrast, plate tectonics and the observation of ductile shear zones in the exposed middle to lower crust show that deformation is often localized, that is, strain (and likely strain rate) is locally very high. This behavior is most easily explained if the material in the shear zone is intrinsically weaker than the reference material forming the wall rocks. Many origins for that weakness have been proposed. They include higher temperature (shear heating), reduced grain size, and fabric. The latter two were shown to be the most effective in the middle crust and upper mantle (given observational limits restricting heating to 50K or less) but they were not very important in the lower crust. They are not sufficient to explain the generation of narrow plate boundaries in the oceans. We evaluate here the importance of metamorphism, especially related to hydration, in weakening the lithosphere. Serpentine is a major player in the dynamics of the oceanic lithosphere. Although its ductile behavior is poorly constrained, serpentine is likely to behave in a brittle or quasi-plastic manner with a reduced coefficient of friction, replacing stronger peridotite. Serpentinization sufficiently weakens the oceanic lithosphere to explain the generation of diffuse plate boundaries and, combined with grain size reduction, the development of narrow plate boundaries. Lower crust outcrops, especially in the Bergen Arc (Norway), display eclogite shear zones hosted in metastable granulites. The introduction of water triggered locally a metamorphic reaction that reduces rock strength and resulted in a ductile shear zone. The presence of these shear zones has been used to explain the weakness of the lower crust perceived from geodesy and seismic activity. We evaluate here how much strain rate may increase as a result of

Experimental Melting Study of Basalt-Peridotite Hybrid Source: Constrains on Chemistry of Recycled Component

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Gao, S.; Takahashi, E.; Matsukage, K. N.; Suzuki, T.; Kimura, J. I.

2015-12-01

It is believed that magma genesis of OIB is largely influenced by recycled oceanic crust component involved in the mantle plume (e.g., Hauri et al., 1996; Takahashi & Nakajima., 2002; Sobolev et al., 2007). Mallik & Dasgupta (2012) reported that the wall-rock reaction in MORB-eclogite and peridotite layered experiments produced a spectrum of tholeiitic to alkalic melts. However, the proper eclogite source composition is still under dispute. In order to figure out the geochemistry of recycled component as well as their melting process, we conducted a series of high-P, high-T experiments. Melting experiments (1~10hrs) were performed under 2.9GPa with Boyd-England type piston-cylinder (1460~1540°C for dry experiments, 1400~1500°C for hydrous experiments) and 5GPa with Kawai-type multi-anvil (1550~1650°C for dry experiments, 1350~1550°C for hydrous experiments), at the Magma Factory, Tokyo Tech. Spinel lherzolite KLB-1 (Takahashi 1986) was employed as peridotite component. Two basalts were used as recycled component: Fe-enriched Columbia River basalt (CRB72-180, Takahashi et al., 1998) and N-type MORB (NAM-7, Yasuda et al., 1994). In dry experiments below peridotite dry solidus, melt compositions ranged from basaltic andesite to tholeiite. Opx reaction band generated between basalt and peridotite layer hindered chemical reaction. On the other hand, alkali basalt was formed in hydrous run products because H2O promoted melting process in both layers. Compared with melts formed by N-MORB-peridotite runs, those layered experiments with CRB are enriched in FeO, TiO2, K2O and light REE at given MgO. In other words, melts produced by CRB-peridotite layered experiments are close to alkali basalts in OIB and tholeiite in Hawaii, while those by layered experiments with N-MORB are poor in above elements. Thus we propose that Fe-rich Archean or Proterozoic tholeiite (BVSP 1980) would be a possible candidate for recycled component in OIB source.

Forty years of TTG research

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Moyen, Jean-François; Martin, Hervé

2012-09-01

TTGs (tonalite-trondhjemite-granodiorite) are one of the archetypical lithologies of Archaean cratons. Since their original description in the 1970s, they have been the subject of many studies and discussions relating to Archaean geology. In this paper, we review the ideas, concepts and arguments brought forward in these 40 years, and try to address some open questions — both old and new. The late 1960s and the 1970s mark the appearance of "grey gneisses" (TTG) in the scientific literature. During this period, most work was focused on the identification and description of this suite, and the recognition that it is a typical Archaean lithology. TTGs were already recognised as generated by melting of mafic rocks. This was corroborated during the next decade, when detailed geochemical TTG studies allowed us to constrain their petrogenesis (melting of garnet-bearing metamafic rocks), and to conclude that they must have been generated by Archaean geodynamic processes distinct from their modern counterparts. However, the geodynamic debate raged for the following 30 years, as many distinct tectonic scenarios can be imagined, all resulting in the melting of mafic rocks in the garnet stability field. The 1990s were dominated by experimental petrology work. A wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to TTG liquids; whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate. From 1990s onwards, one of the key questions became the comparison with modern adakites. As originally defined these arc lavas are reasonably close equivalents to Archaean TTGs. Pending issues largely revolve around definitions, as the name TTG has now been applied to most Archaean plutonic rocks, whether sodic or potassic, irrespective of their HREE contents. This leads to a large range of petrogenetic and tectonic scenarios; a fair number of which may well have operated concurrently, but are

Phase equilibria constraints on models of subduction zone magmatism

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Myers, James D.; Johnston, Dana A.

Petrologic models of subduction zone magmatism can be grouped into three broad classes: (1) predominantly slab-derived, (2) mainly mantle-derived, and (3) multi-source. Slab-derived models assume high-alumina basalt (HAB) approximates primary magma and is derived by partial fusion of the subducting slab. Such melts must, therefore, be saturated with some combination of eclogite phases, e.g. cpx, garnet, qtz, at the pressures, temperatures and water contents of magma generation. In contrast, mantle-dominated models suggest partial melting of the mantle wedge produces primary high-magnesia basalts (HMB) which fractionate to yield derivative HAB magmas. In this context, HMB melts should be saturated with a combination of peridotite phases, i.e. ol, cpx and opx, and have liquid-lines-of-descent that produce high-alumina basalts. HAB generated in this manner must be saturated with a mafic phase assemblage at the intensive conditions of fractionation. Multi-source models combine slab and mantle components in varying proportions to generate the four main lava types (HMB, HAB, high-magnesia andesites (HMA) and evolved lavas) characteristic of subduction zones. The mechanism of mass transfer from slab to wedge as well as the nature and fate of primary magmas vary considerably among these models. Because of their complexity, these models imply a wide range of phase equilibria. Although the experiments conducted on calc-alkaline lavas are limited, they place the following limitations on arc petrologic models: (1) HAB cannot be derived from HMB by crystal fractionation at the intensive conditions thus far investigated, (2) HAB could be produced by anhydrous partial fusion of eclogite at high pressure, (3) HMB liquids can be produced by peridotite partial fusion 50-60 km above the slab-mantle interface, (4) HMA cannot be primary magmas derived by partial melting of the subducted slab, but could have formed by slab melt-peridotite interaction, and (5) many evolved calc

Serpentinite-driven Exhumation of the UHP Lago di Cignana Unit in the Fossil Alpine Plate Interface

Science.gov (United States)

Scambelluri, M.; Gilio, M.; Angiboust, S.; Godard, M.; Pettke, T.

2015-12-01

The Lago di Cignana Unit (LCU) is a coesite- [1] and diamond-bearing [2] slice of oceanic-derived eclogites and metasediments recording Alpine UHP metamorphism at 600 °C-3.2 GPa (~110 km depth) [3]. The LCU is tectonically sandwiched between the eclogitic Zermatt-Saas Zone (ZSZ; 540 °C-3.2 GPa) [4] and the blueschist Combin Zone (400 °C-0.9 GPa) [5] along a tectonic structure joining HP units recording a ~1.2 GPa (40 km) pressure difference. So far, the ZSZ has been attributed to normal HP conditions and the mechanism driving exhumation and accretion of the LCU in its present structural position is not fully understood.We performed petrography and bulk-rock trace element analyses of rocks from LCU and ZSZ serpentinites. We observed that, while serpentinites in the core of the ZSZ show normal subduction zone trace elements and REE's patterns, the Ol+Ti-chu+Chl veins and host serpentinites enveloping the LCU are strongly enriched in sediment-derived fluid-mobile elements (U, Th, Nb, Ta, Ce, Y, As, Sb) and REE's: their patterns well match those of the closely associated LCU-UHP rocks.The presence of extremely enriched Ol+Ti-chu+Chl veins in the serpentinites at direct contact with the UHP Lago di Cignana Unit suggests that fluid exchange between serpentinite and LCU crustal rocks occurred at peak metamorphic conditions. Their coupling therefore occurred during subduction burial and/or peak UHP conditions. As such, the buoyancy force originating from the relatively light serpentinites fuelled the exhumation of the Lago di Cignana Unit. In this contest, the tectonic contact between the Zermatt-Saas Zone and the Combin Zone evolved into a true tectonic plate interface surface.1. Reinecke (1998). Lithos 42(3), 147-189; 2. Frezzotti et al. (2011). Nat. Geosci. 4(10), 703-706; 3. Groppo et al. (2009). J. Metam. Geol. 27(3), 207-231; 4. Angiboust et al. (2009). Terra Nova 21(3), 171-180; 5. Reddy et al. (1999). J. Metam. Geol. 17, 573-590.

Distribution of lithium in the Cordilleran Mantle wedge

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Shervais, J. W.; Jean, M. M.; Seitz, H. M.

2015-12-01

Enriched fluid-mobile element (i.e., B, Li, Be) concentrations in peridotites from the Coast Range ophiolite are compelling evidence that this ophiolite originated in a subduction environment. A new method presented in Shervais and Jean (2012) for modeling the fluid enrichment process, represents the total addition of material to the mantle wedge source region and can be applied to any refractory mantle peridotite that has been modified by melt extraction and/or metasomatism. Although the end-result is attributed to an added flux of aqueous fluid or fluid-rich melt phase derived from the subducting slab, in the range of tens of parts per million - the nature and composition of this fluid could not be constrained. To address fluid(s) origins, we have analyzed Li isotopes in bulk rock peridotite and eclogite, and garnet separates, to identify possible sources, and fluid flow mechanisms and pathways. Bulk rock Li abundances of CRO peridotites (δ7Li = -14.3 to 5.5‰; 1.9-7.5 ppm) are indicative of Li addition and δ7Li-values are lighter than normal upper mantle values. However, Li abundances of clino- and orthopyroxene appear to record different processes operating during the CRO-mantle evolution. Low Li abundances in orthopyroxene (2 ppm) record subsequent interaction with Li-enriched fluids (or melts). The preferential partitioning of lithium in clinopyroxene could be indicative of a particular metasomatic agent, e.g., fluids from a dehydrating slab. Future in-situ peridotite isotope studies via laser ablation will further elucidate the fractionation of lithium between orthopyroxene, clinopyroxene, and serpentine. To obtain a more complete picture of the slab to arc transfer processes, we also measured eclogites and garnet separates to δ7Li= -18 to 3.5‰ (11.5-32.5 ppm) and δ7Li= 1.9 to 11.7‰ (0.7-3.9 ppm), respectively. In connection with previous studies focused on high-grade metamorphic assemblages within the Franciscan complex, an overall framework exists

Regional metamorphism at extreme conditions: Implications for orogeny at convergent plate margins

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Zheng, Yong-Fei; Chen, Ren-Xu

2017-09-01

Regional metamorphism at extreme conditions refers either to Alpine-type metamorphism at low geothermal gradients of geothermal gradients of >30 °C/km. Extreme pressures refer to those above the polymorphic transition of quartz to coesite, so that ultrahigh-pressure (UHP) eclogite-facies metamorphism occurs at mantle depths of >80 km. Extreme temperatures refer to those higher than 900 °C at crustal depths of ≤80 km, so that ultrahigh-temperature (UHT) granulite-facies metamorphism occurs at medium to high pressures. While crustal subduction at the low geothermal gradients results in blueschist-eclogite facies series without arc volcanism, heating of the thinned orogenic lithosphere brings about the high geothermal gradients for amphibolite-granulite facies series with abundant magmatism. Therefore, UHP metamorphic rocks result from cold lithospheric subduction to the mantle depths, whereas UHT metamorphic rocks are produced by hot underplating of the asthenospheric mantle at the crustal depths. Active continental rifting is developed on the thinned lithosphere in response to asthenospheric upwelling, and this tectonism is suggested as a feasible mechanism for regional granulite-facies metamorphism, with the maximum temperature depending on the extent to which the mantle lithosphere is thinned prior to the rifting. While lithospheric compression is associated with subduction metamorphism in accretionary and collisional orogens, the thinned orogenic lithosphere undergoes extension due to the asthenospheric upwelling to result in orogen-parallel rifting metamorphism and magmatism. Thus, the rifting metamorphism provides a complement to the subduction metamorphism and its operation marks the asthenospheric heating of the orogenic lithosphere. Because of the partial melting and melt extraction of the lower continental crust, contemporaneous granite-migmatite-granulite associations may serve as a petrological indicator of rifting orogeny that is superimposed on

Reconstruction of multiple P-T-t stages from retrogressed mafic rocks: Subduction versus collision in the Southern Brasília orogen (SE Brazil)

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Tedeschi, Mahyra; Lanari, Pierre; Rubatto, Daniela; Pedrosa-Soares, Antônio; Hermann, Jörg; Dussin, Ivo; Pinheiro, Marco Aurélio P.; Bouvier, Anne-Sophie; Baumgartner, Lukas

2017-12-01

The identification of markers of subduction zones in orogenic belts requires the estimation of paleo-geothermal gradients through pressure-temperature-time (P-T-t) estimates in mafic rocks that potentially derive from former oceanic units once. However, such markers are rare in supracrustal sequences specially in deeply eroded and weathered Precambrian orogens, and reconstructing their metamorphic history is challenging because they are commonly retrogressed and only preserve a few mineral relicts of high-pressure metamorphism. Metamorphosed mafic rocks from Pouso Alegre region of the Neoproterozoic Southern Brasília Orogen outcrop as rare lenses within continental gneisses. They have previously been classified as retrograde eclogites, based on the presence of garnet and the characteristic symplectitic texture replacing omphacite. These rocks were interpreted to mark the suture zone between the Paranapanema and São Francisco cratons. To test the possible record of eclogitic conditions in the Pouso Alegre mafic rocks, samples including the surrounding felsic rocks have been investigated using quantitative compositional mapping, forward thermodynamic modeling and in-situ dating of accessory minerals to refine their P-T-t history. In the metamorphosed mafic rocks, the peak pressure assemblage of garnet and omphacite (Jd20, reconstructed composition) formed at 690 ± 35 °C and 13.5 ± 3.0 kbar, whereas local retrogression into symplectite or corona occurred at 595 ± 25 °C and 4.8 ± 1.5 kbar. The two reactions were coupled and thus took place at the same time. A zircon U-Pb age of 603 ± 7 Ma was obtained for metamorphic rims and linked to the retrogression stage. Monazite and metamorphic zircon U-Th-Pb ages for the surrounding rocks are at ca. 630 Ma and linked to peak pressure conditions similar to the one recorded by the mafic rocks. The low maximal pressure of 14 kbar and the high geothermal gradient do not necessarily support subduction process

The harzburgites-lherzolite cycle: depletion and refertilization processes

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Dijkstra, A. H.

2011-12-01

Lherzolites or clinopyroxene-rich harzburgites sampled at the ocean floor are now generally interpreted as refractory harzburgites refertilized by melt-rock reaction or melt impregnation at the spreading center, rather than as relatively undepleted bulk upper mantle. The key evidence for a melt refertilization origin is often textural. Critically, the refertilization can mask the underlying very refractory character: oceanic peridotites prior to melt refertilization at the ridge are often too refractory to be simple mantle residues of bulk upper mantle that was melted at the ridge. This suggests that the upper mantle contains large domains that record prior melting histories. This is supported by ancient rhenium-depletion ages that are common in oceanic peridotites. In this presentation, I will discuss some key examples (e.g., Macquarie Island [1], Pindos, Totalp, Lanzarote) of refertilized oceanic peridotites, which all have recorded previous, ancient depletions. I will show the textural and geochemical evidence for melt refertilization. It has often been assumed that melt refertilization occurs by interaction with mantle melts. However, there is now evidence for melt refertilization through a reaction with eclogite-derived melts, probably at the base of the melting column underneath the ridge system. These eclogitic mantle heterogeneities themselves do not normally survive the melting underneath the spreading center, but their isotopic signature can be recognized in the reacted peridotites. In summary, we have moved away from the idea that oceanic mantle rocks are simple melting residues of homogeneous bulk upper mantle. The picture that emerges is a rich and complex one, suggesting that oceanic mantle rocks record dynamic histories of melting and refertilization. In particular, the melting event in refertilized peridotites can be much older than the age of the ridge system at which they are sampled. Many oceanic peridotites contain evidence for a Mesoproterozoic

On the nature and origin of garnet in highly-refractory Archean lithosphere: implications for continent stabilisation

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Gibson, Sally

2014-05-01

The nature and timescales of garnet formation in the Earth's subcontinental lithospheric mantle (SCLM) are important to our understanding of how this rigid outer shell has evolved and stabilised since the Archean. Nevertheless, the widespread occurrence of pyrope garnet in the sub-cratonic mantle remains one of the 'holy grails' of mantle petrology. The paradox is that garnet often occurs in mantle lithologies (dunites and harzburgites) which represent residues of major melting events (up to 40 %) whereas experimental studies on fertile peridotite suggest this phase should be exhausted by years. The garnets display systematic trends from ultra-depleted to enriched compositions that have not been recognised in peridotite suites from elsewhere (Gibson et al., 2013). Certain harzburgite members of the xenolith suite contain the first reported occurrence of pyrope garnets with rare-earth element (REE) patterns similar to hypothetical garnets proposed by Stachel et al. (2004) to have formed in the Earth's SCLM during the Archean, prior to metasomatism. These rare ultra-depleted low-Cr garnets occur in low temperature (~1050 oC) xenoliths derived from depths of ~120 km and coexist in chemical and textural equilibrium with highly-refractory olivine (Fo95.4) and orthopyroxene (Mg#=96.4). These phases are all more magnesian than generally encountered in global samples of depleted mantle, i.e. harzburgites and diamond inclusion suites. The Tanzanian ultra-depleted garnets form interconnecting networks ('necklaces') around grains of orthopyroxene, which is of key importance to their origin. This close spatial relationship of garnet and orthopyroxene together with the major, trace and REE contents of the ultra-depleted garnets, are consistent with an origin by isochemical exsolution. The significance of ultra-depleted low-Cr garnets has not previously been recognised in global suites of mantle xenoliths or diamond inclusions: they appear to have been overlooked, primarily

Source assemblage types for cratonic diamonds from X-ray synchrotron diffraction

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Nestola, F.; Alvaro, M.; Casati, M. N.; Wilhelm, H.; Kleppe, A. K.; Jephcoat, A. P.; Domeneghetti, M. C.; Harris, J. W.

2016-11-01

Three single crystals of clinopyroxene trapped within three different gem-quality diamonds from the Udachnaya kimberlite (Siberia, Russia) were analysed in situ by single-crystal synchrotron X-ray diffraction in order to obtain information on their chemical composition and infer source assemblage type. A non-destructive approach was used with high-energy (≈ 60 keV; λ ≈ 0.206 Å) at I15, the extreme-conditions beamline at Diamond Light Source. A dedicated protocol was used to center the mineral inclusions located deep inside the diamonds in the X-ray beam. Our results reveal that two of the inclusions can be associated with peridotitic paragenesis whereas the third is eclogitic. This study also demonstrates that this non-destructive experimental approach is extremely efficient in evaluating the origin of minerals trapped in their diamond hosts.

Hyperextension in the Caledonian margin of Baltica and its bearing on the structural and metamorphic evolution of Scandinavian Caledonides (Invited)

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Andersen, T. B.

2013-12-01

transitional crust basin with mantle exhumed by hyperextension during the Caledonian Wilson cycle ';kick-off'. Islands formed by serpentinite and clastic serpentinites suggest that hydrated mantle diapirs rose above sea level in the Early Ordovician. A number of solitary peridotites and detrital serpentinites are also typical elements in Seve nappe complex in north-central Sweden and Norway. The Pre-Scandian events affecting the passive Baltican margin show a range of ages and characteristics, but most important are the eclogites of Ordovician age. The oldest (~482 Ma) occur in the northern part of the Seve (Nordbotn). UHP eclogites in Jämtland formed at 446 Ma, and both these occurrences in the Seve are associated with mantle peridotites. In SW Norway, 470-460 Ma eclogites are preserved in continental nappes immediately below the suture near Stavanger. Finally, a lower grade HP-LT Ordovician event (~450 Ma) also dated by unconformable Middle Silurian (Wenlock) sediments has been identified. These pre-Scandian events demonstrate that the margin of Baltica underwent a sequence of geographic and time-separated events in the Ordovician before the Iapetus closed in the Middle Silurian. In this presentation it is suggested that the extension and hyperextension geometry inherited from the Caledonian Wilson-cycle ';kick-off' controlled the sequence of short-lived and local HP-LT events in the Scandinavian Caledonides.

The Geology of the Kivetty area

International Nuclear Information System (INIS)

Anttila, P.; Paulamaeki, S.; Lindberg, A.; Paananen, M.; Koistinen, T.; Front, K.; Pitkaenen, P.

1992-05-01

Teollisuuden Voima Oy (TVO) is preparing for the final disposal of spent nuclear fuel from the Olkiluoto nuclear power plant (TVO-I and TVO-II) deep in the Finnish bedrock. Kivetty in Konginkangas was one of the five areas selected in 1987 for preliminary site investigations for this purpose. The Kivetty area in Central Finland is located in a Svecokarelian granitoid environment consisting of a complex of synorogenic granitoids 1900 - 1860 million years in age. The bedrock consists almost entirely of plutonic rocks, i.e. gabbro, porphyritic granodiorite and granite, eguigranular granodiorite and granite, listed in order of age. The majority of the rock types are porphyritic in character, and supracrustal rocks such as quartz-feldspar schist and gneiss are found occasionally in small xenoliths

Experimental petrology and origin of rocks from the Descartes Highlands

Science.gov (United States)

Walker, D.; Longhi, J.; Grove, T. L.; Stolper, E.; Hays, J. F.

1973-01-01

Petrographic studies of Apollo 16 samples indicate that rocks 62295 and 68415 are crystallization products of highly aluminous melts. 60025 is a shocked, crushed and partially annealed plagioclase cumulate. 60315 is a recrystallized noritic breccia of disputed origin. 60335 is a feldspathic basalt filled with xenoliths and xenocrysts of anorthosite, breccia, and anorthite. The Fe/(Fe+Mg) of plagioclase appears to be a relative crystallization index. Low pressure melting experiments with controlled Po2 indicate that the igneous samples crystallized at oxygen fugacities well below the Fe/FeO buffer. Crystallization experiments at various pressures suggest that the 62295 and 68415 compositions were produced by partial or complete melting of lunar crustal materials, and not by partial melting of the deep lunar interior.

An application of GOCE satellite gravity to resolve mantle heterogeneity in Europe

DEFF Research Database (Denmark)

Herceg, Matija; Artemieva, Irina; Thybo, Hans

2015-01-01

The aim of this study is to obtain new information on the density structure of the European upper mantle by incorporating the state-of-the-art global gravity data derived from the GOCE satellite gravity mission and recently released seismic model for the crustal structure, EUNAseis. The residual ...... by seismic tomography. Furthermore, we compare our regional upper mantle density model with petrological studies of mantle-derived xenoliths from the Baltic shield and the Arkhangelsk region.......The aim of this study is to obtain new information on the density structure of the European upper mantle by incorporating the state-of-the-art global gravity data derived from the GOCE satellite gravity mission and recently released seismic model for the crustal structure, EUNAseis. The residual...

Magnesium isotopic composition of the mantle

Science.gov (United States)

Teng, F.; Li, W.; Ke, S.; Marty, B.; Huang, S.; Dauphas, N.; Wu, F.; Helz, R. L.

2009-12-01

Studies of Mg isotopic composition of the Earth not only are important for understanding its geochemistry but also can shed light on the accretion history of the Earth as well as the evolution of the Earth-Moon system. However, to date, the Mg isotopic composition of the Earth is still poorly constrained and highly debated. There is uncertainty in the magnitude of Mg isotope fractionation at mantle temperatures and whether the Earth has a chondritic Mg isotopic composition or not. To constrain further the Mg isotopic composition of the mantle and investigate the behavior of Mg isotopes during igneous differentiation, we report >200 high-precision (δ26Mg French Polynesian volcanoes (Society island and Cook Austral chain); 3) olivine grains from Hawaiian volcanoes (Kilauea, Koolau and Loihi) and 4) peridotite xenoliths from Australia, China, France, Tanzania and USA. Global oceanic basalts and peridotite xenoliths have a limited (<0.2 ‰) variation in Mg isotopic composition, with an average δ26Mg = -0.25 relative to DSM3. Olivines from Hawaiian lavas have δ26Mg ranging from -0.43 to +0.03, with most having compositions identical to basalts and peridotites. Therefore, the mantle’s δ26Mg value is estimated to be ~ -0.25 ± 0.1 (2SD), different from that reported by Wiechert and Halliday (2007; δ26Mg = ~ 0) but similar to more recent studies (δ26Mg = -0.27 to -0.33) (Teng et al. 2007; Handler et al. 2009; Yang et al., 2009). Moreover, we suggest the Earth, as represented by the mantle, has a Mg isotopic composition similar to chondrites (δ26Mg = ~-0.33). The need for a model such as that of Wiechert and Halliday (2007) that involves sorting of chondrules and calcium-aluminum-rich inclusions in the proto planetary disc is thus not required to explain the Mg isotopic composition of the Earth.

Invasion of Hydrous Fluids Predates Kimberlite Formation

Science.gov (United States)

Kopylova, M. G.; Wang, Q.; Smith, E. M.

2017-12-01

Petrological observations on diamonds and peridotite xenoliths in kimberlites point towards an influx of hydrous metasomatic fluids shortly predating kimberlite formation. Diamonds may grow at different times within the same segment of the cratonic mantle, and diamonds that form shortly before (diamonds typically contain 10-25 wt.% water in fluid inclusions, while older octahedrally-grown diamonds host "dry" N2-CO2 fluids. Our recent studies of fluids in diamond now show that many different kinds of diamonds can contain fluid inclusions. Specifically, we found a new way to observe and analyze fluids in octahedrally-grown, non-fibrous diamonds by examining healed fractures. This is a new textural context for fluid inclusions that reveals a valuable physical record of infiltrating mantle fluids, that postdate diamond growth, but equilibrate within the diamond stability field at depths beyond 150 km. Another sign of the aqueous fluids influx is the formation of distinct peridotite textures shortly predating the kimberlite. Kimberlites entrain peridotite xenoliths with several types of textures: older coarse metamorphic textures and younger, sheared textures. The preserved contrast in grain sizes between porphyroclasts and neoblasts in sheared peridotites constrain the maximum duration of annealing. Experimental estimates of the annealing time vary from 7x107 sec (2 years) to 106 years (1 My) depending on olivine hydration, strain rate, pressure, temperature and, ultimately, the annealing mechanism. Kimberlite sampling of sheared peridotites from the lithosphere- asthenosphere boundary (LAB) implies their formation no earlier than 1 My prior to the kimberlite ascent. Water contents of olivine measured by FTIR spectrometry using polarized light demonstrated contrasting hydration of coarse and sheared samples. Olivine from sheared peridotite samples has the average water content of 78±3 ppm, in contrast to the less hydrated coarse peridotites (33±6 ppm). LAB hydration

Lamprophyres from the Harohalli dyke swarm in the Halaguru and Mysore areas, Southern India: Implications for backarc basin magmatism

Science.gov (United States)

Lanjewar, Shubhangi; Randive, Kirtikumar

2018-05-01

The Bangalore and Harohalli dyke swarms occur in the eastern part of the Dharwar craton. The older Bangalore dyke swarm is made up of dolerites, trending east-west, and the younger contains alkaline dykes that trend approximately north-south. The lamprophyres of the Harohalli dyke swarm occur in the Halaguru and Mysore industrial areas where they are exposed as fresh porphyritic - panidiomorphic dykes, containing crustal xenoliths, and showing chilled contacts with the country rock charnokites. They are chiefly composed of amphiboles which form well-developed phenocrysts. Clinopyroxenes are present in some of the dykes. Compositional zoning is observed in clinopyroxenes and amphiboles; their zoning patterns indicate that the magma experienced cryptic variations and that fractional crystallization was a dominant process in the evolution of the Harohalli Lamprophyres (HRL). The HRL are calc-alkaline with shoshonitic affinity and exhibit a K2O/Na2O ratio of ∼1. They show primitive (MORB-like) trace-element characters. LILE and LREE both show marginally enriched patterns; whereas HFSE and HREE show strongly depleted patterns. In the regional geologic sense, HRL dykes are characterised by two major influences; namely, (i) primary source region characteristics, which are geochemically more primitive, roughly falling within fields of primitive - MORB and enriched- MORB and (ii) the continental lithosphere. The data points for the HRL distinctly show their proximity to N-MORB and scatter towards the continental crust. Moreover, features like xenolith assimilation might influence the trace-element characteristics of the HRL dykes. Such magmas with mixed characters can be formed in a backarc basin environment. Geochemical proxies such as Ba/Nb vs Nb/Yb, Ba/Th vs Th/Nb, and the water content of magmas; which have been effectively used for discriminating backarc basin magmas worldwide, also indicate that the HRL magmas were generated in a backarc environment with inputs from

Cratonic roots and lower crustal seismicity: Investigating the role of deep intrusion in the Western rift, Africa

Science.gov (United States)

Drooff, C.; Ebinger, C. J.; Lavayssiere, A.; Keir, D.; Oliva, S. J.; Tepp, G.; Gallacher, R. J.

2017-12-01

Improved seismic imaging beneath the African continent reveals lateral variations in lithospheric thickness, and crustal structure, complementing a growing crust and mantle xenolith data base. Border fault systems in the active cratonic rifts of East Africa are characterized by lower crustal seismicity, both in magmatic sectors and weakly magmatic sectors, providing constraints on crustal rheology and, in some areas, magmatic fluid migration. We report new seismicity data from magmatic and weakly magmatic sectors of the East African rift zone, and place the work in the context of independent geophysical and geochemical studies to models for strain localization during early rifting stages. Specifically, multidisciplinary studies in the Magadi Natron rift sectors reveal volumetrically large magmatic CO2 degassing along border faults with seismicity along projections of surface dips to the lower crust. The magmatic CO2 degassing and high Vp/Vs ratios and reflectivity of the lower crust implies that the border fault serves a conduit between the lower crustal underplating and the atmospheric. Crustal xenoliths in the Eastern rift sector indicate a granulitic lower crust, which is relatively weak in the presence of fluids, arguing against a strong lower crust. Within magmatic sectors, seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Within some weakly magmatic sectors, lower crustal earthquakes also occur along projections of border faults to the lower crust (>30 km), and they are prevalent in areas with high Vp/Vs in the lower crust. Within the southern Tanganyika rift, focal mechanisms are predominantly normal with steep nodal planes. Our comparative studies suggest that pervasive metasomatism above a mantle plume, and melt extraction in thin zones between cratonic roots, lead to

Isotopic constraints on contamination processes in the Tonian Goiás Stratiform Complex

Science.gov (United States)

Giovanardi, Tommaso; Mazzucchelli, Maurizio; Lugli, Federico; Girardi, Vicente A. V.; Correia, Ciro T.; Tassinari, Colombo C. G.; Cipriani, Anna

2018-06-01

The Tonian Goiás Stratiform Complex (TGSC, Goiás, central Brazil), is one of the largest mafic-ultramafic layered complexes in the world, emplaced during the geotectonic events that led to the Gondwana accretion. In this study, we present trace elements and in-situ U/Pb-Lu-Hf analyses of zircons and 87Sr/86Sr ratios of plagioclases from anorthosites and gabbros of the TGSC. Although formed by three isolated bodies (Cana Brava, Niquelândia and Barro Alto), and characterized by a Lower and Upper Sequence (LS and US), our new U/Pb zircon data confirm recent geochemical, geochronological, and structural evidences that the TGSC has originated from a single intrusive body in the Neoproterozoic. New Hf and Sr isotope ratios construe a complex contamination history for the TGSC, with different geochemical signatures in the two sequences. The low Hf and high Sr isotope ratios of the Lower Sequence (εHf(t) from -4.2 down to -27.5; 87Sr/86Sr = 0.706605-0.729226), suggest the presence of a crustal component and are consistent with contamination from meta-pelitic and calc-silicate rocks found as xenoliths within the Sequence. The more radiogenic Hf isotope ratios and low Sr isotope composition of the Upper Sequence (εHf(t) from 11.3 down to -8.4; 87Sr/86Sr = 0.702368-0.702452), suggest a contamination from mantle-derived metabasalts in agreement with the occurrences of amphibolite xenoliths in the US stratigraphy. The differential contamination of the two sequences is explained by the intrusion of the TGSC in a stratified crust dominated by metasedimentary rocks in its deeper part and metavolcanics at shallower levels. Moreover, the differential thermal gradient in the two crystallizing sequences might have contributed to the preservation and recrystallization of inherited zircon grains in the US and total dissolution or magmatic overgrowth of the LS zircons via melt/rock reaction processes.

Isotopic composition of reduced and oxidized sulfur in the Canary Islands: implications for the mantle S cycle

Science.gov (United States)

Beaudry, P.; Longpre, M. A.; Wing, B. A.; Bui, T. H.; Stix, J.

2017-12-01

The Earth's mantle contains distinct sulfur reservoirs, which can be probed by sulfur isotope analyses of volcanic rocks and gases. We analyzed the isotopic composition of reduced and oxidized sulfur in a diverse range of volcanically derived materials spanning historical volcanism in the Canary Islands. Our sample set consists of subaerial volcanic tephras from three different islands, mantle and sedimentary xenoliths, as well as lava balloon samples from the 2011-2012 submarine El Hierro eruption and associated crystal separates. This large sample set allows us to differentiate between the various processes responsible for sulfur isotope heterogeneity in the Canary archipelago. Our results define an array in triple S isotope space between the compositions of the MORB and seawater sulfate reservoirs. Specifically, the sulfide values are remarkably homogeneous around d34S = -1 ‰ and D33S = -0.01 ‰, while sulfate values peak at d34S = +4 ‰ and D33S = +0.01 ‰. Lava balloons from the El Hierro eruption have highly enriched sulfate d34S values up to +19.3 ‰, reflecting direct interaction between seawater sulfate and the erupting magma. Several sulfate data points from the island of Lanzarote also trend towards more positive d34S up to +13.8 ‰, suggesting interaction with seawater sulfate-enriched lithologies or infiltration of seawater within the magmatic system. On the other hand, the modal values and relative abundances of S2- and S6+ in crystal separates suggest that the Canary Island mantle source has a d34S around +3 ‰, similar to the S-isotopic composition of a peridotite xenolith from Lanzarote. We infer that the S2- and S6+ modes reflect isotopic equilibrium between those species in the magmatic source, which requires 80 % of the sulfide to become oxidized after melting, consistent with measured S speciation. This 34S enrichment of the source could be due to the recycling of hydrothermally-altered oceanic crust, which has been previously suggested

Triassic rejuvenation of unexposed Archean-Paleoproterozoic deep crust beneath the western Cathaysia block, South China

Science.gov (United States)

Li, Xi-Yao; Zheng, Jian-Ping; Xiong, Qing; Zhou, Xiang; Xiang, Lu

2018-01-01

Jurassic (ca. 150 Ma) Daoxian basalts from the western Cathaysia block (South China) entrained a suite of deep-seated crustal xenoliths, including felsic schist, gneiss and granulite, and mafic two-pyroxene granulite and metagabbro. Zircon U-Pb-Hf isotopic, whole-rock elemental and Sr-Nd-Pb isotopic compositions have been determined for these valuable xenoliths to reveal the poorly-known, unexposed deep crust beneath South China. Detrital zircons from the garnet-biotite schists show several populations of ages at 0.65-0.5 Ga, 1.1-0.75 Ga, 1.6-1.4 Ga, 1.8-1.7 Ga, 2.5-2.4 Ga, 2.8 Ga, and 3.5 Ga, representing a multi-sourced, meta-sedimentary origin with deposition time at the early Cambrian. One mafic granulite contains zircons with concordant U-Pb ages of Neoarchean ( 2520 Ma), as well as Hf model ages of 2.8-2.6 Ga and positive εHf(t) values (up to 6.3), suggesting an accretion of juvenile crust in Neoarchean, probably as the main framework of the lower crust. Geochemical and geochronological evidence shows the mafic granulite and metagabbro were produced by underplating of magmas derived from the depleted asthenosphere and mixed with EM2-type materials during the Late Triassic (205-196 Ma). This magmatic underplating also resulted in the widespread metamorphism of the mafic lower crust and felsic middle crust (e.g., the felsic granulite and gneiss) at 202-201 Ma. We suggest the existence of a highly evolved Archean-Paleoproterozoic basement beneath the western Cathaysia block, which experienced episodic accretion and reworking and the strong rejuvenation during the Triassic. A three-layered structure of the lower crust could exist beneath the Daoxian area during the Jurassic time: its upper layer is an evolved Archean-Paleoproterozoic basement; the middle hybrid layer represents a mixture of Archean-Paleoproterozoic basement with newly accreted/reworked Proterozoic to Phanerozoic materials; and the deeper layer consists of mafic granulites derived from the

Trace element partitioning between aqueous fluids and silicate melts measured with a proton microprobe

Energy Technology Data Exchange (ETDEWEB)

Adam, J; Green, T H [Macquarie Univ., North Ryde, NSW (Australia). School of Earth Sciences; Sie, S H [Commonwealth Scientific and Industrial Research Organisation (CSIRO), North Ryde, NSW (Australia). Div. of Exploration Geoscience

1997-12-31

A series of experiments were performed to examine the capacity of H{sub 2}O-fluids to concentrate and transport incompatible elements through peridotitic mantle and metamorphosed (eclogitic) ocean crust. Two naturally occurring rock compositions, trondhjemitic and basanitic, were used in experiments. The proton microprobe was used to determine the trace element concentrations in the solutes from H{sub 2}O-fluids equilibrated at 900-1100 degree C, 2.0 GPa with water saturated melts of trondhjemitic and basanitic compositions. Partitioning data for H{sub 2}O-fluids and silicate melts show that H{sub 2}O-fluids equilibrated with mantle peridotites will not be strongly enriched in trace elements relative to their wallrocks, and thus they melts do not strongly concentrate alkaline earths Th and U, relative to high-field strength elements. 3 refs., 1 tab., 2 figs.

Trace element partitioning between aqueous fluids and silicate melts measured with a proton microprobe

Energy Technology Data Exchange (ETDEWEB)

Adam, J.; Green, T.H. [Macquarie Univ., North Ryde, NSW (Australia). School of Earth Sciences; Sie, S.H. [Commonwealth Scientific and Industrial Research Organisation (CSIRO), North Ryde, NSW (Australia). Div. of Exploration Geoscience

1996-12-31

A series of experiments were performed to examine the capacity of H{sub 2}O-fluids to concentrate and transport incompatible elements through peridotitic mantle and metamorphosed (eclogitic) ocean crust. Two naturally occurring rock compositions, trondhjemitic and basanitic, were used in experiments. The proton microprobe was used to determine the trace element concentrations in the solutes from H{sub 2}O-fluids equilibrated at 900-1100 degree C, 2.0 GPa with water saturated melts of trondhjemitic and basanitic compositions. Partitioning data for H{sub 2}O-fluids and silicate melts show that H{sub 2}O-fluids equilibrated with mantle peridotites will not be strongly enriched in trace elements relative to their wallrocks, and thus they melts do not strongly concentrate alkaline earths Th and U, relative to high-field strength elements. 3 refs., 1 tab., 2 figs.

The anatomy of an andesite volcano: A time-stratigraphic study of andesite petrogenesis and crustal evolution at Ruapehu volcano, New Zealand

DEFF Research Database (Denmark)

Price, R.C.; Gamble, J.A.; Smith, I.E.M.

2012-01-01

Ruapehu, New Zealand’s largest active andesite volcano is located at the southern tip of the Taupo Volcanic Zone (TVZ), the main locus of subduction-related volcanism in the North Island. Geophysical data indicate that crustal thickness transitions from ... Ruapehu. The volcano is built on a basement of Mesozoic metagreywacke and geophysical evidence together with xenoliths contained in lavas indicates that this is underlain by oceanic, meta-igneous lower crust. The present-day Ruapehu edifice has been constructed by a series of eruptive events that produced...... and andesite. Dacite also occurs but only one basalt flow has been identified. There have been progressive changes in the minor and trace element chemistry and isotopic composition of Ruapehu eruptives over time. In comparison with rocks from younger formations, Te Herenga eruptives have lower K2O abundances...

The continental lithosphere

DEFF Research Database (Denmark)

Artemieva, Irina

2009-01-01

The goal of the present study is to extract non-thermal signal from seismic tomography models in order to distinguish compositional variations in the continental lithosphere and to examine if geochemical and petrologic constraints on global-scale compositional variations in the mantle...... are consistent with modern geophysical data. In the lithospheric mantle of the continents, seismic velocity variations of a non-thermal origin (calculated from global Vs seismic tomography data [Grand S.P., 2002. Mantle shear-wave tomography and the fate of subducted slabs. Philosophical Transactions...... and evolution of Precambrian lithosphere: A global study. Journal of Geophysical Research 106, 16387–16414.] show strong correlation with tectono-thermal ages and with regional variations in lithospheric thickness constrained by surface heat flow data and seismic velocities. In agreement with xenolith data...

Radar-Enabled Recovery of the Sutters Mill Meteorite, a Carbonaceous Chondrite Regolith Breccia

Science.gov (United States)

Jenniskens, Petrus M.; Fries, Marc D.; Yin, Qing-Zhu; Zolensky, Michael E.; Krot, Alexander N.; Sandford, Scott A.; Sears, Derek; Beauford, Robert; Ebel, Denton S.; Friedrich, Jon M.;

Basaltic Diatreme To Root Zone Volcanic Processes In Tuzo Kimberlite Pipe (Gahcho Kué Kimberlite Field, NWT, Canada)

Science.gov (United States)

Seghedi, I.; Kurszlaukis, S.; Maicher, D.

2009-05-01

Tuzo pipe is infilled by a series of coherent and fragmental kimberlite facies types typical for a diatreme to root zone transition level. Coherent or transitional coherent kimberlite facies dominate at depth, but also occur at shallow levels, either as dikes or as individual or agglutinated coherent kimberlite clasts (CKC). Several fragmental kimberlite varieties fill the central and shallow portions of the pipe. The definition, geometry and extent of the geological units are complex and are controlled by vertical elements. Specific for Tuzo is: (1) high abundance of locally derived xenoliths (granitoids and minor diabase) between and within the kimberlite phases, varying in size from sub-millimeter to several tens of meters, frequent in a belt-like domain between 120-200 m depth in the pipe; (2) the general presence of CKC, represented by round-subround, irregular to amoeboid-shaped clasts with a macrocrystic or aphanitic texture, mainly derived from fragmentation of erupting magma and less commonly from previously solidified kimberlite, as well as recycled pyroclasts. In addition, some CKC are interpreted to be intersections of a complex dike network. This diversity attests formation by various volcanic processes, extending from intrusive to explosive; (3) the presence of bedded polymict wall- rock and kimberlite breccia occurring mostly in deep levels of the pipe below 345 m depth. The gradational contact relationships of these deposits with the surrounding kimberlite rocks and their location suggest that they formed in situ. The emplacement of Tuzo pipe involved repetitive volcanic explosions alternating with periods of relative quiescence causing at least partial consolidation of some facies. The volume deficit in the diatreme-root zone after each eruption was compensated by gravitational collapse of overlying diatreme tephra and pre-fragmented wall-rock xenoliths. Highly explosive phases were alternating with weak explosions or intrusive phases, suggesting

Depleted and metasomatized oceanic lithosphere beneath La Palma, Canary Islands

Science.gov (United States)

Janisch, Astrid; Ntaflos, Theodoros

2017-04-01

Due to the inaccessibility of Earth's interior, xenoliths became the best possibility to study the chemical composition of the earth mantle as well as its various processes. Three samples out of the sample suite of mantle peridotites from San Antonio Volcano on La Palma, Canary Islands, have been chosen to illustrate three examples of diverse mantle metasomatic events. The first sample, a pyroxene-hornblende-peridotite, was influenced by an alkali-rich, silicic-hydrous undersaturated melt and/or fluid forming a conspicuous cross-cutting amphibole-apatite-dyke with several veins percolating through the rock. Forsterite content in olivine varies between 82.5 - 85.5 and 86.0 - 89.0, suggesting at least two different occurrences of metasomatic overprint. Clinopyroxenes are mostly found in association with amphibole and in textural equilibrium hinting that both minerals may have grown together, while orthopyroxene have only been found as remnant inclusions in olivine. These clinopyroxenes are Cr-Diopsides with En43.40-50.97-Wo43.99-48.64-Fs4.30-8.22 and Mg# between 85.54 and 92.36. Secondary clinopyroxenes are Ti-Augites with En39.86-46.81-Wo46.65-51.98-Fs5.86-8.72 and Mg# of 82.44 - 89.09. The second sample, a sp-dunite, is characterized by haüyne-bearing melt veins which clearly indicate host-basalt infiltration. The haüyne is always in contact with amphibole, spinel and clinopyroxene denoting that they have been formed at the same time because there is no evidence for reaction among these phases. The melt infiltration apparently took place prior to xenolith entrainment in the host basalt. Primary olivine has Fo content of 89.57 - 89.67 with NiO ranging from 0.32 - 0.334, in contrast Fo content in secondary olivine varies from 89.05 - 90.86 and NiO fluctuates between 0.24 - 0.31. Cr-Diopside compositions are in range of En41.63-47.05-Wo47.83-51-90-Fs4.93-6.64 and Mg# between 86.48 - 90.50. The third sample is also a sp-dunite and marked by a network of phlogopite

The relationship between mantle pH and the deep nitrogen cycle

Science.gov (United States)

Mikhail, Sami; Barry, Peter H.; Sverjensky, Dimitri A.

2017-07-01

Nitrogen is distributed throughout all terrestrial geological reservoirs (i.e., the crust, mantle, and core), which are in a constant state of disequilibrium due to metabolic factors at Earth's surface, chemical weathering, diffusion, and deep N fluxes imposed by plate tectonics. However, the behavior of nitrogen during subduction is the subject of ongoing debate. There is a general consensus that during the crystallization of minerals from melts, monatomic nitrogen behaves like argon (highly incompatible) and ammonium behaves like potassium and rubidium (which are relatively less incompatible). Therefore, the behavior of nitrogen is fundamentally underpinned by its chemical speciation. In aqueous fluids, the controlling factor which determines if nitrogen is molecular (N2) or ammonic (inclusive of both NH4+ and NH30) is oxygen fugacity, whereas pH designates if ammonic nitrogen is NH4+ or NH30. Therefore, to address the speciation of nitrogen at high pressures and temperatures, one must also consider pH at the respective pressure-temperature conditions. To accomplish this goal we have used the Deep Earth Water Model (DEW) to calculate the activities of aqueous nitrogen from 1-5 GPa and 600-1000 °C in equilibrium with a model eclogite-facies mineral assemblage of jadeite + kyanite + quartz/coesite (metasediment), jadeite + pyrope + talc + quartz/coesite (metamorphosed mafic rocks), and carbonaceous eclogite (metamorphosed mafic rocks + elemental carbon). We then compare these data with previously published data for the speciation of aqueous nitrogen across these respective P-T conditions in equilibrium with a model peridotite mineral assemblage (Mikhail and Sverjensky, 2014). In addition, we have carried out full aqueous speciation and solubility calculations for the more complex fluids in equilibrium with jadeite + pyrope + kyanite + diamond, and for fluids in equilibrium with forsterite + enstatite + pyrope + diamond. Our results show that the pH of the fluid is

Geochemistry of single diamond crystals by instrumental neutron activation analysis

International Nuclear Information System (INIS)

Damarupurshad, A.

1995-02-01

Neutron activation analysis is probably the most powerful technique, available to date, for the analysis of the trace elements in diamond. In this study the technique of neutron activation analysis has been modified and optimized for the analysis of single, small (0.01-0.5 carat), inclusion-bearing and inclusion-free diamonds. Instrumental neutron activation analysis was used to analyze for up to 40 different elements at the ppb and ppt levels in diamonds from Brazil, South Africa, Colorado and China. The data obtained was used to detect and understand the differences between diamonds from the eclogitic and peridotitic para geneses and between diamonds from the different localities. In this regard, two inter element ratios, i.e. Cr/Sc and Au/Ir ratios were found to be useful. It seems that diamonds from a particular locality or mine have a unique range of Cr/Sc ratios. Furthermore, the identity of the dominant silicate inclusion(s) can be deduced from the Cr/Sc ratio of the diamond, since each type of silicate inclusion has a different range of Cr/Sc ratios. Not only is the Cr/Sc ratio distinctive for silicate inclusions in diamonds, it is also distinctive for minerals co genetic with diamond, such as orange garnet, red garnet, chrome diopside and ortho pyroxene (macrocrysts) which were separated from kimberlites. Sulphide inclusions may also contain detectable quantities of Au and Ir and the ratios of these two elements can also be used to differentiate between diamonds of the two para geneses. Carbon isotope ratios of these eclogitic and peridotitic diamonds were also measured. The comparison of this with the Cr/Sc ratios revealed that the carbon isotope ratios of both para geneses overlap in a narrow range and do not show the clear separations seen with Cr/Sc and Au/Ir ratios. It can be suggested, therefore, on the basis of the suite of 61 diamonds analyzed in this study, that the Cr/Sc and Au/Ir ratios are much more useful tools to distinguish between diamonds

Zircon/fluid trace element partition coefficients measured by recrystallization of Mud Tank zircon at 1.5 GPa and 800-1000 °C

Science.gov (United States)

Ayers, John C.; Peters, Timothy J.

2018-02-01

Hydrothermal zircon grains have trace element characteristics such as low Th/U, high U, and high rare earth element (REE) concentrations that distinguish them from magmatic, metamorphic, and altered zircon grains, but it is unclear whether these characteristics result from distinctive fluid compositions or zircon/fluid fractionation effects. New experiments aimed at measuring zircon/fluid trace element partition coefficients Dz/f involved recrystallizing natural Mud Tank zircon with low trace element concentrations in the presence of H2O, 1 m NaOH, or 1 m HCl doped with ∼1000 ppm of rare earth elements (REE), Y, U and Th and ∼500 ppm of Li, B, P, Nb, Ba, Hf, and Ta. Experiments were run for 168 h at 1.5 GPa, 800-1000 °C, and fO2 = NNO in a piston cylinder apparatus using the double capsule method. LA-ICP-MS analysis shows that run product zircon crystals have much higher trace element concentrations than in Mud Tank zircon starting material. Dz/f values were estimated from run product zircon analyses and bulk composition using mass balance. Most elements behave incompatibly, with median Dz/f being highest for Hf = 8 and lowest for B = 0.02. Addition of NaOH or HCl had little influence on Dz/f values. Dz/f for LREE are anomalously high, likely due to contamination of run product zircon with quenched solutes enriched in incompatible elements, so DLREE were estimated using lattice strain theory. Brice curves for +3 ions yield zircon/fluid DLu/DLa of ∼800-5000. A Brice curve fit to +4 ions yielded DCe4+ values. Estimated concentrations of Ce3+ and Ce4+ show that the average Ce4+/Ce3+ in zircon of 27 is much higher than in fluid of 0.02. Th and U show little fractionation, with median DTh/DU = 0.7, indicating that the low Th/U in natural hydrothermal zircon is inherited from the fluid. Natural fluid compositions estimated from measured Dz/f and published compositions of hydrothermal zircon grains from aplite and eclogite reflect the mineralogy of the host rock, e

Aeromagnetic signatures of Precambrian shield and suture zones of Peninsular India

Directory of Open Access Journals (Sweden)

Mita Rajaram

2014-01-01

Full Text Available In many Precambrian provinces the understanding of the tectonic history is constrained by limited exposure and aeromagnetic data provide information below the surface cover of sediments, water, etc. and help build a tectonic model of the region. The advantage of using the aeromagnetic data is that the data set has uniform coverage and is independent of the accessibility of the region. In the present study, available reconnaissance scale aeromagnetic data over Peninsular India are analyzed to understand the magnetic signatures of the Precambrian shield and suture zones thereby throwing light on the tectonics of the region. Utilizing a combination of differential reduction to pole map, analytic signal, vertical and tilt derivative and upward continuation maps we are able to identify magnetic source distribution, tectonic elements, terrane boundaries, suture zones and metamorphic history of the region. The magnetic sources in the region are mainly related to charnockites, iron ore and alkaline intrusives. Our analysis suggests that the Chitradurga boundary shear and Sileru shear are terrane boundaries while we interpret the signatures of Palghat Cauvery and Achankovil shears to represent suture zones. Processes like metamorphism leave their signatures on the magnetic data: prograde granulites (charnockites and retrograde eclogites are known to have high susceptibility. We find that charnockites intruded by alkali plutons have higher magnetization compared to the retrogressed charnockites. We interpret that the Dharwar craton to the north of isograd representing greenschist to amphibolite facies transition, has been subjected to metamorphism under low geothermal conditions. Some recent studies suggest a plate tectonic model of subduction–collision–accretion tectonics around the Palghat Cauvery shear zone (PCSZ. Our analysis is able to identify several west to east trending high amplitude magnetic anomalies with deep sources in the region from

Early Cretaceous wedge extrusion in the Indo-Burma Range accretionary complex: implications for the Mesozoic subduction of Neotethys in SE Asia

Science.gov (United States)

Zhang, Ji'en; Xiao, Wenjiao; Windley, Brian F.; Cai, Fulong; Sein, Kyaing; Naing, Soe

2017-06-01

The Indo-Burma Range (IBR) of Myanmar, the eastern extension of the Yarlung-Tsangpo Neotethyan belt of Tibet in China, contains mélanges with serpentinite, greenschist facies basalt, chert, sericite schist, silty slate and unmetamorphosed Triassic sandstone, mudstone and siltstone interbedded with chert in the east, and farther north high-pressure blueschist and eclogite blocks in the Naga Hills mélange. Our detailed mapping of the Mindat and Magwe sections in the middle IBR revealed a major 18 km antiformal isocline in a mélange in which greenschist facies rocks in the core decrease in grade eastwards and westwards symmetrically `outwards' to lower grade sericite schist and silty slate, and at the margins to unmetamorphosed sediments, and these metamorphic rocks are structurally repeated in small-scale imbricated thrust stacks. In the Mindat section the lower western boundary of the isoclinal mélange is a thrust on which the metamorphic rocks have been transported over unmetamorphosed sediments of the Triassic Pane Chaung Group, and the upper eastern boundary is a normal fault. These relations demonstrate that the IBR metamorphic rocks were exhumed by wedge extrusion in a subduction-generated accretionary complex. Along strike to the north in the Naga Hills is a comparable isoclinal mélange in which central eclogite lenses are succeeded `outwards' by layers of glaucophane schist and glaucophanite, and to lower grade greenschist facies sericite schist and slate towards the margins. In the Natchaung area (from west to east) unmetamorphosed Triassic sediments overlie quartzites, sericite schists, actinolite schists and meta-volcanic amphibolites derived from MORB-type basalt, which are in fault contact with peridotite. Olivine in the peridotite has undulatory extinction suggesting deformation at 600-700 °C, similar to the peak temperature of the amphibolite; these relations suggest generation in a metamorphic sole. The amphibolites have U/Pb zircon ages of 119

MODELING OF MOVING DEFORMABLE CONTINENTS BY ACTIVE TRACERS: CLOSING AND OPENING OF OCEANS, RECIRCULATION OF OCEANIC CRUST

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A. V. Bobrov

2018-01-01

Full Text Available The evolution of the ‘mantle – moving deformable continents’ system has been studied by numerical experiments. The continents move self-consistently with the mantle flows of thermo-compositional convection. Our model (two-dimensional mantle convection, non-Newtonian rheology, the presence of deformable continents demonstrates the main features of global geodynamics: convergence and divergence of continents; appearance and disappearance of subduction zones; backrolling of subduction zones; restructuring of mantle flows; stretching, breakup and divergence of continents; opening and closing of oceans; oceanic crust recirculation in the mantle, and overriding of hot mantle plumes by continents. In our study, the continental crust is modeled by active markers which transfer additional viscosity and buoyancy, while the continental lithosphere is marked only by increased viscosity with neutral buoyancy. The oceanic crust, in its turn, is modeled by active markers that have only an additional buoyancy. The principal result of our modeling is a consistency between the numerical calculations and the bimodal dynamics of the real Earth: the oceanic crust, despite its positive buoyancy near the surface, submerges in subduction zones and sinks deep into the mantle. (Some part of the oceanic crust remains attached to the continental margins for a long time. In contrast to the oceanic crust, the continental crust does not sink in subduction zones. The continental lithosphere, despite its neutral buoyancy, also remains on the surface due to its viscosity and coupling with the continental crust. It should be noted that when a continent overrides a subduction zone, the subduction zone disappears, and the flows in the mantle are locally reorganized. The effect of basalt-eclogite transition in the oceanic crust on the mantle flow pattern and on the motion of continents has been studied. Our numerical experiments show that the inclusion of this effect in the

Magnesium Isotopic Evidence for Ancient Subducted Oceanic Crust in LOMU-Like Potassium-Rich Volcanic Rocks

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Sun, Yang; Teng, Fang-Zhen; Ying, Ji-Feng; Su, Ben-Xun; Hu, Yan; Fan, Qi-Cheng; Zhou, Xin-Hua

2017-10-01

To evaluate the role of subducted oceanic crust in the genesis of potassium-rich magmas, we report high-precision Mg isotopic data for a set of Cenozoic volcanic rocks from Northeast China. These rocks overall are lighter in Mg isotopic composition than the normal mantle and display considerable Mg isotopic variations, with δ26Mg ranging from -0.61 to -0.23. The covariation of δ26Mg with TiO2 in these rocks suggests that their light Mg isotopic compositions were derived from recycled oceanic crust in the form of carbonated eclogite in the source region. The strong correlations between δ26Mg and (Gd/Yb)N ratio as well as Sr-Pb isotopes further indicate a multicomponent and multistage origin of these rocks. Magnesium isotopes may thus be used as a novel tracer of recycled oceanic crust in the source region of mantle-derived magmas.

Temperature Profile of the Upper Mantle

International Nuclear Information System (INIS)

Anderson, O.L.

1980-01-01

Following the procedure outlined by Magnitsky [1971], thermal profiles of the upper mantle are computed by deriving the thermal gradient from the seismic data given as dv/sub s//drho used along with the values of (dv/sub s//dT9/sub p/ and (dv/sub s//dP)/sub T/ of selected minerals, measured at high temperature. The resulting values of dT/dZ are integrated from 380 km upward toward the surface, where the integrating constant is taken from Akagi and Akimoto's work, T=1400 0 C at 380 km. The resulting geotherms for minerals are used to derive geotherms for an eclogite mantle and a lherzolite mantle, with and without partial melting in the low-velocity zone. The geotherms are all subadiabatic, and some are virtually isothermal in the upper mantle. Some are characterized by a large thermal hump at the lithosphere boundary

Density heterogeneity of the upper mantle beneath Siberia from satellite gravity and a new regional crustal model

DEFF Research Database (Denmark)

Herceg, Matija; Thybo, Hans; Artemieva, Irina

2013-01-01

We present a new regional model for the density structure of the upper mantle below Siberia. The residual mantle gravity anomalies are based on gravity data derived from the GOCE gravity gradients and geopotential models, with crustal correction to the gravity field being calculated from a new...... on regional and global crustal models. We analyze how uncertainties and errors in the crustal model propagate from crustal densities to mantle residual gravity anomalies and the density model of the upper mantle. The new regional density model for the Siberian craton and the West Siberian Basin complements...... regional crustal model. This newly compiled database on the crustal seismic structure, complemented by additional constraints from petrological analysis of near-surface rocks and lower crustal xenoliths, allows for a high-resolution correction of the crustal effects as compared to previous studies based...

Global map of lithosphere thermal thickness on a 1 deg x 1 deg grid - digitally available

DEFF Research Database (Denmark)

Artemieva, Irina

2014-01-01

with no or low quality heat flow data. This analysis requires knowledge oflithosphere age globally.A compilation of tectono-thermal ages of lithospheric terranes on a 1 deg 1 deg grid forms the basis forthe statistical analysis. It shows that, statistically, lithospheric thermal thickness z (in km) depends......This presentation reports a 1 deg 1 deg global thermal model for the continental lithosphere (TC1). The modelis digitally available from the author’s web-site: www.lithosphere.info.Geotherms for continental terranes of different ages (early Archean to present) are constrained by reliabledata...... on borehole heat flow measurements (Artemieva and Mooney, 2001), checked with the original publicationsfor data quality, and corrected for paleo-temperature effects where needed. These data are supplemented bycratonic geotherms based on xenolith data.Since heat flow measurements cover not more than half...

A New Type of Foreign Clast in A Polymict Ureilite: A CAI or AL-Rich Chondrule

Science.gov (United States)

Goodrich, C. A.; Ross, D. K.; Treiman, A. H.

2017-01-01

Introduction: Polymict ureilites are breccias interpreted to represent regolith formed on a ureilitic asteroid [1-3]. They consist of approximately 90-95% clasts of various ureilite types (olivine-pyroxene rocks with Fo 75-95), a few % indigenous feldspathic clasts, and a few % foreign clasts [4-20]. The foreign clasts are diverse, including fragments of H, L, LL and R chondrites, angrites, other achondrites, and dark clasts similar to CC [6,7,9-19]. We report a new type of foreign clast in polymict ureilite DaG 999. Methods: Clast 8 in Dar al Gani (DaG) 999/1 (Museum fur Naturkunde) was discovered during a survey of feldspathic clasts in polymict ureilites [19,20]. It was studied by BEI, EMPA, and X-ray mapping on the JEOL 8530F electron microprobe at ARES, JSC. Petrography and Mineral Compositions: Clast 8 is sub-rounded to irregular in shape, approximately 85 micrometers in diameter, and consists of approximately 68% pyroxene and 32% mesostasis (by area). Part of the pyroxene (top half of clast in Fig. 1a and 2) shows a coarse dendritic morphology; the rest appears massive. Mesostasis may be glassy and contains fine needles/grains of pyroxene. The pyroxene has very high CaO (23.5 wt.%) and Al2O3 (19.7 wt.%), with the formula: (Ca(0.91)Mg(0.63)Fe(0.01)Al(sup VI) (0.38)Cr(0.01)Ti(0.05)1.99 Si2O6. The bulk mesostasis also has very high Al2O3 (approximately 26 wt.%). A bulk composition for the clast was obtained by combining modal abundances with phase compositions (Table 1, Fig. 3). Discussion: The pyroxene in clast 8 has a Ca-Al-(Ti)- rich (fassaitic) composition that is clearly distinct from compositions of pyroxenes in main group ureilites [22] or indigenous feldspathic clasts in polymict ureilites [4-8]. It also has significantly higher Al than fassaite in angrites (up to approximately 12 wt.% [23]), which occur as xenoliths in polymict ureilites. Ca-Al-Ti rich pyroxenes are most commonly found in CAIs, Al-rich chondrules and other types of refractory

The Mafic Lower Crust of Neoproterozoic age beneath Western Arabia: Implications for Understanding African Lower Crust

Science.gov (United States)

Stern, R. J.; Mooney, W. D.

2011-12-01

We review evidence that the lower crust of Arabia - and by implication, that beneath much of Africa was formed at the same time as the upper crust, rather than being a product of Cenozoic magmatic underplating. Arabia is a recent orphan of Africa, separated by opening of the Red Sea ~20 Ma, so our understanding of its lower crust provides insights into that of Africa. Arabian Shield (exposed in W. Arabia) is mostly Neoproterozoic (880-540 Ma) reflecting a 300-million year process of continental crustal growth due to amalgamated juvenile magmatic arcs welded together by granitoid intrusions that make up as much as 50% of the Shield's surface. Seismic refraction studies of SW Arabia (Mooney et al., 1985) reveal two layers, each ~20 km thick, separated by a well-defined Conrad discontinuity. The upper crust has average Vp ~6.3 km/sec whereas the lower crust has average Vp ~7.0 km/sec, corresponding to a granitic upper crust and gabbroic lower crust. Neogene (<30 ma) lava fields in Arabia (harrats) extend over 2500 km, from Yemen to Syria. Many of these lavas contain xenoliths, providing a remarkable glimpse of the lower-crustal and upper-mantle lithosphere beneath W. Arabia. Lower crustal xenoliths brought up in 8 harrats in Saudi Arabia, Jordan, and Syria are mostly 2-pyroxene granulites of igneous (gabbroic, anorthositic, and dioritic) origin. They contain plagioclase, orthopyroxene, and clinopyroxene, and a few contain garnet and rare amphibole and yield mineral-equilibrium temperatures of 700-900°C. Pyroxene-rich and plagioclase-rich suites have mean Al2O3 contents of 13% and 19%, respectively: otherwise the two groups have similar elemental compositions, with ~50% SiO2 and ~1% TiO2, with low K2O (<0.5%) and Na2O (1-3%). Both groups show tholeiitic affinities, unrelated to their alkali basalt hosts. Mean pyroxene-rich and plagioclase-rich suites show distinct mean MgO contents (11% vs. 7%), Mg# (67 vs. 55), and contents of compatible elements Ni (169 vs. 66 ppm

Petrogenesis of Neogene basaltic volcanism associated with the Lut block, eastern Iran: Implication for tectonic and metallogenic evolution

Science.gov (United States)

Saadat, Saeed

This dissertation presents petrochemical data concerning Neogene olivine basalts erupted both along the margins and within the micro-continental Lut block, eastern Iran, which is a part of the active Alpine-Himalayan orogenic belt. These data demonstrate the following: (1) Basalts that erupted from small monogenetic parasitic cones around the Bazman stratovolcano, Makran arc area, in the southern Lut block, are low-Ti sub-alkaline olivine basalts. Enrichments of LILE relative to LREE, and depletions in Nb and Ta relatively to LILE, are similar to those observed for other convergent plate boundary arc magmas around the world and suggest that these basalts formed by melting of subcontinental mantle modified by dehydration of the subducted Oman Sea oceanic lithosphere. (2) Northeast of Iran, an isolated outcrop of Neogene/Quaternary alkali olivine basalt, containing mantle and crustal xenoliths, formed by mixing of small melt fractions from both garnet and spinel-facies mantle. These melts rose to the surface along localized pathways associated with extension at the junction between the N-S right-lateral strike-slip faults and E-W left-lateral strike slip faults. The spinel-peridotite mantle xenoliths contained in the basalts, which equilibrated in the subcontinental lithosphere at depths of 30 to 60 km and temperatures of 965°C to 1065°C, do not preserve evidence of extensive metasomatic enrichment as has been inferred for the mantle below the Damavand volcano further to the west in north-central Iran. (3) Neogene mafic rocks within the central Lut block represent the last manifestation of a much more extensive mid-Tertiary magmatic event. These basalts formed from both OIB-like asthenosphere and subcontinental lithosphere which preserved chemical characteristics inherited from mid-Tertiary subduction associated with the collision of the Arabian with the Eurasian plate and closing of the Neotethys Ocean. Neogene/Quternary alkali olivine basalts erupted mainly along

Did a whole-crustal hydrothermal system generate the Irish Zn-Pb orefield?

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Daly, J. Stephen; Badenszki, Eszter; Chew, David; Kronz, Andreas; O'Rourke, Helen; Whitehouse, Martin; Menuge, Julian; van den Berg, Riana

2016-04-01

Current models[1] for the genesis of the giant Irish Carboniferous-hosted Zn-Pb orefield propose shallow (700°C) metamorphism and melting during the Acadian orogeny at ~390Ma and during separate episodes of extension at ~ 381-373Ma and ~362Ma. Sm-Nd garnet dating shows that the lower crust remained hot or was re-heated to ~600°C at ~341Ma during Lower Carboniferous volcanism, also associated with extension and, in part, coincident with the mineralization[1]. Isotopic data from the xenoliths correspond closely to Sr and Nd isotopic analyses of gangue calcite[8] and galena Pb[9] isotopic data from the major ore deposits. While Zn contents of the xenoliths permit them to be metal sources, their mineralogy and texture provide an enriched template and a plausible extraction mechanism. In situ analyses of modally-abundant biotite and garnet show significant enrichment in Zn (and other relevant metals) as well as order of magnitude depletion of Zn during retrograde alteration, providing a metal-release mechanism and pointing to a hydrothermal fluid system operating at least to depths of ~ 25km. References [1] Wilkinson, J.J. & Hitzman, M.W. 2015. The Irish Pb-Zn orefield: The view from 2014. In: Archibald, S.M. and Piercey, S.J. (eds) Current Perspectives on Zinc deposits. Irish Association for Economic Geology, pp. 59-72.; [2] Davidheiser-Kroll, B., Stuart, F.M. & Boyce, A.J. 2014. Mineralium Deposita, 49, 547-553; [3] Elliott, H. 2015. Unpublished PhD thesis, University of Southampton; [4] Hnatyshin, D., Creaser, R.A., Wilkinson, J.J. & Gleeson, S.A. 2015. Geology, 43, 143-146; [5] McCusker, J. & Reed, C. 2013. Mineralium Deposita, 48, 687-695; [6] Van den Berg, R., Daly, J.S. & Salisbury, M.H. 2005. Tectonophysics, 407(1-2), 81-99; [7] Hauser, F., O'Reilly, B.M., Readman, P.W., Daly, J. S. & Van den Berg, R. 2008. Geophysical Journal International 175, 1254-1272; [8] Walshaw, R.D., Menuge, J.F. & Tyrrell, S. 2006. Mineralium Deposita, 41, 803-819; [9] Everett, C

Dating exhumed peridotite with spinel (U-Th)/He chronometry

Science.gov (United States)

Cooperdock, Emily H. G.; Stockli, Daniel F.

2018-05-01

The timing of cooling and exhumation of mantle peridotites in oceanic and continental settings has been challenging to determine using traditional geo- and thermochronometric techniques. Hence, the timing of the exhumation of mantle rocks to the Earth's surface at mid-ocean ridges, rifted and passive continental margins, and within continental volcanic and orogenic systems has remained largely elusive or only loosely constrained by relative age bracketing. Magmatic spinel [(Mg, Fe)(Al,Cr)2O4] is a ubiquitous primary mineral phase in mantle peridotites and is often the only primary mineral phase to survive surface weathering and serpentinization. This work explores spinel (U-Th)/He thermochronology as a novel tool to directly date the exhumation and cooling history of spinel-bearing mantle peridotite. Samples were chosen from a range of tectonic and petrologic settings, including a mid-ocean ridge abyssal peridotite (ODP Leg 209), an orogenic tectonic sliver of sub-continental mantle (Lherz massif, France), and a volcanic-rock hosted mantle xenolith (Green Knobs, NM). Spinel grains were selected based on grain size and morphology, screened for internal homogeneity using X-ray computed tomography, and air abraded to eliminate effects of alpha ejection/implantation. These case studies yield spinel He age results that are reproducible and generally in good agreement with independent age constraints. For ODP Leg 209, a spinel He age of 1.1 ± 0.3 Ma (2 SE) (n = 8) is consistent with independent U-Pb and magnetic anomaly ages for the exhumation of oceanic crust by detachment faulting along this segment of the slow-spreading ridge. Spinel from the Lherz massif yield He ages from 60-70 Ma (n = 3), which correspond well with independent thermochronometric constraints for cooling associated with Pyrenean collisional exhumation. Spinel from a mantle xenolith within a previously undated kimberlite diatreme at Green Knobs, New Mexico, generate a reproducible mean He age of 11

Crust-Mantle Interactions at Pico de Orizaba (Citlaltepetl) Volcano, Mexico.

Science.gov (United States)

Schaaf, P.; Carrasco, G.

2006-12-01

Pico de Orizaba (Citlaltepetl) volcano constitutes the easternmost and highest stratovolcano of the subduction- related Plio-Quaternary Trans-Mexican Volcanic Belt (TMVB). The volcano can be divided into three main constructional stages. Its activity started during the mid-Pleistocene. The present cone was built on the remnants of the ancestral buildings by eruption of amphibole-two pyroxene dacitic lava flows, the most recent of which was erupted in the seventeenth century. The volcano is surrounded to the SW by monogenetic Quaternary cindercones and maars. All representative units were sampled in this work for geochemical and isotopic purposes, including a small quartzitic xenolith found in the basaltic monogenetic suite. Volcanic products of the stratocone are quite heterogeneous and range from calc-alkaline basaltic andesites to dome rhyolites, also displayed by a wide range of SiO2 and MgO (72.6-53.2 and 7.0-0.3 wt. %, respectively). In comparison to other TMVB stratovolcanoes (e.g., Colima, Nevado de Toluca), Pico de Orizaba shows similar 87Sr/86Sr ratios (0.7037-0.7048) but considerably more evolved Nd-Pb isotopic ratios (eNd: -1.8 to + 1.4; 206Pb/204Pb: 18.61-18.78). Elevated LILE concentrations and depleted HFSE witness the importance of slab- derived aqueous fluids and metasomatic reactions between the subducting lithosphere and overlying mantle wedge. On the other hand, Pico de Orizaba volcano shows additionally high crustal contributions of a source with depleted Sr and enriched Nd and Pb isotopic signatures, best explained by considerable assimilation of the local Grenvillian basement in magma generation processes. In contrast to Popocatépetl volcano with a high-level magma reservoir emplacement (7-8 km) and obvious interaction with the carbonate-dominated shallow basement rocks (e.g. elevated 87Sr/86Sr ratios and CO2 in gas plumes), this effect cannot be observed at Pico de Orizaba volcano, although a regional Cretaceous limestone basement is also

Plagioclase-dominated Seismic Anisotropy in the Basin and Range Lower Crust

Science.gov (United States)

Bernard, R. E.; Behr, W. M.

2017-12-01

Observations of seismic anisotropy have the ability to provide important information on deformation and structures within the lithosphere. While the mechanisms controlling seismic anisotropy in the upper mantle are fairly well understood (i.e., olivine "lattice preferred orientation" or LPO), less is known about the minerals and structures controlling regional lower crustal anisotropy. We use lower crustal xenoliths from young cinder cones in the eastern Mojave/western Basin and Range to investigate mineral LPOs and their effect on seismic anisotropy. Lower crustal gabbros were collected from two areas roughly 80 km apart — the Cima and Deadman Lake Volcanic Fields. Lower crustal fabrics measured using EBSD are dominated by LPOs in plagioclase associated with both plastic deformation and magmatic flow. In all fabric types, plagioclase LPOs produce seismic fast axes oriented perpendicular to the foliation plane. This is in contrast to mantle peridotite xenoliths from the same locations, which preserve olivine LPOs with fast axes aligned parallel to the foliation plane. The orthogonal orientations of mantle and lower crustal fast axes relative to foliation implies that even where fabric development in both layers is coeval and kinematically compatible, their measured anisotropies can be perpendicular to each other, therefore appearing anti-correlated when measured seismically. Furthermore, our observation of plagioclase-dominated LPO and negligible concentrations of mica is at odds with the common assumption that lower crustal anisotropy is dominated by micaceous minerals, whose slow axes reliably align parallel to lineation or flow. In contrast, our data show that for plagioclase, fast axes align perpendicular to flow and the slow axes are variably aligned within the foliation plane. Therefore, for a crustal section dominated by plagioclase LPO with assumed horizontal foliation, there would be a vertical rather than a horizontal axis of symmetry, resulting in a

Tracking silica in Earth's upper mantle using new sound velocity data for coesite to 5.8 GPa and 1073 K: Tracking Silica in Earth's Upper Mantle

Energy Technology Data Exchange (ETDEWEB)

Chen, Ting [Department of Geosciences, Stony Brook University, Stony Brook New York USA; Liebermann, Robert C. [Department of Geosciences, Stony Brook University, Stony Brook New York USA; Mineral Physics Institute, Stony Brook University, Stony Brook New York USA; Zou, Yongtao [Mineral Physics Institute, Stony Brook University, Stony Brook New York USA; State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun China; Li, Ying [Mineral Physics Institute, Stony Brook University, Stony Brook New York USA; Key Laboratory of Earthquake Prediction, Institute of Earthquake Science, China Earthquake Administration, Beijing China; Qi, Xintong [Department of Geosciences, Stony Brook University, Stony Brook New York USA; Li, Baosheng [Department of Geosciences, Stony Brook University, Stony Brook New York USA; Mineral Physics Institute, Stony Brook University, Stony Brook New York USA

2017-08-12

The compressional and shear wave velocities for coesite have been measured simultaneously up to 5.8 GPa and 1073 K by ultrasonic interferometry for the first time. The shear wave velocity decreases with pressure along all isotherms. The resulting contrasts between coesite and stishovite reach ~34% and ~45% for P and S wave velocities, respectively, and ~64% and ~75% for their impedance at mantle conditions. The large velocity and impedance contrasts across coesite-stishovite transition imply that to generate the velocity and impedance contrasts observed at the X-discontinuity, only a small amount of silica would be required. The velocity jump dependences on silica, d(lnVP)/d(SiO2) = 0.38 (wt %)-1 and d(lnVS)/d(SiO2) = 0.52 (wt %)-1, are utilized to place constraints on the amount of silica in the upper mantle and provide a geophysical approach to track mantle eclogite materials and ancient subducted oceanic slabs.

Redox Interactions between Iron and Carbon in Planetary Mantles: Implications for Degassing and Melting Processes

Science.gov (United States)

Martin, A.; Righter, K.

2009-01-01

Carbon stability in planetary mantles has been studied by numerous authors because it is thought to be the source of C-bearing atmospheres and of C-rich lavas observed at the planetary surface. In the Earth, carbonaceous peridotites and eclogites compositions have been experimentally studied at mantle conditions [1] [2] [3]. [4] showed that the fO2 variations observed in martian meteorites can be explained by polybaric graphite-CO-CO2 equilibria in the Martian mantle. Based on thermodynamic calculations [4] and [5] inferred that the stable form of carbon in the source regions of the Martian basalts should be graphite (and/or diamond), and equilibrium with melts would be a source of CO2 for the martian atmosphere. Considering the high content of iron in the Martian mantle (approx.18.0 wt% FeO; [6]), compared to Earth s mantle (8.0 wt% FeO; [7]) Fe/C redox interactions should be studied in more detail.

Comparison of mantle lithosphere beneath early Triassic kimberlite fields in Siberian craton reconstructed from deep-seated xenocrysts

Directory of Open Access Journals (Sweden)

I.V. Ashchepkov

2016-07-01

Kharamai mantle clinopyroxenes represent three geochemical types: (1 harzburgitic with inclined linear REE, HFSE troughs and elevated Th, U; (2 lherzolitic or pyroxenitic with round TRE patterns and decreasing incompatible elements; (3 eclogitic with Eu troughs, Pb peak and high LILE content. Calculated parental melts for garnets with humped REE patterns suggest dissolution of former Cpx and depression means Cpx and garnets extraction. Clinopyroxenes from Ary-Mastakh fields show less inclined REE patterns with HMREE troughs and an increase of incompatible elements. Clinopyroxenes from Kuranakh field show flatter spoon-like REE patterns and peaks in Ba, U, Pb and Sr, similar to those in ophiolitic harzburgites. The PT diagrams for the mantle sections show high temperature gradients in the uppermost SCLM accompanied by an increase of P-Fe#Ol upward and slightly reduced thickness of the mantle keel of the Siberian craton, resulting from the influence of the Permian–Triassic superplume, but with no signs of delamination.

Petrology and geochemistry of the Los Cuartos granite, Tafi del Valle, Tucuman, northwest of Argentina: its integration to the regional magmatic layout; Petrologia y geoquimica del granito Los Cuartos, Tafi del Valle, Tucuman, noroeste de Argentina: integracion al esquema magmatico regional

Energy Technology Data Exchange (ETDEWEB)

Lopez, J.P.; Bellos, L.I.

2010-07-01

Los Cuartos Granite crops out on the western flank of the Cumbres Calchaquies, east of Tafi del Valle, Tucuman Province, Argentina. It has a granitic to granodioritic composition, of two micas, being biotite dominant. The granite is medium grained and inequigranular, with rare K-feldspar megacrysts. It contains host-rock xenoliths with different degrees of assimilation. The granite intrusion produced contact metamorphism in the surrounding basement, with development of andalucite porphyroblasts and poikiloblastic biotite. The granite is calk-alkaline and peraluminous, and is K- and P-rich and Ca- and Napoor compared to other granitoids of the region. The pluton shows late-tectonic features with regard to the regional deformation. The granite possibly intruded during the Ordovician (Famatinian Cycle) and its emplacement was structurally controlled by the Tafi Megafracture. Los Cuartos Granite and other intrusive bodies of the region share similar petrographical, geochemical, structural and geochronological characteristics. (Author).

The evolution of complex type B Allende inclusion - An ion microprobe trace element study

Science.gov (United States)

Macpherson, Glenn J.; Crozaz, Ghislaine; Lundberg, Laura L.

1989-01-01

Results are presented of a detailed trace-element and isotopic analyses of the constituent phases in each of the major textural parts (mantle, core, and islands) of a Type B refractory inclusion, the USNM 5241 inclusion from Allende, first described by El Goresy et al. (1985). The REE data on 5241 were found to be largely consistent with a model in which the mantle and the core of 5241 formed sequentially out of a single melt by fractional crystallization. The numerical models of REE evolution in the 5241 melt, especially that of Eu, require that a significant mass of spinel-free island material was assimilated into the evolving melt during the last half of the solidification history of 5241. The trace element results pbtained thus strongly support the interpretation of El Goresy et al. (1985) that the spinel-free islands in the 5241 are trapped xenoliths.

Petrology and mapping of Fe and Ti occurences in Passira country, PE, Brazil

International Nuclear Information System (INIS)

Silva Filho, A.F. da; Pinho Guimaraes, I. de.

1979-01-01

The results of a geologic mapping realized at the scale of 1:25.000, for graduating in geology, the will show in this communication. The area mapped has 230 square kilometers and it is localized between the south lattitudes of 7 0 52'30'' and 8 0 42'45'' and the west greenwiches of 35 0 26'49'' and 35 0 33'30'', at the districts of Passira and Limoeiro-PE. At the area, are recorgnized migmatites granites, biotite schists, hornblende-gneisses, quartz-diorites and amphigolites. The iron titanium ore are fitted, like xenoliths, inside the amphibolites. The amphibolites have orthorocks type, and they are derived predominatly of the quartz-diorite unit, where they may be individualized cartographicaly. On the other side of the geological mapping, we did many thin sections and polished, chemical analyses of the ore bodyes, and the results will be disciminated [pt

Subsurface Geology of the Fenton Hill Hot Dry Rock Geothermal Energy Site

Energy Technology Data Exchange (ETDEWEB)

Levey, Schon S.

2010-12-01

The Precambrian rock penetrated by wells EE-2A and -3A belongs to one or more granitic to granodioritic plutons. The plutonic rock contains two major xenolith zones of amphibolite, locally surrounded by fine-grained mafic rock of hybrid igneous origin. The granodiorite is cut by numerous leucogranite dikes that diminish in abundance with depth. The most prominent structural feature is the main breccia zone, in which the rock is highly fractured and moderately altered. This zone is at least 75 m thick and is of uncertain but near-horizontal orientation. Fracture abundance decreases with increasing depth below the main breccia zone, and fractures tend to be associated with leucogranite dikes. This association suggests that at least some of the fractures making up the geothermal reservoir are of Precambrian age or have long-range orientations controlled by the presence of Precambrian-age granitic dikes.

The Earth's heterogeneous mantle a geophysical, geodynamical, and geochemical perspective

CERN Document Server

Khan, Amir

2015-01-01

This book highlights and discusses recent developments that have contributed to an improved understanding of observed mantle heterogeneities and their relation to the thermo-chemical state of Earth's mantle, which ultimately holds the key to unlocking the secrets of the evolution of our planet. This series of topical reviews and original contributions address 4 themes. Theme 1 covers topics in geophysics, including global and regional seismic tomography, electrical conductivity and seismic imaging of mantle discontinuities and heterogeneities in the upper mantle, transition zone and lower mantle. Theme 2 addresses geochemical views of the mantle including lithospheric evolution from analysis of mantle xenoliths, composition of the deep Earth and the effect of water on subduction-zone processes. Theme 3 discusses geodynamical perspectives on the global thermo-chemical structure of the deep mantle. Theme 4 covers application of mineral physics data and phase equilibrium computations to infer the regional-scale ...

Carbonatite and silicate melt metasomatism of the mantle surrounding the Hawaiian plume: Evidence from volatiles, trace elements, and radiogenic isotopes in rejuvenated-stage lavas from Niihau, Hawaii

Science.gov (United States)

Dixon, Jacqueline; Clague, David A.; Cousens, Brian; Monsalve, Maria Luisa; Uhl, Jessika

2008-09-01

We present new volatile, trace element, and radiogenic isotopic compositions for rejuvenated-stage lavas erupted on Niihau and its submarine northwest flank. Niihau rejuvenated-stage Kiekie Basalt lavas are mildly alkalic and are isotopically similar to, though shifted to higher 87Sr/86Sr and lower 206Pb/204Pb than, rejuvenated-stage lavas erupted on other islands and marginal seafloor settings. Kiekie lavas display trace element heterogeneity greater than that of other rejuvenated-stage lavas, with enrichments in Ba, Sr, and light-rare earth elements resulting in high and highly variable Ba/Th and Sr/Ce. The high Ba/Th lavas are among the least silica-undersaturated of the rejuvenated-stage suite, implying that the greatest enrichments are associated with the largest extents of melting. Kiekie lavas also have high and variable H2O/Ce and Cl/La, up to 620 and 39, respectively. We model the trace element concentrations of most rejuvenated-stage lavas by small degrees (˜1% to 9%) of melting of depleted peridotite recently metasomatized by a few percent of an enriched incipient melt (0.5% melting) of the Hawaiian plume. Kiekie lavas are best explained by 4% to 13% partial melting of a peridotite source metasomatized by up to 0.2% carbonatite, similar in composition to oceanic carbonatites from the Canary and Cape Verde Islands, with lower proportion of incipient melt than that for other rejuvenated-stage lavas. Primary H2O and Cl of the carbonatite component must be high, but variability in the volatile data may be caused by heterogeneity in the carbonatite composition and/or interaction with seawater. Our model is consistent with predictions based on carbonated eclogite and peridotite melting experiments in which (1) carbonated eclogite and peridotite within the Hawaiian plume are the first to melt during plume ascent; (2) carbonatite melt metasomatizes plume and surrounding depleted peridotite; (3) as the plume rises, silica-undersaturated silicate melts are also

Birch's Mantle

Science.gov (United States)

Anderson, D. L.

2002-12-01

Francis Birch's 1952 paper started the sciences of mineral physics and physics of the Earth's interior. Birch stressed the importance of pressure, compressive strain and volume in mantle physics. Although this may seem to be an obvious lesson many modern paradoxes in the internal constitution of the Earth and mantle dynamics can be traced to a lack of appreciation for the role of compression. The effect of pressure on thermal properties such as expansivity can gravitational stratify the Earth irreversibly during accretion and can keep it chemically stratified. The widespread use of the Boussinesq approximation in mantle geodynamics is the antithesis of Birchian physics. Birch pointed out that eclogite was likely to be an important component of the upper mantle. Plate tectonic recycling and the bouyancy of oceanic crust at midmantle depths gives credence to this suggestion. Although peridotite dominates the upper mantle, variations in eclogite-content may be responsible for melting- or fertility-spots. Birch called attention to the Repetti Discontinuity near 900 km depth as an important geodynamic boundary. This may be the chemical interface between the upper and lower mantles. Recent work in geodynamics and seismology has confirmed the importance of this region of the mantle as a possible barrier. Birch regarded the transition region (TR ; 400 to 1000 km ) as the key to many problems in Earth sciences. The TR contains two major discontinuities ( near 410 and 650 km ) and their depths are a good mantle thermometer which is now being exploited to suggest that much of plate tectonics is confined to the upper mantle ( in Birch's terminology, the mantle above 1000 km depth ). The lower mantle is homogeneous and different from the upper mantle. Density and seismic velocity are very insensitive to temperature there, consistent with tomography. A final key to the operation of the mantle is Birch's suggestion that radioactivities were stripped out of the deeper parts of

The Precambrian/Lower Cambrian pluton from Vila Nova (Central Portugal)

Energy Technology Data Exchange (ETDEWEB)

Reis, A. I. M.; Silva, M. M. V. G.; Antunes, I. M. H. R.

2010-07-01

The Vila Nova pluton is a small, Pre-Variscan granitic body that intruded rocks of the Central Iberian Zone near the contact with the Ossa Morena Zone and is affected by several shear zones and faults. Its contact metamorphic aureole is constituted by micaschist with porphyroblasts in the outer zone and hornfels in the inner zone. Small mainstreamed xenoliths are dispersed all over the body. The pluton has a great mineralogical heterogeneity with pronounced variations in muscovite/biotite and plagioclase/ microcline contents and is classified as granite, granodiorite or tonalite. It is a leuco granite, highly peraluminous (A/CN K = 1.31 - 1.64), magnesian and calc-alkaline to alkaline-calcic. The variation diagrams show curvilinear trends with silica. Eu/Eu* = 0.47 - 0.77 and there is a slight enrichment in LREE relative to HREE. The normalized diagrams indicated dominantly crustal granite, related to subduction. U-Pb isotopic data of zircon and monazite gives 540-542 Ma age. (Author) 19 refs.

Petrographical and geochemical characterization and deformation conditions of the San Cristobal pluton, Sierra de Velasco, La Rioja, Argentina; Caracterizacion petrografica y geoquimica y condiciones de deformacion del pluton San Cristobal, Sierra de Velasco, La Rioja, Argentina

Energy Technology Data Exchange (ETDEWEB)

Bellos, L.I.; Toselli, A.J.; Rossi, J.N.; Grosse, P.; Rosa, J.D. de la; Castro, A.

2010-07-01

The San Cristobal pluton is a 35 km2 granitic body that outcrops at the southestern tip of the Sierra de Velasco, located west of La Rioja city, Argentina. It is formed by monzogranites and syenogranites, together with scarce granodiorites, with medium to fine-grained, equigranular to slightly porphyritic textures. Their mineral assemblage consists of quartz + microcline + plagioclase + biotite {+-} muscovite + zircon + apatite + magnetite. The granite contains dioritic to tonalitic mafic enclaves. The central and eastern parts of the granite have been deformed by the NNW-SSE trending South Mylonitic shear zone formed by mylonitic rocks. The metamorphic host-rock is represented by scarce greenschist facies xenoliths and hornfels with the high T/P assemblage K-feldspar - cordierite - biotite {+-} sillimanite. The granites are calc-alkaline, weak- to moderately peraluminous, and formed as part of a continental magmatic arc developed along the active margin of western Gondwana during the Early Paleozoic. The depth of emplacement of the San Cristobal pluton is estimated at {approx}12 km. (Author).

The Athabasca Granulite Terrane and Evidence for Dynamic Behavior of Lower Continental Crust

Science.gov (United States)

Dumond, Gregory; Williams, Michael L.; Regan, Sean P.

2018-05-01

Deeply exhumed granulite terranes have long been considered nonrepresentative of lower continental crust largely because their bulk compositions do not match the lower crustal xenolith record. A paradigm shift in our understanding of deep crust has since occurred with new evidence for a more felsic and compositionally heterogeneous lower crust than previously recognized. The >20,000-km2 Athabasca granulite terrane locally provides a >700-Myr-old window into this type of lower crust, prior to being exhumed and uplifted to the surface between 1.9 and 1.7 Ga. We review over 20 years of research on this terrane with an emphasis on what these findings may tell us about the origin and behavior of lower continental crust, in general, in addition to placing constraints on the tectonic evolution of the western Canadian Shield between 2.6 and 1.7 Ga. The results reveal a dynamic lower continental crust that evolved compositionally and rheologically with time.

Space imaging of a 300 years old cooling magma chamber: Timanfaya volcano (Lanzarote, Canary Islands)

Science.gov (United States)

Gonzalez, P. J.; Tiampo, K. F.

2010-12-01

Multitemporal space radar interferometry analysis between 1992 and 2000 revealed significantly deforming areas with a magnitude of 4-6 mm/yr of lengthening in the radar line of sight at Timanfaya volcano (Lanzarote, Canary Island). Timanfaya volcano erupted almost 300 years ago (1730-1736), along a 15 km-long fissure-feeding magmatic system, resulting in the longest and largest historical eruption of the Canarian archipelago to date, with >1 km3 of erupted basaltic lavas covering 200 km2. High surficial temperature (600 degrees-C at 13 m) and high heat flux measurements (150 mW/m2) suggest that the remnants of the magmatic chamber that fed the 1730-1736 are still partly molten. Here, we present preliminary models of the subsidence taking into account all available data, including geophysical data (heat flux, seismic, magnetotelluric and gravity), the geochemistry of freshly erupted lavas, upper mantle and crustal xenoliths, and structural geology.

A propagator matrix method for the Rayleigh-Taylor instability of multiple layers: a case study on crustal delamination in the early Earth

Science.gov (United States)

Mondal, Puskar; Korenaga, Jun

2018-03-01

The dispersion relation of the Rayleigh-Taylor instability, a gravitational instability associated with unstable density stratification, is of profound importance in various geophysical contexts. When more than two layers are involved, a semi-analytical technique based on the biharmonic formulation of Stokes flow has been extensively used to obtain such dispersion relation. However, this technique may become cumbersome when applied to lithospheric dynamics, where a number of layers are necessary to represent the continuous variation of viscosity over many orders of magnitude. Here, we present an alternative and more efficient method based on the propagator matrix formulation of Stokes flow. With this approach, the original instability problem is reduced to a compact eigenvalue equation whose size is solely determined by the number of primary density contrasts. We apply this new technique to the stability of the early crust, and combined with the Monte Carlo sensitivity analysis, we derive an empirical formula to compute the growth rate of the Rayleigh-Taylor instability for this particular geophysical setting. Our analysis indicates that the likelihood of crustal delamination hinges critically on the effective viscosity of eclogite.

Chronology and volcanology of the 1949 multi-vent rift-zone eruption on La Palma (Canary Islands)

Science.gov (United States)

Klügel, A.; Schmincke, H.-U.; White, J. D. L.; Hoernle, K. A.

1999-12-01

The compositionally zoned San Juan eruption on La Palma emanated from three eruptive centers located along a north-south-trending rift zone in the south of the island. Seismic precursors began weakly in 1936 and became strong in March 1949, with their foci progressing from the north of the rift zone towards its south. This suggests that magma ascended beneath the old Taburiente shield volcano and moved southward along the rift. The eruption began on June 24, 1949, with phreatomagmatic activity at Duraznero crater on the ridgetop (ca. 1880 m above sea level), where five vents erupted tephritic lava along a 400-m-long fissure. On June 8, the Duraznero vents shut down abruptly, and the activity shifted to an off-rift fissure at Llano del Banco, located at ca. 550 m lower elevation and 3 km to the northwest. This eruptive center issued initially tephritic aa and later basanitic pahoehoe lava at high rates, producing a lava flow that entered the sea. Two days after basanite began to erupt at Llano del Banco, Hoyo Negro crater (ca. 1880 m asl), located 700 m north of Duraznero along the rift, opened on July 12 and produced ash and bombs of basanitic to phonotephritic composition in violent phreatomagmatic explosions ( White and Schmincke, 1999). Llano del Banco and Hoyo Negro were simultaneously active for 11 days and showed a co-variance of their eruption rates indicating a shallow hydraulic connection. On July 30, after 3 days of quiescence at all vents, Duraznero and Hoyo Negro became active again during a final eruptive phase. Duraznero issued basanitic lava at high rates for 12 h and produced a lava flow that descended towards the east coast. The lava contains ca. 1 vol.% crustal and mantle xenoliths consisting of 40% tholeiitic gabbros from the oceanic crust, 35% alkaline gabbros, and 20% ultramafic cumulates. The occurrence of xenoliths almost exclusively in the final lava is consistent with their origin by wall-rock collapse at depth near the end of the eruption

Lithospheric magma dynamics beneath the El Hierro Volcano, Canary Islands: insights from fluid inclusions

Science.gov (United States)

Oglialoro, E.; Frezzotti, M. L.; Ferrando, S.; Tiraboschi, C.; Principe, C.; Groppelli, G.; Villa, I. M.

2017-10-01

At active volcanoes, petrological studies have been proven to be a reliable approach in defining the depth conditions of magma transport and storage in both the mantle and the crust. Based on fluid inclusion and mineral geothermobarometry in mantle xenoliths, we propose a model for the magma plumbing system of the Island of El Hierro (Canary Islands). The peridotites studied here were entrained in a lava flow exposed in the El Yulan Valley. These lavas are part of the rift volcanism that occurred on El Hierro at approximately 40-30 ka. The peridotites are spinel lherzolites, harzburgites, and dunites which equilibrated in the shallow mantle at pressures between 1.5 and 2 GPa and at temperatures between 800 and 950 °C (low-temperature peridotites; LT), as well as at higher equilibration temperatures of 900 to 1100 °C (high-temperature peridotites; HT). Microthermometry and Raman analyses of fluid inclusions reveal trapping of two distinct fluid phases: early type I metasomatic CO2-N2 fluids ( X N2 = 0.01-0.18; fluid density (d) = 1.19 g/cm3), coexisting with silicate-carbonate melts in LT peridotites, and late type II pure CO2 fluids in both LT (d = 1.11-1.00 and 0.75-0.65 g/cm3) and HT ( d = 1.04-1.11 and 0.75-0.65 g/cm3) peridotites. While type I fluids represent metasomatic phases in the deep oceanic lithosphere (at depths of 60-65 km) before the onset of magmatic activity, type II CO2 fluids testify to two fluid trapping episodes during the ascent of xenoliths in their host mafic magmas. Identification of magma accumulation zones through interpretation of type II CO2 fluid inclusions and mineral geothermobarometry indicate the presence of a vertically stacked system of interconnected small magma reservoirs in the shallow lithospheric mantle between a depth of 22 and 36 km (or 0.67 to 1 GPa). This magma accumulation region fed a short-lived magma storage region located in the lower oceanic crust at a depth of 10-12 km (or 0.26-0.34 GPa). Following our model

COMPLEX GEOLOGICAL–GEOPHYSICAL 3D MODEL OF THE CRUST IN THE SOUTHEASTERN FENNOSCANDIAN SHIELD: NATURE OF DENSITY LAYERING OF THE CRUST AND THE CRUST–MANTLE BOUNDARY

Directory of Open Access Journals (Sweden)

V. N. Glaznev

2015-01-01

combinations of crustal thickness and temperature at the level of Moho discontinuity, the crust in a platform region can be transformed into eclogites. In this case, the crust–mantle boundary is determined by quantitative proportions of the rocks that underwent eclogitization or escaped this process and by corresponding density and velocity values. (5 High compaction of rocks in the crust under lithostatic loading cannot be explained by «simple» concepts of metamorphism and/or rock compaction, which are based on laboratory studies of rock samples and mathematical simulations; this is an evidence of the existence of additional, quite strong mechanisms providing for reversible changes of the rocks.

Iron isotopic systematics of oceanic basalts

Science.gov (United States)

Teng, Fang-Zhen; Dauphas, Nicolas; Huang, Shichun; Marty, Bernard

2013-04-01

The iron isotopic compositions of 93 well-characterized basalts from geochemically and geologically diverse mid-ocean ridge segments, oceanic islands and back arc basins were measured. Forty-three MORBs have homogeneous Fe isotopic composition, with δ56Fe ranging from +0.07‰ to +0.14‰ and an average of +0.105 ± 0.006‰ (2SD/√n, n = 43, MSWD = 1.9). Three back arc basin basalts have similar δ56Fe to MORBs. By contrast, OIBs are slightly heterogeneous with δ56Fe ranging from +0.05‰ to +0.14‰ in samples from Koolau and Loihi, Hawaii, and from +0.09‰ to +0.18‰ in samples from the Society Islands and Cook-Austral chain, French Polynesia. Overall, oceanic basalts are isotopically heavier than mantle peridotite and pyroxenite xenoliths, reflecting Fe isotope fractionation during partial melting of the mantle. Iron isotopic variations in OIBs mainly reflect Fe isotope fractionation during fractional crystallization of olivine and pyroxene, enhanced by source heterogeneity in Koolau samples.

Abnormal lithium isotope composition from the ancient lithospheric mantle beneath the North China Craton.

Science.gov (United States)

Tang, Yan-Jie; Zhang, Hong-Fu; Deloule, Etienne; Su, Ben-Xun; Ying, Ji-Feng; Santosh, M; Xiao, Yan

2014-03-04

Lithium elemental and isotopic compositions of olivines in peridotite xenoliths from Hebi in the North China Craton provide direct evidence for the highly variable δ(7)Li in Archean lithospheric mantle. The δ(7)Li in the cores of olivines from the Hebi high-Mg# peridotites (Fo > 91) show extreme variation from -27 to +21, in marked deviation from the δ(7)Li range of fresh MORB (+1.6 to +5.6) although the Li abundances of the olivines are within the range of normal mantle (1-2 ppm). The Li abundances and δ(7)Li characteristics of the Hebi olivines could not have been produced by recent diffusive-driven isotopic fractionation of Li and therefore the δ(7)Li in the cores of these olivines record the isotopic signature of the subcontinental lithospheric mantle. Our data demonstrate that abnormal δ(7)Li may be preserved in the ancient lithospheric mantle as observed in our study from the central North China Craton, which suggest that the subcontinental lithospheric mantle has experienced modification of fluid/melt derived from recycled oceanic crust.

Research by ESS Division for the Nevada Nuclear Waste Storage Investigations: Progress report, January-June 1985

International Nuclear Information System (INIS)

Vaniman, D.

1987-10-01

Petrographic research for the Nevada Nuclear Waste Storage Investigations focused on xenolithic variability in the Topopah Spring Member and on variations of clinoptilolite composition at Yucca Mountain. Zeolite and smectite occurrences were considered in terms of their relation to a disturbed zone beneath the potential repository, and mineral stability experiments have produced a new clinoptilolite structure as a result of prolonged heating at low temperature. Limitations were defined on the abundance of erionite and of sulfur. X-ray diffraction studies lead to improved analytical methods. Progress was made in the comparative study of mineralogy in sand ramps and in faults. Geological modeling considered the differences of the diffusion of nonsorbing tracers in vertically and in horizontally fractured rock. Modeling also treated the diffusion of a nonsorbing tracer in devitrified and in zeolitized rock. The results of these experiments in all cases show relatively symmetrical two-dimensional diffusion patterns. Preliminary calculations compare the dispersion/diffusion of nonsorbing Tc with the dispersion/diffusion/sorption of U. 27 refs., 20 figs., 5 tabs

The formation conditions of the burial site of Late Cretaceous dinosaurs and plants in the Kakanaut River basin (Koryak Highlands, Northeastern Asia)

Science.gov (United States)

Shczepetov, S. V.; Herman, A. B.

2017-07-01

The stratigraphic position of layers containing plant and animal remains in the Koryak Highlands (Northeast Asia) is under discussion. Their age is defined as late Campanian-early Maastrichtian. Plant-bearing and bone-bearing rocks represent cemented basaltic tephra. The former contain a small amount of xenogenic material and slightly rounded volcaniclastic material, which indicates its insignificant transportation. Ash particles in bone-bearing rocks are even less rounded. Among them, there are no rock fragments of other composition. Large bones and their fragments, as xenoliths, are chaotically distributed in the rock matrix as if floating in mass of ash material. This burial site was probably formed in a continental environment as a result of the gravitational and eolian transportation of the terrigenous material. The burial of small dinosaur bones and teeth occurred during the deposition of a small stream of a semiliquid water-ash mixture. This work presents a possible mechanism of the formation of burial sites, taking into consideration proposed conditions of the life and reproduction of dinosaurs in the Late Mesozoic Arctic.

Texture and Elastic Anisotropy of Mantle Olivine

Science.gov (United States)

Nikitin, A. N.; Ivankina, T. I.; Bourilitchev, D. E.; Klima, K.; Locajicek, T.; Pros, Z.

Eight olivine rock samples from different European regions were collected for neu- tron texture analyses and for P-wave velocity measurements by means of ultrasonic sounding at various confining pressures. The orientation distribution functions (ODFs) of olivine were determined and pole figures of the main crystallographic planes were calculated. The spatial P-wave velocity distributions were determined at confining pressures from 0.1 to 400 MPa and modelled from the olivine textures. In dependence upon the type of rock (xenolith or dunite) different behavior of both the P-wave veloc- ity distributions and the anisotropy coefficients with various confining pressures was observed. In order to explain the interdependence of elastic anisotropy and hydrostatic pressure, a model for polycrystalline olivine rocks was suggested, which considers the influence of the crystallographic and the mechanical textures on the elastic behaviour of the polycrystal. Since the olivine texture depends upon the active slip systems and the deformation temperature, neutron texture analyses enable us to estimate depth and thermodynamical conditions during texture formation.

Petrology and isotopic composition of Quaternary basanites dredged from the Bering Sea continental margin near Navarin Basin

Science.gov (United States)

Davis, A.S.; Gunn, S.H.; Gray, L.-B.; Marlow, M. S.; Wong, F.L.

1993-01-01

Quaternary basanites were recovered from the continental margin of the Bering Sea near Navarin Basin. The basanites are highly vesicular flow rock and hyaloclastites similar to other alkalic volcanic rocks erupted repeatedly during the last Cenozoic on islands in the Bering Sea region and in mainland Alaska. K-Ar ages for the basanites indicate at least two episodes of volcanism at about 1.1 and 0.4 Ma. Trace-element data indicate these alkalic lavas have been generated by small, but variable, amounts of partial melting of a metasomatized lherzolite source. The relativley primitive compositions (MgO >9%), presence of mantle-derived xenoliths in some alkalic lavas, and presence of forsteritic olivine with low CaO and high NiO suggest that magma rose rapidly from great depth without spending time in large, long-lived magma chambers. Alkalic volcanism apparently resulted from upwelling and decompressional melting of small isolated mantle diapirs in response to local lithospheric attenuation associated with jostling of blocks during adjustment to regional stresses. -from Authors

STXM-XANES Analysis of Organic Matter in Dark Clasts and Halite Crystals in Zag and Monahans Meteorites

Science.gov (United States)

Kebukawa, Y.; Zolensky, M. E.; Fries, M.; Nakato, A.; Kilcoyne, A. L. D.; Takeichi, Y.; Suga, H.; Miyamoto, C.; Rahman, Z.; Kobayashi, K.;

Significance of mylonites and phyllonites in the Pohorje and Kobansko area

Directory of Open Access Journals (Sweden)

Mirka Trajanova

2002-06-01

Full Text Available The wide distribution of mylonites and phyllonites in the Pohorje and Kobansko areas is the consequence of the extensive northward thrusting caused by the Cretaceous collision of the European and African plates. In the accretionary wedge meta-ultrabasites withparts of the oceanic crust were incorporated between the nappes and tectonically transported to higher levels. Ascending of the cizlakite and granodiorite melts resulted into the core structure of the Pohorje. The highest were emplaced the most metamorphosed rocksof the eclogite facies. Rocks of the Pohorje Formation continue on the other side of the Ribnica-Selnica stair-like structure in the Kobansko area. The influence of the thrusting is noticed also in the Pohorje granodiorite, which is slightly altered in the zeolite faciesconditions. Part of it (Pekrska gorca was detached and probably in the final stage of thrusting displaced for at least 11 km toward NNE. The eastern prolongation of the Pohorje massif subsided toward the Panonian basin. The pivotal Labot fault separated Pohorje and Kobansko areas from Karavanke and Strojna in Neogene. Along it the NW part of the Pohorje block was downthrown.

Origin of unusual HREE-Mo-rich carbonatites in the Qinling orogen, China.

Science.gov (United States)

Song, Wenlei; Xu, Cheng; Smith, Martin P; Kynicky, Jindrich; Huang, Kangjun; Wei, Chunwan; Zhou, Li; Shu, Qihai

2016-11-18

Carbonatites, usually occurring within intra-continental rift-related settings, have strong light rare earth element (LREE) enrichment; they rarely contain economic heavy REE (HREE). Here, we report the identification of Late Triassic HREE-Mo-rich carbonatites in the northernmost Qinling orogen. The rocks contain abundant primary HREE minerals and molybdenite. Calcite-hosted fluid inclusions, inferred to represent a magmatic-derived aqueous fluid phase, contain significant concentrations of Mo (~17 ppm), reinforcing the inference that these carbonatitic magmas had high Mo concentrations. By contrast, Late Triassic carbonatites in southernmost Qinling have economic LREE concentrations, but are depleted in HREE and Mo. Both of these carbonatite types have low δ 26 Mg values (-1.89 to -1.07‰), similar to sedimentary carbonates, suggesting a recycled sediment contribution for REE enrichment in their mantle sources. We propose that the carbonatites in the Qinling orogen were formed, at least in part, by the melting of a subducted carbonate-bearing slab, and that 10 Ma younger carbonatite magmas in the northernmost Qinling metasomatized the thickened eclogitic lower crust to produce high levels of HREE and Mo.

An internally consistent pressure calibration of geobarometers applicable to the Earth’s upper mantle using in situ XRD

Energy Technology Data Exchange (ETDEWEB)

Beyer, Christopher; Rosenthal, Anja; Myhill, Robert; Crichton, Wilson A.; Yu, Tony; Wang, Yanbin; Frost, Daniel J.

2018-02-01

We have performed an experimental cross calibration of a suite of mineral equilibria within mantle rock bulk compositions that are commonly used in geobarometry to determine the equilibration depths of upper mantle assemblages. Multiple barometers were compared simultaneously in experimental runs, where the pressure was determined using in-situ measurements of the unit cell volumes of MgO, NaCl, Re and h-BN between 3.6 and 10.4 GPa, and 1250 and 1500 °C. The experiments were performed in a large volume press (LVPs) in combination with synchrotron X-ray diffraction. Noble metal capsules drilled with multiple sample chambers were loaded with a range of bulk compositions representative of peridotite, eclogite and pyroxenite lithologies. By this approach, we simultaneously calibrated the geobarometers applicable to different mantle lithologies under identical and well determined pressure and temperature conditions. We identified discrepancies between the calculated and experimental pressures for which we propose simple linear or constant correction factors to some of the previously published barometric equations. As a result, we establish internally-consistent cross-calibrations for a number of garnet-orthopyroxene, garnet-clinopyroxene, Ca-Tschermaks-in-clinopyroxene and majorite geobarometers.

Piecewise delamination of Moroccan lithosphere from beneath the Atlas Mountains

Science.gov (United States)

Bezada, M. J.; Humphreys, E. D.; Davila, J. M.; Carbonell, R.; Harnafi, M.; Palomeras, I.; Levander, A.

2014-04-01

The elevation of the intracontinental Atlas Mountains of Morocco and surrounding regions requires a mantle component of buoyancy, and there is consensus that this buoyancy results from an abnormally thin lithosphere. Lithospheric delamination under the Atlas Mountains and thermal erosion caused by upwelling mantle have each been suggested as thinning mechanisms. We use seismic tomography to image the upper mantle of Morocco. Our imaging resolves the location and shape of lithospheric cavities and of delaminated lithosphere ˜400 km beneath the Middle Atlas. We propose discontinuous delamination of an intrinsically unstable Atlas lithosphere, enabled by the presence of anomalously hot mantle, as a mechanism for producing the imaged structures. The Atlas lithosphere was made unstable by a combination of tectonic shortening and eclogite loading during Mesozoic rifting and Cenozoic magmatism. The presence of hot mantle sourced from regional upwellings in northern Africa or the Canary Islands enhanced the instability of this lithosphere. Flow around the retreating Alboran slab focused upwelling mantle under the Middle Atlas, which we infer to be the site of the most recent delamination. The Atlas Mountains of Morocco stand as an example of large-scale lithospheric loss in a mildly contractional orogen.

Fluid heterogeneity during granulite facies metamorphism in the Adirondacks: stable isotope evidence

Science.gov (United States)

Valley, J.W.; O'Neil, J.R.

1984-01-01

premetamorphic isotopic compositions. Such preservation is particularly evident in instances of high ??18O calcites (25.0 to 27.2), low ??18O wollastonites (-1.3 to 3.5), and sharp gradients in ??18O (18 permil/15m between marble and anorthosite, 8 permil/25 m in metasediments, and 6 permil/1 m in skarn). Isotopic exchange is seen across marble-anorthosite and marble-granite contacts only at the scale of a few meters. Small (<5 m) marble xenoliths are in approximate exchange equilibrium with their hosts, but for larger xenoliths and layers of marble there is no evidence of exchange at distances greater than 10 m from meta-igneous contacts. ?? 1984 Springer-Verlag.

Interactions between magma and the lithospheric mantle during Cenozoic rifting in Central Europe

Science.gov (United States)

Meyer, Romain; Elkins-Tanton, Linda T.

2010-05-01

During the Cenozoic, extensive intraplate volcanic activity occurred throughout Central Europe. Volcanic eruptions extend over France (the Massif Central), central Germany (Eifel, Vogelsberg, Rhön; Heldburg), the Czech Republic (the Eger graben) and SW Poland (Lower Silesia), a region ~1,200 km wide. The origin of this predominantly alkaline intraplate magmatism is often genetically linked to one or several mantle plumes, but there is no convincing evidence for this. We have measured Pb isotope ratios, together with major and trace elements, in a representative set of mafic to felsic igneous rocks from the intra-plate Cenozoic Rhön Mts. and the Heldburg dike swarm in order to gain insight into the melting source and petrogenetic history of these melts. Three different mafic rock types (tholeiitic basalt, alkali basalt, basanite) were distinguished based on petrography and geochemistry within the investigated areas. Except for the lherzolite-bearing phonolite from the Veste Heldburg all other evolved magmas are trachytes. REE geochemistry and calculated partial melting modeling experiments for the three mafic magma types point to different degrees of partial melting in a garnet-bearing mantle source. In addition a new version of the ternary Th-Hf-Ta diagram is presented in this study as a useful petrological tool. This diagram is not only able to define potentially involved melting source end-members (e.g. asthenosphere, sub-continental lithospheric mantle and continental crust) but also interactions between these members are illustrated. An advantage of this diagram compared to partial melting degree sensitive multi-element diagrams is that a ternary diagram is a closed system. An earlier version of this diagram has been recently used to establish the nature and extent of crust mantle melt interaction of volcanic rifted margins magmas (Meyer et al. 2009). The Th-Hf-Ta geochemistry of the investigated magmas is similar to spinel and garnet xenoliths from different

Constraints on the Chemistry and Abundance of Hydrous Phases in Sub Continental Lithospheric Mantle: Implications for Mid-Lithospheric Discontinuities

Science.gov (United States)

Saha, S.; Dasgupta, R.; Fischer, K. M.; Mookherjee, M.

2017-12-01

The origins of a 2-10% reduction in seismic shear wave velocity (Vs) at depths of 60-160 km in sub continental lithospheric mantle (SCLM) regions, identified as the Mid Lithospheric Discontinuity (MLD) [e.g., 1] are highly debated [e.g., 2, 3]. One of the proposed explanations for MLDs is the presence of hydrous minerals such as amphibole and phlogopite at these depths [e.g., 2, 4, 5]. Although the stability and compositions of these phases in peridotite + H2O ± CO2 have been widely explored [e.g., 6], their composition and abundance as a function of permissible SCLM chemistry remain poorly understood. We have compiled phase equilibria experiments conducted over a range of pressure (0.5-8 GPa), temperature (680-1300 °C), major element peridotite compositions, and volatiles (H2O: 0.05-13.79 wt.% and CO2: 0.25-5.3 wt.%). The goal was to constrain how compositional parameters such as CaO and alkali/H2O affect the chemistry and abundance of amphibole and phlogopite. We observe that the abundance of amphibole increases with CaO content and decreasing alkali/H2O. The abundance of phlogopite varies directly with K2O content. Unlike phlogopite compositions that remain consistent, amphibole compositions show variability (pargastitic to K-richterite) depending on bulk CaO and Na2O. Mineral modes, obtained by mass balance on a melt/fluid free basis, were used to calculate aggregate shear wave velocity, Vs for the respective assemblages [e.g., 7] and compared with absolute values observed at MLD depths [e.g., 8]. Vs shows a strong inverse correlation with phlogopite and amphibole modes (particularly where phlogopite is absent). For the Mg# range of cratonic xenoliths, 5-10% phlogopite at MLD depths can match the observed Vs values, while CaO contents in cratonic xenoliths limit the amphibole abundance to 10%, which is lower than previous estimates based on heat flow calculations [e.g., 4]. The modes of hydrous and other phases and corresponding Vs values could be used to

Oceanic mantle rocks reveal evidence for an ancient, 1.2-1.3 Ga global melting event

Science.gov (United States)

Dijkstra, A. H.; Sergeev, D.; McTaminey, L.; Dale, C. W.; Meisel, T. C.

2011-12-01

It is now increasingly being recognized that many oceanic peridotites are refertilized harzburgites, and that the refertilization often masks an extremely refractory character of the original mantle rock 'protolith'. Oceanic peridotites are, when the effects of melt refertilization are undone, often too refractory to be simple mantle melting residues after the extraction of mid-ocean ridge basalts at a spreading center. Rhenium-osmium isotope analysis is a powerful method to look through the effects of refertilization and to obtain constraints on the age of the melting that produced the refractory mantle protolith. Rhenium-depletion model ages of such anomalously refractory oceanic mantle rocks - found as abyssal peridotites or as mantle xenoliths on ocean islands - are typically >1 Ga, i.e., much older than the ridge system at which they were emplaced. In my contribution I will show results from two case studies of refertilized anciently depleted mantle rocks (Macquarie Island 'abyssal' peridotites and Lanzarote mantle xenoliths). Interestingly, very refractory oceanic mantle rocks from sites all around the world show recurring evidence for a Mesoproterozoic (~1.2-1.3 Ga) melting event [1]. Therefore, oceanic mantle rocks seem to preserve evidence for ancient melting events of global significance. Alternatively, such mantle rocks may be samples of rafts of ancient continental lithospheric mantle. Laser-ablation osmium isotope 'dating' of large populations of individual osmium-bearing alloys from mantle rocks is the key to better constrain the nature and significance of these ancient depletion events. Osmium-bearing alloys form when mantle rocks are melted to high-degrees. We have now extracted over >250 detrital osmium alloys from placer gold occurrences in the river Rhine. These alloys are derived from outcrops of ophiolitic mantle rocks in the Alps, which include blocks of mantle rocks emplaced within the Tethys Ocean, and ultramafic lenses of unknown

Cumulate xenoliths from St. Vincent, Lesser Antilles Island Arc: a window into upper crustal differentiation of mantle-derived basalts

Science.gov (United States)

Tollan, P. M. E.; Bindeman, I.; Blundy, J. D.

2012-02-01

In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine-gabbros to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those in erupted basaltic lavas from St. Vincent and in published high-pressure (4-10 kb) experimental run products of a St. Vincent high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ18O) of cumulus olivine (4.89-5.18‰), plagioclase (5.84-6.28‰), clinopyroxene (5.17-5.47‰) and hornblende (5.48-5.61‰) and hydrogen isotope composition of hornblende (δD = -35.5 to -49.9‰) are all consistent with closed system magmatic differentiation of a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic compositions reported. δ18OOlivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H2O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at depth within the crust, lowering MgO and Cr2O3 and raising Al2O3 and CaO of residual melt due to suppression of plagioclase. Low density, hydrous basaltic and basaltic andesite melts then ascend rapidly through the crust, stalling at shallow depth upon water saturation where crystallisation of the chemically distinct cumulus phases observed in this study can occur. Deposited crystals armour the shallow magma chamber where oxygen isotope equilibration between minerals is slowly approached, before remobilisation and entrainment by later injections of magma.

Rhyolites from the Roztoky Intrusive Centre, České středohoří Mts.: Xenoliths or Dyke Differentiates?

Czech Academy of Sciences Publication Activity Database

Ulrych, Jaromír; Pivec, Edvín; Höhndorf, A.; Balogh, K.; Bendl, J.; Rutšek, J.

2000-01-01

Roč. 60, č. 4 (2000), s. 327-352 ISSN 0009-2819 R&D Projects: GA AV ČR IAA3111601 Grant - others:OTKA Foundation(HU) 014961; CZ-HU Hungarian-Czech Project(XC) "Comparative volcanostratigraphy of the Neodic volcanism of the Bohemian Massif and the Pannonian Basin"(Institute of Nuclear Research, Debrecen, and Institute of Geology of the Czech Academy of Sciences, Praha Subject RIV: DD - Geochemistry Impact factor: 0.147, year: 2000

The lower-temperature-pressure stability of pyrope in the presence of quartz in the system MgO-Al2O3-SiO2

Science.gov (United States)

Cheng, N.; Jenkins, D. M.

2017-12-01

Pyrope (Mg3Al2Si3O12) is the dominant component in garnets from type A eclogites. Determining the lower-pressure-temperature (P-T) stability of pyrope in the presence of quartz helps put constraints on the stability of quartz-bearing eclogites and therefore the depths to which crustal rocks in high pressure/ultra-high pressure (HP/UHP) terranes can be transferred. It also defines the lower-pressure stability of the nearly pure pyrope-bearing quartzites of the Dora Maira massif of the Western Alps (Chopin, 1984, Contrib. Min. Pet.). Aside from the approximate boundary proposed by Hensen & Essene (1971, Contrib. Min. Pet.), there has been no detailed study of the lower P-T stability of pyrope + quartz. A reversed determination of the reaction 3 enstatite + 2 kyanite = 2 pyrope + 2 quartz has been done in the system MgO-Al2O3-SiO2 over the P-T range of 900-1100 °C and 1.6-2.5 GPa for durations of 24 hours. Double capsules, one using pure enstatite and the other Al-rich (10 wt% Al2O3) enstatite in the starting mixtures, were used to obtain reversals on the Al content in the orthopyroxene (Opx). Experiments were done using a ½-inch diameter piston-cylinder press and NaCl-pyrex-MgO pressure media. Run products were analyzed using powder XRD and electron microprobe. Reaction direction was readily determined from peak height changes on XRD patterns. The reaction has been bracketed at 1.65 GPa at 1100 °C with > 12 wt% Al2O3 in Opx; 2.05 GPa at 1000 °C with 10 wt% Al2O3 in Opx; and 2.4 GPa at 930 °C with 5 wt% Al2O3 in Opx. The reaction boundary is slightly curved to higher P with increasing T caused by increasing Al in Opx. The boundary observed in this study is about 100 °C or 0.4 GPa higher than previously proposed by Hensen & Essene (1971) and 70-170 °C or 0.6-0.7 GPa higher than the boundary calculated in this system using THERMOCALC ds6.22 (Holland & Powell, 2011, J. Meta. Geol.) and about 1-4 wt% higher Al2O3 contents in Opx. Higher pressure runs in the field

The Morro do Resende orthogneiss: mineralogy, petrography, geochemistry and geochronology

International Nuclear Information System (INIS)

Vasconcelos, Fabiana Franco de; Avila, Ciro Alexandre; Bongiolo, Everton Marques; Camara, Beatriz de Oliveira; Menezes, Victor Hugo Riboura; Cunha, Fernanda Caetano de Mattos; Neumann, Reiner; Teixeira, Wilson; Barbosa, Natali

2017-01-01

The Morro do Resende orthogneiss is a hololeucocratic to leucocratic, fine-grained body with monzogranitic to granodioritic composition, cropping out near the Volta Grande mine in Nazareno County, State of Minas Gerais, Brazil. It bears xenoliths of metamafic rocks of the Rio das Mortes metavolcano-sedimentary sequence and yields a U-Pb SHRIMP crystallization age of 2174 ± 4 Ma, relating it to one of the magmatic pulses of the Mineiro Belt. The primary mineralogy includes quartz, albite, microcline, biotite, allanite, zircon, magnetite, titanite and apatite, while sericite, epidote, zoisite, clinozoisite, carbonate and chlorite are metamorphic minerals. Maghemite, barite, fluorite, monazite, xenotime, garnet and REE fluorides (possibly gagarinite) are hydrothermal, as they fill the fractures, intergrow or replace the primary and metamorphic minerals. Fluid interaction was not homogeneous throughout the body. Metamorphic paragenesis points to greenschist facies conditions, which could be related to the Paleoproterozoic II event of the Mineiro Belt, which lasted from 2131 to 2101 Ma. The Morro do Resende orthogneiss is distinguished by significant REE enrichment, as well as a negative Eu anomaly linked to the magmatic crystallization and a negative Ce anomaly related to oxidizing hydrothermal fluids circulation. (author)

New monazite U-Pb age constraints on the evolution of the Paleoproterozoic Vaasa granitoid batholith, western Finland

Directory of Open Access Journals (Sweden)

A.K. Kotilainen

2016-09-01

Full Text Available The Vaasa batholith, western Finland, is a large, peraluminous granitoid pluton that crystallized at 1.88–1.87 Ga during the culmination of the Svecofennian orogeny. The batholith has gradual contacts, through metatexites and diatexites, with the enveloping metasedimentary rocks of the Bothnian Belt. We present ID-TIMS U-Pb age data on monazite from granitoids and xenoliths of the Vaasa batholith and combine these with published U–Pb zircon ages in order to shed further light on the evolution of the Vaasa batholith. The apparent monazite ages for seven of the examined samples are 1870–1863 Ma, and 1855±3 Ma for one further sample from the southern part of the batholith. Combined with pre-existing data, the monazite ages of the granitoids are 9 to 18 Ma (face values or 3 to 9 Ma (external errors considered younger than the U–Pb zircon crystallization ages from respective samples. Our new data suggest slow cooling for the Vaasa batholith – the closure/saturation temperature of the monazite U–Pb system was probably reached in ~10 m.y. after the crystallization of magmatic zircon in the examined rocks.

U-Pb SHRIMP and Sm-Nd geochronology of the Silvania Volcanics and Jurubatuba Granite: juvenile paleoproterozoic crust in the basement of the Neo proterozoic Brasilia Belt, Goias, central Brazil

International Nuclear Information System (INIS)

Fischel, Danielle P.; Pimentel, Marcio M.; Fuck, Reinhardt A.; Armstrong, Richard

2001-01-01

U-Pb SHRIMP and Sm-Nd isotopic ages were determined for felsic meta volcanic rocks from the Silvania Sequence and Jurubatuba Granite in the central part of the Brasilia Belt. Zircon grains from a meta volcanic sample yielded 2115 ± 23 Ma and from the granite yielded 2089 ±14 Ma, interpreted as crystallization ages of these rocks. Six meta volcanic samples of the Silvania Sequence yielded a six-point whole-rock Sm-Nd isochron indicating a crystallization age of 2262 ±110 Ma and positive ε Nd (T) = +3.0 interpreted as a juvenile magmatic event. Nd isotopic analyses on samples from the Jurubatuba Granite have Paleoproterozoic T DM model ages between 2.30 and 2.42 Ga and ε Nd (T) values vary between -0.22 and -0.58. The oldest T DM value refers to a sedimentary xenolith in the granite. These results suggest crystallization ages of Silvania volcanics and Jurubatuba Granite are the first evidence of a ca. 2.14-2.08 juvenile magmatic event in the basement of the central part of the Brasilia Belt that implies the presence of arc/suture hidden in reworked basement of the Brasilia Belt. (author)

The Morro do Resende orthogneiss: mineralogy, petrography, geochemistry and geochronology; Ortognaisse Morro do Resende: mineralogia, petrografia, geoquimica e geocronologia

Energy Technology Data Exchange (ETDEWEB)

Vasconcelos, Fabiana Franco de; Avila, Ciro Alexandre; Bongiolo, Everton Marques; Camara, Beatriz de Oliveira; Menezes, Victor Hugo Riboura; Cunha, Fernanda Caetano de Mattos, E-mail: fma3003@globo.com, E-mail: avila@mn.ufrj.br, E-mail: ebongiolo@geologia.ufrj.br, E-mail: camara_b@hotmail.com, E-mail: vmenezes92@gmail.com, E-mail: fefemattos@gmail.com [Universidade Federal do Rio de Janeiro (UFRJ), RJ (Brazil); Neumann, Reiner, E-mail: reiner.neumann@gmail.com [Centro de Tecnologia Mineral (CETEM), Rio de Janeiro, RJ (Brazil); Teixeira, Wilson; Barbosa, Natali, E-mail: wteixeir@usp.br, E-mail: natali@usp.br [Universidade de Sao Paulo (USP), Sao Paulo, SP (Brazil). Instituto de Geociencias

2017-01-15

The Morro do Resende orthogneiss is a hololeucocratic to leucocratic, fine-grained body with monzogranitic to granodioritic composition, cropping out near the Volta Grande mine in Nazareno County, State of Minas Gerais, Brazil. It bears xenoliths of metamafic rocks of the Rio das Mortes metavolcano-sedimentary sequence and yields a U-Pb SHRIMP crystallization age of 2174 ± 4 Ma, relating it to one of the magmatic pulses of the Mineiro Belt. The primary mineralogy includes quartz, albite, microcline, biotite, allanite, zircon, magnetite, titanite and apatite, while sericite, epidote, zoisite, clinozoisite, carbonate and chlorite are metamorphic minerals. Maghemite, barite, fluorite, monazite, xenotime, garnet and REE fluorides (possibly gagarinite) are hydrothermal, as they fill the fractures, intergrow or replace the primary and metamorphic minerals. Fluid interaction was not homogeneous throughout the body. Metamorphic paragenesis points to greenschist facies conditions, which could be related to the Paleoproterozoic II event of the Mineiro Belt, which lasted from 2131 to 2101 Ma. The Morro do Resende orthogneiss is distinguished by significant REE enrichment, as well as a negative Eu anomaly linked to the magmatic crystallization and a negative Ce anomaly related to oxidizing hydrothermal fluids circulation. (author)

Geochemical and lead isotope evidence for a mid-ocean ridge type mineralization within a polymetamorphic ophiolite complex (Monte del Forno, North Italy/Switzerland)

Energy Technology Data Exchange (ETDEWEB)

Peretti, A; Koeppel, V

1986-11-01

Major, trace element and Pb isotope investigations show the presence of a mid-ocean ridge-type mineralization within the polymetamorphic Monte del Forno Unit. Detailed analysis of the lithostratigraphy of the amphibolites demonstrates a close similarity to recent oceanic crust: a dyke zone at the bottom, a hydrothermally altered zone with a stockwork-type Fe-Cu-Zn mineralization and a pillow zone at the top. Effects of hydrothermal seafloor alteration are restricted to an approximately 50 m thick horizon. Sulfide mineralization is accompanied by Ca and Sr depletion and Mn and minor Na and Mg enrichments. Mineralogically the horizon distinguishes itself from the unmineralized amphibolites by the presence of chlorite and contact metamorphic magnesio-cummingtonite. The chemical imprint of the hydrothermal seafloor alteration survived a regional greenschist and an upper amphibolite facies contact metamorphism. The MORB signature of the Pb isotopes is preserved in the central parts of the approximately 300 m thick amphibolite sequence. During the regional greenschist facies metamorphism the isotope characteristics of the amphibolites were almost completely changed at the contact to the metasediments. The contact metamorphism of the Bregaglia Intrusion produced no obvious Pb contamination even within amphibolite xenoliths in the granodiorite.

U-Pb SHRIMP and Sm-Nd geochronology of the Silvania Volcanics and Jurubatuba Granite: juvenile paleoproterozoic crust in the basement of the Neo proterozoic Brasilia Belt, Goias, central Brazil

Energy Technology Data Exchange (ETDEWEB)

Fischel, Danielle P.; Pimentel, Marcio M.; Fuck, Reinhardt A. [Brasilia Univ., DF (Brazil). Inst. de Geociencias; Armstrong, Richard [Australian National Univ., Canberra (Australia). Research School of Earth Sciences

2001-09-15

U-Pb SHRIMP and Sm-Nd isotopic ages were determined for felsic meta volcanic rocks from the Silvania Sequence and Jurubatuba Granite in the central part of the Brasilia Belt. Zircon grains from a meta volcanic sample yielded 2115 {+-} 23 Ma and from the granite yielded 2089 {+-}14 Ma, interpreted as crystallization ages of these rocks. Six meta volcanic samples of the Silvania Sequence yielded a six-point whole-rock Sm-Nd isochron indicating a crystallization age of 2262 {+-}110 Ma and positive {epsilon}{sub Nd} (T) = +3.0 interpreted as a juvenile magmatic event. Nd isotopic analyses on samples from the Jurubatuba Granite have Paleoproterozoic T{sub DM} model ages between 2.30 and 2.42 Ga and {epsilon}{sub Nd} (T) values vary between -0.22 and -0.58. The oldest T{sub DM} value refers to a sedimentary xenolith in the granite. These results suggest crystallization ages of Silvania volcanics and Jurubatuba Granite are the first evidence of a ca. 2.14-2.08 juvenile magmatic event in the basement of the central part of the Brasilia Belt that implies the presence of arc/suture hidden in reworked basement of the Brasilia Belt. (author)

Reconstruction of the dynamics of the 1800-1801 Hualalai eruption: Implications for planetary lava flows

Science.gov (United States)

Baloga, Stephen; Spudis, Paul

1993-01-01

The 1800-1801 eruption of alkalic basalt from the Hualalai volcano, Hawaii provides a unique opportunity for investigating the dynamics of lava flow emplacement with eruption rates and compositions comparable to those that have been suggested for planetary eruptions. Field observations suggest new considerations must be used to reconstruct the emplacement of these lava flows. These observations are: (1) the flow traversed the 15 km from the vent to the sea so rapidly that no significant crust formed and an observation of the eruption reported that the flow reach the sea from the vent in approximately 1 hour; (2) the drainage of beds of xenolith nodules indicates a highly fluid, low viscosity lava; (3) overspills and other morphologic evidence for a very low viscosity host fluid; (4) no significant longitudinal increase in flow thickness that might be associated with an increase in the rheological properties of the lava; and (5) the relatively large size of channels associated with the flow, up to 80 meters across and several km long. Models for many geologic mass movements and fast moving fluids with various loadings and suspensions are discussed.

Intraplate mantle oxidation by volatile-rich silicic magmas

Energy Technology Data Exchange (ETDEWEB)

Martin, Audrey M.; Médard, Etienne; Righter, Kevin; Lanzirotti, Antonio

2017-11-01

The upper subcontinental lithospheric mantle below the French Massif Central is more oxidized than the average continental lithosphere, although the origin of this anomaly remains unknown. Using iron oxidation analysis in clinopyroxene, oxybarometry, and melt inclusions in mantle xenoliths, we show that widespread infiltration of volatile (HCSO)-rich silicic melts played a major role in this oxidation. We propose the first comprehensive model of magmatism and mantle oxidation at an intraplate setting. Two oxidizing events occurred: (1) a 365–286 Ma old magmatic episode that produced alkaline vaugnerites, potassic lamprophyres, and K-rich calc-alkaline granitoids, related to the N–S Rhenohercynian subduction, and (2) < 30 Ma old magmatism related to W–E extension, producing carbonatites and hydrous potassic trachytes. These melts were capable of locally increasing the subcontinental lithospheric mantle fO2 to FMQ + 2.4. Both events originate from the melting of a metasomatized lithosphere containing carbonate + phlogopite ± amphibole. The persistence of this volatile-rich lithospheric source implies the potential for new episodes of volatile-rich magmatism. Similarities with worldwide magmatism also show that the importance of volatiles and the oxidation of the mantle in intraplate regions is underestimated.

Alkaline and Carbonate-rich Melt Metasomatism and Melting of Subcontinental Lithospheric Mantle: Evidence from Mantle Xenoliths, NE Bavaria, Bohemian Massif

Czech Academy of Sciences Publication Activity Database

Ackerman, Lukáš; Špaček, Petr; Magna, T.; Ulrych, Jaromír; Svojtka, Martin; Hegner, E.; Balogh, K.

2013-01-01

Roč. 54, č. 12 (2013), s. 2597-2633 ISSN 0022-3530 R&D Projects: GA ČR GA205/09/1170 Institutional research plan: CEZ:AV0Z30130516; CEZ:AV0Z30120515 Institutional support: RVO:67985831 ; RVO:67985530 Keywords : peridotite * metasomatism * subcontinental lithosphere * Sr-Nd-Li isotopes * microstructure Subject RIV: DB - Geology ; Mineralogy Impact factor: 4.485, year: 2013

Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing.

Science.gov (United States)

Audet, Pascal; Bostock, Michael G; Christensen, Nikolas I; Peacock, Simon M

2009-01-01

Water and hydrous minerals play a key part in geodynamic processes at subduction zones by weakening the plate boundary, aiding slip and permitting subduction-and indeed plate tectonics-to occur. The seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization. However, seismological observations bearing on the presence of water within the subducting plate itself are less well documented. Here we use converted teleseismic waves to obtain observations of anomalously high Poisson's ratios within the subducted oceanic crust from the Cascadia continental margin to its intersection with forearc mantle. On the basis of pressure, temperature and compositional considerations, the elevated Poisson's ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values. Combined with observations of a strong negative velocity contrast at the top of the oceanic crust, our results imply that the megathrust is a low-permeability boundary. The transition from a low- to high-permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization. These results may have important implications for our understanding of seismogenesis, subduction zone structure and the mechanism of episodic tremor and slip.

Emergence of silicic continents as the lower crust peels off on a hot plate-tectonic Earth

Science.gov (United States)

Chowdhury, Priyadarshi; Gerya, Taras; Chakraborty, Sumit

2017-09-01

The rock record and geochemical evidence indicate that continental recycling has been occurring since the early history of the Earth. The stabilization of felsic continents in place of Earth's early mafic crust about 3.0 to 2.0 billion years ago, perhaps due to the initiation of plate tectonics, implies widespread destruction of mafic crust during this time interval. However, the physical mechanisms of such intense recycling on a hotter, (late) Archaean and presumably plate-tectonic Earth remain largely unknown. Here we use thermomechanical modelling to show that extensive recycling via lower crustal peeling-off (delamination but not eclogitic dripping) during continent-continent convergence was near ubiquitous during the late Archaean to early Proterozoic. We propose that such destruction of the early mafic crust, together with felsic magmatism, may have caused both the emergence of silicic continents and their subsequent isostatic rise, possibly above the sea level. Such changes in the continental character have been proposed to influence the Great Oxidation Event and, therefore, peeling-off plate tectonics could be the geodynamic trigger for this event. A transition to the slab break-off controlled syn-orogenic recycling occurred as the Earth aged and cooled, leading to reduced recycling and enhanced preservation of the continental crust of present-day composition.

Surface heat flow and lithosphere thermal structure of the larger Luxembourg area as a basis for the evaluation of its geothermal potential

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Schintgen, Tom; Förster, Andrea

2014-05-01

The evaluation of the geothermal potential and the type of geothermal use necessitates knowledge of the subsurface temperature distribution in combination with hydraulic properties (e.g. porosity, permeability and hydraulic conductivity). In the larger Luxembourg area, only a few subsurface temperature data are available restricted to shallow depth. This paucity in data required to assess the thermal regime to drillable depths by modeling. The thermal model was constrained by surface heat flow and the lithosphere-asthenosphere boundary (LAB) characterized by the 1300° C isotherm. A surface heat-flow value of 75 ± 7 (2σ) mW m-2 was determined in central Luxembourg, which corroborates most values known from adjacent areas. The conceptual geological model for thermal modeling has a high resolution in the upper 15 km due to a wealth of geological data, while refraction seismic data and xenoliths provide petrological constraints for the lower part of the model down to the crust/mantle boundary. Thermal rock properties assigned to geological units are based on a large set of laboratory data, complemented by some literature data for the lower parts of the crust. The thermal structure is investigated by calculating 2-D steady-state thermal models along three crustal cross sections developed for the study area assuming a purely conductive lithosphere. The location of the LAB at 100 km depth, as typical for the Ardennes, provides the best fit with the measured surface heat flow of about 75 mW m-2. This LAB model provides temperatures at 5 km of 115-118° C on average and of about 600° C at the Moho. The resulting mantle heat flow in this model is 39-40 mW m-2. A reduced lithosphere thickness of 50 km as typical for the Eifel area to the east results in an increase of surface heat flow to 97 mW m-2 and of the mantle heat flow to 65 mW m-2, respectively. If heating from the Eifel plume had reached the surface yet, temperatures at 5 km would be about 20° C higher (and

Experimental measurements of 3He and 4He mobility in olivine and clinopyroxene at magmatic temperatures

Science.gov (United States)

Trull, T. W.; Kurz, M. D.

1993-03-01

In-vacuo heating of 0.5 -0.7 mm olivine and clinopyroxene grains separated from Hawaiian ultramafic xenoliths leads to complete He loss within hours to days for temperatures of 700-1400°C. Diffusivities calculated from the observed release rates assuming spherical grains and initially homogeneous He distributions define Arrhenius relations with activation energies of 420 ± 20 and 290 ± 40 Kj/mol and log 10D0 of +5.1 ± .7 and +2.1 ± 1.2 cm 2/s in olivine and pyroxene, respectively. Values at 1350°C are 5.3 × 10 -9 cm 2/s in olivine and 10 times faster in pyroxene (4.7 × 10 -8 cm 2/s). These values include small corrections for grain size variations and, in the case of olivine, about 15% prior diffusive He loss. However, an important factor that has not been considered in previous studies of this type, is that the xenolith He resides predominantly within CO 2 rich fluid inclusions. Theoretical description of He loss in such a case demonstrates that the diffusivities calculated using the standard approach actually represent the product of the true volume diffusivity ( D) and the helium solubility, as represented by the distribution coefficient KDv (defined by C crystal/C fluid). Although He solubility in crystals is not well determined, estimates based on the CO 2 concentrations in these samples suggest that it is very low ( KDv of 3 × 10 -4 for pyroxene and 6 × 10 -6 for olivine; which also implies low crystal-melt distribution coefficients of .05 and .001). The resultant corrected diffusion rates are significantly faster than those obtained by the standard approach (~ 10 -4 cm 2/s at 1350°C and are thus higher than basaltic melt values). The most reasonable interpretation of this result is that He release is enhanced by internal grain fractures, including the planar healed cracks along which most fluid inclusions are arrayed. This treatment illustrates the difficulties involved in extrapolating laboratory He release measurements to nature, in particular

Greenschist-Facies Pseudotachylytes and Gouge: a Microstructural Study of the Deformation Propagation at the Boundary Between Hp-Metabasite and Calcite Bearing Metasediments

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Crispini, L.; Scambelluri, M.; Capponi, G.

2013-12-01

Recent friction experiments on calcite-bearing systems reproduce pseudotachylyte structures, that are diagnostic of dinamic calcite recrystallization related to seismic slip in the shallow crust. Here we provide the study of a pseudotachylyte (PT) bearing low angle oblique-slip fault. The fault is linked to the exhumation of Alpine HP-ophiolites and it is syn- to post-metamorphic with respect to retrograde greenschist facies metamorphism. The observed microstructures developed at the brittle-ductile transition and suggest that seismic and interseismic slip was enhanced by interaction with fluids. The fault zone is in-between high-pressure eclogite-facies metabasites (hangingwall) and calcite bearing metasediments (footwall). The mafic rocks largely consist of upper greenschist facies hornblende, albite, chlorite, epidote with relict eclogitic garnet, Na-pyroxene and rutile; metasediments correspond to calcschist and micaschist with quartz, phengite, zoisite, chlorite, calcite and relics of garnet. Key features of the oucrop are: the thickness and geometry of the PT and gouge; the multiple production of PT characterized by overprinting plastic and brittle deformation; the occurrence in footwall metasediments of mm-thick bands of finely recrystallized calcite coeval with PT development in the hangingwall. The damage zone is ca. 2 m-thick and is characterized by two black, ultra-finegrained straight and sharp Principal Slip Zones (PSZ) marked by PT. The damage zone shows a variety of fault rocks (cataclasite and ultracataclasite, gouge and PT) with multiple crosscutting relationships. Within the two main PSZ, PT occurs in 10-20 cm thick layer, in small scale injection veins and in microfractures. In the mafic hanging wall, the PT is recrystallized and does not preserve glass: it shows flow structures with subrounded, embayed and rebsorbed quartz in a fine grained matrix composed of isotropic albite + chlorite + quartz + epidote + titanite, suggesting recrystallization

Strain-dependent evolution of garnets in a high pressure ductile shear zone using Synchroton x-ray microtomography

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Macente, Alice; Fusseis, Florian; Menegon, Luca; John, Timm

2016-04-01

Synkinematic reaction microfabrics carry important information on the kinetics, timing and rheology of tectonometamorphic processes. Despite being routinely interpreted in metamorphic and structural studies, reaction and deformation microfabrics are usually described in two dimensions. We applied Synchrotron-based x-ray microtomography to document the evolution of a pristine olivine gabbro into a deformed omphacite-garnet eclogite in 3D. In the investigated samples, which cover a strain gradient into a shear zone from the Western Gneiss Region (Norway) previously described by John et al., (2009), we focused on the spatial transformation of garnet coronas into elongated clusters of garnets. Our microtomographic data allowed us to quantify changes to the garnet volume, their shapes and their spatial arrangement. We combined microtomographic observations with light microscope- and backscatter electron images as well as electron microprobe- (EMPA) and electron backscatter diffraction (EBSD) analyses to correlate mineral composition and orientation data with the x-ray absorption signal of the same mineral grains. This allowed us to extrapolate our interpretation of the metamorphic microfabric evolution to the third dimension, effectively yielding a 4-dimensional dataset. We found that: - The x-ray absorption contrast between individual mineral phases in our microtomographic data is sufficient to allow the same petrographic observations than in light- and electron microscopy, but extended to 3D. - Amongst the major constituents of the synkinematic reactions, garnet is the only phase that can be segmented confidently from the microtomographic data. - With increasing deformation, the garnet volume increases from about 9% to 25%. - Garnet coronas in the gabbros never completely encapsulate olivine grains. This may indicate that the reaction progressed preferentially in some directions, but also leaves pathways for element transport to and from the olivines that are

The Friningen Garnet Peridotite (central Swedish Caledonides). A good example of the characteristic PTt path of a cold mantle wedge garnet peridotite

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Gilio, Mattia; Clos, Frediano; van Roermund, Herman L. M.

2015-08-01

We present pseudosections of Cr-bearing garnet peridotite that together with new mineral-chemical data allow quantification of the early PT conditions of the original lithospheric mantle assemblage (M1) of the Friningen Garnet Peridotite (FGP) located in the central/middle belt of the Seve Nappe Complex in central Sweden. Results indicate that the early, coarse grained, olivine + orthopyroxene + clinopyroxene + "high Cr" garnet assemblage (M1a) was formed at 1100 ± 100 °C and 5.0 ± 0.5 GPa. These metamorphic conditions were followed by an inferred late Proterozoic exhumation event down to 850-900 °C and 1.5 GPa (M1b). The latter PT estimate is based on the breakdown of high-Cr M1a garnet (Cr# = 0.065) + olivine into an orthopyroxene + clinopyroxene + spinel (Cr# = 0.15-0.25) ± pargasite kelyphite (M1b) and the exsolution of garnet from Al-rich orthopyroxene and clinopyroxene. The M1b kelyphite is overprinted by an early-Caledonian UHPM mineral assemblage (M2; T = 800 °C and P = 3.0 GPa), equivalent to the earlier discovered UHP assemblage within an eclogitic dyke that cross-cuts FGP. In the garnet peridotite M2 is displayed by low-Cr garnet (Cr# = 0.030) growing together with spinel (Cr# = 0.35-0.45), both these minerals form part of the olivine + orthopyroxene + clinopyroxene + garnet + spinel + pargasite M2 assemblage. The formation of plagioclase + diopside symplectites after omphacite and breakdown of kyanite to sapphirine + albite in internal eclogite and the breakdown of M2 olivine + garnet to amphibole + orthopyroxene + spinel assemblages (M3) in garnet peridotite indicate post-UHP isothermal decompression down to 750-800 °C and 0.8-1.0 GPa (= M3). Multiphase solid-and fluid inclusion assemblages composed of Sr-bearing magnesite, dolomite or carbon decorate linear defect structures within M1a-b minerals and/or form subordinate local assemblages together with M2 minerals. The latter are interpreted as evidence for infiltration of early-Caledonian COH

Geochemical and Os–Hf–Nd–Sr Isotopic Characterization of North Patagonian Mantle Xenoliths: Implications for Extensive Melt Extraction and Percolation Processes

Czech Academy of Sciences Publication Activity Database

Mundl, A.; Ntaflos, T.; Ackerman, Lukáš; Bizimis, M.; Bjerg, E. A.; Wegner, W.; Hauzenberger, Ch. A.

2016-01-01

Roč. 57, č. 4 (2016), s. 685-715 ISSN 0022-3530 Institutional support: RVO:67985831 Keywords : metasomatism * Patagonia * radiogenic isotopes * Re–Os * SCLM Subject RIV: DD - Geochemistry Impact factor: 3.280, year: 2016

Retraction notice to "Sapphirine and fluid inclusions in Tel Thanoun mantle xenoliths, Syria" [J. Afr. Earth Sci. 116 (2015) 105-113

Science.gov (United States)

Bilal, Ahmad

2018-06-01

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). This article has been retracted at the request of the Editor. Following complaints raised, it appears to be a reworking of a former manuscript submitted by a group of five authors but not published. The present author has published here alone, without acknowledging the work of the four other co-authors of the initial submission. This poses a problem of integrity as this new article contains material which was submitted for publication initially by all five authors. By retracting this article, we encourage all the authors of the initial manuscript to resubmit together a new version which respects authorship and scientific integrity.

Lead isotope evolution across the Neoproterozoic boundary between craton and juvenile crust, Bayuda Desert, Sudan

Science.gov (United States)

Evuk, David; Lucassen, Friedrich; Franz, Gerhard

2017-11-01

Metaigneous mafic and ultramafic rocks from the juvenile Neoproterozoic Arabian Nubian Shield (ANS) and the Proterozoic, reworked Saharan Metacraton (SMC) have been analysed for major- and trace elements and Sr, Nd, and Pb isotopes. Most of the rocks are amphibolites metamorphosed at amphibolite facies conditions, some with relicts of a granulite facies stage. The other rocks are metapyroxenites, metagabbros, and some ultramafic rocks. Trace element compositions of the metabasaltic (dominantly tholeiitic) rocks resemble the patterns of island arcs and primitive lavas from continental arcs. Variable Sr and Nd isotope ratios indicate depleted mantle dominance for most of the samples. 207Pb/204Pb signatures distinguish between the influence of high 207Pb/204Pb old SMC crust and depleted mantle signatures of the juvenile ANS crust. The Pb isotope signatures for most metabasaltic rocks, metapyroxenites and metagabbros from SMC indicate an autochthonous formation. The interpretation of the new data together with published evidence from mafic xenoliths on SMC and ophiolite from ANS allows an extrapolation of mantle evolution in time. There are two lines of evolution in the regional mantle, one, which incorporates potential upper crust material during Neoproterozoic, and a second one with a depleted mantle signature since pre-Neoproterozoic that still is present in the Red Sea and Gulf of Aden spreading centres.

Carbon isotope composition of CO2-rich inclusions in cumulate-forming mantle minerals from Stromboli volcano (Italy)

Science.gov (United States)

Gennaro, Mimma Emanuela; Grassa, Fausto; Martelli, Mauro; Renzulli, Alberto; Rizzo, Andrea Luca

2017-10-01

We report on measurements of concentration and carbon isotope composition (δ13CCO2) of CO2 trapped in fluid inclusions of olivine and clinopyroxene crystals separated from San Bartolo ultramafic cumulate Xenoliths (SBX) formed at mantle depth (i.e., beneath a shallow Moho supposed to be at 14.8 km). These cumulates, erupted about 2 ka ago at Stromboli volcano (Italy), have been already investigated by Martelli et al. (2014) mainly for Sr-Nd isotopes and for their noble gases geochemistry. The concentration of CO2 varies of one order of magnitude from 3.8·10- 8 mol g- 1 to 4.8·10- 7 mol g- 1, with δ13C values between - 2.8‰ and - 1.5‰ vs V-PDB. These values overlap the range of measurements performed in the crater gases emitted at Stromboli (- 2.5‰ residence within the volcano plumbing system. Such δ13C values are higher than those commonly reported for MORB-like upper mantle (- 8 ÷ - 4‰) and likely reflect the source contamination of the local mantle wedge by CO2 coming from the decarbonation of the sediments carried by the subducting Ionian slab with a contribution of organic carbon up to 7%.

A review of noble gas geochemistry in relation to early Earth history

Science.gov (United States)

Kurz, M. D.

1985-01-01

One of the most fundamental noble gas constraints on early Earth history is derived from isotopic differences in (129)Xe/(130)Xe between various terrestrial materials. The short half life (17 m.y.) of extinct (129I, parent of (129)Xe, means that these differences must have been produced within the first 100 m.y. after terrestrial accretion. The identification of large anomalies in (129)Xe/(130)Xe in mid ocean ridge basalts (MORB), with respect to atmospheric xenon, suggests that the atmosphere and upper mantle have remained separate since that time. This alone is a very strong argument for early catastrophic degassing, which would be consistent with an early fractionation resulting in core formation. However, noble gas isotopic systematics of oceanic basalts show that the mantle cannot necessarily be regarded as a homogeneous system, since there are significant variations in (3)He/(4)He, (40)Ar/(36)Ar, and (129)Xe/(130)Xe. Therefore, the early degassing cannot be considered to have acted on the whole mantle. The specific mechanisms of degassing, in particular the thickness and growth of the early crust, is an important variable in understanding present day noble gas inventories. Another constraint can be obtained from rocks that are thought to be derived from near the lithosphere asthenosphere boundary: ultramafic xenoliths.

Nannofossils in 2011 El Hierro eruptive products reinstate plume model for Canary Islands

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Zaczek, Kirsten; Troll, Valentin R.; Cachao, Mario; Ferreira, Jorge; Deegan, Frances M.; Carracedo, Juan Carlos; Soler, Vicente; Meade, Fiona C.; Burchardt, Steffi

2015-01-01

The origin and life cycle of ocean islands have been debated since the early days of Geology. In the case of the Canary archipelago, its proximity to the Atlas orogen led to initial fracture-controlled models for island genesis, while later workers cited a Miocene-Quaternary east-west age-progression to support an underlying mantle-plume. The recent discovery of submarine Cretaceous volcanic rocks near the westernmost island of El Hierro now questions this systematic age-progression within the archipelago. If a mantle-plume is indeed responsible for the Canaries, the onshore volcanic age-progression should be complemented by progressively younger pre-island sedimentary strata towards the west, however, direct age constraints for the westernmost pre-island sediments are lacking. Here we report on new age data obtained from calcareous nannofossils in sedimentary xenoliths erupted during the 2011 El Hierro events, which date the sub-island sedimentary rocks to between late Cretaceous and Pliocene in age. This age-range includes substantially younger pre-volcanic sedimentary rocks than the Jurassic to Miocene strata known from the older eastern islands and now reinstate the mantle-plume hypothesis as the most plausible explanation for Canary volcanism. The recently discovered Cretaceous submarine volcanic rocks in the region are, in turn, part of an older, fracture-related tectonic episode.

U-Pb SHRIMP and Sm-Nd geochronology of the Silvânia Volcanics and Jurubatuba Granite: juvenile Paleoproterozoic crust in the basement of the Neoproterozoic Brasília Belt, Goiás, central Brazil

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FISCHEL DANIELLE P.

2001-01-01

Full Text Available U-Pb SHRIMP and Sm-Nd isotopic ages were determined for felsic metavolcanic rocks from the Silvânia Sequence and Jurubatuba Granite in the central part of the Brasília Belt. Zircon grains from a metavolcanic sample yielded 2115 ± 23 Ma and from the granite yielded 2089 ± 14 Ma, interpreted as crystallization ages of these rocks. Six metavolcanic samples of the Silvânia Sequence yielded a six-point whole-rock Sm-Nd isochron indicating a crystallization age of 2262 ± 110 Ma and positive epsilonNd(T = +3.0 interpreted as a juvenile magmatic event. Nd isotopic analyses on samples from the Jurubatuba Granite have Paleoproterozoic T DM model ages between 2.30 and 2.42 Ga and epsilonNd(T values vary between -0.22 and -0.58. The oldest T DM value refers to a sedimentary xenolith in the granite. These results suggest crystallization ages of Silvânia volcanics and Jurubatuba Granite are the first evidence of a ca. 2.14-2.08 juvenile magmatic event in the basement of the central part of the Brasília Belt that implies the presence of arc/suture hidden in reworked basement of the Brasília Belt.

Preservation of Permian allanite within an Alpine eclogite facies shear zone at Mt Mucrone, Italy: Mechanical and chemical behaviour of allanite during mylonitization

DEFF Research Database (Denmark)

Cenki-Tok, Benedicte; Oliot, E.; Berger, Alfons

2011-01-01

This study addresses the mechanical and cehmical behavior of allanite during shear zone formation under high-pressure metamorphism. Understanding physico-chemical processes related to the retention or resetting of Pb isotopes in allanite during geological processes is essential for robust......, and they were thus chemically and mechanically shielded during Alpine mylonitization. In undeformed samples (8a and 8b), two populations of epidote group minerals were found. Allanite forms either coronas around Permianmonazite or individual grains with patchy zoning. Both types yield Permian ages (208Pb/232Th...... age: 291±5 Ma). On the other hand, grains of REE-rich clinozoisite of Cretaceous age are found in undeformed rocks. These grains appear as small fragments with embayed surface outlines and minute satellites or rims around Permian allanite. These (re)crystallized grains are Sr-rich and show mosaic...

Genesis of diamond inclusions: An integrated cathodoluminescence (CL) and Electron backscatter diffraction (EBSD) study on eclogitic and peridotitic inclusions and their diamond host.

Science.gov (United States)

van den Heuvel, Quint; Matveev, Sergei; Drury, Martyn; Gress, Michael; Chinn, Ingrid; Davies, Gareth

2017-04-01

Diamond inclusions are potentially fundamental to understanding the formation conditions of diamond and the volatile cycles in the deep mantle. In order to fully understand the implications of the compositional information recorded by inclusions it is vital to know whether the inclusions are proto-, syn-, or epigenetic and the extent to which they have equilibrated with diamond forming fluids. In previous studies, the widespread assumption was made that the majority of diamond inclusions are syngenetic, based upon observation of cubo-octahedral morphology imposed on the inclusions. Recent work has reported the crystallographic relationship between inclusions and the host diamond to be highly complex and the lack of crystallographic relationships between inclusions and diamonds has led some to question the significance of imposed cubo-octahedral morphology. This study presents an integrated EBSD and CL study of 9 diamonds containing 20 pyropes, 2 diopsides, 1 forsterite and 1 rutile from the Jwaneng and Letlhakane kimberlite clusters, Botswana. A new method was developed to analyze the crystallographic orientation of the host diamond and the inclusions with EBSD. Diamonds plates were sequentially polished to expose inclusions at different levels in the diamond. CL imaging at different depths was performed in order to produce a 3D view of diamond growth zones around the inclusions. Standard diamond polishing techniques proved too aggressive for silicate inclusions as they were damaged to such a degree that EBSD measurements on the inclusions were impossible. The inclusions were milled with a Ga+ focused ion beam (FIB) at a 12° angle to clean the surface for EBSD measurements. Of the 24 inclusions, 9 have an imposed cubo-octahedral morphology. Of these inclusions, 6 have faces orientated parallel to diamond growth zones and/or appear to have nucleated on a diamond growth surface, implying syngenesis. In contrast, other diamonds record resorption events such that inclusions now cut diamond growth zones. In most cases, the growth zonation around inclusions is not well defined due to CL haloes but some inclusions clearly disrupt diamond growth. Crystallographic orientations of diamond and the inclusions, determined using EBSD, revealed that each inclusion has a homogeneous orientation and record no compositional zonation. The diamonds also showed no angular deviations despite many having multiple growth and resorption zones; implying epitaxial growth of diamond. Crystallographic alignment between diamond and inclusions was not recorded for the principle planes and limited to 3 possible coincidences on minor planes from the 24 inclusions studied. The CL data show no evidence of syngenesis for these 3 inclusions. Analyses of two diamonds with inclusion clusters in different growth zones, 400 µm apart, revealed the same chemical composition and orientation, potentially implying they originated from an original larger inclusion. Combined EBSD and CL data suggest that there is no direct orientational correlation (epitaxial growth) between silicate inclusions and the host diamond, even when the mineral phases are of the same symmetry group. The presentation will provide a detailed evaluation of the genesis of individual inclusions.

Carbonation by fluid-rock interactions at high-pressure conditions: Implications for carbon cycling in subduction zones

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Piccoli, Francesca; Vitale Brovarone, Alberto; Beyssac, Olivier; Martinez, Isabelle; Ague, Jay J.; Chaduteau, Carine

2016-07-01

Carbonate-bearing lithologies are the main carbon carrier into subduction zones. Their evolution during metamorphism largely controls the fate of carbon, regulating its fluxes between shallow and deep reservoirs. Recent estimates predict that almost all subducted carbon is transferred into the crust and lithospheric mantle during subduction metamorphism via decarbonation and dissolution reactions at high-pressure conditions. Here we report the occurrence of eclogite-facies marbles associated with metasomatic systems in Alpine Corsica (France). The occurrence of these marbles along major fluid-conduits as well as textural, geochemical and isotopic data indicating fluid-mineral reactions are compelling evidence for the precipitation of these carbonate-rich assemblages from carbonic fluids during metamorphism. The discovery of metasomatic marbles brings new insights into the fate of carbonic fluids formed in subducting slabs. We infer that rock carbonation can occur at high-pressure conditions by either vein-injection or chemical replacement mechanisms. This indicates that carbonic fluids produced by decarbonation reactions and carbonate dissolution may not be directly transferred to the mantle wedge, but can interact with slab and mantle-forming rocks. Rock-carbonation by fluid-rock interactions may have an important impact on the residence time of carbon and oxygen in subduction zones and lithospheric mantle reservoirs as well as carbonate isotopic signatures in subduction zones. Furthermore, carbonation may modulate the emission of CO2 at volcanic arcs over geological time scales.

A Comparative Analysis of Seismological and Gravimetric Crustal Thicknesses below the Andean Region with Flat Subduction of the Nazca Plate

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Mario E. Gimenez

2009-01-01

Full Text Available A gravimetric study was carried out in a region of the Central Andean Range between 28∘ and 32∘ south latitudes and from 72∘ and 66∘ west longitudes. The seismological and gravimetrical Moho models were compared in a sector which coincides with the seismological stations of the CHARGE project. The comparison reveals discrepancies between the gravity Moho depths and those obtained from seismological investigations (CHARGE project, the latter giving deeper values than those resulting from the gravimetric inversion. These discrepancies are attenuated when the positive gravimetric effect of the Nazca plate is considered. Nonetheless, a small residuum of about 5 km remains beneath the Cuyania terrane region, to the east of the main Andean chain. This residuum could be gravimetrically justified if the existence of a high density or eclogitized portion of the lower crust is considered. This result differed from the interpretations from Project “CHARGE” which revealed that the entire inferior crust extending from the Precordillera to the occidental “Sierras Pampeanas” could be “eclogitized”. In this same sector, we calculated the effective elastic thickness (Te of the crust. These results indicated an anomalous value of Te = 30 km below the Cuyania terrane. This is further conclusive evidence of the fact that the Cuyania terrane is allochthonous, for which also geological evidences exist.

Monazite behaviours during high-temperature metamorphism: a case study from Dinggye region, Tibetan Himalaya

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Wang, Jia-Min; Wu, Fu-Yuan; Rubatto, Daniela; Liu, Shi-Ran; Zhang, Jin-Jiang

2017-04-01

Monazite is a key accessory mineral for metamorphic geochronology, but its growth mechanisms during melt-bearing high-temperature metamorphism is not well understood. Therefore, the petrology, pressure-temperature and timing of metamorphism have been investigated in pelitic and psammitic granulites from the Greater Himalayan Crystalline Complex (GHC) in Dinggye, southern Tibet. These rocks underwent an isothermal decompression process from pressure conditions of >10 kbar to armour effect of matrix crystals (biotite and quartz). Most monazite grains formed at the M3-stage (21-19 Ma) through either dissolution-reprecipitation or recrystallization that was related to biotite dehydration melting reaction. These monazite grains record HREE and Y signatures in local equilibrium with different reactions involving either garnet breakdown or peritectic garnet growth. Another peak of monazite growth occurs during melt crystallization ( 15 Ma), and these monazites are unzoned and have homogeneous compositions. Our results documented the widespread recrystallization to account for monazite growth during high-temperature metamorphism and related melting reactions that trigger monazite recrystallization. In a regional sense, our P-T-t data along with published data indicate that the pre-M1 eclogite-facies metamorphism occurred at 39-30 Ma in the Dinggye Himalaya. Our results are in favour of a steady exhumation of the GHC rocks since Oligocene that was contributed by partial melting. Key words: U-Th-Pb geochronology, Monazite, Recrystallization, Pelitic granulite, Himalaya

Geology of the ultrabasic to basic Uitkomst complex, eastern Transvaal, South Africa: an overview

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Gauert, C. D. K.; De Waal, S. A.; Wallmach, T.

1995-11-01

The Uitkomst complex in eastern Transvaal, South Africa, is a mineralized, layered ultrabasic to basic intrusion of Bushveld complex age (2.05-2.06 Ga) that intruded into the sedimentary rocks of the Lower Transvaal Supergroup. The complex is situated 20 km north of Badplaas. It is elongated in a northwesterly direction and is exposed over a total distance of 9 km. The intrusion is interpreted to have an anvil-shaped cross-section with a true thickness of approximately 800 m and is enveloped by metamorphosed and, in places, brecciated country rocks. Post-Bushveld diabase intrusions caused considerable vertical dilation of teh complex. The complex consists of six lithological units (from bottom to top): Basal Gabbro, Lower Harzburgite, Chromitiferous Harzburgite, Main Harzburgite, Pyroxenite and Gabbronorite. The Basal Gabbro Unit, developed at the base of the intrusion and showing a narrow chilled margin of 0.2 to 1.5 m against the floor rocks, has an average thickness of 6 m and grades upwards into the sulphide-rich and xenolith-bearing sequence of the Lower Harzburgite Unit. The latter unit averages 50 m in thickness and is gradationally overlain by the chromite-rich harzburgite of the Chromitiferous Harzburgite Unit (average thickness 60 m). Following on from the Chromitiferous Harzburgite Unit is the 330 m thick Main Harzburgite Unit. The Pyroxenite and Gabbronorite Units (total combined thickness of 310 m) form the uppermost formations of the intrusion. The three lower lithological units, Basal Gabbro to Chromitiferous Harzburgite, are highly altered by late magmatic, hydrothermal processes causing widespread serpentinization, steatitization, saussuritization and uralitization. Field relations, petrography and mineral and whole rock chemistry suggest the following sequence of events, The original emplacement of magma took place from northwest to southeast. The intrusion was bounded between two major fracture zones that gave rise to an elongated body, which

Multiphotonic Confocal Microscopy 3D imaging: Application to mantle sulfides in sub-arc environment (Avacha Volcano, Kamchatka)

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Antoine, Bénard; Luc-Serge, Doucet; Sabine, Palle; Dmitri A., Ionov

2010-05-01

Petrogenetic relations in igneous rocks are usually studied in natural samples using classical optical microscopy and subsequent geochemical data acquisition. Multiphotonic Laser Scanning Confocal Microscopy (MLSCM) can be a powerful tool to section geological materials optically with sub-micrometric resolution and then generate a three-dimensional (3D) reconstruction (ca. 106 μm3 stack). MLSCM is used here to investigate textural relations of Monosulfide Solid Solution (MSS) with silicate phases in fresh spinel harzburgite xenoliths from the andesitic Avacha volcano (Kamchatka, Russia). The xenoliths contain MSS disseminated in olivine and orthopyroxene (opx) neoblasts as well as MSS-rich quenched magmatic opx veins [1]. First, Reflection Mode (RM) was tested on vein sulfides in resin-impregnated thick (120 μm) polished rock sections. Then we used a combination of Differential Interference Contrast (DIC) with a transmitted light detector, two photons-excited fluorescence (2PEF) and Second Harmonic Generation (SHG). Sequential imaging feature of the Leica TCS-SP2 software was applied. The excitation laser used for 2PEF was a COHERENT MIRA 900 with a 76Hz repetition rate and 800nm wavelength. Image stacks were analysed using ImageJ software [2]. The aim of the tests was to try to discriminate sulfides in silicate matrix as a tool for a better assessment of equilibrium conditions between the two phases. Preliminary results show that Fe-Ni rich MSS from vein and host rock have a strong auto-fluorescence in the Near UV-VIS domain (392-715 nm) whereas silicate matrix is only revealed through DIC. SHG is obtained only from dense nanocentrosymmetrical structures such as embedded medium (organic matter like glue and resin). The three images were recorded sequentially enabling efficient discrimination between the different components of the rock slices. RM permits reconstruction of the complete 3D structure of the rock slice. High resolution (ca. 0.2 μm along X-Y axis vs

The UHP metamorphic Seve Nappe Complex of the Swedish Caledonides - a new occurrence of the microdiamond-bearing gneisses and their exhumation

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Klonowska, Iwona; Janák, Marian; Majka, Jarosław; ‎ Froitzheim, Nikolaus; Gee, David G.

2015-04-01

The ultra-high pressure metamorphism (UHPM) in the Seve Nappe Complex of the Swedish Caledonides has been recently recognized within several lithologies including gneisses, eclogites and garnet pyroxenites (e.g. Janák et al. 2013, Klonowska et al. 2014a, Majka et al. 2014). Thermodynamic modelling and thermobarometric calculations indicate peak pressure conditions of >3GPa at c. 800-900°C (reaching the diamond stability field) for eclogites and garnet pyroxenites from northern Jämtland (e.g. Klonowska et al. 2014b). In addition to this, the first microdiamonds were found in paragneisses from the Snasahögarna Mt. in central Jämtland (Majka et al. 2014). Here we report a new discovery of microdiamond together with moissanite (SiC) from one of the world's most famous localities for thrusting, Mount Åreskutan, where long transport distances were recognized already in the 19th century (Törnebohm 1888). Garnet porphyroblasts in gneisses from the Åreskutan Mt. contain abundant mineral inclusions, mainly graphite, carbonates and quartz, together with fluid inclusions of CO2 concentrated in swarms. Among these inclusions three microdiamonds were found in two gneiss samples. In one of the samples moissanite was also discovered. Both minerals were identified by micro-Raman spectroscopy. In addition to these 'swarm' inclusions, biotite, kyanite, rutile, feldspars, zircon, monazite, ±phengite, ±muscovite, ±spinel, ±ilmenite, ±apatite occur in garnets. Phase equilibrium modelling for the phengite-bearing gneiss confirms its UHP history at temperatures of c. 800°C. Recent discoveries of UHP metamorphism within the Seve Nappe Complex derived from the Baltican outer margin (part of the Middle Allochthon) challenged us to present a new tectonic model incorporating exhumation of the deeply subducted continental rocks together with mantle lithosphere peridotites. Majka et al. (2014) introduced a new "under-pressure"-driven exhumation mechanism of rocks buried in

Mineralogical and isotopic data on two hydrothermal uranium deposits located in the Permian volcano-sedimentary basin of Collio Orobico (Bergamasc Alps): occurrence of a Cretaceous U mobilization phase

International Nuclear Information System (INIS)

Philippe, S.; Lancelot, J.R.; Girod, M.; Mercadier, H.; Villemaire, C.

1987-01-01

The U deposits of Novazza and Val Vedello are located close to the unconformity with the South-Alpine basement. The ignimbrites adjacent to the Novazza deposit have undergone a pervasive hydrothermal alteration. For this deposit, the study of the micas provides crystallization temperatures ranging from 540 0 C to 350 0 C. These micas do not show a zonal distribution with respect to the mineralized bodies. In the neighbouring barren basin, the mica crystallization temperatures at 200 0 C suggest a post-magmatic evolution very different. The U-Pb data on zircons were made for ignimbrites collected in the Novazza mine and in the barren basin. They allow to propose a multi-episodic evolution model taking into account a mixing of two populations of zircons: a small amount of Precambrian zircons located in basement xenoliths within the ignimbrites, and a large proportion of zircons having crystallized in the ignimbrites, which are supposed to have been emplaced about 280 My ago. The U-Pb data suggest a phase of U concentration, during Cretaceous times. For each deposit, this age does not seem to be related to the ages of fault motions. Different hypothesis concerning the genesis of Novazza and Val Vedello deposits are discussed which take into account the paleotemperature data on micas, the ore paragenesis and the U-Pb data obtained on U-mineralizations [fr

Spreading continents kick-started plate tectonics.

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Rey, Patrice F; Coltice, Nicolas; Flament, Nicolas

2014-09-18

Stresses acting on cold, thick and negatively buoyant oceanic lithosphere are thought to be crucial to the initiation of subduction and the operation of plate tectonics, which characterizes the present-day geodynamics of the Earth. Because the Earth's interior was hotter in the Archaean eon, the oceanic crust may have been thicker, thereby making the oceanic lithosphere more buoyant than at present, and whether subduction and plate tectonics occurred during this time is ambiguous, both in the geological record and in geodynamic models. Here we show that because the oceanic crust was thick and buoyant, early continents may have produced intra-lithospheric gravitational stresses large enough to drive their gravitational spreading, to initiate subduction at their margins and to trigger episodes of subduction. Our model predicts the co-occurrence of deep to progressively shallower mafic volcanics and arc magmatism within continents in a self-consistent geodynamic framework, explaining the enigmatic multimodal volcanism and tectonic record of Archaean cratons. Moreover, our model predicts a petrological stratification and tectonic structure of the sub-continental lithospheric mantle, two predictions that are consistent with xenolith and seismic studies, respectively, and consistent with the existence of a mid-lithospheric seismic discontinuity. The slow gravitational collapse of early continents could have kick-started transient episodes of plate tectonics until, as the Earth's interior cooled and oceanic lithosphere became heavier, plate tectonics became self-sustaining.

Petrogenesis and U-Pb and Sm-Nd geochronology of the Taquaral granite: record of an orosirian continental magmatic arc in the region of Corumba - MS

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Letícia Alexandre Redes

Full Text Available The Taquaral Granite is located on southern Amazon Craton in the region of Corumbá, westernmost part of the Brazilian state of Mato Grosso do Sul (MS, near Brazil-Bolivia frontier. This intrusion of batholitic dimensions is partially covered by sedimentary rocks of the Urucum, Tamengo Bocaina and Pantanal formations and Alluvial Deposits. The rock types are classified as quartz-monzodiorites, granodiorites, quartz-monzonites, monzo and syenogranites. There are two groups of enclaves genetically and compositionally different: one corresponds to mafic xenoliths and the second is identified as felsic microgranular enclave. Two deformation phases are observed: one ductile (F1 and the other brittle (F2. Geochemical data indicate intermediate to acidic composition for these rocks and a medium to high-K, metaluminous to peraluminous calk-alkaline magmatism, suggesting also their emplacement into magmatic arc settings. SHRIMP zircon U-Pb geochronological data of these granites reveals a crystallization age of 1861 ± 5.3 Ma. Whole rock Sm-Nd analyses provided εNd(1,86 Ga values of -1.48 and -1.28 and TDM model ages of 2.32 and 2.25 Ga, likely indicating a Ryacian crustal source. Here we conclude that Taquaral Granite represents a magmatic episode generated at the end of the Orosirian, as a part of the Amoguija Magmatic Arc.

Calcium Isotopic Composition of Bulk Silicate Earth

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Kang, J.; Ionov, D. A.; Liu, F.; Zhang, C.; Zhang, Z.; Huang, F.

2016-12-01

Ca isotopes are used to study the accretion history of the Earth and terrestrial planets, but, Ca isotopic composition of the Bulk Silicate Earth (BSE) remains poorly constrained [1]. To better understand the Ca isotopic composition of BSE, we analyzed 22 well studied peridotite xenoliths from Tariat (Mongolia), Vitim (southern Siberia) and Udachnaya (Siberian Craton). These samples include both fertile and highly depleted garnet and spinel peridotites that show no or only minor post-melting metasomatism or alteration. Ca isotope measurements were done on a Triton-TIMS using double spike method at the Guangzhou Institute of Geochemistry, CAS. The data are reported as δ44/40Ca (relative to NIST SRM 915a). Results for geostandards are consistent with those from other laboratories. 2 standard deviations of SRM 915a analyses are 0.13‰ (n=48). δ44/40Ca of both and fertile and refractory peridotites range from 0.79 to 1.07‰ producing an average of 0.93±0.12‰ (2SD). This value defines the Ca isotopic composition of the BSE, which is consistent with the average δ44/40Ca of oceanic basalts ( 0.90‰)[2,3]. [1] Huang et al (2010) EPSL 292; [2] Valdes et al (2014) EPSL 394; [3]DePaolo (2004) RMG 55.

Three-Dimensional Magnetotelluric Imaging of the Cascadia Subduction Zone with an Amphibious Array

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Egbert, G. D.; Yang, B.; Bedrosian, P.; Kelbert, A.; Key, K.; Livelybrooks, D.; Parris, B. A.; Schultz, A.

2017-12-01

We present results from three-dimensional inversion of an amphibious magnetotelluric (MT) array consisting of 71 offshore and 75 onshore sites in the central part of Cascadia, to image down-dip and along strike variations of electrical conductivity, and to constrain the 3D distribution of fluids and melt in the subduction zone. The array is augmented by EarthScope TA MT data and legacy 2D profiles providing sparser coverage of western WA, OR, and northern CA. The prior model for the inversion includes ocean bathymetry, conductive marine sediments, and a resistive subducting plate, with geometry derived from the model of McCrory et al. (2012) and seismic tomography. Highly conductive features appear just above the interface with the a priori resistive plate in three zones. (1) In the area with marine MT data a conductive layer, which we associate with fluid-rich decollement and subduction channel sediments, extends eastward from the trench to underthrust the seaward edge of Siletzia, which is clearly seen as a thick crustal resistor. The downdip extent of the underthrust conductive layer is a remarkably uniform 35 km. (2) High conductivities, consistent with metamorphic fluids associated with eclogitization, occur near the forearc mantle corner. Conductivity is highly variable along strike, organized in a series of E-W to diagonal elongated conductive/resistive structures, whose significance remains enigmatic. (3) High conductivities associated with fluids and melts are found in the backarc, again exhibiting substantial along strike variability.

The behavior of iron and zinc stable isotopes accompanying the subduction of mafic oceanic crust: A case study from Western Alpine ophiolites

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Inglis, Edward C.; Debret, Baptiste; Burton, Kevin W.; Millet, Marc-Alban; Pons, Marie-Laure; Dale, Christopher W.; Bouilhol, Pierre; Cooper, Matthew; Nowell, Geoff M.; McCoy-West, Alex J.; Williams, Helen M.

2017-07-01

Arc lavas display elevated Fe3+/ΣFe ratios relative to MORB. One mechanism to explain this is the mobilization and transfer of oxidized or oxidizing components from the subducting slab to the mantle wedge. Here we use iron and zinc isotopes, which are fractionated upon complexation by sulfide, chloride, and carbonate ligands, to remark on the chemistry and oxidation state of fluids released during prograde metamorphism of subducted oceanic crust. We present data for metagabbros and metabasalts from the Chenaillet massif, Queyras complex, and the Zermatt-Saas ophiolite (Western European Alps), which have been metamorphosed at typical subduction zone P-T conditions and preserve their prograde metamorphic history. There is no systematic, detectable fractionation of either Fe or Zn isotopes across metamorphic facies, rather the isotope composition of the eclogites overlaps with published data for MORB. The lack of resolvable Fe isotope fractionation with increasing prograde metamorphism likely reflects the mass balance of the system, and in this scenario Fe mobility is not traceable with Fe isotopes. Given that Zn isotopes are fractionated by S-bearing and C-bearing fluids, this suggests that relatively small amounts of Zn are mobilized from the mafic lithologies in within these types of dehydration fluids. Conversely, metagabbros from the Queyras that are in proximity to metasediments display a significant Fe isotope fractionation. The covariation of δ56Fe of these samples with selected fluid mobile elements suggests the infiltration of sediment derived fluids with an isotopically light signature during subduction.

Peeling back the lithosphere: Controlling parameters, surface expressions and the future directions in delamination modeling

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Göğüş, Oğuz H.; Ueda, Kosuke

2018-06-01

Geodynamical models investigate the rheological and physical properties of the lithosphere that peels back (delaminates) from the upper-middle crust. Meanwhile, model predictions are used to relate to a set of observations in the geological context to the test the validity of delamination. Here, we review numerical and analogue models of delamination from these perspectives and provide a number of first-order topics which future modeling studies may address. Models suggest that the presence of the weak lower crust that resides between the strong mantle lithosphere (at least 100 times more viscous/stronger) and the strong upper crust is necessary to develop delamination. Lower crustal weakening may be induced by melt infiltration, shear heating or it naturally occurs through the jelly sandwich type strength profile of the continental lithosphere. The negative buoyancy of the lithosphere required to facilitate the delamination is induced by the pre-existing ocean subduction and/or the lower crustal eclogitization. Surface expression of the peeling back lithosphere has a distinct transient and migratory imprint on the crust, resulting in rapid surface uplift/subsidence, magmatism, heating and shortening/extension. New generation of geodynamical experiments can explain how different types of melting (e.g hydrated, dry melting) occurs with delamination. Reformation of the lithosphere after removal, three dimensional aspects, and the termination of the process are key investigation areas for future research. The robust model predictions, as with other geodynamic modeling studies should be reconciled with observations.

Detachments of the subducted Indian continental lithosphere based on 3D finite-frequency tomographic images

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Liang, X.; Tian, X.; Wang, M.

2017-12-01

Indian plate collided with Eurasian plate at 60 Ma and there are about 3000 km crustal shortening since the continental-continental collision. At least one third of the total amount of crustal shortening between Indian and Eurasian plates could not be accounted by thickened Tibetan crust and surface erosion. It will need a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion. Based on the lithosphere-asthenosphere boundary images beneath the Tibetan plateau, there is also at least the same amount deficit for lithospheric mantle subducted into upper/lower mantle or lateral extrusion with the crust. We have to recover a detailed Indian continental lithosphere image beneath the plateau in order to explain this deficit of mass budget. Combining the new teleseismic body waves recorded by SANDWICH passive seismic array with waveforms from several previous temporary seismic arrays, we carried out finite-frequency tomographic inversions to image three-dimensional velocity structures beneath southern and central Tibetan plateau to examine the possible image of subducted Indian lithosphere in the Tibetan upper mantle. We have recovered a continuous high velocity body in upper mantle and piece-wised high velocity anomalies in the mantle transition zone. Based on their geometry and relative locations, we interpreted these high velocity anomalies as the subducted and detached Indian lithosphere at different episodes of the plateau evolution. Detachments of the subducted Indian lithosphere should have a crucial impact on the volcanism activities and uplift history of the plateau.

Piecewise Delamination Drives Uplift in the Atlas Mountains Region of Morocco

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Bezada, M. J.; Humphreys, E.; Martin Davila, J.; mimoun, H.; Josep, G.; Palomeras, I.

2013-12-01

The elevation of the intra-continental Atlas Mountains of Morocco and surrounding regions requires a mantle component of buoyancy, and there is consensus that this buoyancy results from an abnormally thin lithosphere. Lithospheric delamination under the Atlas Mountains and thermal erosion caused by upwelling mantle have each been suggested as thinning mechanisms. We use seismic tomography to image the upper mantle of Morocco by inverting teleseimic p-wave delay times, complemented with local delays, recorded on a dense array of stations in the Iberian peninsula and Morocco. A surface wave model provides constraint on the shallower layers. We determine the geometry of lithospheric cavities and mantle upwelling beneath the Middle Atlas and central High Atlas, and image delaminated lithosphere at ~400 km beneath the Middle Atlas. We propose discontinuous delamination of an intrinsically unstable Atlas lithosphere, enabled by the presence of anomalously hot mantle, as a mechanism for producing the imaged structures. The Atlas lithosphere was made unstable by a combination of tectonic shortening and eclogite loading during Mesozoic rifting and Cenozoic magmatism. The presence of hot mantle, sourced from regional upwellings in northern Africa or the Canary Islands, enabled the mobilization of this lithosphere. Flow around the retreating Alboran slab focused upwelling mantle under the Middle Atlas, where we image the most recent delamination. The Atlas Mountains of Morocco stand as an example of mantle-generated uplift and large-scale lithospheric loss in a mildly contractional orogen.

Adakite-like volcanism of Ecuador: lower crust magmatic evolution and recycling

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Chiaradia, Massimo; Müntener, Othmar; Beate, Bernardo; Fontignie, Denis

2009-11-01

In the Northern Andes of Ecuador, a broad Quaternary volcanic arc with significant across-arc geochemical changes sits upon continental crust consisting of accreted oceanic and continental terranes. Quaternary volcanic centers occur, from west to east, along the Western Cordillera (frontal arc), in the Inter-Andean Depression and along the Eastern Cordillera (main arc), and in the Sub-Andean Zone (back-arc). The adakite-like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt-mantle interactions or of lower crustal melting, fractional crystallization, and assimilation processes. In this paper, we present petrographic, geochemical, and isotopic (Sr, Nd, Pb) data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers (Pululagua, Pichincha, Ilalo, Chacana, Sumaco) forming a NW-SE transect at about 0° latitude and encompassing the frontal (Pululagua, Pichincha), main (Ilalo, Chacana), and back-arc (Sumaco) chains. All rocks display typical subduction-related geochemical signatures, such as Nb and Ta negative anomalies and LILE enrichment. They show a relative depletion of fluid-mobile elements and a general increase in incompatible elements from the front to the back-arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab. We observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene- and/or amphibole-bearing cumulates and of magma mixing at all frontal and main arc volcanic centers. Within each volcanic center, rocks display correlations between evolution indices and radiogenic isotopes, although absolute variations of radiogenic isotopes are small and their values are overall rather primitive (e.g., ɛNd = +1.5 to +6, 87Sr/86Sr = 0

Origin and significance of the 2011 El Hierro xeno-pumice

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Zaczek-Pedroza, Kirsten; Troll, Valentin R.; Deegan, Frances M.; Meade, Fiona C.; Burchardt, Steffi; Carracedo, Juan C.; Klügel, Andreas; Harris, Chris; Wiesmaier, Sebastian; Berg, Sylvia E.; Barker, Abigail K.

2014-05-01

During the first week of the 2011/2012 submarine eruption off El Hierro (Canary Islands), peculiar light-coloured pumiceous rocks (xeno-pumice) were found floating on the sea. The appearance of these rocks led to a potentially inappropriate response from the authorities, because the rocks were viewed as likely indicators of high-silica magma and possible explosive eruptive behaviour. However, the eruption remained a relatively minor and dominantly effusive event and the origin and significance of these peculiar xeno-pumice rocks for volcanic monitoring remains unresolved. Three contrasting models have been put forward, describing them as: a) recycled hydrothermally altered felsic magmatic rocks (Meletlidis et al., 2012, Geophys. Res. Lett., 39:L17302), b) sediment-contaminated high-silica magma (Sigmarsson et al., 2013, Contrib. Mineral. Petrol., 165:601-622) or c) frothy xenoliths from pre-island sedimentary strata that were melted while immersed in magma (Troll et al., 2012, Solid Earth, 3:97-110). Here, we combine the three available datasets to assess the origin of El Hierro xeno-pumice in the light of texture, mineralogy, major and trace element composition, and oxygen isotope characteristics in order to discuss their significance. We note that: 1) Sedimentary relicts occur frequently in xeno-pumice samples with occasionally observed relict bedding. 2) Vesicle sizes are extremely variable, which documents multiple degassing events. The vesicles are biggest especially close to sedimentary relicts, likely the result of a complex mix of minerals and porewaters originally present. 3) The mineral assemblage of xeno-pumice includes quartz, smectite, illite, wollastonite, jasper and mica (XRD) and is akin to marine sedimentary rocks in the region (Hoernle, 1998, J. Petrol.,39:859-880; Robertson & Stillman, 1979, J. Geol. Soc., 136:47 -60; Aparicio et al., 2006, Geol. Mag. 143:181 -193). 4) CIPW norms calculated from xeno-pumice major element compositions show the

Collision-induced basalt eruptions at Pleiku and Buôn Mê Thuột, south-central Viet Nam

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Hoàng, Nguyễn; Flower, Martin F. J.; Chí, Cung Thu'ọ'ng; Xuân, Phạm Tích; Quý, Hoàng Văn; Sơn, Trần Thanh

2013-09-01

Neogene-Quaternary basalts occur as dispersed volcanic clusters in the vicinity of the Tethyan tectonic belt, possibly representing 'far-field' effects of the Early Tertiary collisions of Gondwana fragments with the southern margin of Eurasia. In Indochina, such a 'Diffuse Igneous Province' post-dates the 45-42 Ma 'hard' India-Asia collision and southeastward, collision induced (c. 30-17 Ma.), extrusion of Indochina. Extrusion was accommodated by left-lateral strike-slip shearing on the Ailao Shan-Red River Fault, coeval with seafloor spreading in the East Viet Nam (South China) Sea. The Indochina basalts mostly comprise shield-building tholeiites capped by small-volume undersaturated types, the latter often bearing mantle xenoliths and 'exotic' xenocrysts such as sapphire, zircon. They appeared at c. 17 Ma, more-or-less coinciding with the cessation of both continental extrusion and seafloor spreading. At this point extensional stress appears to have shifted westwards to continental Indochina, with magmatic activity appearing, characteristically, at 'pull-apart' basins. However, the relationship of mantle melting beneath this region to its geodynamic setting is controversial, being variously attributed to mantle plumes, extreme lithospheric stretching, and lateral asthenospheric displacement. There is little or no definitive evidence for regional mantle upwelling while lithosphere stretching alone appears to be insufficient to allow for melting, Here, we present geochemical and Sr, Nd, and Pb isotopic (and paleomagnetic data), for cored sections from the Pleiku and Buon Mê Thuột plateaus in south-central Viet Nam, representative in most respects of the Indochina province as a whole. In the Pleiku shield olivine tholeiite flows are intercalated with quartz tholeiites while, in contrast, alkali basalts predominate over olivine tholeiite in the Buon Mê Thuột (BMT) shield. The first of these features (in Pleiku) probably reflects crustal wall-rock reaction while

Comparative Magma Oceanography

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Jones, J. H.

1999-01-01

and, perhaps, mostly molten. The Giant Impact hypothesis for the origin of the Moon offers a tremendous input of thermal energy and the same could be true for core formation. And current solar system models favor the formation of a limited number of large (about 1000 km) planetesimals that, upon accreting to Earth, would cause great heating, being lesser versions of the Giant Impact. Several lines of geochemical evidence do not favor this hot early Earth scenario. (i) Terrestrial man-tle xenoliths are sometimes nearly chondritic in their major element compositions, suggesting that these rocks have never been much molten. Large degrees of partial melting probably promote differentiation rather than homogenization. (ii) Unlike the case of Mars, the continental crust probably did not form as a highly fractionated residual liquid from a magma ocean (about 99% crystallization), but, rather, formed in multiple steps. [The simplest model for the formation of continental crust is complicated: (a) about 10% melting of a primitive mantle, making basalt; (b) hydrothermal alteration of that basalt, converting it to greenstone; and (c) 10% partial melting of that greenstone, producing tonalite.] This model is reinforced by the recent observation from old (about 4.1 b.y.) zircons that the early crust formed from an undepleted mantle having a chondritic Lu/Hf ratio. (iii) If the mantle were once differentiated by a magma ocean, the mantle xenolith suite requires that it subsequently be homogenized. The Os isotopic compositions of fertile spinel lherzolites place constraints on the timing of that homogenization. The Os isotopic composition of spinel lherzolites approaches that of chondrites and correlates with elements such as Lu and Al. As Lu and Al concentrations approach those of the primitive mantle, Os isotopic compositions approach chondritic. The Re and Os in these xenoliths were probably added as a late veneer. Thus, the mantle that received the late veneer must have been

The LATEA metacraton (Central Hoggar, Tuareg shield, Algeria): behaviour of an old passive margin during the Pan-African orogeny

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Liégeois, Jean Paul; Latouche, Louis; Boughrara, Mustapha; Navez, Jacques; Guiraud, Michel

2003-10-01

Historically, the Tuareg shield is divided into three parts bordered by mega-shear zones with the centre, the Central Polycyclic Hoggar, characterized by Archaean and Palaeoproterozoic lithologies. Nearly 10 years ago, the Tuareg shield was shown to be composed of 23 displaced terranes [Geology 22 (1994) 641] whose relationships were deciphered in Aı̈r to the SE [Precambr. Res. 67 (1994) 59]. The Polycyclic Central Hoggar terranes were characterized by the presence of well preserved Archaean/Palaeoproterozoic and Neoproterozoic lithologies. We show here that the terranes from Central Hoggar (Laouni, Azrou-n-Fad, Tefedest, Egéré-Aleksod) belonged to a single old passive margin, to which we gave the acronym name LATEA, which behaved as a craton during the Mesoproterozoic and the Early-Middle Neoproterozoic but was partly destabilized and dissected during the Late Neoproterozoic as a consequence of its involvement as a passive margin in the Pan-African orogen. An early Pan-African phase consisted of thrust sheets including garnet-bearing lithologies (eclogite, amphibolite, gneiss) that can be mapped and correlated in three LATEA terranes. In the Tin Begane area, P- T- t paths have been established from >15 kbar--790 °C (eclogite) to 4 kbar--500 °C (greenschist retrogression) through 12 kbar--830 °C (garnet amphibolite) and 8 kbar--700 °C (garnet gneiss), corresponding to the retrograde path of a Franciscan-type loop. Sm-Nd geochronology on minerals and laser ablation ICP-MS on garnet show the mobility of REE, particularly LREE, during the retrograde greenschist facies that affects, although slightly, some of these rocks. The amphibolite-facies metamorphism has been dated at 685 ± 19 Ma and the greenschist facies at 522 ± 27 Ma. During the thrust phase, the Archaean-Palaeoproterozoic basement was only locally affected by the Pan-African tectonics. LATEA behaved as a craton. Other juvenile terranes were also thrust early onto LATEA: the Iskel island arc at �

Oxo-amphiboles in mantle xenoliths: evidence for H2O-rich melt interacting with the lithospheric mantle of Harrow Peaks (Northern Victoria Land, Antarctica)

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Gentili, S.; Bonadiman, C.; Biagioni, C.; Comodi, P.; Coltorti, M.; Zucchini, A.; Ottolini, L.

2015-12-01

Amphiboles are the most widespread hydrous metasomatic phases in spinel-bearing mantle peridotites from Harrow Peaks (HP), Northern Victoria Land (Antarctica). They occur both in veinlets and disseminated in the peridotite matrix (preferentially associated with clinopyroxene and spinel grains). Four amphibole crystals were investigated by single-crystal X-ray diffraction (SC-XRD), electron microprobe analysis (EMPA), secondary ion mass spectrometry (SIMS) and micro-Mössbauer spectroscopy; these crystal-chemical data allow to constrain upper mantle conditions during growth of these amphiboles and the role of volatile circulation during metasomatic processes in the Antarctic region. The HP amphiboles have low Mg# values (69.3-84.1), high TiO2 (2.74-5.30 wt%) and FeOtot contents (3.40 to 6.90 wt%). The Fe3+/Fetot ratios are significantly high (0.53-0.66). The W-site is mainly occupied by O2- (0.984-1.187 apfu) plus OH (H2O: 0.70-1.01 wt%) and minor F (0.04-0.24 wt%) and Cl (0.03-0.08 wt%). Consequently, HP amphiboles are actually characterized by a significant oxo component. The aH2O values were calculated at 1.5 GPa by dehydration equilibrium equations written as H2O-buffering equilibria among end-member components of amphibole and coexisting peridotite phases. Three out of four HP amphibole-bearing peridotites have values of aH2O ranging from 0.122 to 0.335; whereas one sample has aH2O remarkably higher (0.782) approaching an ideal H2O basalt solubility. The HP fO2 values, determined by the olivine-spinel-orthopyroxene oxygeobarometer (ΔQFM = -1.77 : +0.01), are remarkably different from those calculated on the basis of the amphibole dehydration equilibrium and the application of the dissociation reaction (ΔQFM = -2.60 : +6.8). The high aH2O and the extremely high fO2 values, determined by the oxy-amphibole equilibrium with respect to the redox conditions recorded by the co-existing anhydrous minerals (close to QFM buffer), revealed that: i) the amphibole-forming reaction is a relatively recent process with the new phases far from having reached a potential equilibrium with the peridotite matrix; ii) amphibole seems to be formed by the precipitation of migrating H2O-rich melts with a negligible contribution of the peridotite system.

Volcanic Characteristics of Kueishantao in Northeast Taiwan and Their Implications

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Ching-Lung Chiu

2010-01-01

Full Text Available Kueishantao (KST is a small offshore volcanic island located at the southernmost part of the Okinawa Trough. In this study, we conducted a detailed mapping incorporating the new high resolution LiDAR DTM laser scanning device to accurately construct a volcanic sequence. A new 1/5000 geological map was established. One primary volcanic cone, composed of layers of both lava flows and pyroclastic rocks constituted the major edifice of KST. The other minor volcanic cone, which consists of volcanic lapillis and blocks, is seated to the east of the main cone. The escarped and nearly straight coast in the southern part of the KST indicates that the volcano suffered a large post-volcanic edifice collapse erasing nearly one half of the volume of both volcanic cones. The increase in the abundance of the xenoliths of sedimentary rocks from the lower to the upper part of the volcanic sequence indicates that the formation of volcanic rocks of the KST involved an intensification of crustal contamination. The possibility of volcanic eruption can not be excluded in the future based on the present thermolu¬minescene age data of 7 ka. The associated eruptive ash fall and tsunami induced by the further collapse of the KST volcanic edifice might have great influence to the adjacent inland. Thus, long-term monitoring of volcanic activities around KST should be required for future hazard assessments.

Crustal contamination processes traced by helium isotopes: Examples from the Sunda arc, Indonesia

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Gasparon, M.; Hilton, D. R.; Varne, R.

1994-08-01

Helium isotope data have been obtained on well-characterised olivine and clinopyroxene phenocrysts and xenocrysts from thirteen volcanic centres located between central Sumatra and Sumbawa in the Sunda arc of Indonesia. Olivine crystals in mantle xenoliths (Iherzolite) from Bukit Telor basalts are primitive (Mg# = 90), and their He-3/He-4 value (R/R(sub A) = 8.8) indicates that the Sumatran mantle wedge is MORB-like in helium isotope composition. All other samples have lower He-3/He-4 ratios ranging from 8.5R(sub A) to 4.5R(sub A), with most (thirteen out of eighteen) following a trend of more radiogenic He-3/He-4 values with decreasing Mg#. The only exceptions to this trend are phenocrysts from Batur, Agung and Kerinci, which have MORB-like He-3/He-4 values but relatively low Mg# (Mg# = 70-71), and two highly inclusion-rich clinopyroxenes which have He-3/He-4 values lower than other samples of similar Mg#. The results indicate that crustal contamination unrelated to subduction in the Sunda arc is clearly recorded in the He-3/He-4 characteristics of mafic phenocrysts of subaerial volcanics, and that addition of radiogenic helium is related to low-pressure differentiation processes affecting the melts prior to eruption. These conclusions may have widespread applicability and indicate that helium isotope variations can act as an extremely sensitive tracer of upper crustal contamination.

Eudialyte-group minerals from the Monte de Trigo alkaline suite, Brazil: composition and petrological implications

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Gaston Eduardo Enrich Rojas

Full Text Available ABSTRACT: The Monte de Trigo alkaline suite is a SiO2-undersaturated syenite-gabbroid association from the Serra do Mar alkaline province. Eudialyte-group minerals (EGMs occur in one nepheline microsyenite dyke, associated with aegirine-augite, wöhlerite, låvenite, magnetite, zircon, titanite, britholite, and pyrochlore. Major compositional variations include Si (25.09- 25.57 apfu , Nb (0.31- 0.76 apfu , Fe (1.40-2.13 apfu , and Mn (1.36- 2.08 apfu . The EGMs also contain relatively high contents of Ca (6.13- 7.10 apfu , moderate enrichment of rare earth elements (0.38-0.67 apfu , and a relatively low Na content (11.02-12.28 apfu , which can be correlated with their transitional agpaitic assemblage. EGM compositions indicate a complex solid solution that includes eudialyte, kentbrooksite, feklichevite, zirsilite-(Ce, georgbarsanovite, and manganoeudialyte components. EGM trace element analyses show low Sr and Ba contents and a negative Eu/Eu* anomaly, which are interpreted as characteristic of the parental magma due to the previous fractionation of plagioclase and/or alkali feldspar. The EGMs from the dyke border have higher contents of Fe, Sr (2,161-2,699 ppm, Mg (1,179-3,582 ppm, and Zn (732- 852 ppm than those at the dyke center. These differences are related to the incorporation of xenoliths and xenocrysts of melatheralitic host rock into the nepheline-syenitic magma followed by crystal-melt diffusive exchange.

3-D Structure of the Slave and Rae Cratons Provides Clues to Their Construction

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Snyder, D. B.

2013-12-01

Deep geologic structures within cratons that make up continental cores were long neglected. Recently acquired geophysical data from large observational arrays and geochemical data resulting from exploration for diamond has now made possible co-registration of large-scale (400-km depth), truly 3-dimensional data sets. P-waves, surface waves and magnetotelluric observations provide 3-D wavespeed and conductivity models. Multi-azimuthal receiver functions map seismic discontinuity surfaces in 3-D. Xenolith suites erupted in kimberlites provide rock samples at key lithospheric depths, albeit at sparsely distributed locations. These multi-disciplinary models are becoming available for several key cratons worldwide; here the deep structure of the Slave and Rae cratons of the Canadian Shield is described. Lithospheric layers with tapered, wedge-shaped margins are common. Slave craton layers are sub-horizontal and indicate construction of the craton core at 2.7 Ga by underthrusting and flat stacking of lithosphere. The central Rae craton has predominantly dipping discontinuities that indicate construction at 1.9 Ga by thrusting similar to that observed in crustal ';thick-skinned' fold-and-thrust belts. 3-D mapping of conductivity and metasomatism, the latter via mineral recrystallization and resetting of isotopic ages, overprints primary structures in both cratons. Distribution of more conductivitve mantle suggests that assumed causative pervasive metasomatism occurs at 100-200 km depths with ';chimneys' reaching to shallower depths, typically in locations where kimberlites or mineralization has occurred.

Mesozoic mafic dikes from the Shandong Peninsula, North China Craton: Petrogenesis and tectonic implications

International Nuclear Information System (INIS)

Liu Shen; Hu Ruizhong; Zhao Junhong; Feng Caixia; Zou, Haibo

2006-01-01

Mesozoic mafic dikes are widely distributed in Luxi (Mengyin and Zichuan) and Jiaodong regions of the Shandong Peninsula, China, providing an opportunity of investigating the nature of the lost lithospheric mantle beneath the North China Craton (NCC). The mafic dikes are characterized by strong depletion in high field strength elements (HFSE), enrichment in light rare earth elements (LREE), highly variable Th/U ratios, high initial ( 87 Sr/ 86 Sr) i (0.7050-0.7099) and negative ε Nd (T) (-6.0 to -17.6). They were derived from melting of metasomatized portions of the subcontinental lithospheric mantle, followed by fractionation of clinopyroxenes. The similarity in Nd isotopic compositions between the Mengyin gabbro dikes and the Paleozoic peridotite xenoliths suggests that ancient lithospheric mantle was still retained at 120 Ma below Mengyin, although the ancient lithospheric mantle in many other places beneath NCC had been severely modified. There might be multiple enrichment events in the lithospheric mantle. An early-stage (before or during Paleozoic) rutile-rich metasomatism affected the lithospheric mantle below Mengyin, Jiaodong and Zichuan. Since then, the lithospheric mantle beneath Mengyin was isolated. A late-stage metasomatism by silicate melts modified the lithospheric mantle beneath Jiaodong and Zichuan but not Mengyin. The removal of the enriched lithospheric mantle and the generation of the mafic dikes may be mainly related to the convective overturn accompanying Jurassic-Cretaceous subduction of the paleo-Pacific plate. (author)

Building stones can be of geoheritage significance

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Brocx, Margaret; Semeniuk, Vic

2017-04-01

Building stones have generally been assigned values according to their cultural, aesthetic, and rarity significance, amongst other criteria, but they also may have geoheritage significance. This is akin to the geoheritage significance ascribed to minerals and fossils housed as ex situ specimens in museums. We proffer the notion that building stones can be of geoheritage value particularly where they comprise permanent buildings, they illustrate significant windows into the history of the Earth, and they can be visited as an ex situ museum locality (e.g., the "Blue Granite" of Iceland) for education as part of building-stone tours. For some rocks the quarries that supplied the building stone are no longer in existence and hence the building stones provide the only record of that type of material; for other rocks, the building stone may illustrate features in the lithology no longer present in the quarry itself (e.g., rare and large xenoliths). Building stones are particularly significant as they are often polished and manifest structures, fabrics, and textures not evident in outcrop. We illustrate here examples of building stone of geoheritage significance using Australian and International examples. Australian designated stones could include the "Sydney Sandstone" or "Victorian Bluestone". For international examples, there is the famous "Carrara Marble" in Italy and the widely known "Portland Limestone" from southern England, the latter having been utilized for St Pauls Cathedral in London and the UN building in New York City.

Plume-stagnant slab-lithosphere interactions: Origin of the late Cenozoic intra-plate basalts on the East Eurasia margin

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Kimura, Jun-Ichi; Sakuyama, Tetsuya; Miyazaki, Takashi; Vaglarov, Bogdan S.; Fukao, Yoshio; Stern, Robert J.

2018-02-01

Intra-plate basalts of 35-0 Ma in East Eurasia formed in a broad backarc region above the stagnant Pacific Plate slab in the mantle transition zone. These basalts show regional-scale variations in Nd-Hf isotopes. The basalts with the most radiogenic Nd-Hf center on the Shandong Peninsula with intermediate Nd-Hf at Hainan and Datong. The least radiogenic basalts occur in the perimeters underlain by the thick continental lithosphere. Shandong basalts possess isotopic signatures of the young igneous oceanic crust of the subducted Pacific Plate. Hainan and Datong basalts have isotopic signatures of recycled subduction materials with billions of years of storage in the mantle. The perimeter basalts have isotopic signatures similar to pyroxenite xenoliths from the subcontinental lithospheric mantle beneath East Eurasia. Hainan basalts exhibit the highest mantle potential temperature (Tp), while the Shandong basalts have the lowest Tp. We infer that a deep high-Tp plume interacted with the subducted Pacific Plate slab in the mantle transition zone to form a local low-Tp plume by entraining colder igneous oceanic lithosphere. We infer that the subducted Izanagi Plate slab, once a part of the Pacific Plate mosaic, broke off from the Pacific Plate slab at 35 Ma to sink into the lower mantle. The sinking Izanagi slab triggered the plume that interacted with the stagnant Pacific slab and caused subcontinental lithospheric melting. This coincided with formation of the western Pacific backarc marginal basins due to Pacific Plate slab rollback and stagnation.

Matrix mineralogy of the Lance CO3 carbonaceous chondrite: A transmission electron microscope study

International Nuclear Information System (INIS)

Keller, L.P.; Buseck, P.R.

1990-01-01

The Lance CO3 carbonaceous chondrite (CC) is less altered than the CI and CM chondrites and so provides a view of the mineralogy and textures resulting from the earliest stages of aqueous alteration of CCs. Matrix olivine in Lance has been partly altered to fine-grained, Fe-bearing serpentine and poorly crystalline Fe 3+ oxide, a process that required both hydration and oxidation. Serpentine occurs as discrete packets separated from the olivine surfaces by the Fe 3+ oxide. The Fe released during the dissolution of olivine was partly incorporated into the serpentine; the remainder was oxidized to form Fe 3+ oxide. Matrix metal was also altered to produce Fe oxides, leaving the residual metal enriched in Ni. Olivine grains in Lance matrix contain channels along their [100] and [001] directions. The formation and convergence of such channels resulted in a grain-size reduction of the olivine. The alteration was pervasive but incomplete, suggesting a limited availability of fluid. A brief study of two other CO chondrites, Kainsaz and Warrenton, shows that these meteorites do not contain phyllosilicates in their matrices, although both contain Fe 3+ oxide between olivine grains. Prior to its alteration, Lance probably resembled Kainsaz, an unaltered CO3 chondrite. The alteration assemblage in Lance is only slightly different from that in Mokoia and essentially the same as that in C3 xenoliths from Murchison. Alteration products in Lance show greater similarities to CI than to CM chondrites

Erosion by flowing lava: Geochemical evidence in the Cave Basalt, Mount St. Helens, Washington

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Williams, D.A.; Kadel, S.D.; Greeley, R.; Lesher, C.M.; Clynne, M.A.

2004-01-01

We sampled basaltic lava flows and underlying dacitic tuff deposits in or near lava tubes of the Cave Basalt, Mount St. Helens, Washington to determine whether the Cave Basalt lavas contain geochemical evidence of substrate contamination by lava erosion. The samples were analyzed using a combination of wavelength-dispersive X-ray fluorescence spectrometry and inductively-coupled plasma mass spectrometry. The results indicate that the oldest, outer lava tube linings in direct contact with the dacitic substrate are contaminated, whereas the younger, inner lava tube linings are uncontaminated and apparently either more evolved or enriched in residual liquid. The most heavily contaminated lavas occur closer to the vent and in steeper parts of the tube system, and the amount of contamination decreases with increasing distance downstream. These results suggest that erosion by lava and contamination were limited to only the initially emplaced flows and that erosion was localized and enhanced by vigorous laminar flow over steeper slopes. After cooling, the initial Cave Basalt lava flows formed an insulating lining within the tubes that prevented further erosion by later flows. This interpretation is consistent with models of lava erosion that predict higher erosion rates closer to sources and over steeper slopes. A greater abundance of xenoliths and xenocrysts relative to xenomelts in hand samples indicates that mechanical erosion rather than thermal erosion was the dominant erosional process in the Cave Basalt, but further sampling and petrographic analyses must be performed to verify this hypothesis. ?? Springer-Verlag 2003.

Detailed Configuration of the Underthrusting Indian Lithosphere Beneath Western Tibet Revealed by Receiver Function Images

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Xu, Qiang; Zhao, Junmeng; Yuan, Xiaohui; Liu, Hongbing; Pei, Shunping

2017-10-01

We analyze the teleseismic waveform data recorded by 42 temporary stations from the Y2 and ANTILOPE-1 arrays using the P and S receiver function techniques to investigate the lithospheric structure beneath western Tibet. The Moho is reliably identified as a prominent feature at depths of 55-82 km in the stacked traces and in depth migrated images. It has a concave shape and reaches the deepest location at about 80 km north of the Indus-Yarlung suture (IYS). An intracrustal discontinuity is observed at 55 km depth below the southern Lhasa terrane, which could represent the upper border of the eclogitized underthrusting Indian lower crust. Underthrusting of the Indian crust has been widely observed beneath the Lhasa terrane and correlates well with the Bouguer gravity low, suggesting that the gravity anomalies in the Lhasa terrane are induced by topography of the Moho. At 20 km depth, a midcrustal low-velocity zone (LVZ) is observed beneath the Tethyan Himalaya and southern Lhasa terrane, suggesting a layer of partial melts that decouples the thrust/fold deformation of the upper crust from the shortening and underthrusting in the lower crust. The Sp conversions at the lithosphere-asthenosphere boundary (LAB) can be recognized at depths of 130-200 km, showing that the Indian lithospheric mantle is underthrusting with a ramp-flat shape beneath southern Tibet and probably is detached from the lower crust immediately under the IYS. Our observations reconstruct the configuration of the underthrusting Indian lithosphere and indicate significant along strike variations.

A planet in transition: The onset of plate tectonics on Earth between 3 and 2 Ga?

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Kent C. Condie

2018-01-01

Full Text Available Many geological and geochemical changes are recorded on Earth between 3 and 2 Ga. Among the more important of these are the following: (1 increasing proportion of basalts with “arc-like” mantle sources; (2 an increasing abundance of basalts derived from enriched (EM and depleted (DM mantle sources; (3 onset of a Great Thermal Divergence in the mantle; (4 a decrease in degree of melting of the mantle; (5 beginning of large lateral plate motions; (6 appearance of eclogite inclusions in diamonds; (7 appearance and rapid increase in frequency of collisional orogens; (8 rapid increase in the production rate of continental crust as recorded by zircon age peaks; (9 appearance of ophiolites in the geologic record, and (10 appearance of global LIP (large igneous province events some of which correlate with global zircon age peaks. All of these changes may be tied directly or indirectly to cooling of Earth's mantle and corresponding changes in convective style and the strength of the lithosphere, and they may record the gradual onset and propagation of plate tectonics around the planet. To further understand the changes that occurred between 3 and 2 Ga, it is necessary to compare rocks, rock associations, tectonics and geochemistry during and between zircon age peaks. Geochemistry of peak and inter-peak basalts and TTGs needs to be evaluated in terms of geodynamic models that predict the existence of an episodic thermal regime between stagnant-lid and plate tectonic regimes in early planetary evolution.

Petrology and Wavespeeds in Central Tibet Indicate a Partially Melted Mica-Bearing Crust

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Hacker, B. R.; Ritzwoller, M. H.; Xie, J.

2013-12-01

S-wave speeds and Vp/Vs ratios in the middle to deep crust of Tibet are best explained by a partially melted, mica-bearing middle to lower crust with a subhorizontal to gently dipping foliation. Surface-wave tomography [e.g., Yang et al., 2012; Xie et al., 2013] shows that the central Tibetan Plateau (the Qiangtang block) is characterized by i) slow S-wave speeds of 3.3-3.5 km/s at depths from 20-25 km to 45-50 km, ii) S-wave radial anisotropy of at least 4% (Vsh > Vsv) with stronger anisotropy in the west than the east [Duret et al., 2010], and iii) whole-crust Vp/Vs ratios in the range of 1.73-1.78 [Xu et al., 2013]. The depth of the Curie temperature for magnetite inferred from satellite magnetic measurements [Alsdorf and Nelson, 1999], the depth of the α-β quartz transition inferred from Vp/Vs ratios [Mechie et al., 2004], and the equilibration pressures and temperatures of xenoliths erupted from the mid-deep crust [Hacker et al., 2000] indicate that the thermal gradient in Qiangtang is steep, reaching 1000°C at 30-40 km depth. This thermal gradient crosses the dehydration-melting solidi for crustal rocks at 20-30 km depth, implying the presence or former presence of melt in the mid-deep crust. These temperatures do not require the wholesale breakdown of mica at these depths, because F and Ti can stabilize mica to at least 1300°C [Dooley and Patino Douce, 1996]. Petrology suggests, then, that the Qiangtang middle to deep crust consists of a mica-bearing residue from which melt has been extracted or is being extracted. Wavespeeds calculated for mica-bearing rocks with a subhorizontal to gently dipping foliation and minor silicate melt are the best match to the wavespeeds and anisotropy observed by seismology. Alsdorf, D., and D. Nelson, The Tibetan satellite magnetic low: Evidence for widespread melt in the Tibetan crust?, Geology, 27, 943-946, 1999. Dooley, D.F., and A.F. Patino Douce, Fluid-absent melting of F-rich phlogopite + rutile +quartz, American