Sample records for depleted mantle peridotite

  1. Mantle depletion and metasomatism recorded in orthopyroxene in highly depleted peridotites

    DEFF Research Database (Denmark)

    Scott, James; Liu, Jingao; Pearson, D. Graham;


    Although trace element concentrations in clinopyroxene serve as a useful tool for assessing the depletion and enrichment history of mantle peridotites, this is not applicable for peridotites in which the clinopyroxene component has been consumed (~ 25% partial melting). Orthopyroxene persists.......6), high spinel Cr# (commonly > 45) and low orthopyroxene Al2O3 (generally compositions shows that all samples, even the most refractory, have undergone metasomatism by small volume light rare earth element-bearing agents. Measured...

  2. Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle (United States)

    Lassiter, J. C.; Byerly, B. L.; Snow, J. E.; Hellebrand, E.


    The Lena Trough is a highly oblique, sparsely magmatic, ultra-slow spreading center located at the smallest distance between North America and Eurasia in the Arctic basin. Competing models suggest that it is either floored by oceanic mantle abyssal peridotites (APs) exposed by lithospheric necking, or by subcontinental mantle exposed in a still juvenile rift. To distinguish between these hypotheses, we have examined mineral major and trace element and whole rock Os-isotope variations in Lena Trough peridotites. Lena Trough peridotites are predominantly LREE-depleted, similar to other AP suites, and have 187Os/188Os ranging from ∼0.118 to 0.130 (Ave.=0.1244). This distribution is nearly identical to that of abyssal peridotites globally. Both the REE patterns and the Os-isotope distribution of the Lena Trough peridotites differ starkly from subcontinental mantle xenoliths sampled at Svalbard adjacent to Lena Trough. This suggests that Lena Trough is a site of oceanic spreading, although mid-ocean ridge volcanism as such has not yet begun. Highly refractory APs from several settings have Os- and Hf-isotope compositions indicating ancient (>1 Ga) melt depletion. Some researchers have proposed that at least some APs do not directly sample the convecting upper mantle source of MORB, but instead sample highly melt-depleted residues either entrained in the convecting mantle or present as a buoyant “slag” floating atop the less-depleted MORB-source mantle. However, ocean island peridotite xenoliths and APs reveal an essentially identical, non-Gaussian distribution of Os-isotopes and also span a similar range in Hf-isotopes. The similar mean and distribution of Os-isotopes between APs and ocean island xenoliths indicate that these two sample types derive from the same heterogeneous mantle reservoir. This similarity is inconsistent with the AP “slag hypothesis” due to the significantly greater depth of origin of ocean island xenoliths with respect to APs. Global

  3. The fate of sulfide during decompression melting of peridotite - implications for sulfur inventory of the MORB-source depleted upper mantle (United States)

    Ding, Shuo; Dasgupta, Rajdeep


    Magmatism at mid ocean ridges is one of the main pathways of S outflux from deep Earth to the surface reservoirs and is a critical step in the global sulfur cycle, yet our understanding of the behavior of sulfide during decompression melting of the upper mantle is incomplete. In order to constrain the sulfur budget of the mantle and reconcile the sulfur and chalcophile element budget of mantle partial melts parental to primitive mid-ocean ridge basalts (MORBs), here we developed a model to describe the behavior of sulfide and Cu during decompression melting by combining the pMELTS thermodynamic model and empirical sulfur contents at sulfide concentration (SCSS) models, taking into account the effect of the presence of Ni and Cu in sulfides on SCSS of mantle-derived melts. Calculation of SCSS along melting adiabat at mantle potential temperature of 1380 °C with variable initial S content in the mantle indicates that the complete consumption or partial survival of sulfide in the melting residue depends on initial S content and degree of melting. Primitive MORBs (Mg# > 60) with S and Cu mostly concentrated in 800-1000 ppm and 80-120 ppm are likely mixture of sulfide undersaturated high degree melts and sulfide saturated low degree melts derived from depleted peridotite containing 100-200 ppm S. Model calculations to capture the effects of variable mantle potential temperatures (1280-1420 °C) indicate that for a given abundance of sulfide in the mantle, hotter mantle consumes sulfide more efficiently than colder mantle owing to the effect of temperature in enhancing sulfide solubility in silicate melt, and higher mantle temperature stabilizing partial melt with higher FeO* and lower SiO2 and Al2O3, all of which generally enhance sulfide solubility. However, sulfide can still be exhausted by ∼ 10- 15% melting with bulk S of 100-150 ppm in the mantle when TP is as low as 1300 °C. We also show that although variation of DCuperidotite/ melt and initial Cu in the

  4. Water concentrations in mantle peridotite minerals (United States)

    Warren, J. M.; Hauri, E. H.


    The concentration and distribution of volatiles in the mantle is important for constraining many key properties, including melting systematics at ridges and subduction zones. We present measurements of water concentrations in nominally anhydrous minerals from abyssal, orogenic and xenolith peridotites. Analyses of fresh and altered samples from a variety of locations are used to assess the extent to which mineral water concentrations reflect primary mantle compositions, versus diffusive loss and/or hydration due to secondary processes. Water concentrations were measured in olivine (Ol), orthopyroxene (Opx) and clinopyroxene (Cpx) by ion microprobe, using mineral specific standards and monitoring background concentrations by analysis of synthetic forsterite. Analytical reproducibility, based on 11 repeat analyses of an Ol grain, is 10%, while background H2O levels varied from 7-19 ppm. Samples include xenoliths from Pali Aike, Samoa and Spitsbergen, along with unusually fresh oceanic peridotites from the Gakkel Ridge and the Tonga Trench. In addition, samples were analyzed from the Southwest Indian Ridge (SWIR) and the Josephine Peridotite, both of which have moderate degrees of alteration. In olivine, water concentrations are Pali Aike xenoliths, which have water concentrations of 16-33 ppm. On average, peridotite Opx have 187 ppm and Cpx have 474 ppm. Pyroxenite veins from the Southwest Indian Ridge have systematically lower concentrations, with an average of 12 ppm in Opx and 55 ppm in Cpx. Water partition coefficients for Opx/Ol have an average value of 28 and Cpx/Ol of 57, significantly higher than previous estimates (e.g., Hirth and Kohlstedt, 1996). Excluding the pyroxenites, the average Cpx/Opx partition coefficient is 2, in agreement with published estimates. This suggests that Cpx and Opx preserve mantle water concentrations, whereas Ol has undergone hydrogen loss. Mineral rims have water concentrations that are within error of core concentrations. The

  5. An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions (United States)

    Warren, J. M.; Shimizu, N.; Sakaguchi, C.; Dick, H. J. B.; Nakamura, E.


    Abyssal peridotites, the depleted solid residues of ocean ridge melting, are the most direct samples available to assess upper oceanic mantle composition. We present detailed isotope and trace element analyses of pyroxene mineral separates from Southwest Indian Ridge abyssal peridotites and pyroxenites in order to constrain the size and length scale of mantle heterogeneity. Our results demonstrate that the mantle can be highly heterogeneous to account adequately for the complexities of ancient and recent melting processes.

  6. 87Sr/86Sr in spinel peridotites from Borée, Massif Central, France: melt depletion and metasomatism in the sub-continental lithospheric mantle (United States)

    Barnett, Caroline; Harvey, Jason


    Radiogenic isotopes and elemental concentrations in peridotite xenoliths may be used to model the timing and degree of partial melting in the upper mantle, but this primary melt depletion signature may be overwritten by subsequent episodes of melt or fluid infiltration. Spinel peridotites from the Maar de Borée, Massif Central, France have mainly poikilitic protogranular textures and clear petrographic evidence of a melt phase apparently unrelated to host basalt infiltration. Bulk rock major and compatible trace element concentrations are consistent with varying degrees of partial melting but incompatible trace element concentrations indicate cryptic metasomatism in some samples. Lithophile trace element mass balance cannot always be reconciled by the inclusion of the chemically characterized melt phase and suggest a contribution from a trace abundance grain boundary phase1. 87Sr/86Sr values for unleached bulk rocks and clinopyroxene mineral separates are higher than those for their leached equivalents, consistent with the removal of a radiogenic grain boundary phase. While unleached bulk rock 87Sr/86Sr is sometimes indistinguishable (within error) from its constituent unleached clinopyroxene, in two samples they show distinct patterns, as do the REE trends in these two xenoliths. BO01-01 bulk-rock is LREE-enriched (La/YbN = 3.6)2, and constituent clinopyroxene shows a similar relative enrichment trend. Bulk-rock 87Sr/86Sr is 0.70342±1 while that of clinopyroxene is lower at 0.70332±2. Clinopyroxene modal abundance is 11%. BO01-03 bulk-rock is only slightly LREE-enriched (La/YbN = 1.2) and both bulk-rock and clinopyroxene show a generally flatter profile. Bulk-rock 87Sr/86Sr is 0.70285±1 while that of clinopyroxene is in this case higher at 0.70296±2. Clinopyroxene modal abundance is also higher at 15%, consistent with a greater contribution by clinopyroxene to the bulk-rock Sr-isotope budget. The results appear to be inconsistent with a simple model of single

  7. Mantle melting and melt refertilization beneath the Southwest Indian Ridge: Mineral composition of abyssal peridotites (United States)

    Chen, Ling; Zhu, Jihao; Chu, Fengyou; Dong, Yan-hui; Liu, Jiqiang; Li, Zhenggang; Zhu, Zhimin; Tang, Limei


    As one of the slowest spreading ridges of the global ocean ridge system, the Southwest Indian Ridge (SWIR) is characterized by discontinued magmatism. The 53°E segment between the Gallieni fracture zone (FZ) (52°20'E) and the Gazelle FZ (53°30'E) is a typical amagmatic segment (crustal thickness directly. We examine the mineral compositions of 17 peridotite samples from the 53°E amagmatic segment. The results show that the peridotites can be divided into two groups. The Group 1 peridotites are characterized by clinopyroxenes having LREE depleted patterns that is typical for the abyssal peridotite, thus are thought to be the residue of the mantle melting. The Group 2 peridotites show the lowest HREE content within the SWIR peridotites but are anomaly enriched in LREE, with flat or U-type REE patterns, thus cannot be the pure residue of mantle melting. Mineral compositions of the Group 2 peridotites are more depleted than that of peridotites sampled near the Bouvet hot spot (Johnson et al., 1990), implying that the depleted mantle beneath the 53°E segment may be the residue of ancient melting event. This hypothesis is supported by the the low Ol/Opx ratios, coarse grain sizes (>1cm) Opx, and Mg-rich mineral compositions akin to harzburgite xenoliths that sample old continental lithospheric mantle (Kelemen et al., 1998). Melt refertilization model shows that Group 2 peridotites were affected by an enriched low-degree partial melt from the garnet stability field. These results indicate that depleted mantle which experiences ancient melting event are more sensitive to melt refertilization, thus may reduce the melt flux, leading to extremely thin crust at 53°E segment. This research was granted by the National Basic Research Programme of China (973 programme) (grant No. 2013CB429705) and the Fundamental Research Funds of Second Institute of Oceanography, State Oceanic Administration (JG1603, SZ1507). References: Johnson K T M, Dick H J B, Shimizu N. Melting in the

  8. Petrological constraints on the mantle peridotites from the Cretaceous ophiolites in southern Turkey and northern Cyprus (United States)

    Yıldız Yüksekol, Özlem; Aldanmaz, Ercan; Güçtekin, Aykut; van Hinsbergen, Douwe J. J.; Mason, Paul R. D.


    In this study we present geochemical variations of peridotites from the ophiolite suites exposed within the Tauride Belt of southern Turkey and in Northern Cyprus with an aim to document the nature of mantle melting and possible effects of melt movement on element behavior in supra-subduction zone (SSZ) mantle. The ultramafic rocks representing the mantle sections of these ophiolites are variably serpentinized spinel-bearing harzburgites and dunites with major element compositions indicating variable depletions in basaltic components. Major and trace element systematics of primary mantle minerals indicate that the peridotites are likely the residual products left behind after relatively high-degree of mantle melting (16 - 23%). These mantle relicts, however, display also compositional and textural evidence indicative of extensive melt-rock interaction. Olivine-orthopyroxene-spinel equilibria indicate that the peridotites are characterized by high oxygen fugacity (QFM+2), which may be indicative of extensive interaction of the peridotites with highly oxidized melts. Precise determination of trace elements from in situ measurements of primary mantle minerals by laser-ablation ICP-MS reveals important features about the petrogenetic evolution of these mantle representatives. Trace element signatures in clinopyroxenes indicate that the peridotites are strongly depleted in Ti and HREE relative to Zr and MLREE, respectively. Uneven distribution of REE among coexisting opx - cpx pairs in the peridotites reflects chemical disequilibrium, which can be interpreted to have resulted from either diffusive exchange during melt movement or interaction with metasomatizing agents. Based on Ga concentrations and Ga-Ti-Fe+3# variations in chrome-spinels the peridotites have been inferred to have experienced significant compositional modification through melt-solid interaction following partial melting. Overall, mineral chemical variations in the peridotites indicate that the

  9. Abundances of Ag and Cu in mantle peridotites and the implications for the behavior of chalcophile elements in the mantle (United States)

    Wang, Zaicong; Becker, Harry


    Silver abundances in mantle peridotites and the behavior of Ag during high temperature mantle processes have received little attention and, as a consequence, the abundance of Ag in the bulk silicate Earth (BSE) has been poorly constrained. In order to better understand the processes that fractionate Ag and other chalcophile elements in the mantle, abundances of Ag and Cu in mantle peridotites from different geological settings (n = 68) have been obtained by isotope dilution ICP-MS methods. In peridotite tectonites and in a few suites of peridotite xenoliths which display evidence for variable extents of melt depletion and refertilization by silicate melts, Ag and Cu abundances show positive correlations with moderately incompatible elements such as S, Se, Te and Au. The mean Cu/Ag in fertile peridotites (3500 ± 1200, 1s, n = 38) is indistinguishable from the mean Cu/Ag of mid ocean ridge basalts (MORB, 3600 ± 400, 1s, n = 338) and MORB sulfide droplets. The constant mean Cu/Ag ratios indicate similar behavior of Ag and Cu during partial melting of the mantle, refertilization and magmatic fractionation, and thus should be representative of the Earth's upper mantle. The systematic fractionation of Cu, Ag, Au, S, Se and Te in peridotites and basalts is consistent with sulfide melt-silicate melt partitioning with apparent partition coefficients of platinum group elements (PGE) > Au ⩾ Te > Cu ≈ Ag > Se ⩾ S. Because of the effects of secondary processes, the abundances of chalcophile elements, notably S, Se, but also Cu and the PGE in many peridotite xenoliths are variable and lower than in peridotite massifs. Refertilization of peridotite may change abundances of chalcophile and lithophile elements in peridotite massifs, however, this seems to mostly occur in a systematic way. Correlations with lithophile and chalcophile elements and the overlapping mean Cu/Ag ratios of peridotites and ocean ridge basalts are used to constrain abundances of Ag and Cu in the BSE

  10. The Sidi Mohamed peridotites (Edough Massif, NE Algeria): Evidence for an uppe mantle origin

    Indian Academy of Sciences (India)

    Soraya Hadj Zobir; Roland Oberhänsli


    The Hercynian Edough massif is the easternmost crystalline massif of the Algerian coast. It consists of two tectonically superposed units composed of micaschists, gneisses, and peridotite. This study concentrates on the small and isolated Sidi Mohamed peridotite outcrop area (0.03 km2). The Sidi Mohamed peridotite is composed mainly of harzburgites (Mg-rich olivine and orthopyroxene as major minerals). The Ni (2051–2920 ppm), Cr (2368–5514 ppm) and MgO (∼28–35 wt.%) whole-rock composition and the relative depletion in Nb make these harzburgites comparable to depleted peridotites related to a subduction zone. We suggest that the Sidi Mohamed ultramafic body was derived directly from the upper mantle and tectonically incorporated into the gneiss units of the Edough metamorphic core complex in a subduction environment.

  11. How Depleted is the MORB mantle? (United States)

    Hofmann, A. W.; Hart, S. R.


    Knowledge of the degree of mantle depletion of highly incompatible elements is critically important for assessing Earth's internal heat production and Urey number. Current views of the degree of MORB source depletion are dominated by Salters and Stracke (2004), and Workman and Hart (2005). The first is based on an assessment of average MORB compositions, whereas the second considers trace element data of oceanic peridotites. Both require an independent determination of one absolute concentration, Lu (Salters & Stracke), or Nd (Workman & Hart). Both use parent-daughter ratios Lu/Hf, Sm/Nd, and Rb/Sr calculated from MORB isotopes combined with continental-crust extraction models, as well as "canonical" trace element ratios, to boot-strap the full range of trace element abundances. We show that the single most important factor in determining the ultimate degree of incompatible element depletion in the MORB source lies in the assumptions about the timing of continent extraction, exemplified by continuous extraction versus simple two-stage models. Continued crust extraction generates additional, recent mantle depletion, without affecting the isotopic composition of the residual mantle significantly. Previous emphasis on chemical compositions of MORB and/or peridotites has tended to obscure this. We will explore the effect of different continent extraction models on the degree of U, Th, and K depletion in the MORB source. Given the uncertainties of the two most popular models, the uncertainties of U and Th in DMM are at least ±50%, and this impacts the constraints on the terrestrial Urey ratio. Salters, F.J.M. and Stracke, A., 2004, Geochem. Geophys. Geosyst. 5, Q05004. Workman, R.K. and Hart, S.R., 2005, EPSL 231, 53-72.

  12. Garnet peridotites from the Sulu UHP terrane, China: A window insights into mantle and subduction processes (United States)

    Zhang, R.; Liou, J.; Yang, J.; Tainfu, L.


    Garnet peridotites from the Sulu UHP terrane of China occur as block and lens within granulite-amphibolite facies gneiss and contain lenses of clinopyroxenite and coesite-bearing eclogite. In addition to the assemblage Ol + Opx + Cpx + Grt, some peridotites contain additional hydrous phases (e.g., phlogopite, Ti-clinohumite and pargasite) and magnesite. The abundances of "fertile elements" (TiO2, Al2O3, CaO and FeO) of the Sulu peridotites are lower than those of the primitive mantle and show negative correlation with MgO content. The garnet peridotites and pyroxenites preserved mantle δ 18O values: +4.8 to +5.7 for Grt, +4.7 to +5.4 for Ol, +4.5 to 5.6 for Cpx and +5.7 to +5.8 ‰ for Opx. These characteristics suggest that the Sulu peridotites represent pieces of a depleted, residual mantle. Most peridotites with LREE-enrichment patterns, high 87Sr/86Sr (0.7071-0.7100) and low 143Nd/144Nd ratios (0.5122-0.5128) have been subjected to the effects of mantle metasomatism and crustal contamination. In Donghai, the site area of the Chinese Continental Scientific Drelling (CCSD), peridotites with or without magnesite are enriched in Th, U, Sr, Ba and LREE, and show Hf and Ti negative anomalies; these may be attributed to carbonatitic metasomatism. Zircon separates from peridotitic cores of the PP1 drill hole show concentric zoning without inherited cores indicating a metamorphic origin; they yield SHRIMP U-Pb age of 221 ± 3 Ma (weighted mean), consistent with the Dabie-Sulu regional UHP metamorphism. Polymorph transformation (OREN to CLEN) of orthopyroxene and exsolution microstructures in peridotitic minerals are common; these include Ilm in olivine, Qtz, Ilm, Phl and Grt + Ilm ± Amp in diopside, and Rt ± Cpx + Ap in garnet. These petrochemical and age data, microstructures and P-T estimates conclude the mantle-derived Sulu garnet peridotites were inserted into a Triassic continental subduction zone and experienced in situ UHP metamorphism together with subducted

  13. Modeling the subsolidus evolution of melt-depleted peridotite residues beneath the continents (United States)

    Brown, E. L.; Lesher, C. E.; Baker, M. B.; Schutt, D.


    The origin and stabilization of subcontinental lithospheric mantle is a consequence of 1) partial melting of peridotitic mantle in mid-ocean ridge, intraplate and/or subduction zone settings, 2) underplating of the melt-depleted residue beneath the continents as a consequence of either plume-head melting or tectonic mechanisms, 3) subsolidus evolution of the residual solid, and 4) metasomatic transformations. The evolution of phase compositions and modes within the lithospheric mantle as a consequence of these processes, and the resulting density structure, are essential to understanding the preservation or convective removal of lithosphere. Whereas melting experiments provide constraints on the range of compositions possible for either fertile or depleted peridotite, the effects of pressure and temperature changes below the solidus on peridotite mineralogy and density remain challenging to quantify. These difficulties have important implications, particularly with regard to the density structure of the lithosphere and the mechanisms enabling long-term stabilization of subcontinental lithospheric mantle. To calculate the effects of changes in P, T, and bulk composition on the subsolidus mineralogy and density of peridotite lithologies, we have coupled the algorithm of [1], which calculates modes and phase compositions of subsolidus peridotite by mass balance constrained by experimentally-determined mineral-mineral exchange and distribution coefficients with the algorithm of [2], which calculates densities at P and T conditions suitable to the upper mantle using mineral physics data. We benchmark our approach against the thermodynamic models pMELTS and PERPLE_X using recently published peridotite melting experimental data, and show that our estimates of mineral modes and compositions typically provide improvements to the fits of experimental results. We also show that our density calculations are similar to those predicted by these thermodynamic models when using

  14. Petrography and Geochemistry of Peridotite Xenoliths from Hannuoba and Significance for Lithospheric Mantle Evolution

    Institute of Scientific and Technical Information of China (English)


    The compositions of the whole rocks and trace elements of minerals in peridotites can reflect the characteristics of the lithospheric mantle. The nature and evolution of the Cenozoic lithospheric mantle beneath Hannuoba (汉诺坝), located on the north edge of the intra-North China orogenic belt,are discussed based on the in-situ LAM-ICPMS detected trace element compositions of clinopyroxenes in the Hannuoba peridotitic xenoliths combined with detailed petrography and geochemistry studies. The Hannuoba lithospheric mantle was formed by different partial meltings of the primitive mantle. Most of the samples reflect the partial melting degree of lower than 5% with a few samples of 15%-20%.Major element compositions of the whole rocks and geochemical compositions of clinopyroxenes reveal the coexistence of both fertile and depleted mantle underneath the Hannuoba region during the Cenozoic.This was probably caused by the asthenospheric mantle replacing the aged craton mantle through erosion,intermingling and modification. Our conclusion is further supported by the existence of both carbonatitic magmatic material and silicate melt/fluid metasomatism as magnified by the trace elements of the clinopyroxenes from the Hannuoba lithospheric mantle.

  15. Long-term cycling of mantle Pb: A trace element study of the major mantle mineral phases in abyssal peridotites (United States)

    D'Errico, M. E.; Warren, J. M.; Godard, M.; Ildefonse, B.


    Peridotites from ultraslow-spreading ridges preserve signatures of the depleted mantle, while also reflecting the fine scale compositional variability present in the mantle. Traditional analyses of these depleted rocks have focused on clinopyroxene, the main trace element host in spinel peridotites. However, key isotopic systems, such as lead and osmium, are hosted in other phases at low but significant concentration levels. The amount of lead contained within mantle mineral phases is of critical importance to understanding the long-term evolution of the Earth, because the radiogenic isotopes of lead are sensitive to past material cycling and melt-rock interaction. Sulfides have long been suggested as the main host for lead (Pb) in the mantle, but recent studies have demonstrated that Pb is not exclusively hosted in this trace phase. Therefore, the Pb contents of the major peridotite mineral phases (olivine, orthopyroxene, and clinopyroxene) need to be reassessed. Lead concentration data is available for orogenic and xenolith peridotite samples, which are typically more enriched than abyssal peridotites, but these do not provide direct information on the oceanic upper mantle. Direct measurement of Pb in abyssal peridotites has so far been limited because of its extremely low concentration (often plasma mass spectrometry (LA-ICP-MS). The LA-ICP-MS technique achieves high spatial resolution combined with detection of low elemental abundances. External precision varied from 6% to 17%, with a precision of 6% for Pb, based on 14 repeat analyses of BIR-1G standard basalt glass. Laser spot size varied from 102-163 microns, which produced a detection limit of 0.42-0.81 ppb for Pb. This study focused on abyssal peridotites from the ultra-slow spreading Gakkel and Southwest Indian Ridges (SWIR), with samples coming from segments with full spreading rates fresh samples from Gakkel. A total of five Gakkel and six SWIR peridotites were analyzed by LA-ICP-MS, with a subset of

  16. Electrical conductivity of mantle peridotite at the uppermost lower mantle condition

    Energy Technology Data Exchange (ETDEWEB)

    Yoshino, T; Katsura, T; Yamazaki, D; Ito, E, E-mail: [Institute for Study of the Earth' s Interior, Okayama University, 827 Yamada, Misasa, Tottori 682-1093 (Japan)


    Electrical conductivity of mantle peridotite was measured at 25 GPa and temperature up to 1800 K in a Kawai-type multi-anvil apparatus. The starting material was gel with a composition of fertile spinel lherzolite (KLB1). After the conductivity measurement, mineral phases of run products are composed of magnesium silicate perovskite, ferro-periclase and Ca perovskite. The conductivity value of the peridotite is distinctly higher than those of post-spinel and magnesian silicate perovskite with a composition of (Mg{sub 0.9},Fe{sub 0.1})SiO{sub 3}, but lower than that of ferro-periclase. Both absolute values and change in activation enthalpy for the conductivity of the mantle peridotite are similar to those for the silicate perovskite. A presence of aluminous perovskite with substantial amount of ferric iron in crystal structure would enhance bulk conductivity of the lower mantle.

  17. Platinum-group elements for the mantle peridotites in the Dazhuka ophiolite, Tibet, China

    Institute of Scientific and Technical Information of China (English)


    The total PGE amounts of mantle peridotites in the Dazhuka ophiolite, Tibet, are 28.37-50.67 ng/g, slightly higher than those of mantle peridotites in the primitive mantle, and typical ophiolites in the world, and the Alps-type mantle peridotites. The PGE distribution patterns in the Dazhuka mantle peridotites are also different from those of the mantle peridotites of partial melting relict origin. The Dazhuka mantle peridotites have relatively high total PGE amounts and are enriched in Pt, Pd, and Ru. Their PGE distribution patterns belong to the positively inclined- or swallow-type patterns. The PGE distribution patterns in the mantle peridotites of partial melting relict origin belong to the negative-slope patterns or flat patterns. This reflects the unique features of the upper mantle in this region. Relative enrichment in Pt and Pd, as well as in the incompatible ele-ments Cu, Au, Cs, Rb, Ba, Th, U and LREE, indicates that the partial melting-derived relict mantle peridotites in the Dazhuka ophiolite had experienced intensive permeating and mixing processes of the melt and fluid both containing abundant incompatible elements.

  18. Garnet - Spinel Peridotites from Potrok Aike: An insight into the Patagonian Lithospheric Mantle (United States)

    Schrott, C.; Ntaflos, T.; Bjerg, E. A.; Tschegg, C.


    The mantle-xenolith bearing hyaloclastic tephra from Potrok Aike, located 68 km SW of Río Gallegos, Argentina, provide good opportunities for studying the lithospheric mantle beneath southern Patagonia. The Potrok Aike maar belongs to the Pliocene to Holocene Pali Aike volcanic field located east of the Andean volcanic arc. The studied samples are spinel-bearing and garnet + spinel-bearing lherzolites and harzburgites. The entire suite of the studied mantle-xenoliths have protogranular to protogranular-equigranular textures. The most interesting textural features are the kelyphitic rims around both garnet and spinel, that clearly demonstrate break-down of garnet in spinel bearing xenoliths. Detailed petrographic investigations showed different degrees of kelyphitisation. Another feature is the intergranular growth of clinopyroxene, representing a late metasomatic event. No hydrous minerals were so far found in the studied xenoliths. The Potrok Aike clinopyroxenes are Cr-diopsides with Al2O3 contents ranging from 3.2 to 7.1 wt %, whereas the garnets show pyrope composition (Alm16.3Py69.3Gros13.7 Spess0.7). The spinel shows broad range of compositional variation with Cr# varying between 0.1 and 0.5. The bulk chemistry points out the fertile character of the lherzolithes (Al2O3 2.6 - 3.6 wt % and CaO 2.5 - 2.9 wt %), while the harzburgites indicate a depleted character (Al2O3 0.7 - 2.5 wt % and CaO 0.33 - 2.14 wt %). According to the REE patterns, the Protok Aike peridotites can be divided into three groups: group I, non-metasomatised peridotites with Lan/Smn from 0.76 to 0.91 and Tbn/Ybn from 0.70 to 0.71; group II, slightly metasomatised peridotites with Lan/Smn from 0.95 to 1.27 and Tbn/Ybn from 0.96 to 1.18; and group 3, metasomatised peridotites with Lan/Smn from 1.36 to 3.2 and Tbn/Ybn from 1.06 to 2.12. LA-ICP-MS analyses on clinopyroxenes from spinel-peridotites have convex upward REE patterns resembling those from the spinel-garnet-bearing peridotites

  19. Petrology and geochemistry of mantle peridotites from the Kalaymyo and Myitkyina ophiolites (Myanmar): Implications for tectonic settings (United States)

    Liu, Chuan-Zhou; Zhang, Chang; Xu, Yang; Wang, Jian-Gang; Chen, Yi; Guo, Shun; Wu, Fu-Yuan; Sein, Kyaing


    Ophiolites within Myanmar have been commonly divided into two belts, i.e., the Eastern Belt and Western Belt. The Kalaymyo ophiolite from the Western Belt crops out at the eastern margin of the Indo-Burma Range and was formed during the Early Cretaceous, whereas the Myitkyina ophiolite from the Eastern Belt was formed during the Middle Jurassic. Both ophiolites are characterized by the occurrence of big massifs of mantle peridotites. Mantle peridotites of the Kalaymyo ophiolite are mainly composed of spinel lherzolites and harzburgites, with minor plagioclase peridotites. Mantle peridotites of the Myitkyina ophiolite mainly consist of spinel harzburgites, with minor dunites. Spinel lherzolites from the Kalaymyo ophiolite have relatively fertile compositions, with 40.12-45.19 wt.% MgO, 1.1-2.74 wt.% Al2O3 and 0.67-2.67 wt.% CaO. Their spinels have Cr# values of 0.12-0.4, yielding fractional melting degrees of 3-15%. In comparison, spinel harzburgites from the Kalaymyo ophiolite are more refractory, with 42.08-48.73 wt.% MgO, 0.09-0.99 wt.% Al2O3 and 0.07-0.8 wt.% CaO. Their spinels have Cr# values of 0.3-0.73, giving 12-21% degrees of fractional melting. Plagioclase peridotites from the Kalaymyo ophiolite have compositions intermediate between spinel lherzolites and harzburgites. Compared to the spinel peridotites, spinels in the plagioclase peridotites have relatively higher TiO2 contents. Harzburgites from the Myitkyina ophiolite, containing 40.88-48.16 wt.% MgO, 0.13-1.65 wt.% Al2O3 and 0.1-1.68 wt.% CaO, have refractory compositions similar to the Kalaymyo harzburgites. Spinels in the Myitkyina harzburgites with low TiO2 contents (i.e., < 0.2 wt.%) have variable Cr# values of 0.28-0.72, yielding 11-21% degrees of fractional melting. Clinopyroxenes in all Kalaymyo peridotites display flat patterns in MREE and HREE, but variably LREE-depleted patterns. They also show remarkably negative Sr and Zr anomalies. Plagioclases in the Kalaymyo plagioclase peridotites

  20. Os-isotopic Compositions of Peridotite Xenoliths from the Oceanic Mantle: Implications for the Age of Isotopic Domains in the Oceanic Mantle. (United States)

    Jackson, M. G.; Shirey, S. B.; Hauri, E. H.; Kurz, M. D.


    Os-isotopic compositions of abyssal peridotites and peridotite xenoliths from oceanic hotpots that sample the convecting mantle extend to relatively unradiogenic compositions. However, they do not preserve a record of early-formed (Hadean and Archean) depleted mantle domains, either by earlier cycles of ridge-related depletion, continent extraction, or subcontinental lithospheric mantle erosion. The lack of preservation of early-formed (Hadean and Archean) depleted Os-isotopic compositions is consistent with the lack of preservation of Hadean 142Nd/144Nd variability in the modern convecting mantle, but is in stark contrast to the existence of early-formed (early-Hadean), heterogeneous 129Xe/130Xe isotopic anomalies in the modern mantle. Mukhopadhay (Nature, 2012) suggested that the erasure of 142Nd/144Nd, but not 129Xe/130Xe, anomalies from the convecting mantle may be due to the small magnitude of the 142Nd/144Nd anomalies (10% variability observed) because the smaller 142Nd/144Nd anomalies would have been more easily erased than the larger magnitude 129Xe/130Xe anomalies. This model does not work for the Re-Os system because the magnitude of Os-isotopic heterogeneities in the mantle are large, yet early-formed Os-isotopic signatures have been erased. For example, the 187Os/188Os of early-formed depleted mantle at 4.55 Ga was ~0.095, which is >25% lower than the modern mantle. Given that there were substantial amounts of mantle partial melting throughout the Hadean and Archean, it is hard to understand how some refractory residues retaining some vestige of these low, early-formed Os isotopic compositions have not have been preserved in the modern mantle. Therefore, the lack of preservation of early-formed, large magnitude 187Os/188Os excursions in the modern convecting mantle suggests that the preservation of early geochemical heterogeneities was not necessarily a function of the original geochemical anomaly. We explore alternative solutions to the paradox of

  1. Mantle refertilization by melts of crustal-derived garnet pyroxenite: Evidence from the Ronda peridotite massif, southern Spain (United States)

    Marchesi, Claudio; Garrido, Carlos J.; Bosch, Delphine; Bodinier, Jean-Louis; Gervilla, Fernando; Hidas, Károly


    Geochemical studies of primitive basalts have documented the presence of crustal-derived garnet pyroxenite in their mantle sources. The processes whereby melts with the signature of garnet pyroxenite are produced in the mantle are, however, poorly understood and somewhat controversial. Here we investigate a natural example of the interaction between melts of garnet pyroxenite derived from recycled plagioclase-rich crust and surrounding mantle in the Ronda peridotite massif. Melting of garnet pyroxenite at ˜1.5 GPa generated spinel websterite residues with MREE/HREE fractionation and preserved the positive Eu anomaly of their garnet pyroxenite precursor in whole-rock and clinopyroxene. Reaction of melts from garnet pyroxenite with depleted surrounding peridotite generated secondary fertile spinel lherzolite. These secondary lherzolites differ from common spinel lherzolite from Ronda and elsewhere by their lower-Mg# in clinopyroxene, orthopyroxene and olivine, lower-Cr# in spinel and higher whole-rock Al2O3, CaO, Sm/Yb and FeO* at a given SiO2. Remarkably, secondary spinel lherzolite shows the geochemical signature of ghost plagioclase in the form of positive Eu and Sr anomalies in whole-rock and clinopyroxene, reflecting the transfer of a low-pressure crustal imprint from recycled pyroxenite to hybridized peridotite. Garnet pyroxenite melting and melt-peridotite interaction, as shown in the Ronda massif, may explain how the signature of subducted or delaminated crust is transferred to the mantle and how a garnet pyroxenite component is introduced into the source region of basalts. The efficiency of these processes in conveying the geochemical imprint of crustal-derived garnet pyroxenite to extruded lavas depends on the reactivity of pyroxenite melt with peridotite and the mantle permeability, which may be controlled by prior refertilization reactions similar to those documented in the Ronda massif. Highly fertile heterogeneities produced by pyroxenite-peridotite

  2. Melting experiments on peridotite to lowermost mantle conditions (United States)

    Tateno, Shigehiko; Hirose, Kei; Ohishi, Yasuo


    Melting experiments on a pyrolitic mantle material were performed in a pressure range from 34 to 179 GPa based on laser-heated diamond-anvil cell (DAC) techniques. The textural and chemical characterizations of quenched samples were made by using field-emission-type electron microprobe (FE-EPMA). Melts formed by 46 to 77 wt.% partial melting in this study were ultrabasic in composition and became more depleted in SiO2 and more enriched in FeO with increasing pressure. Melting textures indicate that the liquidus phase changed from ferropericlase to MgSiO3-rich perovskite at least above 34 GPa and further to post-perovskite. The first phase to melt (disappear) changed from CaSiO3 perovskite to (Mg,Fe)O ferropericlase between 68 and 82 GPa. The stability of ferropericlase above solidus temperature shrinks with increasing pressure (melting last below 34 GPa and first 82 GPa), resulting in higher (MgO + FeO)/SiO2 ratio in partial melt at higher pressure. Additionally, the Fe-Mg distribution coefficients (KD) between perovskite/post-perovskite and melt decreased considerably with increasing pressure, leading to strong Fe-enrichment in partial melts. It supports dense partial melts in a deep lower mantle, which migrate downward to the core mantle boundary (CMB).

  3. Melt variability in percolated peridotite: an experimental study applied to reactive migration of tholeiitic basalt in the upper mantle (United States)

    van den Bleeken, Greg; Müntener, Othmar; Ulmer, Peter


    Melt-rock reaction in the upper mantle is recorded in a variety of ultramafic rocks and is an important process in modifying melt composition on its way from the source region towards the surface. This experimental study evaluates the compositional variability of tholeiitic basalts upon reaction with depleted peridotite at uppermost-mantle conditions. Infiltration-reaction processes are simulated by employing a three-layered set-up: primitive basaltic powder (`melt layer') is overlain by a `peridotite layer' and a layer of vitreous carbon spheres (`melt trap'). Melt from the melt layer is forced to move through the peridotite layer into the melt trap. Experiments were conducted at 0.65 and 0.8 GPa in the temperature range 1,170-1,290°C. In this P-T range, representing conditions encountered in the transition zone (thermal boundary layer) between the asthenosphere and the lithosphere underneath oceanic spreading centres, the melt is subjected to fractionation, and the peridotite is partially melting ( T s ~ 1,260°C). The effect of reaction between melt and peridotite on the melt composition was investigated across each experimental charge. Quenched melts in the peridotite layers display larger compositional variations than melt layer glasses. A difference between glasses in the melt and peridotite layer becomes more important at decreasing temperature through a combination of enrichment in incompatible elements in the melt layer and less efficient diffusive equilibration in the melt phase. At 1,290°C, preferential dissolution of pyroxenes enriches the melt in silica and dilutes it in incompatible elements. Moreover, liquids become increasingly enriched in Cr2O3 at higher temperatures due to the dissolution of spinel. Silica contents of liquids decrease at 1,260°C, whereas incompatible elements start to concentrate in the melt due to increasing levels of crystallization. At the lowest temperatures investigated, increasing alkali contents cause silica to increase

  4. Paleo-Asian oceanic subduction-related modification of the lithospheric mantle under the North China Craton: Evidence from peridotite xenoliths in the Datong basalts (United States)

    Wang, Chengyuan; Liu, Yongsheng; Min, Ning; Zong, Keqing; Hu, Zhaochu; Gao, Shan


    In-situ major and trace elements and Sr isotopic compositions of peridotite xenoliths of the Datong Quaternary alkaline basalt were analyzed to evaluate the influences of the southward subduction of the Paleo-Asian oceanic plate (PAOP) on the lithospheric mantle transformation of the North China Craton (NCC). These peridotite xenoliths including spinel harzburgites and lherzolites were classified into three groups. The type 1 peridotites have the lowest temperatures (961-1007 °C). Clinopyroxenes in these peridotites exhibit LREE-depleted REE patterns and have the lowest 87Sr/86Sr ratios of 0.70243-0.70411. The type 2 and 3 peridotites show higher temperatures (1017-1022 °C). Clinopyroxenes in the type 2 peridotite have V-shaped REE patterns and relatively higher 87Sr/86Sr ratios of 0.70418-0.70465. Clinopyroxenes in the type 3 peridotite have concave-downward REE patterns and unusually high 87Sr/86Sr ratios of 0.70769-0.70929. Carbonatitic veinlets are found in the type 1 peridotites. They show steep LREE-enriched REE patterns with enrichment in LILE and depletion in HFSE, and have the highest 87Sr/86Sr ratios of 0.71145-0.71285. The mineral chemistries and modal calculations suggest that the protolith of these peridotites experienced a variable degree of partial melting. The type 2 and 3 peridotites sampled from deeper depth experienced latter cryptic carbonatitic metasomatism. The carbonatitic veinlets have generally consistent trace element patterns and Sr isotopic ratios with the calculated melts equilibrated with clinopyroxenes in the type 3 peridotite, which may represent the percolated carbonatitic melt quickly solidified in the relatively cold and shallow mantle. The remarkable negative Eu anomalies (0.37-0.61) and highly radiogenic Sr isotopic compositions of the calculated metasomatic agents preclude indicate melt derived from carbonated peridotite or carbonated eclogite but point to a crustal sedimentary origin. Considering the tectonic setting and

  5. The longevity of subcontinental lithospheric mantle beneath Jiangsu-Anhui Region ——The Os isotope model age of mantle-derived peridotite xenoliths

    Institute of Scientific and Technical Information of China (English)


    The basalt-borne peridotite xenoliths from Jiangsu-Anhui provinces were analyzed for whole rock Os isotopic compositions in two laboratories of USTC, China and CRPG, France, respectively. The 187Os/188Os ratio of the sample set ranges from 0.119 to 0.129 (25 samples, USTC)and from 0.117 to 0.131 (17 samples, CRPG). The Os isotopic compositions of most samples are less than 0.129 and depleted relatively to the primitive mantle, showing a good correlation with the major element compositions. With the 187Os/188Os-Al2O3 alumichron, the samples yield a model age of 2.5 ± 0.1 Ga (data of USTC) and 1.9 ± 0.1 Ga (data of CRPG), late Archean to early Proterozoic, The two samples with the lowest 187Os/188Os ratio (0.119 and 0.117) have the TRD (Re depleted age) of 1.1 Ga (USTC) and 1.4 Ga (CRPG), mid-Proterozoic. The Os isotope model age shows that the peridotite xenoliths from Cenozoic alkali basalt in Jiangsu-Anhui provinces have an old formation age (early- to mid- Proterozoic). They are not newly produced mantle after the Phanerozoic replacement of the lithosphere mantle, but residual fractions of Proterozoic mantle.

  6. Extremely depleted lithospheric mantle and diamonds beneath the southern Zimbabwe Craton (United States)

    Smith, Chris B.; Pearson, D. Graham; Bulanova, Galina P.; Beard, Andrew D.; Carlson, Richard W.; Wittig, Nadine; Sims, Keith; Chimuka, Lovemore; Muchemwa, Ellah


    Inclusion-bearing diamonds, mantle xenoliths, and kimberlite concentrates from the Cambrian-aged Murowa and Sese kimberlites have been studied to characterise the nature of the lithospheric mantle beneath the southern Zimbabwe Craton. The diamonds are mostly octahedral, moderately rich in nitrogen with moderate to high aggregation, and contain mainly dunite-harzburgite mineral inclusions. Similarly, dunite xenoliths predominate over harzburgite and lherzolite and carry olivines with Mg/Mg + Fe (Mg#) values of 0.92-0.95, spanning the average signatures for Kaapvaal Craton peridotites. Eclogitic xenoliths are extremely rare, in contrast to the Kaapvaal mantle lithosphere. The Zimbabwe mantle assemblage has been only slightly affected by later silicic metasomatism and re-fertilisation with re-introduction of pyroxenes in contrast to the Kaapvaal and many cratonic lithospheric blocks elsewhere where strong metasomatism and re-fertilisation is widespread. Pyroxene, garnet and spinel thermobarometry suggests an ambient 40 mW m - 2 geotherm, with the lithosphere extending down to 210 km at the time of kimberlite eruption. Whole rock peridotite Re-Os isotope analyses yield T RD model ages of 2.7 to 2.9 Ga, providing minimum estimates of the time of melt depletion, are slightly younger in age than the basement greenstone formation. These model ages coincide with the mean T RD age of > 200 analyses of Kaapvaal Craton peridotites, whereas the average Re-Os model age for the Zimbabwe peridotites is 3.2 Ga. The Os data and low Yb n/Lu n ratios suggest a model whereby thick lithospheric mantle was stabilised during the early stages of crustal development by shallow peridotite melting required for formation of residues with sufficiently high Cr/Al to stabilise chromite which then transforms to low Ca, high Cr garnet. Sulphide inclusions in diamond produce minimum T RD model ages of 3.4 Ga indicating that parts of the lithosphere were present at the earliest stages of crust

  7. 186Os-187Os and highly siderophile element abundance systematics of the mantle revealed by abyssal peridotites and Os-rich alloys (United States)

    Day, James M. D.; Walker, Richard J.; Warren, Jessica M.


    Abyssal peridotites are oceanic mantle fragments that were recently processed through ridges and represent residues of both modern and ancient melting. To constrain the nature and timing of melt depletion processes, and the composition of the mantle, we report high-precision Os isotope data for abyssal peridotites from three ocean basins, as well as for Os-rich alloys, primarily from Mesozoic ophiolites. These data are complemented by whole-rock highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), trace- and major-element abundances for the abyssal peridotites, which are from the Southwest Indian (SWIR), Central Indian (CIR), Mid-Atlantic (MAR) and Gakkel Ridges. The results reveal a limited role for melt refertilization or secondary alteration processes in modifying abyssal peridotite HSE compositions. The abyssal peridotites examined have experienced variable melt depletion (2% to >16%), which occurred >0.5 Ga ago for some samples. Abyssal peridotites typically exhibit low Pd/Ir and, combined with high-degrees of estimated total melt extraction, imply that they were relatively refractory residues prior to incorporation into their present ridge setting. Recent partial melting processes and mid-ocean ridge basalt (MORB) generation therefore played a limited role in the chemical evolution of their precursor mantle domains. The results confirm that many abyssal peridotites are not simple residues of recent MORB source melting, having a more complex and long-lived depletion history. Peridotites from the Gakkel Ridge, SWIR, CIR and MAR indicate that the depleted MORB mantle has 186Os/188Os of 0.1198356 ± 21 (2SD). The Phanerozoic Os-rich alloys yield an average 186Os/188Os within uncertainty of abyssal peridotites (0.1198361 ± 20). Melt depletion trends defined between Os isotopes and melt extraction indices (e.g., Al2O3) allow an estimate of the primitive mantle (PM) composition, using only abyssal peridotites. This yields 187Os/188Os (0.1292 ± 25), and 186Os

  8. Origin of mantle peridotite: Constraints from melting experiments to 16.5 GPa (United States)

    Herzberg, Claude; Gasparik, Tibor; Sawamoto, Hiroshi


    Experimental data are reported for the melting of komatiite, peridotite, and chondrite compositions in the pressure range 5-16.5 GPa. All experiments were run using the multiple-anvil apparatus facilities at Nagoya and Stony Brook. Equilibrium between coexisting crystals and liquid is demonstrated to occur in less than 3 min in the 2100°C range. The anhydrous solidus in CaO-MgO-Al2O3-SiO2 has been calibrated and is shown to be about 100° higher than that for naturally occurring peridotite (KLB1). All melting curves have positive dT/dP. The effect of pressure is to expand the crystallization field of garnet at the expense of all other phases, resulting in a change in the liquidus phase from olivine to garnet at high pressures. The melting of rocks which contain the four crystalline phases olivine, orthopyroxene, clinopyroxene, and garnet is restricted to enstatite-rich compositions such as chondrite. For these it is demonstrated that melting is peritectic, rather than eutectic, and takes the form L+Opx = Ol+Cpx+Gt. Partial melting yields liquids with the following properties: 5 GPa for komatiite; and 10-15 GPa for liquid peridotite with about 40% MgO, but one that is unlike mantle peridotite in that it is distinctly enriched in silica. These results provide a test and refutation of the model that upper mantle peridotite originated by direct initial melting of a chondritic mantle (Herzberg and O'Hara, 1985). Unlike chondrite, partial melting of peridotite does not usually involve orthopyroxene. Instead, it occurs by the generation of ultrabasic liquids along a cotectic involving L+Ol+Cpx+Gt. Although the thermal and compositional characteristics of this cotectic have not been fully calibrated, it is very likely that it will degenerate into a thermal minimum (L+Ol+Cpx+Gt), compositionally similar to komatiite at 5 GPa and mantle peridotite at 10-15 GPa. Peridotite liquids that occupy a thermal minimum can be derived from those formed from the melting of chondrite by

  9. Mineral chemistry and petrology of mantle peridotites from the Guleman ophiolite (SE Anatolia, Turkey): Evidence of a forearc setting (United States)

    Rizeli, Mustafa Eren; Beyarslan, Melahat; Wang, Kuo-Lung; Bingöl, A. Feyzi


    The mantle section of Guleman ophiolite, southeast (SE) Turkey consists mainly of harzburgites and dunite lenses and large chromitite pods. The average Cr ratio = [100 × Cr/(Cr + Al) atomic ratio] of Cr-spinels in harzburgites and dunites is remarkably high (>63). The forsterite (Fo) content of olivine is between 90.9 and 92.3 in harzburgites and dunites. These features indicate that the harzburgites and dunites resulted from >35% of partial melting of a depleted mantle source. Discriminant geochemical diagrams based on the mineral chemistry of harzburgites indicate a supra-subduction zone (SSZ) origin. Orthopyroxene and clinopyroxene from the Guleman harzburgites have low CaO, Al2O3 and TiO2 contents, resembling those of depleted harzburgites from modern forearcs and contrasting with moderately depleted abyssal peridotites. Consequently, we propose that the Guleman peridotites formed in a forearc setting during the subduction initiation that developed as a result of northward subduction of the southern branch of the Neo-Tethys in response to the convergence between the Arabian and Anatolian plates.

  10. Peridotite-melt interaction: A key point for the destruction of cratonic lithospheric mantle

    Institute of Scientific and Technical Information of China (English)

    ZHANG HongFu


    This paper presents an overview of recent studies dealing with different ages of mantle peridotitic xenoliths and xenocrysts from the North China Craton, with aim to provide new ideas for further study on the destruction of the North China Craton. Re-Os isotopic studies suggest that the lithospheric mantle of the North China Craton is of Archean age prior to its thinning. The key reason why such a low density and highly refractory Archean lithospheric mantle would be thinned is changes in composition, thermal regime, and physical properties of the lithospheric mantle due to interaction of peridotites with melts of different origins. Inward subducUon of circum craton plates and collision with the North China Craton provided not only the driving force for the destruction of the craton, but also continuous melts derived from partial melting of subducted continental or oceanic crustal materials that resulted in the compositional change of the lithospheric mantle. Regional thermal anomaly at ca. 120 Ma led to the melting of highly modified iithospheric mantle. At the same time or subsequently lithospheric extension and asthenospheric upwelling further reinforced the melting and thinning of the lithospheric mantle. Therefore, the destruction and thinning of the North China Craton is a combined result of peridotite-melt interaction (addition of volatile), enhanced regional thermal anomaly (temperature increase) and lithospheric extension (decompression). Such a complex geological process finally produced a "mixed" lithospheric mantle of highly chemical heterogeneity during the Mesozoic and Cenozoic. It also resulted in significant difference in the composition of mantle peridotitic xenoliths between different regions and times.

  11. Micro—PIXE analysis of trace element composition and their distribution in minerals of mantle peridotite

    Institute of Scientific and Technical Information of China (English)

    陈友红; 张旗; 等


    The proton microprobe and the quantitative micro-PIXE analysis technique are used to determine trace element composition and distribution in minerals of mantle peridotite,therefore,additional useful information is provided on the study of upper mantle mineral phase transformation.An example reported here is the study on the geochemical behavior of trace elements in minerals of Suoluoshu mantle peridotite from Shandong province,which was in the process of mineral phase transformation from spinel to garnet.The study shows that trace element composition and distribution display obvious change in minerals while the mineral phase is transformed from spinel to garnet.Most of the trace elements analyzed reside in clinopyroxene and spinel.However,garnet is nearly poor in all of these elements.This phenomenon is associated with the infiltration of soultion and the existence of micropgrained inclusions in minerals in the process of mantle metasomatism.

  12. Mantle processes during ocean formation:Petrologic records in peridotites from the Alpine-Apennine ophiolites

    Institute of Scientific and Technical Information of China (English)



    Mantle peridotites were early exposed at the sea-floor of the Jurassic Tethys derived from the subcontinental mantle of the Europe-Adria system. During continental rifting and oceanic spreading, these lithospheric peri-dotites were percolated via diffuse reactive porous flowby melt fractions produced by near-fractional melting of the upwelling asthenosphere. Ascending melts inter-acted with the lower lithosphere, dissolving pyroxenes and precipitating olivine, and crystallized at shallower levels in the mantle column causing melt impregnation.Subsequent focused porous flow formed replacive dunitechannels, cutting the impregnated oeridotites, which were conduits for upward migration of MORB-type liq-uids. Melt migration produced depletionlrefertilization and significant heating of the percolatedlimpregnated mantle, i.e the thermochemical erosion of the litho-sphere. Impregnated and thermally modified lithos-pheric mantle was cooled by conductive heat loss dur-ing progressive lithosphere thinning and was intrudeaby MORB magmas, which formed Mg-rich and Fe-richgabbroic dykes and bodies. Alpine-Apennine ophiolitic peridotites record the deep-seated migration of melts which changed their compositions and dynamics during the rift evolution. The thermochemical erosion of the lithospheric mantle by the ascending asthenospheric melts, which induces significant compositional and rhe-ological changes in the lower lithosphere, is a major process in the evolution of the continent-ocean transi-tion towards a slow spreading oceanic system.

  13. Highly Siderophile Elements as Tracers for the Subcontinental Mantle Evolution Beneath the Southwestern USA: The San Carlos and Kilbourne Hole Peridotite Xenoliths Revisited (United States)

    van Acken, D.; Brandon, A. D.; Peslier, A. H.; Lee, C.


    Peridotite xenoliths from San Carlos, Arizona, and Kilbourne Hole, New Mexico, have been studied since the 1970s to give insights into melting and metasomatism in the subcontinental mantle beneath the southwestern USA. More recently, the highly siderophile elements (HSE; Os, Ir, Ru, Rh, Pt, Pd, and Re) and the included Re-Os isotope system have been established as powerful tools for the study of mantle processes because of their range in compatibility during mantle melting and their siderophile and chalcophile geochemical behavior. Model aluminachron Re-Os ages for San Carlos and Kilbourne Hole, as well as for the nearby Dish Hill and Vulcan’s Throne sites, give consistent depletion ages of around 2.2 Ga. This age can be interpreted as a single large scale mantle melting event linked to crustal formation and continental growth under the southwestern USA. However, recent studies showed that HSE may be added to depleted peridotites via melt-rock interaction, especially the more incompatible and hence mobile Pt, Pd, and Re. This may result in overprinting of the signature of melt extraction, thus abating the usefulness of Re-Os mantle extraction model ages. A comprehensive characterization of the suite of mantle xenoliths from the SW USA in terms of HSE concentrations is thus necessary to re-assess the Re-Os system for dating purposes. San Carlos peridotites are depleted to moderately fertile, as indicated by their bulk Al2O3 contents between 0.66 wt% and 3.13 wt%. Bulk 187Os/188Os in San Carlos peridotites range from 0.1206 to 0.1357. In contrast, Kilbourne Hole peridotites tend to be more fertile with Al2O3 between 2.11 and 3.78 wt%, excluding one extremely depleted sample with 0.30 wt% Al2O3, and have 187Os/188Os between 0.1156 and 0.1272, typical for mantle peridotites. No large fractionation between the more compatible HSE Os, Ir, and Ru is observed. The more incompatible HSE Re, Pd, and to a minor extent, Pt, however, are depleted in a number of samples by

  14. Petrogenesis of spinel peridotite suite xenoliths from northern Santa Cruz province, Argentina; implication for the Patagonian Lithospheric Mantle (United States)

    Ntaflos, Theodoros; Mundl, Andrea; Bjerg, Ernesto; Tschegg, Cornelius; Kosler, Jan


    Mantle xenoliths from Don Camilo, an area located on the North margin of the Deseado Masiff in Patagonia, comprise spinel bearing lherzolites, harburgites and dunites, wehrlites, clinopyroxenites and gabbros. The most common rock type in our collection is spinel-lherzolite followed by dunites. Harzurgites, wehrlites and gabbros are less widespread. Spinel-lherzolites and harzburgites have protogranular textures whereas dunites have equigranular to equigranular tabular textures. There are two kinds of dunites: mantle dunites and cumulate dunites. The olivine mg# in the mantle dunites vary within a narrow range, from 90.5 to 91.5 and the NiO content from 0.39 to 0.42 wt%, whereas in the cumulate dunites the mg# ranges from 87 to 90.5 and the NiO content from 0.22 to 0.40 wt%. Both types of dunites contain fine grained interstitial diopside. Hydrous phases, besides one sample that contains amphibole, were so far not found. The spinel peridotites have whole rock REE abundances depleted in LREE [(La/Yb)N=0.34-0.85)] and the dunites are LREE enriched [(La/Yb)N=3.49]. LA-ICP-MS analyses of cpx show that a number of the studied spinel peridotite xenoliths experienced cryptic metasomatism. Three groups of xenoliths have been recognized according to REE and other incompatible trace element patterns in cpx: group I has depleted LREE abundances, group II is highly enriched in LREE (20-30 x PM) and group 3 has moderate LREE enrichments. The core of some clinopyroxenes in group II has depleted LREE similar to those in group I, apparently representing relictic cores not affected by metasomatism. In addition the metasomatized clinopyroxenes are significantly enriched in Sr, Th and U. Evidently, the metasomatic agent was a H2O-rich fluid (high LREE, Sr, Th and U). Clinopyroxene Sr and Nd isotopic ratios vary largely from 0.702671 to 0.705788 and from 0.51229 to 0.513251 respectively. Mantle and cumulate dunites have experienced modal metasomatism. In both types of dunites the

  15. The aluminum-in-olivine thermometer for mantle peridotites - Experimental versus empirical calibration and potential applications (United States)

    Bussweiler, Y.; Brey, G. P.; Pearson, D. G.; Stachel, T.; Stern, R. A.; Hardman, M. F.; Kjarsgaard, B. A.; Jackson, S. E.


    This study provides an experimental calibration of the empirical Al-in-olivine thermometer for mantle peridotites proposed by De Hoog et al. (2010). We report Al concentrations measured by secondary ion mass spectrometry (SIMS) in olivines produced in the original high-pressure, high-temperature, four-phase lherzolite experiments by Brey et al. (1990). These reversed experiments were used for the calibration of the two-pyroxene thermometer and Al-in-orthopyroxene barometer by Brey and Köhler (1990). The experimental conditions of the runs investigated here range from 28 to 60 kbar and 1000 to 1300 °C. Olivine compositions from this range of experiments have Al concentrations that are consistent, within analytical uncertainties, with those predicted by the empirical calibration of the Al-in-olivine thermometer for mantle peridotites. Fitting the experimental data to a thermometer equation, using the least squares method, results in the expression: This version of the Al-in-olivine thermometer appears to be applicable to garnet peridotites (lherzolites and harzburgites) well outside the range of experimental conditions investigated here. However, the thermometer is not applicable to spinel-bearing peridotites. We provide new trace element criteria to distinguish between olivine from garnet-, garnet-spinel-, and spinel-facies peridotites. The estimated accuracy of the thermometer is ± 20 °C. Thus, the thermometer could serve as a useful tool in settings where two-pyroxene thermometry cannot be applied, such as garnet harzburgites and single inclusions in diamond.

  16. Mantle-derived peridotites in southwestern Oregon: relation to plate tectonics. (United States)

    Medaris, L G; Dott, R H


    A group of peridotites in southwestern Oregon contains high-pressure mineral assemblages reflecting recrystallization at high temperatures (1100 degrees to 1200 degrees C) over a range of pressure decreasing from 19 to 5 kilobars. It is proposed that the peridotites represent upper-mantle material brought from depth along the ancestral Gorda-Juan de Fuca ridge system, transported eastward by the spreading Gorda lithosphere plate, and then emplaced by thrust-faulting in the western margin of the Cordillera during late Mesozoic time.

  17. Geochronological Constraints on the Exhumation and Emplacement of Subcontinental Lithospheric Mantle Peridotites in the Westernmost Mediterranean (United States)

    Garrido, Carlos J.; Hidas, Károly; Marchesi, Claudio; Varas-Reus, María Isabel; Booth-Rea, Guillermo


    Exhumation of subcontinental mantle peridotite in the Western Mediterranean has been attributed to different tectonic processes including pure extension, transpression, or alternating contractive and extensional processes related with continental subduction followed by extension, before final their contractive intracrustal emplacement. Any model trying to explain the exhumation and emplacement of subcontinental lithospheric mantle peridotites in the westernmost Mediterranean should take into account the available geochronological constraints, as well as the petrological and geochemical processes that lead to internal tectono-magmatic zoning so characteristic of the Betic and Rif orogenic peridotites. Different studies have suggested a Hercynian, Cenozoic-Mesozoic or an Alpine age for the late tectono-magmatic evolution and intra-crustal emplacement of Betic-Rif peridotites. The pervasive presence of Mesozoic U-Pb zircon ages in Ronda UHP and HP garnet pyroxenites does not support a Hercynian age for the intracrustal emplacement of the peridotite. A hyper-extended margin setting for is in good agreement with the Jurassic extensional event that pervasively affected ALKAPECA terrains (i.e. the Alboran, Kabylides, Peloritani, and Calabria domains) in the western Mediterranean due to the opening of the Piemonte-Ligurian Ocean. However, a Jurassic age and a passive margin tectonic setting do not account, among other observations, for the late Miocene thermochronological ages recorded in zircons rims (U-Pb) and garnets (Lu-Hf) in garnet pyroxenites from the Betic-Rif peridotites, the pervasive Miocene resetting of U-Pb zircon and monazite ages in the overlying Jubrique crustal section, the supra-subduction radiogenic signature of late pyroxenite intrusive dikes in the Ronda peridotite, and the arc tholeiitic affinity of late mantle-derived, gabbroic dykes intruding in the Ronda and Ojen plagioclase lherzolites. These data are more consistent with a supra

  18. The compositions of Hydrous Fluids in equilibrium with the Peridotitic Mantle (United States)

    Adam, J.; Locmelis, M.; Fiorentini, M.; Rushmer, T. A.


    The compositional characteristics of H2O-rich fluids are critical to determine because migrating hydrous fluids have the capacity to impart many of the compositional characteristics of arc magmas and the continental crust/hydrosphere, to mantle source regions. However, the compositions of H2O-fluids produced by near-solidus experiments on peridotite are intrinsically difficult to determine. In this study we avoided many previously encountered problems by using an indirect approach. This involved H2O-saturated experiments on a hydrous nepheline basanite for which conditions of equilibrium with garnet-lherzolite (~ 1200 °C and 2.6 GPa) had been independently established using near-liquidus phase equilibria. Following experiments in single Ag70Pd30 and Au capsules, the quenched products of melts and H2O-fluids could be easily distinguished and directly analysed by EDS and LAM ICP-MS. Solute concentrations in the fluid phase were then calculated from mass-balances using estimated H2O-solubilities in melts. Because mineral/melt partition coefficients had already been determined for trace and minor elements in the basanite, we indirectly obtained peridotite/fluid partition coefficients for the same elements. In addition, we were able to directly analyse co-existing fluid-solutes and peridotite phases (clinopyroxene + orthopyroxene + olivine + amphibole + mica + spinel) from one sub-solidus experiment at 950 °C and 2.0 GPa. At 2.0 GPa and 950-1100 °C the H2O-fluid contains between 10 and 20 wt. % of dissolved solute, which has a peralkaline phonolite composition. Relative to the co-existing basanite melt, the H2O-fluid is enriched in Cs, Rb, K, Pb, Ba and V, but otherwise has lower concentrations of most incompatible elements. It is not depleted in HFSE relative to REE. As pressure and temperature increase, the fluid becomes more solute-rich until at 4.0 GPa and 1100 °C no clear distinction between the fluid and melt phase is apparent. At 2-3 GPa and 1100

  19. Hf–Zr anomalies in clinopyroxene from mantle xenoliths from France and Poland: implications for Lu–Hf dating of spinel peridotite lithospheric mantle


    Downes, Hilary; Vries, C. de; Wittig, N.


    \\ud \\ud Clinopyroxenes in some fresh anhydrous spinel peridotite mantle xenoliths from the northern Massif Central (France) and Lower Silesia (Poland), analysed for a range of incompatible trace elements by laser ablation inductively coupled plasma mass spectrometry, show unusually strong negative anomalies in Hf and Zr relative to adjacent elements Sm and Nd, on primitive mantle-normalised diagrams. Similar Zr–Hf anomalies have only rarely been reported from clinopyroxene in spinel peridotit...

  20. High Water Contents in the Siberian Cratonic Mantle: An FTIR Study of Udachnaya Peridotite Xenoliths (United States)

    Doucet, Luc S.; Peslier, Anne H.; Ionov, Dimitri A.; Brandon, Alan D.; Golovin, Alexander V.; Ashchepkov, Igor V.


    Water is believed to be a key factor controlling the long-term stability of cratonic lithosphere, but mechanisms responsible for the water content distribution in the mantle remain poorly constrained. Water contents were obtained by FTIR in olivine, pyroxene and garnet for 20 well-characterized peridotite xenoliths from the Udachnaya kimberlite (central Siberian craton) and equilibrated at 2-7 GPa. Water contents in minerals do not appear to be related to interaction with the host kimberlite. Diffusion modeling indicates that the core of olivines preserved their original water contents. The Udachnaya peridotites show a broad range of water contents in olivine (6.5 +/- 1.1 to 323 +- 65 ppm H2O (2 sigma)), and garnet (0 - 23 +/- 6 ppm H2O). The water contents of olivine and garnet are positively correlated with modal clinopyroxene, garnet and FeO in olivine. Water-rich garnets are also rich in middle rare earth elements. This is interpreted as the result of interaction between residual peridotites and water rich-melts, consistent with modal and cryptic metasomatism evidenced in the Siberian cratonic mantle. The most water-rich Udachnaya minerals contain 2 to 3 times more water than those from the Kaapvaal craton, the only craton with an intact mantle root for which water data is available. The highest water contents in olivine and orthopyroxene in this study (>= 300 ppm) are found at the bottom of the lithosphere (> 6.5 GPa). This is in contrast with the Kaapvaal craton where the olivines of peridotites equilibrated at > 6.4 GPa have 6 GPa is lower or similar (8.4× 10(exp 16) to 8.0× 10(exp 18) Pa./s) to that of the asthenosphere (<= 3.7x10(exp 18) Pa./s ). Such lithologies would not be able to resist delamination by the convecting asthenosphere. However, seismology studies as well as the high equilibration pressures of our samples indicate that the Udachnaya cratonic lithosphere is 220-250 km thick. Consequently, the water-rich peridotites are likely not

  1. Peridotite xenoliths from the Shiribeshi Seamount, Japan Sea: insights into mantle processes in a back-arc basin (United States)

    Ichiyama, Yuji; Morishita, Tomoaki; Tamura, Akihiro; Arai, Shoji


    Orthopyroxene-rich and orthopyroxene-poor peridotite xenoliths were sampled from quaternary basaltic to andesitic lava flows of the Shiribeshi seamount, Japan Sea. These xenoliths were affected by reactions with the host magma during transportation to the surface, which caused partial orthopyroxene dissolution and intergrowth with vermicular spinel. Chromian spinel and clinopyroxene major element compositions in the Shiribeshi peridotite are similar to those in abyssal peridotites. REE modeling indicates that the Opx-rich peridotite experienced decompression partial melting from the garnet to the spinel peridotite stability field. Rare earth element (REE) patterns of clinopyroxene in the Opx-rich peridotite show various degrees of enrichment in light REE, which resulted from melt percolation through the reaction with host magma. Comparison with peridotite xenoliths from two other localities (Seifu and Oshima-Ōshima) in the Japan Sea suggests that the Oshima-Ōshima peridotite record higher degree of partial melting than the Shiribeshi and Seifu peridotites. Oxygen fugacities calculated from chromian spinel in the Japan Sea peridotites are comparable to those of arc peridotites. The high degree of partial melting of the Oshima-Ōshima peridotite was possibly caused by the infiltration of a H2O-bearing flux released from the subducted slab. The Shiribeshi peridotite is interpreted as the residue formed after the extraction of depleted back-arc basalts during a later stage of the Japan Sea opening in the Middle Miocene, whereas the Oshima-Ōshima peridotite is residual after the extraction of enriched basalts during an earlier stage of the opening of the Japan Sea.

  2. Origin and evolution of rare amphibole-bearing mantle peridotites from Wilcza Góra (SW Poland), Central Europe (United States)

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


    Mantle xenoliths in the 20 Ma Wilcza Góra basanite (Lower Silesia, NE Bohemian Massif) are mostly harzburgites, some with amphibole which is exceptional in the region. Forsterite content in olivine defines two Groups of peridotites: Group A (Fo89.1-91.5) and Group B (Fo84.2-89.2). Hornblende-clinopyroxenite, websterite and one composite xenolith consisting of dunite, olivine-hornblendite and pyroxene-hornblende-peridotite contain olivine with Fo77.3-82.5 and are classified as Group C. Group A xenoliths contain Al-poor orthopyroxene and some contain LREE-enriched clinopyroxene with negative Ti, Zr-Hf and Nb-Ta anomalies. Spinel (Cr# 0.57-0.68) is scarce in Group A, and Cr-rich pargasite occurs in only two xenoliths. Group B xenoliths contain less magnesian orthopyroxene and clinopyroxene. The REE patterns of Group B clinopyroxene are convex downward, less enriched in LREE and have smaller negative Ti, Zr-Hf and Nb-Ta anomalies than those in Group A. The Cr# in Group B spinel is 0.26-0.56, while pargasite is Ti-rich and Cr-poor. Clinopyroxene from Group C is low magnesian, slightly enriched in LREE and has no negative Ti, Zr-Hf and Nb-Ta anomalies. Group C pargasite is rich in Ti and poor in Cr. Equilibration temperatures recorded in all groups vary within the range of 905-970 °C. Xenoliths from Wilcza Góra record a polyphase lithospheric mantle evolution, starting with melting which extracted ca. 30% melt from the protolith and left a harzburgite residuum depleted in Al, lacking clinopyroxene and containing rare Cr-rich spinel. This residuum was later overprinted by chromatographic metasomatism by carbonated hydrous silicate melt related to Cenozoic volcanism. The metasomatic agent was locally hydrous enough to enable amphibole to crystallize. The Group C pyroxenites formed directly from the metasomatic melt or during peridotite-melt reactions at high melt-rock ratio. The melt is inferred to have percolated through the wall-rock peridotite, decreasing its amount

  3. Complex metasomatism of lithospheric mantle by asthenosphere-derived melts: Evidence from peridotite xenoliths in Weichang at the northern margin of the North China Craton (United States)

    Zou, Dongya; Zhang, Hongfu; Hu, Zhaochu; Santosh, M.


    The petrology, in situ analyses of major and trace elements as well as Sr isotopic compositions of spinel-facies lherzolite and harzburgite xenoliths from Weichang within the northern margin of the North China Craton (NCC) are reported for the first time in this study to evaluate the nature and evolution of the lithospheric mantle. These peridotite xenoliths display porphyroclastic texture and can be subdivided into two groups. Group I peridotites have slightly higher forsterite contents (Fo) (90.6-91.2) and 87Sr/86Sr ratios (0.7025-0.7043) in the cores than those in the rims (89.8-90.8; 0.7025-0.7038) of olivines and clinopyroxenes, respectively. The clinopyroxenes in these rocks exhibit uniform LREE-depleted patterns. These geochemical features suggest that the Group I peridotites were weakly metasomatized by recent asthenospheric melts. In contrast, Group II peridotites show sieve-texture and clear compositional zoning in minerals. The olivines and clinopyroxenes from these rocks have higher Fo (86.9-91.3) and 87Sr/86Sr ratios (0.7035-0.7049) in the cores than those in the rims (76.9-90.6; 0.7021-0.7046). The clinopyroxenes show three types of REE patterns: LREE-enriched, convex-upward and spoon-shaped. The LREE-enriched clinopyroxenes have the highest (La/Yb)N and lowest Ti/Eu and those with spoon-shaped REE patterns show an increase in LREE, Ba, Th and U contents from the cores to the rims. These features indicate that the Group II peridotites witnessed a high degree of refertilization by recent asthenosphere-derived silicate and carbonatite melts or their mixture. Compared with the data of the Mesozoic NCC lithospheric mantle, the heterogeneous and low 87Sr/86Sr ratios (0.7025-0.7049) in the LREE-depleted clinopyroxenes reveal that the ancient lithospheric mantle could have been modified by asthenospheric melts before the recent metasomatism event. We conclude that the lithospheric mantle beneath Weichang underwent multiple modifications through asthenosphere

  4. Experimental calibration of Forsterite-Anorthite-Ca-Tschermak-Enstatite (FACE) geobarometer for mantle peridotites (United States)

    Fumagalli, P.; Borghini, G.; Rampone, E.; Poli, S.


    The crystallization of plagioclase-bearing assemblages in mantle rocks is witness of mantle exhumation at shallow depth. Previous experimental works on peridotites have found systematic compositional variations in coexisting minerals at decreasing pressure within the plagioclase stability field. In this experimental study we present new constraints on the stability of plagioclase as a function of different Na2O/CaO bulk ratios, and we present a new geobarometer for mantle rocks. Experiments have been performed in a single-stage piston cylinder at 5-10 kbar, 1050-1150 °C at nominally anhydrous conditions using seeded gels of peridotite compositions (Na2O/CaO = 0.08-0.13; X Cr = Cr/(Cr + Al) = 0.07-0.10) as starting materials. As expected, the increase of the bulk Na2O/CaO ratio extends the plagioclase stability to higher pressure; in the studied high-Na fertile lherzolite (HNa-FLZ), the plagioclase-spinel transition occurs at 1100 °C between 9 and 10 kbar; in a fertile lherzolite (FLZ) with Na2O/CaO = 0.08, it occurs between 8 and 9 kbar at 1100 °C. This study provides, together with previous experimental results, a consistent database, covering a wide range of P- T conditions (3-9 kbar, 1000-1150 °C) and variable bulk compositions to be used to define and calibrate a geobarometer for plagioclase-bearing mantle rocks. The pressure sensitive equilibrium: Mg_{2}SiO_{4}^Ol\\limits_{Forsterite} + CaAl_{2}Si_{2}O_{8}^{Pl\\limits_{Anorthite} = CaAl_{2}SiO_{6}^{Cpx}\\limits_{Ca-Tschermak} + Mg_{2}Si_{2}O_{6}^{Opx}\\limits_{Enstatite}, has been empirically calibrated by least squares regression analysis of experimental data combined with Monte Carlo simulation. The result of the fit gives the following equation: P=7.2( ± 2.9)+0.0078( ± 0.0021)T{{ }}+0.0022( ± 0.0001)T ln K, {R^2}=0.93, where P is expressed in kbar and T in kelvin. K is the equilibrium constant K = a CaTs × a en/ a an × a fo, where a CaTs, a en, a an and a fo are the activities of Ca-Tschermak in

  5. Petrography and mineral chemistry of metamorphosed mantle peridotites of Nain Ophiolite (Central Iran

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    Nargess Shirdashtzadeh


    Full Text Available Introduction Study of the petrology of the ophiolites as the relics of ancient oceanic lithosphere, is a powerful tool to reconstruct Earth’s history. Mantle peridotites have mostly undergone alteration and serpentinization to some extent. Thus, the relics of metamorphic signatures from the upper mantle and crustal processes from most of the peridotites have been ruined. Several recent papers deal with the mantle peridotites of Nain Ophiolite (e.g. Ghazi et al., 2010. However, no scientific work has been carried out on the metamorphosed mantle peridotites. The study area of the Darreh Deh that is located in the east of the Nain Ophiolite, is composed of huge massifs of metamorphosed mantle peridotites (i.e. lherzolite, clinopyroxene-bearing harzburgite, and harzburgite, and small volumes of dunite, characterized by darker color, higher topographic relief, smaller number of basic intrusives, lower serpentinization degree, and amphibolite-facies metamorphism. In this study, the petrography and mineralogy of metamorphosed peridotites in the Darreh Deh has been considered based on geochemical data. Geological Setting The Mesozoic ophiolitic mélange of Nain is located in the west of CEIM, along the Nain-Baft fault. As a part of a metamorphosed oceanic crust, it is mainly composed of harzburgite, lherzolite, dunite and their serpentinized varieties, chromitite, pyroxenite, gabbro, diabasic dike, spilitized pillow lava, plagiogranite, amphibolite, metaperidotites, schist, skarn, marble, rodingite, metachert and listwaenite (Shirdashtzadeh et al., 2010, 2014a, 2014b. Geochemical investigations indicate a suprasubduction zone in the eastern branch of the Neo-Tethys Ocean (Ghasemi and Talbot, 2006; Shirdashtzadeh et al., 2010, 2014a, 2014b. Materials and Methods Chemical analyses of mineral compositions were carried out using a JEOL JXA8800R wavelength-dispersive electron probe micro-analyzer (accelerating voltage of 15 kV and a beam current of 15 n

  6. Zircon syenite pegmatites in the Finero peridotite (Ivrea zone): evidence for a syenite from a mantle source

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    Staehle, V.; Frenzel, G. (Mineralogisch-Petrographisches Inst., Univ. Heidelberg (Germany, F.R.)); Kober, B. (Lab. fuer Geochronologie, Univ. Heidelberg (Germany, F.R.)); Michard, A. (Centre de Recherches Petrographiques et Geochimiques, 54 - Vandoeuvre (France)); Puchelt, H. (Inst. fuer Petrographie und Geochemie, Univ. Karlsruhe (Germany, F.R.)); Schneider, W. (Inst. fuer Geologie und Palaeontologie, Technische Univ. Braunschweig (Germany, F.R.))


    Zircon syenite pegmatite dikes in the phlogopite peridotite of Finero are rich in volatiles, incompatible elements and in rare earths with (La/Yb){sub N} ratios in the range of 14-15. The main mineral constituents in the alkaline dikes (Na>K) are albite and magnesium-rich biotites whereas nepheline, cancrinite, sodalite, zircon and apatite are minor or accessory. With their initial {sup 87}Sr/{sup 86}Sr ratio of 0.70371 the non-contaminated syenite pegmatites originate from partial melts of the upper mantle. The magma is derived from a depleted mantle source with isotopic signatures ({epsilon}Nd(T)=5.7 and 5.1) of ocean island basalts (OIB). The ascent of the syenite pegmatites and the marginal uplift of the granulitic crust in the northern Ivrea zone may be correlated with the intrusion of alkaline melts at depth. The Triassic zircon age of 225+-13 Ma is interpreted as the time of the dikes' emplacement. From geochemistry and setting it is concluded, that the syenites are related to a rift magmatic stage at the early Mesozoic in the southern Alps. (orig.).

  7. Generation of mantle heterogeneity via reaction between MORB-pyroxenite derived partial melts and peridotite (United States)

    Mallik, A.; Dasgupta, R.


    Characterization of mineralogy and lithology of mantle heterogeneities is important as they affect melting processes and physical properties of the mantle in turn influencing mantle dynamics. Direct evidence of such heterogeneities are found as lithospheric xenoliths and in tectonically exposed mantle sections. However, possible major element compositions of heterogeneities that can be produced in the convective mantle remain unclear. Reaction between silica-rich melt from subducted oceanic crust and peridotite has been deemed responsible for generation of secondary pyroxenite and olivine-free mantle beneath Hawaii [1]. However, no studies till date have systematically characterized the evolution of mineralogy and lithology owing to such a melt-rock reaction. We performed reaction experiments between two MORB-pyroxenite derived basaltic andesites and a volatile-free lherzolite using piston-cylinder apparatus and Pt/Gr capsules, at 2.5-3 GPa, 1375°C and 1440°C. The fraction of melt was varied from ~8 to 50%. Melt was introduced either in a layered geometry or mixed homogeneously with peridotite to simulate channelized and porous flow, respectively. Layered experiments produced a zone of (± garnet-) websterite separating a pool of reacted melt from four-phase lherzolite. They show that olivine-exhaustion is possible only locally along melt channels and the peridotite domains away from melt bands remain effectively unaltered in terms of modal mineralogy although slight diffusive enrichment in TiO2, Al2O3 and FeO is observed. In the mixed experiments, olivine mode diminishes at the expense of orthopyroxene producing a continuous spectrum from lherzolite to olivine websterite and finally websterite with increasing melt-rock ratio. With increasing orthopyroxene modal enrichment, orthopyroxene composition displays increase in TiO2 (~0.2 to ~0.9 wt%), Al2O3 (~4.5 to ~6.7 wt%), FeO (~6 to ~8 wt%), Na2O (~0.2 to 0.8 wt%) and decrease in Cr2O3 (~0.35 to 0.15 wt%) compared

  8. Subduction-related cryptic metasomatism in fore-arc to nascent fore-arc Neoproterozoic mantle peridotites beneath the Eastern Desert of Egypt: mineral chemical and geochemical evidences (United States)

    Hamdy, Mohamed; Salam Abu El-Ela, Abdel; Hassan, Adel; Kill, Youngwoo; Gamal El Dien, Hamed


    Mantle spinel peridotites beneath the Arabian Nubian Shield (ANS) in the Eastern Desert (ED) of Egypt were formed in arc stage in different tectonic setting. Thus they might subject to exchange with the crustal material derived from recycling subducting oceanic lithosphere. This caused metasomatism enriching the rocks in incompatible elements and forming non-residual minerals. Herein, we present mineral chemical and geochemical data of four ophiolitic mantle slice serpentinized peridotites (W. Mubarak, G. El-Maiyit, W. Um El Saneyat and W. Atalla) widely distributed in the ED. These rocks are highly serpentinized, except some samples from W. Mubarak and Um El-Saneyat, which contain primary olivine (Fo# = 90-92 mol %) and orthopyroxene (En# = 86-92 mol %) relics. They have harzburgite composition. Based on the Cr# and Mg# of the unaltered spinel cores, all rocks formed in oceanic mantle wedge in the fore-arc setting, except those from W. Atalla formed in nascent fore-arc. This implies that the polarity of the subduction during the arc stage was from the west to the east. These rocks are restites formed after partial melting between 16.58 in W. Atalla to 24 % in G-El Maiyit. Melt extraction occurred under oxidizing conditions in peridotites from W. Mubarak and W. Atalla and under reducing conditions in peridotites from G. El-Maiyit and Um El-Saneyat. Cryptic metasomatism in the studied mantle slice peridotites is evident. This includes enrichment in incompatible elements in minerals and whole rocks if compared with the primitive mantle (PM) composition and the trend of the depletion in melt. In opx the Mg# doesn't correlate with TiO2, CaO, MnO, NiO and Cr2O3concentrations. In addition, in serpentinites from W. Mubarak and W. Atalla, the TiO2spinel is positively correlated with the TiO2 whole-rock, proposing enrichment by the infiltration of Ti-rich melts, while in G. El- Maiyit and Um El-Saneyat serpentinites they are negatively correlated pointing to the reaction

  9. Origin of the Luobusa diamond-bearing peridotites from the sub-arc mantle (United States)

    Liu, Chuanzhou; Zhang, Chang; Wu, Fuyuang; Chung, Sunlin


    Ophiolites are the remnants of ancient oceanic lithosphere that were emplaced onto continental margins. Ophiolites along the E-W trending Yarlung-Tsangpo Suture (YTS), which separates the Indian plate from the Eurasian plate, have been regarded as relics of the Neo-Tethys Ocean. The Luobusa ophiolite outcrops at the eastern YTS and mainly consists of harzburgites and dunites that have been intruded by gabbroic/diabase dykes at ca 130 Ma (Zhang et al., 2015). Basaltic lavas are rarely outcropped, and volumetrically minor (ocean ridge and subsequently reacted with boninitic magmas in subduction zone, which gave rise to podifiorm chromitites (Zhou et al., 1996). However, such a shallow depth origin fails to explain the occurrence of diamond and other ultra-high pressure (UHP) minerals in both peridotites and chromitites (Yang et al., 2007, 2014). A mantle plume origin has been proposed for the Luobusa ophiolite to explain the UHP minerals. However, this model is not reconciled with the occurrence of low-pressure crustal minerals in both chromitites and peridotites (Robinson et al., 2015). Here we report whole-rock Re-Os isotope data, which suggest that most Luobusa peridotites have subjected to ancient melting events older than 1.9 billion years. High contents of heavy rare earth elements in clinopyroxenes support the occurrence of ancient melting in garnet stability field. Hf-Nd isotopes of clinopyroxenes, which yield young model ages as 110 Ma, with one showing the lowest ɛNd(T) value of -3, do not preserve the signatures of ancient melting but record metasomatism by subduction-related agents. Consequently, we argue that protoliths of the Luobusa peridotites originated from ancient domains in the transition zone and, together with diamond-bearing chromitites, were accreted through a plume activity (Yang et al., 2014) up to the lithospheric mantle beneath the Lhasa terrane, where they have been metasomatized by Neo-Tethyan subduction processes. This sub

  10. Microstructural evolution of the Yugu peridotites in the Gyeonggi Massif, Korea: Implications for olivine fabric transition in mantle shear zones (United States)

    Park, Munjae; Jung, Haemyeong


    Large-scale emplaced peridotite bodies may provide insights into plastic deformation process and tectonic evolution in the mantle shear zone. Due to the complexity of deformation microstructures and processes in natural mantle rocks, the evolution of pre-existing olivine fabrics is still not well understood. In this study, we examine well-preserved transitional characteristics of microstructures and olivine fabrics developed in a mantle shear zone from the Yugu peridotite body, the Gyeonggi Massif, Korean Peninsula. The Yugu peridotite body predominantly comprises spinel harzburgite together with minor lherzolite, dunite, and clinopyroxenite. We classified highly deformed peridotites into four textural types based on their microstructural characteristics: proto-mylonite; proto-mylonite to mylonite transition; mylonite; and ultra-mylonite. Olivine fabrics changed from A-type (proto-mylonite) via D-type (mylonite) to E-type (ultra-mylonite). Olivine fabric transition is interpreted as occurring under hydrous conditions at low temperature and high strain, because of characteristics such as Ti-clinohumite defects (and serpentine) and fluid inclusion trails in olivine, and a hydrous mineral (pargasite) in the matrix, especially in the ultra-mylonitic peridotites. Even though the ultra-mylonitic peridotites contained extremely small (24-30 μm) olivine neoblasts, the olivine fabrics showed a distinct (E-type) pattern rather than a random one. Analysis of the lattice preferred orientation strength, dislocation microstructures, recrystallized grain-size, and deformation mechanism maps of olivine suggest that the proto-mylonitic, mylonitic, and ultra-mylonitic peridotites were deformed by dislocation creep (A-type), dislocation-accommodated grain-boundary sliding (D-type), and combination of dislocation and diffusion creep (E-type), respectively.

  11. Petrogenesis of fertile mantle peridotites from the Monte del Estado massif (southwest Puerto Rico): a preserved section of Proto-Caribbean oceanic lithospheric mantle? (United States)

    Marchesi, Claudio; Jolly, Wayne T.; Lewis, John F.; Garrido, Carlos J.; Proenza, Joaquín. A.; Lidiak, Edward G.


    The Monte del Estado massif is the largest and northernmost serpentinized peridotite belt in southwest Puerto Rico. It is mainly composed of spinel lherzolite and minor harzburgite with variable clinopyroxene modal abundances. Mineral and whole rock major and trace element compositions of peridotites coincide with those of fertile abyssal peridotites from mid ocean ridges. Peridotites lost 2-14 wt% of relative MgO and variable amounts of CaO by serpentinization and seafloor weathering. HREE contents in whole rock indicate that the Monte del Estado peridotites are residues after low to moderate degrees (2-15%) of fractional partial melting in the spinel stability field. However, very low LREE/HREE and MREE/HREE in clinopyroxene cannot be explained by melting models of a spinel lherzolite source and support that the Monte del Estado peridotites experienced initial low fractional melting degrees (~ 4%) in the garnet stability field. The relative enrichment of LREE in whole rock is not due to secondary processes but probably reflects the capture of percolating melt fractions along grain boundaries or as microinclusions in minerals, or the presence of exotic micro-phases in the mineral assemblage. We propose that the Monte del Estado peridotite belt represents a section of ancient Proto-Caribbean (Atlantic) lithospheric mantle originated by seafloor spreading between North and South America in the Late Jurassic-Early Cretaceous. This portion of oceanic lithospheric mantle was subsequently trapped in the forearc region of the Greater Antilles paleo-island arc generated by the northward subduction of the Caribbean plate beneath the Proto-Caribbean ocean. Finally, the Monte del Estado peridotites belt was emplaced in the Early Cretaceous probably as result of the change in subduction polarity of the Greater Antilles paleo-island arc without having been significantly modified by subduction processes.

  12. Infiltration of Refractory Melts into the Sub-Oceanic Mantle: Evidence from Major and Minor Element Compositions of Minerals from the 53° E Amagmatic Segment Abyssal Peridotites at the Southwest Indian Ridge (United States)

    Gao, C.; Dick, H. J.; Zhou, H.; Liu, Y.; Wang, J.


    Elevated sodium and titanium in pyroxene and spinel with high TiO2 (> 0.2 wt%) are suggested as the geochemical characteristic for the MORB-like melt infiltration of peridotites. The petrological and geochemical results of melt infiltrating in mantle peridotites are controlled by not only the melt composition but also the melt/rock ratio. Large discordant dunite bodies in the mantle transition zone are the direct observation of large volume melt (high melt/rock ratio) infiltrating by channeled porous flow in the shallow mantle (1). In addition to dunites, melt infiltrating results in a large variety of vein lithologies in mantle, and the occurrence of plagioclases are considered as a petrological signal of melt-reaction at shallow depth (2, 3) with a medium melt/rock ratio. Because the lacking of obviously petrological and geochemical variation of peridotites, melt infiltration of peridotites with a low melt/rock ratio are rarely reported. Peridotites in this study are from the 53° E amagmatic segment at the Southwest Indian Ridge. These peridotites are suggested as highly depleted buoyant mantle drawn up from the asthenosphere beneath southern Africa during the breakup of Gondwanaland (4) and are residues of multi-stage melt extracting in both spinel and garnet field. We present a detailed analysis of mineral compositions by both the EMPA and LA-ICPMS. Mineral phases in 53°E peridotites have mantle major element compositions, although minerals show variations with the crystal size and the location from cores to rims (Fig.1). In conjunction with the profile analysis of large clinopyroxene crystals, our results document the melt infiltration occurred at the ultraslow-spreading environment. At least two kinds of percolation melts are distinguished. They are normally MORB-like melt and ultra-depleted melt. Reference1.P. B. Kelemen, H. J. B. Dick, Journal of Geophysical Research-Solid Earth 100, 423 (Jan, 1995). 2.J. M. Warren, N. Shimizu, Journal of Petrology 51

  13. Flow in the uppermost mantle during back-arc spreading revealed by Ichinomegata peridotite xenoliths, NE Japan (United States)

    Satsukawa, Takako; Michibayashi, Katsuyoshi


    Spinel peridotite xenoliths from the Ichinomegata Volcano (NE Japan) have distinct foliations defined by compositional layering between olivine-rich and pyroxene-rich layers as well as lineations defined by elongated spinel grains. Crystallographic preferred orientations (CPOs) of olivine are consistent with slip on (010)[100] and {0kl}[100]. The angles between the foliation and the olivine slip planes decrease with increasing values of the J-index (i.e. CPO strength). Such composite planar relationships within the peridotite xenoliths could result from shearing in the uppermost mantle, so that shear strains can be estimated by the angles between the foliation and the olivine slip plane in terms of simple shear strain (0.31-4.26). From these observations, we argue that a suite of the peridotite xenoliths recorded a rare snapshot of uppermost-mantle flow related to back-arc spreading during the opening of the Japan Sea. The peridotite xenoliths with higher J-indices (i.e. higher shear strain) tend to have slightly lower minimum temperatures, possibly defining a vertical strain gradient in the uppermost mantle section at the time of the volcano's eruption. The CPO data have been used to calculate the seismic properties of the xenoliths at PT conditions obtained from geothermobarometry, and are compared to field geophysical data from the literature. Our results are consistent with a roughly EW-oriented fastest P-wave propagation direction in the uppermost mantle beneath the northeast part of the Japan arc. Average samples are calculated based on three different structural reference frames; horizontal plane parallel to 1) foliation, 2) the plane containing the maximum concentration of olivine [100], and 3) P-wave maximum direction. S-wave anisotropy deduced from CPOs requires a reasonable thickness of the anisotropic layer (24.1-26.6 km), and the structural reference frame does not have significant effect on the estimation of thickness. Consequently, Ichinomegata

  14. Timing of Precambrian melt depletion and Phanerozoic refertilization events in the lithospheric mantle of the Wyoming Craton and adjacent Central Plains Orogen (United States)

    Carlson, R.W.; Irving, A.J.; Schulze, D.J.; Hearn, B.C.


    Garnet peridotite xenoliths from the Sloan kimberlite (Colorado) are variably depleted in their major magmaphile (Ca, Al) element compositions with whole rock Re-depletion model ages generally consistent with this depletion occurring in the mid-Proterozoic. Unlike many lithospheric peridotites, the Sloan samples are also depleted in incompatible trace elements, as shown by the composition of separated garnet and clinopyroxene. Most of the Sloan peridotites have intermineral Sm-Nd and Lu-Hf isotope systematics consistent with this depletion occurring in the mid-Proterozoic, though the precise age of this event is poorly defined. Thus, when sampled by the Devonian Sloan kimberlite, the compositional characteristics of the lithospheric mantle in this area primarily reflected the initial melt extraction event that presumably is associated with crust formation in the Proterozoic-a relatively simple history that may also explain the cold geotherm measured for the Sloan xenoliths. The Williams and Homestead kimberlites erupted through the Wyoming Craton in the Eocene, near the end of the Laramide Orogeny, the major tectonomagmatic event responsible for the formation of the Rocky Mountains in the late Cretaceous-early Tertiary. Rhenium-depletion model ages for the Homestead peridotites are mostly Archean, consistent with their origin in the Archean lithospheric mantle of the Wyoming Craton. Both the Williams and Homestead peridotites, however, clearly show the consequences of metasomatism by incompatible-element-rich melts. Intermineral isotope systematics in both the Homestead and Williams peridotites are highly disturbed with the Sr and Nd isotopic compositions of the minerals being dominated by the metasomatic component. Some Homestead samples preserve an incompatible element depleted signature in their radiogenic Hf isotopic compositions. Sm-Nd tie lines for garnet and clinopyroxene separates from most Homestead samples provide Mesozoic or younger "ages" suggesting

  15. Peridotite xenoliths from Ethiopia: inferences on mantle processes from Plume to Rift settings (United States)

    Beccaluva, Luigi; Bianchini, Gianluca; Ellam, Robert Mark; Natali, Claudio; Santato, Alessandro; Siena, Franca; Stuart, Finlay


    A comprehensive petrological study has been carried out on Ethiopian mantle xenoliths entrained in Neogene-Quaternary alkaline lavas both overlying the Continental Flood Basalt area (Dedessa River - Wollega Region, Injibara - Gojam Region) and from southern Main Ethiopian Rift (MER - Mega, Sidamo Region) in order to investigate the mantle evolution from plume to rift settings. Mantle xenoliths from the plateau area (Injibara, Dedessa River) range in composition from spinel lherzolite to harzburgite and olivine websterite, showing P-T equilibration conditions in the range of 1-2 GPa/950-1050 °C. These xenoliths show flat chondrite-normalized bulk-rock REE patterns, with only few LREE-enriched samples (LaN/YbN up to 5). Clinopyroxene (cpx) REE patterns are generally flat or LREE depleted (LaN/YbN down to 0.6). Sr-Nd isotopes on separated cpx mainly show compositions (87Sr/86Sr 0.5132) approaching the Depleted Mantle end-member, or displaced (87Sr/86Sr 0.7033-0.7034; 143Nd/144Nd 0.5129-0.5128) toward the Enriched Mantle components which also characterize the Ethiopian Oligocene plateau basalts. These characteristics indicate that most xenoliths reflect complex asthenosphere/lithosphere interactions due to plume-related refertilization processes, whose agents may be envisaged as mafic subalkaline melts that infiltrated and reacted with the pristine parageneses ultimately leading to the formation of olivine-websterite domains. On the other hand, mantle xenoliths from southern MER (Mega) consist of spinel lherzolite to harzburgites showing various degree of deformation and recrystallization coupled with an extremely wide incompatible element distribution. Bulk rock Rare Earth Element (REE) patterns show generally flat HREE ranging from 0.1 x chondrite (ch) in harzburgites up to 2 x ch in fertile lherzolites, and are variably enriched in LREE, with LaN/YbN up to 41.5. The constituent clinopyroxenes have flat HREE distribution and LaN/YbN between 0.1 and 55, in general

  16. Evidence of Arc Magma Genesis in a Paleo-Mantle Wedge, the Higashi-Akaishi Peridotite, Japan (United States)

    Till, C. B.; Guild, M. R.; Grove, T. L.; Carlson, R. W.


    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

  17. Petrogenesis and tectonic implications of gabbro and plagiogranite intrusions in mantle peridotites of the Myitkyina ophiolite, Myanmar (United States)

    Xu, Yang; Liu, Chuan-Zhou; Chen, Yi; Guo, Shun; Wang, Jian-Gang; Sein, Kyaing


    Centimeter-size intrusions of gabbros and plagiogranites occur in mantle peridotites of the Myitkyina ophiolite, Myanmar. The gabbros mainly consist of plagioclase and clinopyroxene, whereas orthopyroxene occasionally occurs. The plagiogranites are mainly composed of plagioclase, quartz and amphibole, with small amount of accessory minerals, such as zircon, apatite and rutile. Plagioclase in the gabbros varies from andesine to anorthite (An37-91), whereas plagioclase in the plagiogranites is less calcic (An1-40). Clinopyroxene in the gabbros is pervasively altered to hornblende. The gabbros contain 42.97-52.88 wt% SiO2, which show negative correlations with Al2O3, CaO and MgO, but positive correlations with Na2O, P2O5 and TiO2. Microtextural relations reveal the crystallization of clinopyroxene prior to plagioclase in the Myitkyina gabbros. This suggests that the gabbros were crystallized from hydrous melts, which is also supported by the occurrence of orthopyroxene and anorthitic plagioclase in some gabbros. The gabbros have slightly enriched Sr-Nd isotopes, with initial 87Sr/86Sr ratios of 0.703938-0.706609 and εNd(t) values of + 2.4-+7.2, and relatively variable Hf isotopes, with εHf(t) values of + 13.4-+24.9. A subduction component is required to explain the decoupled Nd-Hf isotopes of the gabbros. Binary mixing suggests that addition of ca 2% subducted sediments to a depleted mantle can account for the Nd-Hf decoupling. Therefore, both petrological and geochemical data of the gabbros support that the Myitkyina ophiolite was originated in a supra-subduction zone setting. The plagiogranites have compositions of tonalites and trondhjemites, containing 56.93-77.93 wt% SiO2, 1.27-10.79 wt% Na2O and 0.05-0.71 wt% K2O. They are slightly enriched in LREE over HREE and display positive anomalies in Eu, Zr, Hf but negative Nb anomalies. Very low TiO2 contents (0.03-0.2 wt%) of the plagiogranites suggest that they were not products of fractional crystallization of MORB

  18. Petrography and mineral chemistry of wehrlites in contact zone of gabbro intrusions and mantle peridotites of the Naein ophiolite

    Directory of Open Access Journals (Sweden)

    Farhad Ghaseminejad


    Full Text Available Introduction Geological background Ophiolites have played a major role in our understanding of Earth’s processes ranging from seafloor spreading, melt evolution and magma transport in oceanic spreading centers, and hydrothermal alteration and mineralization of oceanic crust to collision tectonics, mountain building processes, and orogeny. They provide the essential structural, petrological, geochemical, and geochronological evidence to document the evolutionary history of ancient continental margins and ocean basin. Ophiolites include a peridotitic mantle sequence, generally characterized by high-temperature plastic deformation and residual chemistry, and a comagmatic crustal sequence (gabbros, diabase dikes, and submarine basalts, weakly or not deformed. According to this interpretation, ophiolites were allochthonous with respect to their country rocks. They were assembled during a primary accretion stage at an oceanic spreading center, and later tectonically emplaced on a continental margin or island arc (Dilek, 2003. The indigenous dikes of pyroxenites and gabbros that were injected into a melting peridotite, or intrusive dikes of pyroxenite and gabbro that injected when the peridotite was fresh and well below its solidus, are discussed in different ophiolite papers. Pyroxenite formation and contact of gabbro and mantle peridotite are discussed in different articles (Dilek, 2003. When a gabbro intrude a fresh mantle peridotite could not significantly react with it, but if intrusion occurs during the serpentinization, the gabbro will change to rodingite. Geological setting The Naein ophiolitic melanges comprise the following rock units: mantle peridotites (harzburgite, lherzolite, dunite, with associated chromitite, gabbro, pyroxenite, sheeted and swarm dikes, massive basalts, pillow lava, plagiogranite, radiolarian chert, glaubotruncana limestone, rodingite, listvenite, and metamorphic rocks (foliated amphibolitic dike, amphibolite, skarn

  19. Recrystallisation, phase mixing and strain localisation in peridotite during rapid extrusion of sub-arc mantle lithosphere (United States)

    Czertowicz, T. A.; Toy, V. G.; Scott, J. M.


    The Anita Peridotite, in southwestern New Zealand, is a ∼1 × 20 km ultramafic massif that was rapidly extruded from beneath a Cretaceous arc within the 4 km wide mylonitic Anita Shear Zone. The peridotitic body contains a spectacular array of textures that preserve evidence for changing temperature, stress, and deformation mechanisms during the exhumation process. Olivine and orthopyroxene microstructures and lattice-preferred orientations (LPO) record a three-phase deformation history. Dislocation glide on the C- and E-type slip systems is recorded by coarse pre-mylonitised olivine grains, and occurred under hydrous conditions at T ∼650 °C, stress ∼200-700 MPa and strain rate ∼10-15 s-1, probably within hydrated sub-arc mantle lithosphere. Rare protomylonite pods record deformation by dislocation creep in porphyroclasts and dislocation-accommodated grain boundary sliding in the matrix on {0kl}[100] in olivine and (100)[001] in orthopyroxene, under conditions of T ∼730-770 °C, stress ∼52-700 MPa and strain rate ∼10-15 s-1. The massif, however, is dominated by mylonite and ultramylonite that wrap the protomylonite pods, comprising mostly fine-grained olivine neoblasts that lack internal distortions and have uniform LPOs. These textures indicate deformation occurred by grain-size sensitive (GSS) creep at T ∼650 °C, stress ∼69-137 MPa and strain rate ∼10-15 s-1, and thus during conditions of cooling and decreasing stress. GSS creep became more dominant with time, as the proportion of randomly-oriented neoblasts increased and formed interlinked networks that accommodated much of the strain. Grain boundary pinning allowed GSS creep to be maintained in polyphase regions, following mixing of olivine and orthopyroxene, which may have occurred by grain boundary transport in a fluid phase during a "creep cavitation" process. The results indicate that the Anita Peridotite recrystallised and underwent rheological weakening at a constant strain rate

  20. Peridotite xenoliths from the Polynesian Austral and Samoa hotspots: Implications for the destruction of ancient 187Os and 142Nd isotopic domains and the preservation of Hadean 129Xe in the modern convecting mantle (United States)

    Jackson, M. G.; Shirey, S. B.; Hauri, E. H.; Kurz, M. D.; Rizo, H.


    The Re-Os systematics in 13 peridotite xenoliths hosted in young (<0.39 myr) rejuvenated lavas from the Samoan island of Savai'i and 8 peridotite xenoliths from 6 to 10 myr old lavas from the Austral island of Tubuai have been examined to evaluate the history of the oceanic mantle in this region. Modal mineralogy, trace element compositions and 187Os/188Os ratios suggest that these peridotites are not cognate or residual to mantle plumes but rather samples of Pacific oceanic lithosphere created at the ridge. Savai'i and Tubuai islands lie along a flow line in the Pacific plate, and provide two snapshots (separated by over 40 Ma in time) of Pacific mantle that originated in the same region of the East Pacific rise. Tubuai xenoliths exhibit 187Os/188Os from 0.1163 to 0.1304, and Savai'i (Samoa) xenoliths span a smaller range from 0.1173 to 0.1284. The 187Os/188Os ratios measured in Tubuai xenoliths are lower than (and show no overlap with) basalts from Tubuai. The 187Os/188Os of the Savai'i xenoliths overlap the isotopic compositions of lavas from the island of Savai'i, but also extend to lower 187Os/188Os than the lavas. 3He/4He measurements of a subset of the xenoliths range from 2.5 to 6.4 Ra for Tubuai and 10.8 to 12.4 Ra for Savai'i. Like abyssal peridotites and xenoliths from oceanic hotspots that sample the convecting mantle, Os isotopes from the Savai'i and Tubuai xenolith suites are relatively unradiogenic, but do not preserve a record of depleted early-formed (Hadean and Archean) mantle domains expected from earlier cycles of ridge-related depletion, continent extraction, or subcontinental lithospheric mantle erosion. The lack of preservation of early-formed, geochemically-depleted Os-isotopic and 142Nd/144Nd domains in the modern convecting mantle contrasts with the preservation of early-formed (early-Hadean) 129Xe/130Xe isotopic heterogeneities in the convecting mantle. This can be explained if the initial isotopic signatures in Re-Os and Sm-Nd systems

  1. Highly refractory peridotites in Songshugou, Qinling orogen: Insights into partial melting and melt/fluid-rock reactions in forearc mantle (United States)

    Cao, Yi; Song, Shuguang; Su, Li; Jung, Haemyeong; Niu, Yaoling


    The Songshugou ultramafic massif is located in the eastern segment of the Qinling orogenic belt, central China. It is a large spinel peridotite body dominated by coarse-grained, porphyroclastic, and fine-grained dunite with minor harzburgite, olivine clinopyroxenite, and banded/podiform chromitite. The compositions of the bulk-rock dunite and harzburgite, and the constituent olivine and spinel, together with the textures and chemical characteristics of multiphase mineral inclusions, point to the highly refractory nature of these rocks with complex histories of high-temperature boninite melt generation and boninitic melt-rock reaction, probably in a young, warm, and volatile-rich forearc lithospheric mantle setting. Additionally, a subsequent low-temperature fluid-rock reaction is also recorded by TiO2-rich spinel with Ti solubility/mobility enhanced by chloride- or fluoride-rich subduction-zone fluids as advocated by Rapp et al. (2010). The olivine clinopyroxenite, on the other hand, was likely crystallized from a residual boninitic melt that had reacted with harzburgitic residues. The ubiquitous occurrences of hydrous minerals, such as anthophyllite, tremolite, Cr-chlorite, and serpentine (stable at lower P-T crustal conditions) in the matrix, suggest that further low-temperature fluid-rock reaction (or retrograde metamorphism) has affected the original volatile-poor peridotites either in a mature and cool subduction zone, or in a continental crust during their exhumation into the Qinling collisional orogeny at early Paleozoic era, or both. The prolonged and intense ductile/brittle deformation can decrease the mineral grain size through dynamic recrystallization and fracturing, and thus aid the fluid-rock reaction or retrograde metamorphism and mineral chemical re-equilibration processes. Therefore, the Songshugou peridotites present a good example for understanding the petrogenesis and evolution of the mantle wedge, with the emphasis on the complex partial

  2. Fluid-mediated slab-mantle interaction during (U)HP and HT metamorphism of garnet peridotites and their hosting crustal rocks (Monte Duria, Central Alps, N Italy) (United States)

    Pellegrino, Luca; Malaspina, Nadia; Zanchetta, Stefano; Tumiati, Simone


    The Adula-Cima Lunga nappe represents the highest of the Lower Penninic units of the Central Alps. It consists of orthogneiss and paragneiss of pre-Mesozoic origin hosting lenses of metacarbonates, partly retrogressed eclogites and garnet/chlorite peridotites. The garnet peridotite bodies cropping out in the southern area of the nappe complex (Alpe Arami, Cima di Gagnone and Monte Duria) record the highest metamorphic conditions (P>3.0 GPa, T=800-850°C. In the study area garnet peridotite lenses are in contact with biotite-bearing migmatites or hosted in amphibole-bearing migmatites and K-feldspar gneisses. Petrographic and minerochemical data indicate a previously unknown HT stage during peridotite and eclogite exhumation. Peridotites show orthopyroxene with symplectites of tiny crystals of baddeleyte (ZrO2) and srilankite (ZrTi2O6), whereas only zircon was found in the symplectites after garnet. Bulk rock analyses of peridotites show REE content 3 to 5 times lower than PM but with a clear selective enrichment in LREE (spoon-like pattern), and with a Eu positive anomaly. The same "spoon like" pattern in the LREE field is displayed also by eclogites, shifted to values 2 to 5 times higher than PM. Due to the similarities of the REE patterns of peridotite and eclogite and the occurrence of the Eu positive anomaly in most of the peridotite analyses that points to a contamination by a "basaltic" source, we suggest that eclogites are the source of the metasomatic agent that enriched the original garnet peridotite. The Monte Duria area thus represents a natural laboratory where is possible to study "in situ" the mantle-crust interaction. Metasomatic agents and processes, relationships between UHP-UHT metamorphism and metasomatism, and mechanisms of emplacement of mantle rocks into crustal slab can be here addressed and framed in the context of the well known geological background of the Alps.

  3. Carbon isotope fractionation during diamond growth in depleted peridotite: Counterintuitive insights from modelling water-maximum CHO fluids as multi-component systems (United States)

    Stachel, T.; Chacko, T.; Luth, R. W.


    Because of the inability of depleted cratonic peridotites to effectively buffer oxygen fugacities when infiltrated by CHO or carbonatitic fluids, it has been proposed recently (Luth and Stachel, 2014) that diamond formation in peridotites typically does not occur by rock-buffered redox reactions as previously thought but by an oxygen-conserving reaction in which minor coexisting CH4 and CO2 components in a water-rich fluid react to form diamond (CO2 + CH4 = 2C + 2H2O). In such fluid-buffered systems, carbon isotope fractionation during diamond precipitation occurs in the presence of two dominant fluid carbon species. Carbon isotope modelling of diamond precipitation from mixed CH4- and CO2-bearing fluids reveals unexpected fundamental differences relative to diamond crystallization from a single carbon fluid species: (1) irrespective of which carbon fluid species (CH4 or CO2) is dominant in the initial fluid, diamond formation is invariably associated with progressive minor (water-rich fluids responsible for diamond formation. Specifically, precipitation of diamonds with δ13C values in the range -4 to -6‰ from mantle-derived fluids with an average δ13C value of -5‰ (derived from evidence not related to diamonds) requires that diamond-forming fluids were relatively reduced and had methane as the dominant carbon species (XCO2 = 0.1-0.5). Application of our model to a recently published set of in-situ carbon isotope analyses for peridotitic diamonds from Marange, Zimbabwe (Smit et al., 2016), which contain CH4 fluid inclusions, allows us to perfectly match the observed co-variations in δ13 C, δ15 N and N content and at the same time explain the previously counter-intuitive observation of progressive 13C enrichment in diamonds that appear to have grown from a fluid with methane as the dominant carbon species. Similarly, the almost complete absence in the published record of progressive 13C depletion trends within diamonds likely reflects ubiquitous

  4. Osmium isotope compositions of detrital Os-rich alloys from the Rhine River provide evidence for a global late Mesoproterozoic mantle depletion event (United States)

    Dijkstra, Arjan H.; Dale, Christopher W.; Oberthür, Thomas; Nowell, Geoffrey M.; Graham Pearson, D.


    We report osmium isotopic compositions for 297 mantle-derived detrital Ru-Os-Ir alloy grains found in gold and platinum-group mineral bearing placers of the Rhine River. These alloys were likely formed as a result of high degree melting in the convective mantle and derived from residual Paleozoic mantle peridotites in the Alps of Central Europe that were accreted as part of a collage of Gondwana-derived 'Armorican' terranes before the Variscan Orogeny. The 187Os/188Os isotope ratios of the Os-rich alloys show a wide distribution, with two modes at 0.1244 and 0.1205. These two modes correspond to rhenium depletion ages, interpreted to correspond with episodes of high-degree mantle melting, at ∼0.5 and ∼1.1 Ga. The data confirm the ability of the oceanic mantle to preserve evidence of ancient melting events. Our new data, in combination with published data on Os-rich alloys from the Urals and Tasmania and with data for abyssal peridotites, indicate a geographically widespread record of a major global Late Mesoproterozoic (1.0-1.2 Ga) high-degree melting event in Paleozoic oceanic mantle rocks. This model age peak is essentially absent from the crustal record of Central-Western Europe, but does coincide with the apparent peak in global continental crust zircon ages at this time. Thus, high-degree mantle melting peaking in the 1.0-1.2 Ga interval may have affected a large part of Earth's mantle. This interval occurred during a period of relative super-continental stability, which may have been accompanied in the oceanic realm by rapid seafloor spreading and extensive subduction, and by unusually high activity of mantle plumes forming two active mantle superswells.

  5. Fragments of deeper parts of the hanging wall mantle preserved as orogenic peridotites in the central belt of the Seve Nappe Complex, Sweden

    NARCIS (Netherlands)

    Clos, F.; Gilio, M.; van Roermund, H.L.M.


    Formation conditions of olivine microstructures are investigated in the Kittelfjäll spinel peridotite (KSP), a fragment of lithospheric mantle which occurs as an isolated body within high grade metamorphic crustal rocks of the Seve Nappe Complex (SNC), southern Västerbotten, central Sweden. The KSP

  6. The Acasta Gneisses revisited: Evidence for an early depleted mantle (United States)

    Scherer, E. E.; Sprung, P.; Bleeker, W.; Mezger, K.


    The oldest known mineral samples crystallized on the Earth are the up to 4.4 Ga zircon grains from the Jack Hills, Australia [e.g., 1,2]. Zircon, which is datable by U-Pb, contains ca. 1 wt% Hf, and has very low Lu/Hf, is well suited to recording the initial 176Hf/177Hf of its parent magma. It has therefore been widely used to track Earth’s crust-mantle differentiation over time and to estimate the relative amounts of juvenile and recycled components that contributed to Archean and Hadean crust. [e.g., 3,4,5,6]. Zircon studies may be subject to sampling bias, however: Juvenile mafic magmas are likely to stem from depleted sources, but are less likely to crystallize zircon. Processes such as host-rock metamorphism, remelting, weathering of the host rock, and sedimentary transport of grains may have further biased the zircon population. Metamict grains or those with high aspect ratios are likely to be destroyed by these processes, potentially biasing the zircon Hf record toward enriched compositions such that the degree of mantle depletion remains poorly defined before 4 Ga. In addition, incorrect age assignments to Hf analyses result in spurious initial ɛHf values. Here, we attempt to overcome these issues by investigating the bulk rock Lu-Hf and Sm-Nd systematics of some of the oldest rocks on Earth, the Acasta Gneisses (Northwest Territories, Canada). Earlier studies showed that zircon grains in these gneisses tend to come from enriched sources [e.g, 3,7,8] and are thus of little use for directly tracking the degree of mantle depletion. Furthermore, the gneisses themselves have been multiply metamorphosed and are often affected by mixing: The banded gneisses in particular comprise several magmatic precursor rocks of different age that have been repeatedly folded into each other. This promted questions of whether zircon ages should be used in the calculation of bulk rock initial epsilon Nd, and whether linear trends on Sm-Nd isochron represented meaningful

  7. Chemical and mineralogical analyses of a weathering mantle developing on peridotite of the mining area for nickel in Cerro Matoso, Colombia

    Energy Technology Data Exchange (ETDEWEB)

    Goncalves, C.M.; Fabris, J.D. [Departamento de Quimica, UFMG (Brazil)], E-mail:, E-mail:; Pacheco Serrano, W.A. [Departamento de Fisica (Colombia)], E-mail:


    Four samples were collected at several depths in a weathering mantle developing on peridotite, found in a mining area for nickel of Cerro Matoso, about 20 km southwest of Monte Libano, Department of Cordoba, Colombia. Samples represent the main stratigraphy of the mantle. Powder X-ray diffractometry, RT Moessbauer spectroscopy, saturation magnetization measurements and conventional chemical analysis on the whole and chemically treated samples, and on some of their magnetic extracts, were used to give some details of the main occurring mineralogical phases in the mantle, particular of the magnetic iron oxides.

  8. Ancient melt depletion overprinted by young carbonatitic metasomatism in the New Zealand lithospheric mantle

    DEFF Research Database (Denmark)

    Scott, James M.; Hodgkinson, A.; Palin, J.M.;


    removal of clinopyroxene. LREE-enriched, low-Ti/Eu and Al2O3 clinopyroxene and rare F-rich apatite reflect subsequent interaction between peridotite and carbonatite. The clearest metasomatic overprint occurs in the formerly depleted samples because there was little or no pre-existing clinopyroxene...

  9. Gravimetric structure for the abyssal mantle massif of Saint Peter and Saint Paul peridotite ridge, Equatorial Atlantic Ocean, and its relation to active uplift

    Directory of Open Access Journals (Sweden)



    Full Text Available This paper presents gravimetric and morphologic analyses based on the satellite-derived data set of EGM2008 and TOPEX for the area of the oceanic mantle massif of the Saint Peter and Saint Paul peridotite ridge, Equatorial Atlantic Ocean. The free-air anomaly indicates that the present plate boundary is not situated along the longitudinal graben which cuts peridotite ridge, but about 20 km to the north of it. The high Bouguer anomaly of the peridotite ridge suggests that it is constituted mainly by unserpentinised ultramafic rocks. The absence of isostatic compensation and low-degree serpentinisation of the ultramafic rocks indicate that the peridotite ridge is sustained mainly by active tectonic uplift. The unparallel relation between the transform fault and the relative plate motion generates near north-south compression and the consequent tectonic uplift. In this sense, the peridotite massif is a pressure ridge due to the strike-slip displacement of the Saint Paul Transform Fault.

  10. Composition of garnet and clinopyroxene in peridotite xenoliths from the Grib kimberlite pipe, Arkhangelsk diamond province, Russia: Evidence for mantle metasomatism associated with kimberlite melts (United States)

    Kargin, A. V.; Sazonova, L. V.; Nosova, A. A.; Tretyachenko, V. V.


    Here we present major and trace element data for garnet and clinopyroxene from mantle-derived peridotite xenoliths of the Grib kimberlite, the Arkhangelsk diamond province, Russia, and provide new insights into the metasomatic processes that occur within the subcontinental lithospheric mantle (SCLM) during the kimberlite generation and ascent. The mantle xenoliths examined in this study are both coarse and sheared garnet peridotites and consist of olivine, orthopyroxene, clinopyroxene, garnet with minor ilmenite, magnetite, and Cr-spinel. Based on garnet and clinopyroxene composition, two groups of peridotite are recognized. One group contains high-Ti, light rare earth elements (LREE) enriched garnets and low-Mg# clinopyroxenes with low (La/Sm)n (C1 chondrite-normalized) values. This mineral assemblage was in equilibrium with a high-temperature carbonate-silicate metasomatic agent, presumably, a protokimberlite melt. Pressure-temperature (P-T) estimates (T = 1220 °C and P = 70 kbar) suggest that this metasomatic event occurred at the base of the SCLM. Another group contains low-Ti garnet with normal to sinusoidal rare earth elements (REE) distribution patterns and high-Mg# clinopyroxenes with wide range of (La/Sm)n values. The geochemical equilibrium between garnet and clinopyroxene coupled with their REE composition indicates that peridotite mantle experienced metasomatic transformation by injection of a low-Ti (after crystallizations of the ilmenite megacrysts) kimberlite melt that subsequently percolated through a refractory mantle column. Peridotites of this group show a wide range of P-T estimates (T = 730-1070 °C and P = 22-44 kbar). It is suggested that evolution of a kimberlite magma from REE-enriched carbonate-bearing to carbonate-rich ultramafic silicate compositions with lower REE occurs during the ascent and interaction with a surrounding lithospheric mantle, and this process leads to metasomatic modification of the SCLM with formation of both high

  11. The Anita Peridotite, New Zealand

    DEFF Research Database (Denmark)

    Czertowicz, Tom; Scott, James; Waight, Tod Earle;


    –93, spinel Cr# of 70, orthopyroxene with low Al2O3, and extremely depleted whole-rock geochemical characteristics indicate that the peridotite body experienced >30% melt extraction, probably within the spinel facies. Mineral compositions show some similarity to those of cratonic peridotitic mantle. Rare Cr....../86Sr (0.705–0.706), eNd ( +6.3 to +11.1), 208Pb/204Pb (37.8–38.9) and eHf ( +5.6 to 36.9) indicate that the metasomatic agent, which caused crystallization of clinopyroxene and plagioclase, had an isotopic composition similar to ocean island basalt. On the basis of isotopic data and mineral chemistry...

  12. Genesis of Ultra-High Pressure Garnet Pyroxenite in Orogenic Peridotites and its bearing on the Isotopic Chemical Heterogeneity in the Mantle Source of Oceanic Basalts (United States)

    Varas Reus, María Isabel; Garrido, Carlos J.; Marchesi, Claudio; Bosch, Delphine; Hidas, Károly


    The genesis of ultra-high pressure (UHP) garnet pyroxenites in orogenic peridotite massifs and its implications on the formation of chemical heterogeneities in the mantle and on basalt petrogenesis are still not fully understood. Some UHP (diamond-bearing) garnet pyroxenites have isotopic, and major and trace element compositions similar to the recycled oceanic crustal component observed in oceanic basalts [1-6]. These pyroxenites hence provide an exceptional opportunity to investigate in situ the nature and scale of the Earth's mantle chemical heterogeneities. Here, we present an integrated geochemical study of UHP garnet pyroxenites from the Ronda (Betic Belt, S. Spain) and Beni Bousera (Rif Belt, N. Morocco) peridotite massifs. This investigation encompasses, in the same sample, bulk rock major and trace elements, as well as Sr-Nd-Pb-Hf isotopic analyses. According to their Al2O3 content, we classify UHP garnet pyroxenites into three groups that have distinct trace elements and Sr-Nd-Pb-Hf isotopic signatures. Group A pyroxenites (Al2O3: 15 - 17.5 wt. %) are characterized by low initial 87Sr/86Sr, relatively high 143Nd/144Nd, 206Pb/204Pb and 176Hf/177Hf ratios, and highly variable 207Pb/204Pb and 208Pb/204Pb ratios. Group B pyroxenites (Al2O3 intimately mixed with peridotites by mantle convection. The present study reveals, however, that besides this exotic component of ancient recycled oceanic crust, the genesis of these pyroxenites requires a previously unnoticed component of recycled lower continental crust akin to the lower crustal section of the lithosphere where these UHP garnet pyroxenites now reside in. The results of this study provide a new recipe for the marble cake hypothesis for the genesis of UHP garnet pyroxenites in orogenic peridotites. Furthermore, it establishes a connection between the genesis of UHP pyroxenites, the composition of the continental crust and the generation of Earth's mantle heterogeneities. References: [1] Pearson, D. G

  13. Mantle metasomatism did not modify the initial H2O content in peridotite xenoliths from the Tianchang basalts of eastern China (United States)

    Hao, Yan-Tao; Xia, Qun-Ke; Tian, Zhen-Zhen; Liu, Jia


    Metasomatism induced by melts/fluids is ubiquitous in the lithospheric mantle and can potentially modify the initial water content of the mantle. However, the preservation of correlations between H2O content and partial melting indices (e.g., Yb content in clinopyroxene, Cr / (Cr + Al) in spinel) and the lack of correlations between H2O content and metasomatic indices (e.g., La/Yb in clinopyroxene) in peridotite xenoliths from several localities suggest that variations in the initial H2O content were controlled by partial melting processes rather than by subsequent metasomatic event(s) (Hao et al., 2014; Denis et al., 2015). However, the applied partition coefficients of H2O between peridotite and melt (Dperidotite/melt = 0.1-0.3) in the partial melting models cast doubts on the reasonability of such explanations. Whether metasomatism always modifies the initial H2O content of the lithospheric mantle remained a topic of debate. In this paper, we measure major and trace element concentrations and H2O contents of minerals in the peridotite xenoliths hosted by the Tianchang Cenozoic basalts in eastern China by using electron microprobe, laser-ablation ICP-MS and Fourier transform infrared spectroscopy, respectively. The H2O contents (weight in ppm) of clinopyroxene, orthopyroxene and olivine are 70-280 ppm, 35-140 ppm and below detection limit (partitioning between clinopyroxene and orthopyroxene. The calculated whole-rock H2O contents range from 14 to 93 ppm (average 52 ± 25 ppm) assuming 0.1 for the H2O partition coefficient between olivine and clinopyroxene. Although no hydrous minerals are found, the enrichment in light rare earth elements and large ion lithophile elements of clinopyroxene indicates cryptic mantle metasomatism. However, variations between the H2O contents of the whole rocks and the metasomatic index (La/Yb ratio in clinopyroxene) are not correlated, suggesting that mantle metasomatism did not modify the initial H2O contents after the melting

  14. Mantle Composition and Temperature of Western North America Revealed from Direct P and S Wave Velocities of KLB-1 Peridotite to the Condition of Transition Zone (United States)

    Wang, X.; Chen, T.; Qi, X.; Zou, Y.; Liebermann, R. C.; Li, B.


    Comparing the elasticity of candidate compositional models with seismic profiles (e.g., PREM and AK135) is one of the most important geophysical approaches to constrain the mineralogical composition of the mantle. However in such averaging schemes (e.g., Voigt-Reuss-Hill), it is difficult to take into account all of the mineralogical and chemical complexities; we therefore undertook elasticity study of a natural mantle rock sample at high pressures and temperatures. In this study, a series of polycrystalline aggregates of peridotite KLB-1 (from Kilbourne Hole, New Mexico) were hot-pressed at pressures of 3-15 GPa and temperatures of 1200-1400°C. Scanning electron microscopy (SEM), Electron Microprobe Analysis (EPMA) and X-ray diffraction (XRD) were used to characterize the texture, grain size, and composition of these well-sintered specimens. For the first time in history, the P and S wave velocities of a pyrolitic multiphase aggregate were directly measured at mantle transition zone pressures and temperatures using ultrasonic interferometry. Based on the phase fractions from EPMA and the P and S wave velocities from in situ measurement at high pressure and high temperature, the velocities of the KLB-1 peridotite along 1200-1400 oC adiabatic mantle geotherms were obtained and compare well with the seismic models of western North America, the region where these peridotite KLB-1 samples were collected. The comparison with regional seismic models of western North America (e.g., GCA and TNA/TNA2) as well as global seismic models (PREM and AK135) place unprecedented constraints on the composition, temperature and density profiles for the upper mantle in this region, which can help us understand the nature of thermal and tectonic processes of the Rio Grande Rift.

  15. Aqueous fluids and sedimentary melts as agents for mantle wedge metasomatism, as inferred from peridotite xenoliths at Pinatubo and Iraya volcanoes, Luzon arc, Philippines (United States)

    Yoshikawa, Masako; Tamura, Akihiro; Arai, Shoji; Kawamoto, Tatsuhiko; Payot, Betchaida D.; Rivera, Danikko John; Bariso, Ericson B.; Mirabueno, Ma. Hannah T.; Okuno, Mitsuru; Kobayashi, Tetsuo


    Mantle xenoliths entrained in subduction-zone magmas often record metasomatic signature of the mantle wedge. Such xenoliths occur in magmas from Iraya and Pinatubo volcanoes, located at the volcanic front of the Luzon arc in the Philippines. In this study, we present the major element compositions of the main minerals, trace element abundances in pyroxenes and amphiboles, and Nd-Sr isotopic compositions of amphiboles in the peridotite xenoliths from Pinatubo volcano. The data indicate enrichment in fluid-mobile elements, such as Rb, Ba, U, Pb, and Sr, and Nd-Sr isotopic ratios relative to those of mantle. The results are considered in terms of mixing of asthenospheric mantle and subducting oceanic crustal components. The enrichments observed in the Pinatubo mantle xenoliths are much less pronounced than those reported for the Iraya mantle xenoliths. This disparity suggests differences in the metasomatic agents contributing to the two suites; i.e., aqueous fluids infiltrated the mantle wedge beneath the Pinatubo volcano, whereas aqueous fluids and sediment-derived melts infiltrated the mantle wedge beneath the Iraya volcano.

  16. Amphibole—Bearing Peridotite Xenoliths from Nushan,Anhui Province:Evidence for Melt Percolation Process in the Upper mantle and Lithospheric uplift

    Institute of Scientific and Technical Information of China (English)

    徐义刚; J.C.MERCIER


    The spinel peridotite xenoliths of Group I in Quaternary basanites from Nushan,Anhui province,can be classified as two suites:a hydrous suite characterized by the ubiquitous occurrence of (Ti-) pargasite and an anhydrous suite.The nineral chemistry reveals that the anhydrous suite and one associated phlogopite-bearing lherzolite are equilibrated under temperature conditions of 1000-1100℃,whereas amplhibole-bearing peridotites display distinct disequilibrum features,indicating partial reequilibration from 1050 to 850℃ and locally down to 750℃. The amplhbole-bearing peridotites were probably the uppermost part of the high temperature anhydrous suite which was modally modifed by fractionating H2O-rich metasomatic agent during regional upwelling.This relatively recent lithospheric uplift event followed an older uplift event recognized from pyroxene unmixing of domains in local equilibrium,as well as the dominant deformation texture in the anhydrous suite.The first thermal disturbance can be linked with the regional extension and widespread basaltic volcanism in Jiangsu-Anhui provinces since Early Tertiary and the formation of the nearby Subei(North Jiangsu) fault-depression basin during the Eocene,while the second event in association with the formation of amphiboles probably indicates the continuation but diminution of upwared mantle flux since Neogene in response to the change in tectonic regime for eastern china.

  17. Metasomatically altered peridotite xenoliths from the Hessian Depression (Northwest Germany)

    Energy Technology Data Exchange (ETDEWEB)

    Hartmann, G.; Wedepohl, K.H. (Univ. of Goettingen (Germany, F.R.))


    A set of 36 spinel lherzolite and spinel harzburgite xenoliths from 7 occurrences of Tertiary alkali olivine basalt and olivine nephelinite have been investigated for modal composition, major elements, and 39 minor elements. The periodotites were depleted during former partial melting events. The proportion of clinopyroxene, elements indicating depletion, and compatible trace elements are on average close to the means of the worldwide subcontinental lithosphere. The metasomatic alteration of the sampled mantle varies in a complicated three-dimensional pattern without vertical layering. Mobilization of elements in mantle fluids and accumulation in certain peridotites is a complex open-system process. The exceptional behavior of high field-strength elements might be controlled by the formation of Ti minerals. The metasomatic transport can be explained by extraction of the incompatible elements either from mantle minerals or from subducted crustal sources into water-rich fluids. Chlorine in phlogopite has equilibrated with fluids containing chloride concentrations close to seawater. Values of {delta}{sup 18}O in bulk peridotites and in phlogopite exceeding typical mantle compositions probably reflect a crustal contribution to the metasomatic fluids. Nd and Sr isotopic ratios of both peridotites and alkali basalts received a crustal signature from an environment with time-integrated LREE accumulation and Rb depletion.

  18. In situ Re-Os isotope ages of sulfides in Hannuoba peridotitic xenoliths: Significance for the frequently-occurring mantle events beneath the North China Block

    Institute of Scientific and Technical Information of China (English)


    In situ Re-Os isotopes of sulfides in peridotitic xenoliths from Cenozoic Hannuoba basalts were analyzed by LAM-MC-ICPMS. The suifides developed two types of occurrences including silicate-enclosed and interstitial. In the enclosed sulfides, 187Os/188Os vary from 0.1124 to 0.1362 and 187Re/188Os from 0.0026 to 1.8027. In the interstitial ones, 187Os/188Os have a range from 0.1174 to 0.1354 and 187Re/188Os from 0.0365 to 1.4469. The oldest age, calculated by TRD for the enclosed sulfides, is 2.1 Ga. An isochron age of 2.3±1.2 Ga is obtained by five grains of enclosed sulfides and primitive mantle. The sulfides used have lower Re-Os isotopic ratio than primitive mantle. Meanwhile, an isochron age of 645±225 Ma is given by all interstitial sulfides and the enclosed sulfides with higher Re-Os isotopic ratio due to Re addition after mantle formation. In addition, the model age of 1.3 Ga recorded by one interstitial sulfide, having similar TDM and TRD, should be meaningful to deep thermal event. The coexistence of different ages, revealed by in situ Re-Os isotope, indicates frequently-occurring mantle events beneath Hannuoba area.

  19. Destruction of the peridotite-eclogite liquidus barrier during mantle magma differentiation (experiment at 7.0 GPa). (United States)

    Butvina, Dr


    system where a total olivine vanishing takes place. Thus, the performed experimental investigations of the model system forsterite-diospside-jadeite at pressure 7 GPa testify to the fact that forsterite (olivine) is a stable phase in the boundary system forsterite-diopside (olivine-clinopyroxene). While introducing rather low contents of jadeite component into the composition of this system the reaction of jadeite component with forsterite takes place in the melt. As a result, garnet appears as liquidus phase. With the increase of the jadeite component concentration in the system the field of liquidus garnet expands, but a physico-chemical control of crystallization differentiation of the remnant melts transforms from the monovariant cotectics Fo + DiSS + L through the invariant peritectic point Fo + DiSS + Grt + L to the monovariant cotectics Grt + Cpx + L, which is responsible for crystallization of bimineral garnet-omphazite eclogite parageneses. The obtained experimental results testify unambiguously to the fact that in the system Fo-Di-Jd a physico-chemical mechanism of overcoming liquidus peridotite-eclogite barrier at mantle magma differentiation is realized. Thus, a gradual transition from olivine-bearing assemblages to those close by their characteristics to bimineral eclogites is provided.

  20. Miocene-Pliocene mantle depletion event in the northern Fossa Magna, western NE Japan (United States)

    Okamura, Satoshi; Inaba, Mitsuru; Adachi, Yoshiko; Shinjo, Ryuichi


    New isotopic and trace element data presented here imply a temporal change in magma sources and thermal conditions beneath the northern Fossa Magna of NE Japan from the Miocene to the Pliocene. Less radiogenic 176Hf/177Hf and 143Nd/144Nd, high Zr/Hf, and little or no Hf anomaly characterize the Early Miocene volcanism in the northern Fossa Magna region. The mantle wedge consisted of chemically heterogeneous mantle source. Based on out isotope proxies, we propose that during the onset of subduction, influx of hot asthenospheric mantle provided sufficient heat to partially melt newly subducting sediment. Geochemical modeling demonstrates that slab-derived melt mixed with mantle wedge produces the observed isotopic and trace elemental characteristics. In the Middle Miocene, the injection of hot and depleted asthenospheric material replaced the mantle beneath the northern Fossa Magna region of NE Japan. This caused the isotopic signature of the rocks to change from enriched to depleted. Then, the mantle wedge was gradually cooled during the Middle Miocene to the Pliocene with back-arc opening ending in the Late Miocene. Slab surface temperatures were still high enough for sediments to melt but not too high (<∼780 °C) to lose zircon as a residual phase. The Late Miocene and Pliocene volcanism at the post stage of the back-arc opening is best explained by a partial melting of subducted metasediment saturated with trace quantities of zircon and rutile.

  1. The Texture,Mineral Assemblages and Origin of Metamorphic Peridotite from Kuda Ophiolite,Xinjiang

    Institute of Scientific and Technical Information of China (English)

    沈步明; 周云生; 等


    There are two grades of network-type texture in metamorphic peridotite from the Kuda ophiolite,Xinjiang,China,The first graduation consists of coarse-grained olivine with plastic deformation fabrics,orthopyroxene and minor clinopyroxene and Cr-spinel.The second graduation consists of small minerals filling among the mineral grains of the first graduation.Four kinds of assemblages containing tremolite have been discovered in the second graduation,which were formed during mantle partial melting and mantle metasomatism of a highly depleted peridotite.

  2. Petrology of periodotites as a tool of insight into mantle processes: a review. Jobu mantle kanrangan no gansekigaku (ikani mantle process wo yomitoruka)

    Energy Technology Data Exchange (ETDEWEB)

    Arai, S. (Kanazawa University, Ishikawa (Japan). Faculty of Science)


    Petrological studies of the upper mantle materials are reviewed mainly in terms of recent topics. Along with on increase in depletion degree (degree of slipped-out magma) of resistate which remains after magma slipped out from the upper mantle materials, a series from lherzolite to Harzburgites is obtained and as an example, the Horoman peridotite complex which intrudes into the Hidaka metamorphic belt (Hokkaido) is shown. In this connection, a theory is introduced where the bedded structure of this peridotite complex was formed by a series of processes accompanied by the cracks generated in the mantle peridotite. Then, the following topics are explained: strongly incompatible trace elements can be enriched in resistate with high depletion degree (REE paradox); and the recent research trend on issues which relates to the interaction between the magma and mantle, such as the formation of dunites, the origin of picrite basalt, and the origin of chromitite are described. 40 refs., 13 figs.

  3. Depletion of Vandium in Planetary Mantles: Controlled by Metal, Oxide, or Silicate? (United States)

    Righter, Kevin


    Vanadium concentrations in planetary mantles can provide information about the conditions during early accretion and differentiation. Because V is a slightly siderophile element, it is usually assumed that any depletion would be due to core formation and metal-silicate equilibrium. However, V is typically more compatible in phases such as spinel, magnesiowuestite and garnet. Fractionation of all of these phases would cause depletions more marked than those from metal. In this paper consideration of depletions due to metal, oxide and silicate are critically evaluated.

  4. Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway. (United States)

    Spengler, Dirk; van Roermund, Herman L M; Drury, Martyn R; Ottolini, Luisa; Mason, Paul R D; Davies, Gareth R


    The buoyancy and strength of sub-continental lithospheric mantle is thought to protect the oldest continental crust (cratons) from destruction by plate tectonic processes. The exact origin of the lithosphere below cratons is controversial, but seems clearly to be a residue remaining after the extraction of large amounts of melt. Models to explain highly melt-depleted but garnet-bearing rock compositions require multi-stage processes with garnet and clinopyroxene possibly of secondary origin. Here we report on orogenic peridotites (fragments of cratonic mantle incorporated into the crust during continent-continent plate collision) from Otrøy, western Norway. We show that the peridotites underwent extensive melting during upwelling from depths of 350 kilometres or more, forming a garnet-bearing cratonic root in a single melting event. These peridotites appear to be the residue after Archaean aluminium depleted komatiite magmatism.

  5. Paradoxical co-existing base metal sulphides in the mantle: The multi-event record preserved in Loch Roag peridotite xenoliths, North Atlantic Craton (United States)

    Hughes, Hannah S. R.; McDonald, Iain; Loocke, Matthew; Butler, Ian B.; Upton, Brian G. J.; Faithfull, John W.


    The role of the subcontinental lithospheric mantle as a source of precious metals for mafic magmas is contentious and, given the chalcophile (and siderophile) character of metals such as the platinum-group elements (PGE), Se, Te, Re, Cu and Au, the mobility of these metals is intimately linked with that of sulphur. Hence the nature of the host phase(s), and their age and stability in the subcontinental lithospheric mantle may be of critical importance. We investigate the sulphide mineralogy and sulphide in situ trace element compositions in base metal sulphides (BMS) in a suite of spinel lherzolite mantle xenoliths from northwest Scotland (Loch Roag, Isle of Lewis). This area is situated on the margin of the North Atlantic Craton which has been overprinted by a Palaeoproterozoic orogenic belt, and occurs in a region which has undergone magmatic events from the Palaeoproterozoic to the Eocene. We identify two populations of co-existing BMS within a single spinel lherzolite xenolith (LR80) and which can also be recognised in the peridotite xenolith suite as a whole. Both populations consist of a mixture of Fe-Ni-Cu sulphide minerals, and we distinguished between these according to BMS texture, petrographic setting (i.e., location within the xenolith in terms of 'interstitial' or within feldspar-spinel symplectites, as demonstrated by X-ray Computed Microtomography) and in situ trace element composition. Group A BMS are coarse, metasomatic, have low concentrations of total PGE (relationships of the symplectites, it is possible to infer the relative ages of each metasomatic BMS population. We tally these with major tectono-magmatic events for the North Atlantic region by making comparisons to carbonatite events recorded in crustal and mantle rocks, and we suggest that the Pt-enrichment was associated with a pre-Carboniferous carbonatite episode. This method of mantle xenolith base metal sulphide documentation may ultimately permit the temporal and spatial mapping of

  6. Geochemistry and tectonic significance of the Gongzhu peridotites in the northern branch of the western Yarlung Zangbo ophiolitic belt, western Tibet (United States)

    Lian, Dongyang; Yang, Jingsui; Liu, Fei; Wu, Weiwei; Zhang, Li; Zhao, Hui; Huang, Jian


    The Gongzhu ophiolite is situated in the northern branch of the western Yarlung Zangbo ophiolitic belt. This massif consists of a strongly dismembered ophiolitic sequence dominated by mantle peridotites. The peridotites comprise lherzolite with low- to moderately-depleted mineral and bulk rock compositions. The degree of partial melting deduced from Cr# values of the Gongzhu peridotites varies between 7% and 10%. The mineral and whole rock compositions of the Gongzhu peridotites are comparable to those of abyssal peridotites. The chondrite normalized REE compositions of the peridotites typically display U-shaped or spoon-shaped patterns, and primitive mantle-normalized PGEs patterns show Ir depletion relative to Os and Ru, and Pt enrichment relative to Rh and Pd. On the basis of the petrological, mineralogical and geochemical data, we concluded that the Gongzhu peridotites either formed in the back-arc setting of an intra-oceanic subduction system or the Gongzhu and Dajiweng peridotites both formed in the in the same incipient forearc/proto-forearc environment of an intra-oceanic subduction zone.

  7. Seawater-derived rare earth element addition to abyssal peridotites during serpentinization (United States)

    Frisby, Carl; Bizimis, Michael; Mallick, Soumen


    Serpentinized abyssal peridotites are evidence for active communication between the Earth's hydrosphere and the upper mantle, where exchange and retention of both major and trace elements occur. Bulk rock Nd isotopes in serpentinized abyssal peridotites imply interaction of seawater with the peridotite. In contrast, the Nd isotopes of clinopyroxenes from serpentinized abyssal peridotites retain their primary magmatic signature. It is currently unclear if, how and where seawater-derived Nd and other REE are being added or exchanged with the mantle peridotite minerals during serpentinization. To remedy this knowledge gap, we present in situ trace and major element concentrations, bulk rock and sequential leaching experiment trace element concentrations as well as Nd, Sr isotope data on refertilized and depleted serpentinized abyssal peridotites from the Southwest Indian Ridge. The secondary serpentine matrix and magnetite veins in these peridotites have elevated LREE concentrations, with variable negative Ce anomalies and large Rb, Sr, Pb and U enrichments that resemble seawater trace element patterns. The LREE concentrations in the serpentine phase are higher than those expected for the primary mantle mineralogy (olivine, orthopyroxene) based on data from relic clinopyroxenes and equilibrium partition coefficients. These data are consistent with seawater-derived REE addition to the peridotite during serpentinization. The bulk rocks have more radiogenic Sr and more unradiogenic Nd isotopes than their clinopyroxene (up to 8 εNd units lower than clinopyroxene). Sequential leaching experiments designed to mobilize secondary carbonates and Fe-oxides show even more unradiogenic Nd isotope ratios in the leachates than the bulk rock and clinopyroxene, approaching seawater compositions (up to 15 εNd units lower than clinopyroxene). Mass balance calculations using trace elements or Nd isotopes suggest that up to 30% of the bulk peridotite Nd budget is of seawater origin and

  8. Si- and alkali-rich melt inclusions in minerals of mantle peridotites from eastern China: Implication for lithospheric evolution

    Institute of Scientific and Technical Information of China (English)

    FAN; Qicheng; SUI; Jianli; XU; Ping; LI; Ni; SUN; Qian; WANG; Tuanhua


    Minerals of spinel- and garnet-facies mantle xenoliths entrained in Cenozoic basalts from eastern China (North China, Northeastern China and Southeastern China coastal area) contains lots of melt inclusions. Studies on these melt inclusions show that the glass inclusions are rich in SiO2 (60%―68%) and alkalis (K2O+Na2O=5%―11%, especially for K2O) as well as volatiles such as H2O and CO2 (2%―7%), which belong to dacites and andesites of the high-K calcic alkali series rocks with few shoshonites. High Al and Ca diopside in melt inclusion is the product of melt crystallization at high temperature and pressure, rather than the product of devitrification. Results show that these K-rich (in general K2O>3%) intermediate-acidic silicate melt inclusions have characteristics of continent without a genetical link to host basalts and their phenocrystic minerals. Thus, these trapped melt inclusions represent melts of Mesozoic lithospheric mantle-crust interaction and imply that the continental lithospheric mantle beneath eastern China had undergone fragmentation and recreation processes during the Mesozoic and Cenozoic periods. This result undoubtly provides important implication for the evolution of sub-continental lithosphere beneath eastern China. We propose that these Si- and alkalis-rich melts should be responsible for the mantle chemical heterogeneity underneath eastern China.

  9. Serpentinization Changes Nd, but not Hf Isotopes of Abyssal Peridotites (United States)

    Bizimis, M.; Frisby, C. P.; Mallick, S.


    Serpentinization of the oceanic lithosphere is a known sink for fluid mobile elements (B, Cl, Li, Sr, etc.), while high field strength elements (HFSE: e.g., Hf, Zr, Ti, Nb) are thought to be unaffected by it. In contrast, the fate of REE during serpentinization is equivocal. Correlations between REE and HFSE concentrations in abyssal peridotites suggest control by magmatic processes (Niu, 2004, J. Pet), while some LREE enrichments in serpentinized peridotites compared to their clinopyroxene (cpx) and Nd, Sr isotope data (Delacour et al., 2008, Chem. Geol.) imply seawater-derived REE addition to the mantle protolith (Paulick et al., 2006, Chem. Geol). To further constrain peridotite-seawater interaction during serpentinization we compare bulk rock and cpx Hf and Nd isotope data in partially (up to ~70%) serpentinized abyssal peridotites (9-16°E South West Indian Ridge). We also present a new method that improves yields in Hf, Nd and Pb separations from depleted (90% of Hf, Zr, Ti are retained in the residue. LA-ICPMS data shows that serpentine after olivine typically has higher LREE/HREE ratios than cpx, pronounced negative Ce anomalies, high U, Sr concentrations and low HFSE, unlike the coexisting cpx. These data are consistent with some seawater-derived LREE addition to peridotite during serpentinization, localized in the serpentine and other secondary phases, while cpx retains the magmatic value. This process will lower the Sm/Nd relative to Lu/Hf ratio in the peridotite and can lead to decoupled radiogenic Hf and unradiogenic Nd isotopes upon recycling and aging. Our data further testifies to the fidelity of Hf isotopes in tracing mantle processes, even in serpentinized rocks.

  10. Cyclin D1 depletion induces DNA damage in mantle cell lymphoma lines. (United States)

    Mohanty, Suchismita; Mohanty, Atish; Sandoval, Natalie; Tran, Thai; Bedell, Victoria; Wu, Jun; Scuto, Anna; Murata-Collins, Joyce; Weisenburger, Dennis D; Ngo, Vu N


    Elevated cyclin D1 (CCND1) expression levels in mantle cell lymphoma (MCL) are associated with aggressive clinical manifestations related to chemoresistance, but little is known about how this important proto-oncogene contributes to the resistance of MCL. Here, we showed that RNA interference-mediated depletion of CCND1 increased caspase-3 activities and induced apoptosis in the human MCL lines UPN-1 and JEKO-1. In vitro and xenotransplant studies revealed that the toxic effect of CCND1 depletion in MCL cells was likely due to increase in histone H2AX phosphorylation, a DNA damage marker. DNA fiber analysis suggested deregulated replication initiation after CCND1 depletion as a potential cause of DNA damage. Finally, in contrast to depletion or inhibition of cyclin-dependent kinase 4, CCND1 depletion increased chemosensitivity of MCL cells to replication inhibitors hydroxyurea and cytarabine. Our findings have an important implication for CCND1 as a potential therapeutic target in MCL patients who are refractory to standard chemotherapy.

  11. Isotopic contrasts within the Internal Liguride ophiolite (N. Italy): the lack of a genetic mantle-crust link (United States)

    Rampone, Elisabetta; Hofmann, Albrecht W.; Raczek, Ingrid


    It is widely accepted that oceanic lithosphere is generated by partial melting of fertile mantle peridotites producing basaltic melts and residual peridotites. This theory implies a cogenetic relationship between peridotites and associated crustal rocks, but the actual existence of such a genetic link has been tested in very few places. In this paper, we use Sr and Nd isotopes to test the relationship between mafic and ultramafic mantle rocks from a MORB-type ophiolite in the Internal Liguride Units of the Northern Apennines. This ophiolite is a remnant of the oceanic lithosphere of the Jurassic Ligurian Tethys, and consists of depleted mantle peridotites intruded by a gabbroic complex and covered by pillow lavas and ophiolitic breccias. Whole rocks and mineral separates from the gabbroic rocks yield a Sm-Nd isochron with an age of 164±14 Ma. The whole rock data for pillow lavas are also consistent with this isochron, yielding an initial value of ɛ Nd (164)=8.6±0.3 (1σ). The mantle peridotites, by contrast, have ɛ Nd(164) values ranging from 11.9 to 14.8, indicating an extreme depletion unlike that seen in modern oceanic mafic and ultramafic rocks. These results demonstrate that some ophiolites consist of mantle and crustal sections that are not genetically linked by a simple melt-residue relationship, and consequently do not represent mature oceanic lithosphere. Similar evidence has previously been available only from the Xigaze and Trinity ophiolites. The Internal Liguride peridotites yield Sm-Nd model ages of about 270 Ma, assuming a normally depleted mantle source with ɛ Nd = 9, indicating a Permian time of `extra' depletion. The ophiolite thus consists of mantle peridotites which were depleted during Permian time, and were later intruded and covered by MORB-type magmas during Jurassic time. On a regional scale, this interpretation is consistent with widespread evidence that extensional processes leading to asthenospheric upwelling and magma production

  12. Pyroxenes as tracers of mantle water variations (United States)

    Warren, Jessica M.; Hauri, Erik H.


    The concentration and distribution of volatiles in the Earth's mantle influence properties such as melting temperature, conductivity, and viscosity. To constrain upper mantle water content, concentrations of H2O, P, and F were measured in olivine, orthopyroxene, and clinopyroxene in mantle peridotites by secondary ion mass spectrometry. Analyzed peridotites are xenoliths (Pali Aike, Spitsbergen, Samoa), orogenic peridotites (Josephine Peridotite), and abyssal peridotites (Gakkel Ridge, Southwest Indian Ridge, Tonga Trench). The comparison of fresh and altered peridotites demonstrates that low to moderate levels of alteration do not affect H2O concentrations, in agreement with mineral diffusion data. Olivines have diffusively lost water during emplacement, as demonstrated by disequilibrium between olivine and coexisting pyroxenes. In contrast, clinopyroxene and orthopyroxene preserve their high-temperature water contents, and their partitioning agrees with published experiments and other xenoliths. Hence, olivine water concentrations can be determined from pyroxene concentrations using mineral-mineral partition coefficients. Clinopyroxenes have 60-670 ppm H2O, while orthopyroxenes have 10-300 ppm, which gives calculated olivine concentrations of 8-34 ppm. The highest olivine water concentration translates to an effective viscosity of 6 × 1019 Pa s at 1250°C and ~15 km depth, compared to a dry effective viscosity of 2.5 × 1021 Pa s. Bulk rock water concentrations, calculated using mineral modes, are 20-220 ppm and correlate with peridotite indices of melt depletion. However, trace element melt modeling indicates that peridotites have too much water relative to their rare earth element concentrations, which may be explained by late-stage melt addition, during which only hydrogen diffuses fast enough for reequilibration.

  13. Melt extraction and mantle source at a Southwest Indian Ridge Dragon Bone amagmatic segment on the Marion Rise (United States)

    Gao, Changgui; Dick, Henry J. B.; Liu, Yang; Zhou, Huaiyang


    This paper works on the trace and major element compositions of spatially associated basalts and peridotites from the Dragon Bone amagmatic ridge segment at the eastern flank of the Marion Platform on the ultraslow spreading Southwest Indian Ridge. The rare earth element compositions of basalts do not match the pre-alteration Dragon Bone peridotite compositions, but can be modeled by about 5 to 10% non-modal batch equilibrium melting from a DMM source. The Dragon Bone peridotites are clinopyroxene-poor harzburgite with average spinel Cr# ~ 27.7. The spinel Cr# indicates a moderate degree of melting. However, CaO and Al2O3 of the peridotites are lower than other abyssal peridotites at the same Mg# and extent of melting. This requires a pyroxene-poor initial mantle source composition compared to either hypothetical primitive upper mantle or depleted MORB mantle sources. We suggest a hydrous melting of the initial Dragon Bone mantle source, as wet melting depletes pyroxene faster than dry. According to the rare earth element patterns, the Dragon Bone peridotites are divided into two groups. Heavy REE in Group 1 are extremely fractionated from middle REE, which can be modeled by ~ 7% fractional melting in the garnet stability field and another ~ 12.5 to 13.5% in the spinel stability field from depleted and primitive upper mantle sources, respectively. Heavy REE in Group 2 are slightly fractionated from middle REE, which can be modeled by ~ 15 to 20% fractional melting in the spinel stability field from a depleted mantle source. Both groups show similar melting degree to other abyssal peridotites. If all the melt extraction occurred at the middle oceanic ridge where the peridotites were dredged, a normal ~ 6 km thick oceanic crust is expected at the Dragon Bone segment. However, the Dragon Bone peridotites are exposed in an amagmatic ridge segment where only scattered pillow basalts lie on a partially serpentinized mantle pavement. Thus their depletion requires an

  14. Tracing Proterozoic arc mantle Hf isotope depletion of southern Fennoscandia through coupled zircon U-Pb and Lu-Hf isotopes (United States)

    Petersson, Andreas; Bjärnborg, Karolina; Scherstén, Anders; Gerdes, Axel; Næraa, Tomas


    Constraints on the composition of the depleted mantle Sm-Nd and Lu-Hf crust formation ages have a long history of scientific debate. When calculating mantle extraction ages, and constructing crustal growth models, a linear evolution of incompatible trace elements in a depleted mantle since > 4 Ga is routinely used. Mantle depletion however varies regionally and over time and subduction of sediments and oceanic crust renders a mantle-wedge variously enriched relative to a modelled depleted mantle. Here we show that primitive mantle-derived subduction related gabbroic intrusions from southern Fennoscandia have Hf isotope compositions that are enriched relative to a MORB-like linear depleted mantle evolution curve. Extrapolation of primitive Paleoproterozoic gabbro suites enables the construction of a regional mantle evolution curve, providing improved constraints on model ages, crustal residence times and the fraction of juvenile versus reworked continental crust. Convergent margins are assumed to be one of the main sites of continental crust growth, and using an overly depleted mantle source yield model ages that are too old, and hence cumulative crustal growth models show too much crust generation early in the Earth's history. The approach of using the Hf isotope composition of zircon from primitive subduction related gabbroic intrusions as a proxy for mantle Hf isotope composition, piloted in this study, can be applied to other convergent margins.

  15. Melt-rock interactions and fabric development of peridotites from North Pond in the Kane area, Mid-Atlantic Ridge: Implications of microstructural and petrological analyses of peridotite samples from IODP Hole U1382A (United States)

    Harigane, Yumiko; Abe, Natsue; Michibayashi, Katsuyoshi; Kimura, Jun-Ichi; Chang, Qing


    North Pond is an isolated sedimentary pond on the western flank of the Kane area along the Mid-Atlantic Ridge. Drill-hole U1382A of IODP Expedition 336 recovered peridotite and gabbro samples from a sedimentary breccia layer in the pond, from which we collected six fresh peridotite samples. The peridotite samples came from the southern slope of the North Pond where an oceanic core complex is currently exposed. The samples were classified as spinel harzburgite, plagioclase-bearing harzburgite, and a vein-bearing peridotite that contains tiny gabbroic veins. No obvious macroscopic shear deformation related to the formation of a detachment fault was observed. The spinel harzburgite with a protogranular texture was classified as refractory peridotite. The degree of partial melting of the spinel harzburgite is estimated to be ˜17%, and melt depletion would have occurred at high temperatures in the uppermost mantle beneath the spreading axis. The progressive melt-rock interactions between the depleted spinel harzburgite and the percolating melts of Normal-Mid Ocean Ridge Basalt (N-MORB) produced the plagioclase-bearing harzburgite and the vein-bearing peridotite at relatively low temperatures. This implies that the subsequent refertilization occurred in an extinct spreading segment of the North Pond after spreading at the axis. Olivine fabrics in the spinel and plagioclase-bearing harzburgites are of types AG, A, and D, suggesting the remnants of a mantle flow regime beneath the spreading axis. The initial olivine fabrics appear to have been preserved despite the later melt-rock interactions. The peridotite samples noted above preserve evidence of mantle flow and melt-rock interactions beneath a spreading ridge that formed at ˜8 Ma.

  16. Transition from ultra-enriched to ultra-depleted primary MORB melts in a single volcanic suite (Macquarie Island, SW Pacific): Implications for mantle source, melting process and plumbing system (United States)

    Husen, Anika; Kamenetsky, Vadim S.; Everard, John L.; Kamenetsky, Maya B.


    Compositional diversity of basalts forming the oceanic floor is attributed to a variety of factors such as mantle heterogeneities, melting conditions, mixing of individual melt batches, as well as fractionation and assimilation processes during magma ascent and emplacement. In this study the compositional range and origin of mid-ocean ridge basalts (MORB) is approached by petrological, mineralogical and geochemical studies of the Miocene Macquarie Island ophiolite, an uplifted part of the Macquarie Ridge at the boundary between the Australian and Pacific plates. In this study, earlier results on the enriched to ultra-enriched (La/Sm 1.4-7.9), isotopically homogeneous basaltic glasses are complemented by the compositions of olivine-phyric rocks, principal phenocrystic minerals and Cr-spinel hosted melt inclusions. Studied olivine, clinopyroxene and Cr-spinel phenocrysts are among the most primitive known for MORB (85-91 mol% forsterite in olivine, 81-91 Mg# in clinopyroxene, and 66-77 Mg# and 34-60 Cr# in spinel) and represent primary and near-primary compositions of their parental melts. Geochemical characteristics of the liquids parental to clinopyroxene (La/Sm 0.8-6.3) and Cr-spinel (La/Sm 0.4-5) partly overlap with those of the basaltic glasses, but also strongly advocate the role of depleted to ultra-depleted primary melts in the origin of the Macquarie Island porphyritic rocks. The trace element composition of olivine phenocrysts and the systematics of rare-earth elements in glasses, melt inclusions, and clinopyroxene provide evidence for a peridotitic composition of the source mantle. Our data supports the mechanism of fractional "dynamic" melting of a single mantle peridotite producing individual partial melt batches with continuously changing compositions from ultra-enriched towards ultra-depleted. The incipient enriched melt batches, represented by basaltic glasses in this study, may erupt without significant modification, whereas consecutively derived

  17. Evolution of the South African mantle — A case study of garnet peridotites from the Finsch diamond mine (Kaapvaal craton); part 1: Inter-mineral trace element and isotopic equilibrium (United States)

    Lazarov, Marina; Brey, Gerhard P.; Weyer, Stefan


    A thorough assessment of inter-mineral equilibrium is essential for the understanding of trace element partitioning and also for the interpretation of isotopic data. Here we investigated high temperature (~ 1200 °C and 6 GPa) garnet peridotite xenoliths from the Kaapvaal craton (Finsch mine, South Africa), with exceptionally well equilibrated mineral major element compositions, for their trace element and isotopic inter-mineral equilibrium. Trace element compositions for all major mineral phases, i.e. olivine, orthopyroxene (opx), clinopyroxene (cpx) and garnet, were analysed by laser ablation inductively coupled plasma mass spectrometry (ICP-MS). Garnet, cpx and opx of selected samples were analysed for their Sm-Nd and Lu-Hf isotope compositions by multi collector ICP-MS. Most important mineral characteristics include: a) olivines from most samples are enriched in high-field-strength elements relative to other incompatible trace elements. Their lithium content correlates negatively with Mg#, indicating a depletion signature; b) all other minerals are depleted in heavy and middle rare earth elements (H- and M-REE) and enriched in light REE and large ion lithophile elements. This implies a complex history of depletion and metasomatic overprint for the Finsch cratonic mantle; c) orthopyroxene has similarly shaped trace element patterns as cpx, with one to two orders of magnitude lower abundances; and d) both, garnet and cpx, display variable, mostly positive ɛHf coupled with moderate variations in ɛNd. Trace element partitioning between garnet/cpx, cpx/opx and garnet/opx, displays only a weak pressure and temperature dependency. However, equilibrium partitioning of most trace elements between garnet and cpx shows a strong compositional dependency, i.e. on the Cr- (and Ca-) content of the garnets. Garnet-cpx partition coefficients follow a second grade polynomial correlation with Cr2O3 of garnet, whereby high chromium garnets (Cr2O3 > 6 wt.%) have generally higher

  18. Peridotites and mafic igneous rocks at the foot of the Galicia Margin: an oceanic or continental lithosphere? A discussion

    Energy Technology Data Exchange (ETDEWEB)

    Korprobst, J.; Chazot, G.


    An ultramafic/mafic complex is exposed on the sea floor at the foot of the Galicia Margin (Spain and Portugal). It comprises various types of peridotites and pyroxenites, as well as amphibole-diorites, gabbros, dolerites and basalts. For chronological and structural reasons (gabbros were emplaced within peridotites before the continental break-up) this unit cannot be assigned to the Atlantic oceanic crust. The compilation of all available petrological and geochemical data suggests that peridotites are derived from the sub-continental lithospheric mantle, deeply transformed during Cretaceous rifting. Thus, websterite dykes extracted from the depleted MORB mantle reservoir (DMM), were emplaced early within the lithospheric harzburgites; subsequent boudinage and tectonic dispersion of these dykes in the peridotites, during deformation stages at the beginning of rifting, resulted in the formation of fertile but isotopically depleted lherzolites. Sterile but isotopically enriched websterites, would represent melting residues in the peridotites, after significant partial melting and melt extraction related to the thermal erosion of the lithosphere. The latter melts are probably the source of brown amphibole metasomatic crystallization in some peridotites, as well as of the emplacement of amphibole-diorite dykes. Melts directly extracted from the asthenosphere were emplaced as gabbro within the sub-continental mantle. Mixing these DMM melts together with the enriched melts extracted from the lithosphere, provided the intermediate isotopic melt-compositions - in between the DMM and Oceanic Islands Basalts reservoir - observed for the dolerites and basalts, none of which are characterized by a genuine N-MORB signature. An enriched lithospheric mantle, present prior to rifting of the Galicia margin, is in good agreement with data from the Messejana dyke (Portugal) and more generally, with those of all continental tholeiites of the Central Atlantic Magmatic Province (CAMP

  19. Global variations in abyssal peridotite compositions (United States)

    Warren, Jessica M.


    Abyssal peridotites are ultramafic rocks collected from mid-ocean ridges that are the residues of adiabatic decompression melting. Their compositions provide information on the degree of melting and melt-rock interaction involved in the formation of oceanic lithosphere, as well as providing constraints on pre-existing mantle heterogeneities. This review presents a compilation of abyssal peridotite geochemical data (modes, mineral major elements, and clinopyroxene trace elements) for > 1200 samples from 53 localities on 6 major ridge systems. On the basis of composition and petrography, peridotites are classified into one of five lithological groups: (1) residual peridotite, (2) dunite, (3) gabbro-veined and/or plagioclase-bearing peridotite, (4) pyroxenite-veined peridotite, and (5) other types of melt-added peridotite. Almost a third of abyssal peridotites are veined, indicating that the oceanic lithospheric mantle is more fertile, on average, than estimates based on residual peridotites alone imply. All veins appear to have formed recently during melt transport beneath the ridge, though some pyroxenites may be derived from melting of recycled oceanic crust. A limited number of samples are available at intermediate and fast spreading rates, with samples from the East Pacific Rise indicating high degrees of melting. At slow and ultra-slow spreading rates, residual abyssal peridotites define a large (0-15% modal clinopyroxene and spinel Cr# = 0.1-0.6) compositional range. These variations do not match the prediction for how degree of melting should vary as a function of spreading rate. Instead, the compositional ranges of residual peridotites are derived from a combination of melting, melt-rock interaction and pre-existing compositional variability, where melt-rock interaction is used here as a general term to refer to the wide range of processes that can occur during melt transport in the mantle. Globally, ~ 10% of abyssal peridotites are refractory (0

  20. Mantle-crust differentiation of chalcophile elements in the oceanic lithosphere (United States)

    Ciążela, J.; Dick, H. J.; Koepke, J.; Kuhn, T.; Muszynski, A.; Kubiak, M.


    The chalcophile elements, as associated with sulfides, are believed mainly from the study of ophiolites to be generally enriched in the upper mantle, but depleted by magmatic processes in the lower and upper ocean crust. However, studies of some orogenic lherzolites suggest a copper depletion of peridotites in relation to the primitive mantle, suggesting that a portion of the sulfides is melted during decompression and incorporated into the ascending magmas. The rarity of abyssal peridotites and the high degree of their alteration have not allowed these results to be verified in situ in the oceans.Here, we present the first complete study of chalcophile elements based on a suite of rocks from an oceanic core complex (OCC), the Kane Megamullion at 22°30'N at the Mid-Atlantic Ridge. OCCs provide large exposures of mantle and lower crustal rocks on the seafloor on detachment fault footwalls at slow and ultraslow spreading ridges. The Kane Megamullion is one of the best sampled OCCs in the world, with 1342 rocks from 28 dredge sites and 14 dives. We have made XRF, TD-MS and INAA analyses of 129 representative peridotites, gabbroic rocks, diabases and basalts. Our results suggest a depletion of some peridotites in relation to the primitive mantle (28 ppm Cu). Dunites, troctolites and olivine gabbros are relatively enriched in chalcophile elements. The amount of sulfides decreases gradually with progressive differentiation, reaching a minimum in gabbronorites and diabases. The highest bulk abundance of chalcophile elements in our sample suite was observed in dunites (up to ~ 300 ppm Cu in several samples) and a contact zone between residual peridotite and a mafic vein (294 ppm Cu). Plagioclase-bearing harzburgites, generally formed by late-stage melt impregnation in the mantle, are typically more enriched in Cu than unimpregnated residual peridotites. For these reasons, our initial results indicate sulfide melting during mantle melting, and their local precipitation in

  1. Mineralogy and composition of the oceanic mantle (United States)

    Putirka, Keith; Ryerson, F.J.; Perfit, Michael; Ridley, W. Ian


    contents, however, that are enriched compared with melts from natural peridotites and magmas derived from the Siqueiros depleted mantle, and consequently may require an enriched source. TiO2 is not the only element that is enriched relative to melts of natural peridotites. Moderately incompatible elements, such as Ti, Zr, Hf, Y, and Eu, and compatible elements, such as Yb and Lu, are all enriched at the Hawaiian Islands. Such enrichments can be explained by adding 5-10% mid-ocean ridge basalt (crust) to depleted mantle; when the major element composition of such a mixture is recast into mineral components, the result is a fertile peridotite mineralogy.

  2. Trace element and Sr and Nd isotope geochemistry of peridotite xenoliths from the Eifel (West Germany) and their bearing on the evolution of the subcontinental lithosphere

    Energy Technology Data Exchange (ETDEWEB)

    Stosch, H.G.; Lugmair, G.W.


    Peridotite xenoliths from the Eifel can be divided into incompatible element-depleted and -enriched members. The depleted group is restricted to dry lherzolites whereas the enriched group encompasses dry harzburgites, dry websterite and amphibole and/or phlogopite-bearing peridotites. Isotopically the depleted group is very diverse with /sup 143/Nd//sup 144/Nd ranging from proportional to 0.51302 to 0.51355 and /sup 87/Sr//sup 86/Sr from proportional to 0.7041 to 0.7019, thus occupying a field larger than expected for oceanic-type subcontinental mantle. These xenoliths are derived from a mantle which appears to have diverged from a bulk-earth Nd and Sr isotopic evolution path proportional to 2 Ga ago as a consequence of partial melting. The combination of high /sup 143/Nd//sup 144/Nd with high /sup 87/Sr//sup 86/Sr in some members of depleted-xenoliths suite is likely to be the result of incipient reaction with incompatible element-enriched fluids in the mantle. In the enriched group such reactions have proceeded further and erased any pre-enriched isotope memory resulting in a smaller isotopic diversity (/sup 143/Nd//sup 144/Nd proportional to 0.51256-0.51273, /sup 87/Sr//sup 86/Sr proportional to 0.7044-0.7032). An evaluation of Sm-Hf and Yb-Hf relationships suggests that the amphibole-bearing lherzolites and harzburgites acquired their high enrichment of light rare earth elements by fluid infiltration into previously depleted peridotite rather than by silicate melt-induced metasomatism. Upper mantle composed of such metasomatized peridotites does not represent a potential source for the basanites and nephelinites from the Eifel. The isotopic and chemcial diversity of the subcontinental lithospheric part of the mantle may result from it having remained isolated from the convecting mantle for times >1 Ga.

  3. Isotopic decoupling during porous melt flow: A case-study in the Lherz peridotite (United States)

    Le Roux, V.; Bodinier, J.-L.; Alard, O.; O'Reilly, S. Y.; Griffin, W. L.


    Most peridotite massifs and mantle xenoliths show a wide range of isotopic variations, often involving significant decoupling between Hf, Nd and Sr isotopes. These variations are generally ascribed either to mingling of individual components of contrasted isotopic compositions or to time integration of parent-element enrichment by percolating melts/fluids, superimposed onto previous depletion event(s). However, strong isotopic decoupling may also arise during porous flow as a result of daughter-elements fractionation during solid-liquid interaction. Although porous flow is recognized as an important process in mantle rocks, its effects on mantle isotopic variability have been barely investigated so far. The peridotites of the Lherz massif (French Pyrenees) display a frozen melt percolation front separating highly refractory harzburgites from refertilized lherzolites. Isotopic signatures observed at the melt percolation front show a strong decoupling of Hf from Nd and Sr isotopes that cannot be accounted for by simple mixing involving the harzburgite protolith and the percolating melt. Using one dimensional percolation-diffusion and percolation-reaction modeling, we show that these signatures represent transient isotopic compositions generated by porous flow. These signatures are governed by a few critical parameters such as daughter element concentrations in melt and peridotite, element diffusivity, and efficiency of isotopic homogenization rather than by the chromatographic effect of melt transport and the refertilization reaction. Subtle variations in these parameters may generate significant inter-isotopic decoupling and wide isotopic variations in mantle rocks.

  4. Magma Supply at the Arctic Gakkel Ridge: Constraints from Peridotites and Basalts (United States)

    Sun, C.; Dick, H. J.; Hellebrand, E.; Snow, J. E.


    Crustal thickness in global ridge systems is widely believed to be nearly uniform (~7 km) at slow- and fast-spreading mid-ocean ridges, but appears significantly thinner (ancient mantle depletion, lower mantle temperature, ridge obliquity, and melt retention/focusing. To better understand the magma supply at ultraslow-spreading ridges, we examined melting dynamics by linking peridotites and basalts dredged along the Gakkel Ridge. We analyzed rare earth elements in clinopyroxene from 84 residual peridotites, and estimated melting parameters for individual samples through nonlinear least squares analyses. The degrees of melting show a large variation but mainly center at around 7% assuming a somewhat arbitrary but widely used depleted MORB mantle starting composition. Thermobarometry on published primitive basaltic glasses from [2] indicates that the mantle potential temperature at the Gakkel Ridge is ~50°C cooler than that at the East Pacific Rise. The ridge-scale low-degree melting and lower mantle potential temperature place the final depth of melting at ~30 km and a melt thickness of 1.0 or 2.9 km for a triangular or trapezoidal melting regime, respectively. The final melting depth is consistent with excess conductive cooling and lithosphere thickening suggested by geodynamic models, while the estimated melt thickness is comparable to the seismic crust (1.4 - 2.9 km; [1]). The general agreement among geochemical analyses, seismic measurements, and geodynamic models supports that lower mantle potential temperature and thick lithosphere determine the ridge-scale low-degree melting and thin crust at the Gakkel Ridge, while melt retention/focusing and excess ancient mantle depletion are perhaps locally important at short length scales (e.g., < 50 - 100 km). [1] Jokat and Schmidt-Aursch (2007) Geophys. J. Int. (2007) 168, 983-998. [2] Gale et al. (2012) J. Petrology, 55, 1051-1082.

  5. Probing Archean lithosphere using the Lu-Hf isotope systematics of peridotite xenoliths from Somerset Island kimberlites, Canada (United States)

    Schmidberger, Stefanie S.; Simonetti, Antonio; Francis, Don; Gariépy, Clément


    A knowledge of the Hf isotopic composition of the subcontinental lithosphere beneath Archean cratons is essential to constrain the Hf isotope budget of the Earth's mantle. Hf isotopic measurements were obtained by MC-ICP-MS for a suite of refractory peridotite xenoliths and constituent garnets from the Nikos kimberlite (100 Ma) on Somerset Island in order to constrain the isotopic composition and age of the lithosphere beneath the northern Canadian craton. The low-temperature Nikos peridotites (Somerset lithosphere, are characterized by higher 176Lu/ 177Hf ratios (0.03-0.05) and Hf isotopic values ( 176Hf/ 177Hf (0.1Ga)=0.28296-0.28419) than the deep-seated high-temperature peridotites (>1100°C; 0.004-0.03, 0.28265-0.28333, respectively). These differences in Hf isotope signatures suggest that shallow and deep subcontinental lithosphere beneath Somerset Island represent isotopically distinct domains and do not share a common petrogenetic history. The Lu-Hf isotope systematics of the shallow low-temperature peridotites define a positively sloped line that plot along a 2.8 Ga reference isochron. A number of these peridotites are characterized by highly radiogenic Hf isotopic compositions suggestive of long-term radiogenic ingrowth (billions of years). These findings are consistent with an interpretation that the shallow Somerset lithosphere (to depths of ˜150 km) stabilized in the Archean. The majority of the high-temperature peridotites plot closer to the composition of the host kimberlite. Although the observed isotopic variation may be attributed in part to kimberlite-related Hf addition, it is possible that these deep-seated xenoliths represent younger mantle. The superchondritic 176Lu/ 177Hf ratios observed for a number of the shallow low-temperature peridotites indicate strong fractionation of Lu and Hf, suggesting mantle root formation in the garnet stability field (depths >80 km). The Hf isotope compositions for the Somerset low-temperature peridotites

  6. Tracking the Depleted Mantle Signature in Melt Inclusions and Residual Glass of Basaltic Martian Shergottites using Secondary Ionization Mass Spectrometry (United States)

    Peters, Timothy J.; Simon, Justin I.; Jones, John H.; Usui, Tomohiro; Economos, Rita C.; Schmitt, Axel K.; McKeegan, Kevin D.


    Trace element abundances of depleted shergottite magmas recorded by olivine-hosted melt inclusions (MI) and interstitial mesostasis glass were measured using the Cameca ims-1270 ion microprobe. Two meteorites: Tissint, an olivine-­phyric basaltic shergottite which fell over Morocco July 18th 2001; and the Antarctic meteorite Yamato 980459 (Y98), an olivine-phyric basaltic shergottite with abundant glassy mesostasis have been studied. Chondrite-­normalized REE patterns for MI in Tissint and Y98 are characteristically LREE depleted and, within analytical uncertainty, parallel those of their respective whole rock composition; supporting each meteorite to represent a melt composition that has experienced closed-­system crystallization. REE profiles for mesostasis glass in Y98 lie about an order of magnitude higher than those from the MI; with REE profiles for Tissint MI falling in between. Y98 MI have the highest average Sm/Nd and Y/Ce ratios, reflecting their LREE depletion and further supporting Y98 as one of our best samples to probe the depleted shergotitte mantle. In general, Zr/Nb ratios overlap between Y98 and Tissint MI, Ce/Nb ratios overlap between Y98 MI and mesostasis glass, and Sm/Nd ratios overlap between Y98 mesostasis glass and Tissint MI. These features support similar sources for both, but with subtle geochemical differences that may reflect different melting conditions or fractionation paths during ascent from the mantle. Interestingly, the REE patterns for both Y98 bulk and MI analyses display a flattening of the LREE that suggests a crustal contribution to the Y98 parent melt. This observation has important implications for the origins of depleted and enriched shergottites.

  7. Isotopic (Pb, Sr, Nd, C, O) evidence for plume-related sampling of an ancient, depleted mantle reservoir (United States)

    Chen, Wei; Simonetti, Antonio


    The exact mantle source for carbonatite melts remains highly controversial. Despite their predominant occurrence within continental (lithospheric) domains, the radiogenic isotope data from young (oceanic island basalts (OIBs). This feature suggests an intimate petrogenetic relationship with asthenospheric mantle. New Pb, Sr, C, and O isotopic data are reported here for constituent minerals from the Oka carbonatite complex, which is associated with the Cretaceous Monteregian Igneous Province (MIP), northeastern North America. The Pb isotope data define linear arrays in Pb-Pb isotope diagrams, with the corresponding Sr isotope ratios being highly variable (0.70314-0.70343); both these features are consistent with open system behavior involving at least three distinct mantle reservoirs. Compared to the isotope composition of known mantle sources for OIBs and carbonatite occurrences worldwide, the least radiogenic 207Pb/204Pb (14.96 ± 0.07) and 208Pb/204Pb (37.29 ± 0.15) isotopic compositions relative to their corresponding 206Pb/204Pb ratios (18.86 ± 0.08) reported here are distinct, and indicate the involvement of an ancient depleted mantle (ADM) source. The extremely unradiogenic Pb isotope compositions necessitate U/Pb fractionation early in Earth's history (prior to 4.0 Ga ago) and growth via a multi-stage Pb evolution model. The combined stable (C and O) and radiogenic isotopic compositions effectively rule out crustal/lithosphere contamination during the petrogenetic history of the Oka complex. Instead, the isotopic variations reported here most likely result from the mixing of discrete, small volume partial melts derived from a heterogeneous plume source characterized by a mixed HIMU-EM1-ADM signature.

  8. Role of the mantle exhumation channel in the formation of ultramafic seafloor (United States)

    Muntener, O.; Lavier, L. L.


    Mantle peridotites from ocean-continent transition zones (OCT's) illustrate the distribution of the scale of upper mantle heterogeneity in extensional systems that evolve from rifting to (ultra-) slow seafloor spreading. We summarize research on mantle processes of the conjugate Iberia-Newfoundland rift and from the Alps that show that the basement of OCT's consists of 3 mantle domains. Thermally undisturbed, cold subcontinental mantle formed the ocean floor next to thinned continental crust. This 'subcontinental domain' is separated by ductile shear zones from an infiltrated (e.g. hot) domain dominated by plagioclase peridotite. The footwall of these mantle shear zones display complex refertilization processes and high-temperature deformation. These rocks are highly heterogeneous and are juxtaposed with depleted lherzolites and dunites (extraction domain). Upwelling of partial melts that enter the conductive lithospheric mantle inevitably leads to freezing of the melt and to the formation of a chemical and rheological barrier, which we term mantle exhumation channel. We will discuss an example that displays km-scale refertilization with active deformation (and melt focusing?) on top, and the formation of a domain that represents focused melt extraction at the bottom. We show that an actively deforming refertilization front in Alpine plagioclase peridotites and in the Iberia Newfoundland rift moved ahead of a melting front. Melt lubricated shear zones (or melt bands) focus melt flow. Continuous uplift leads to crystallization and deformation will prevail in the subsolidus state. Final exposure of infiltrated peridotites on the seafloor is accommodated by faults in which hydrous phases crystallize (chlorite, serpentine, talc). We combine petrologic data and numerical models to illustrate that these processes play a key role in the rejuvenation and erosion of the lithospheric mantle and ultimately form ultramafic seafloor in ocean-continent transitions and ultra

  9. Geochemical heterogeneity in the Arctic mantle at Gakkel Ridge (United States)

    D'Errico, M. E.; Warren, J. M.; Godard, M.


    Conductive cooling due to ultraslow spreading has been suggested to limit partial melting of the mantle and crustal production at Gakkel Ridge. In addition, the thick lithosphere induced by cooling should significantly control melt migration and extraction. To explore these effects at ultraslow spreading rates, major and trace element concentrations in pyroxene minerals are presented for 14 dredged Gakkel abyssal peridotites. Samples from the same dredge and among dredges reveal wide compositional variation. Trace element compositions of lherzolites reflect 4-6% non-modal fractional mantle melting. However, these high degrees of melting without a corresponding amount of oceanic crust suggest the occurrence of infertile mantle due to ancient melting event(s). In addition, high degrees of melt depletion at short length-scales (earth elements that can be fit by 6 to ≥13% non-modal melting, but this results in modeled light rare earth element contents that are too low relative to observed concentrations. Instead, harzburgite trace element patterns require open-system melting involving interaction with a percolating melt. Extreme enrichments in highly incompatible elements also suggest the occurrence of late-stage refertilization and melt entrapment. Modeling of several different source melt compositions indicates that the trapped melt was generated from garnet field-equilibrated peridotite. Overall, the compositional variability in Gakkel peridotite samples reflects a heterogeneous mantle resulting from inherited depletion and recent melt percolation and entrapment.

  10. Coupled Pb isotopic and trace element systematics of the Tissint meteorite: Geochemical signatures of the depleted shergottite source mantle (United States)

    Moriwaki, Ryota; Usui, Tomohiro; Simon, Justin I.; Jones, John H.; Yokoyama, Tetsuya; Tobita, Minato


    The application of Martian meteorite U-Th-Pb isotope systematics to track the geochemical evolution of the Martian mantle has had limited success because of the difficulty in discriminating an indigenous magmatic Pb component from secondary near-surface components that have additionally been overprinted by terrestrial contamination. To mitigate this challenge, a successive acid-leaching experiment was conducted on the Tissint meteorite, the freshest, witnessed fall of a primitive, olivine-bearing Martian basalt. Trace element concentration analyses of acid leachates and residues indicate that secondary terrestrial contaminants were effectively removed by the early steps in the leaching experiments and that the acid residues contain pristine Pb from Tissint. The acid residue, which shows the most depleted REE signature, also has the least radiogenic Pb isotopic composition (206Pb/204Pb = 10.948, 207Pb/204Pb = 11.187, 208Pb/204Pb = 30.228). A two-stage mantle evolution model based on this composition indicates that the Tissint mantle has the lowest μ-value (238U/204Pb = 1.62 ± 0.09) among the shergottite sources.

  11. Petrology and geochemistry of a peridotite body in Central- Carpathian Paleogene sediments (Sedlice, eastern Slovakia

    Directory of Open Access Journals (Sweden)

    Koppa Matúš


    Full Text Available We studied representative samples from a peridotite body situated NE of Sedlice village within the Central- Carpathian Paleogene sediments in the Central Western Carpathians. The relationship of the peridotite to the surrounding Paleogene sediments is not clear. The fractures of the brecciated peridotite margin are healed with secondary magnesite and calcite. On the basis of the presented bulk-rock and electron microprobe data, the wt. % amounts of mineral phases were calculated. Most of calculated “modal” compositions of this peridotite corresponds to harzburgites composed of olivine (∼70-80 wt. %, orthopyroxene (∼17-24 wt. %, clinopyroxene ( < 5 wt. % and minor spinel ( < 1 wt. %. Harzburgites could originate from lherzolitic protoliths due to a higher degree of partial melting. Rare lherzolites contain porphyroclastic 1-2 mm across orthopyroxene (up to 25 wt. %, clinopyroxene (∼ 5-8 wt. % and minor spinel ( < 0.75 wt. %. On the other hand, rare, olivine-rich dunites with scarce orthopyroxene porphyroclasts are associated with harzburgites. Metamorphic mineral assemblage of low-Al clinopyroxene (3, tremolite, chrysotile, andradite, Cr-spinel to chromite and magnetite, and an increase of fayalite component in part of olivine, indicate low-temperature metamorphic overprint. The Primitive Mantle normalized whole-rock REE patterns suggest a depleted mantle rock-suite. An increase in LREE and a positive Eu anomaly may be consequence of interactive metamorphic fluids during serpentinization. Similar rocks have been reported from the Meliatic Bôrka Nappe overlying the Central Western Carpathians orogenic wedge since the Late Cretaceous, and they could be a potential source of these peridotite blocks in the Paleogene sediments.

  12. The origin of reaction textures in mantle peridotite xenoliths from Sal Island, Cape Verde: the case for ``metasomatism'' by the host lava (United States)

    Shaw, Cliff S. J.; Heidelbach, Florian; Dingwell, Donald B.


    Reaction zones around minerals in mantle xenoliths have been reported from many localities worldwide. Interpretations of the origins of these textures fall into two groups: mantle metasomatic reaction or reaction during transport of the xenoliths to the surface. A suite of harzburgitic mantle xenoliths from Sal, Cape Verde show clear evidence of reaction during transport. The reactions resulted in the formation of olivine-clinopyroxene and Si- and alkali-rich glass reaction zones around orthopyroxene and sieve-textured clinopyroxene and sieve textured spinel, both of which are associated with a Si- and alkali-rich glass similar to that in the orthopyroxene reaction zones. Reaction occurred at pressures less than the mantle equilibration pressure and at temperatures close to the liquidus temperature of the host magma. In addition, there is a clear spatial relation of reaction with the host lava: reaction is most intense near the lava/xenolith contact. The residence time of the xenoliths in the host magma, determined from Fe-Mg interdiffusion profiles in olivine, was approximately 4 years. Our results cannot be reconciled with a recent model for the evolution of the mantle below the Cape Verde Archipelago involving mantle metasomatism by kimberlitic melt. We contend that alkali-rich glasses in the Sal xenoliths are not remnants of a kimberlitic melt, but rather they are the result of reaction between the host lava or a similar magma and xenolith minerals, in particular orthopyroxene. The formation of a Si- and alkali-rich glass by host magma-orthopyroxene reaction appears to be a necessary precursor to formation of sieve textured spinel and clinopyroxene.

  13. Big insights from tiny peridotites: Evidence for persistence of Precambrian lithosphere beneath the eastern North China Craton (United States)

    Liu, Jingao; Rudnick, Roberta L.; Walker, Richard J.; Xu, Wen-liang; Gao, Shan; Wu, Fu-yuan


    Previous studies have shown that the eastern North China Craton (NCC) lost its ancient lithospheric mantle root during the Phanerozoic. The temporal sequence, spatial extent, and cause of the lithospheric thinning, however, continue to be debated. Here we report olivine compositions, whole-rock Re-Os isotopic systematics, and platinum-group element abundances of small ( 92) lithospheric mantle is largely absent. Osmium isotopic data suggest the Wudi peridotites experienced melt depletion primarily during the Paleoproterozoic (~ 1.8 Ga), although an Archean Os model age for one xenolith indicates incorporation of a minor component of Archean lithospheric mantle. These data suggest that a previously unrecognized Paleoproterozoic orogenic event removed and replaced the original Archean lithospheric mantle beneath the sedimentary basin at the southern edge of the Bohai Sea. By contrast, the Fuxin peridotites, entrained in Cretaceous basalts that crop out along the northern edge of the eastern NCC, document the coexistence of both ancient (≥ 2.3 Ga) and modern lithospheric mantle components. Here, the original Late Archean-Early Paleoproterozoic lithospheric mantle was, at least partially, removed and replaced prior to 100 Ma. Combined with literature data, our results show that removal of the original Archean lithosphere occurred within Proterozoic collisional orogens, and that replacement of Precambrian lithosphere during the Mesozoic may have been spatially associated with the collisional boundaries and the strike-slip Tan-Lu fault, as well as the onset of Paleo-Pacific plate subduction.

  14. Trace element mass balance in hydrous adiabatic mantle melting: The Hydrous Adiabatic Mantle Melting Simulator version 1 (HAMMS1) (United States)

    Kimura, Jun-Ichi; Kawabata, Hiroshi


    numerical mass balance calculation model for the adiabatic melting of a dry to hydrous peridotite has been programmed in order to simulate the trace element compositions of basalts from mid-ocean ridges, back-arc basins, ocean islands, and large igneous provinces. The Excel spreadsheet-based calculator, Hydrous Adiabatic Mantle Melting Simulator version 1 (HAMMS1) uses (1) a thermodynamic model of fractional adiabatic melting of mantle peridotite, with (2) the parameterized experimental melting relationships of primitive to depleted mantle sources in terms of pressure, temperature, water content, and degree of partial melting. The trace element composition of the model basalt is calculated from the accumulated incremental melts within the adiabatic melting regime, with consideration for source depletion. The mineralogic mode in the primitive to depleted source mantle in adiabat is calculated using parameterized experimental results. Partition coefficients of the trace elements of mantle minerals are parameterized to melt temperature mostly from a lattice strain model and are tested using the latest compilations of experimental results. The parameters that control the composition of trace elements in the model are as follows: (1) mantle potential temperature, (2) water content in the source mantle, (3) depth of termination of adiabatic melting, and (4) source mantle depletion. HAMMS1 enables us to obtain the above controlling parameters using Monte Carlo fitting calculations and by comparing the calculated basalt compositions to primary basalt compositions. Additionally, HAMMS1 compares melting parameters with a major element model, which uses petrogenetic grids formulated from experimental results, thus providing better constraints on the source conditions.

  15. Multi-stage melt-rock interaction in the Mt. Maggiore (Corsica, France) ophiolitic peridotites: microstructural and geochemical evidence (United States)

    Rampone, Elisabetta; Piccardo, Giovanni B.; Hofmann, Albrecht W.


    Spinel and plagioclase peridotites from the Mt.Maggiore (Corsica, France) ophiolitic massif record a composite asthenosphere-lithosphere history of partial melting and subsequent multi-stage melt-rock interaction. Cpx-poor spinel lherzolites are consistent with mantle residues after low-degree fractional melting ( F = 5-10%). Opx + spinel symplectites at the rims of orthopyroxene porphyroclasts indicate post-melting lithospheric cooling ( T = 970-1,100°C); this was followed by formation of olivine embayments within pyroxene porphyroclasts by melt-rock interaction. Enrichment in modal olivine (up to 85 wt%) at constant bulk Mg values, and variable absolute REE contents (at constant LREE/HREE) indicate olivine precipitation and pyroxene dissolution during reactive porous melt flow. This stage occurred at spinel-facies depths, after incorporation of the peridotites in the thermal lithosphere. Plagioclase-enriched peridotites show melt impregnation microtextures, like opx + plag intergrowths replacing exsolved cpx porphyroclasts and interstitial gabbronoritic veinlets. This second melt-rock interaction stage caused systematic chemical changes in clinopyroxene (e.g. Ti, REE, Zr, Y increase), related to the concomitant effects of local melt-rock interaction at decreasing melt mass, and crystallization of small (<3%) trapped melt fractions. LREE depletion in minerals of the gabbronoritic veinlets indicates that the impregnating melts were more depleted than normal MORB. Preserved microtextural evidence of previous melt-rock interaction in the impregnated peridotites suggests that they were progressively uplifted in response to lithosphere extension and thinning. Migrating melts were likely produced by mantle upwelling and melting related to extension; they were modified from olivine-saturated to opx-saturated compositions, and caused different styles of melt-rock interaction (reactive spinel harzburgites, vs. impregnated plagioclase peridotites) depending on the

  16. Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle (United States)

    Picazo, S.; Müntener, O.; Manatschal, G.; Bauville, A.; Karner, G.; Johnson, C.


    Systematic differences in mineral composition of mantle peridotites are observed in ophiolites and peridotitic bodies from the Alpine Tethys, the Pyrenean domain, the Dinarides and Hellenides, and the Iberia-Newfoundland rifted margins. These differences can be understood in the context of the evolution of rifted margins and allow the identification of 3 major mantle domains: an inherited domain, a refertilized domain and a depleted domain. Most clinopyroxene from the inherited domain equilibrated in the spinel peridotite field and are too enriched in Na2O and Al2O3 to be residues of syn-rift melting. Clinopyroxene from the refertilized domain partially equilibrated with plagioclase and display lower Na2O and Al2O3, and elevated Cr2O3 contents. The refertilized domain is a hybrid zone, which locally preserves remnants from the inherited domain and overlapping chemical compositions. Depleted domains with clinopyroxene similar to abyssal peridotites are rare and Nd-isotopic studies indicate that they represent ancient periods of melting unrelated to the opening of the Jurassic and Cretaceous oceanic basins of the Alpine Tethys and southern North Atlantic. In many studied sections of mantle rocks in exposed ophiolites a systematic spatial distribution of the different domains with respect to the evolution of rifted margins can be identified. This new approach integrates observations from exposed and drilled mantle rocks and proposes that the mantle lithosphere evolved and was modified during an extensional cycle from post-orogenic collapse through several periods of rifting to seafloor spreading. The defined chemical and petrological characteristics of mantle domains based on clinopyroxene and spinel compositions are compiled on present-day and paleogeographic maps of Western and Central Europe. These maps show that the observed distribution of mantle domains are linked to processes related to late post-Variscan extension, rift evolution and refertilization associated

  17. Ultra-reducing conditions in average mantle peridotites and in podiform chromitites: a thermodynamic model for moissanite (SiC) formation (United States)

    Golubkova, Anastasia; Schmidt, Max W.; Connolly, James A. D.


    Natural moissanite (SiC) is reported from mantle-derived samples ranging from lithospheric mantle keel diamonds to serpentinites to podiform chromitites in ophiolites related to suprasubduction zone settings (Luobusa, Dongqiao, Semail, and Ray-Iz). To simulate ultra-reducing conditions and the formation of moissanite, we compiled thermodynamic data for alloys (Fe-Si-C and Fe-Cr), carbides (Fe3C, Fe7C3, SiC), and Fe-silicides; these data were augmented by commonly used thermodynamic data for silicates and oxides. Computed phase diagram sections then constrain the P- T- fO2 conditions of SiC stability in the upper mantle. Our results demonstrate that: Moissanite only occurs at oxygen fugacities 6.5-7.5 log units below the iron-wustite buffer; moissanite and chromite cannot stably coexist; increasing pressure does not lead to the stability of this mineral pair; and silicates that coexist with moissanite have X Mg > 0.99. At upper mantle conditions, chromite reduces to Fe-Cr alloy at fO2 values 3.7-5.3 log units above the moissanite-olivine-(ortho)pyroxene-carbon (graphite or diamond) buffer (MOOC). The occurrence of SiC in chromitites and the absence of domains with almost Fe-free silicates suggest that ultra-reducing conditions allowing for SiC are confined to grain scale microenvironments. In contrast to previous ultra-high-pressure and/or temperature hypotheses for SiC origin, we postulate a low to moderate temperature mechanism, which operates via ultra-reducing fluids. In this model, graphite-/diamond-saturated moderately reducing fluids evolve in chemical isolation from the bulk rock to ultra-reducing methane-dominated fluids by sequestering H2O into hydrous phases (serpentine, brucite, phase A). Carbon isotope compositions of moissanite are consistent with an origin of such fluids from sediments originally rich in organic compounds. Findings of SiC within rocks mostly comprised by hydrous phases (serpentine + brucite) support this model. Both the hydrous phases

  18. Subduction zone mantle enrichment by fluids and Zr-Hf-depleted crustal melts as indicated by backarc basalts of the Southern Volcanic Zone, Argentina (United States)

    Holm, Paul M.; Søager, Nina; Alfastsen, Mads; Bertotto, Gustavo W.


    We aim to identify the components metasomatizing the mantle above the subducting Nazca plate under part of the Andean Southern Volcanic Zone (SVZ). We present new major and ICP-MS trace element and Sr, Nd and high-precision Pb isotope analyses of primitive olivine-phyric alkali basalts from the Northern Segment Volcanic Field, part of the Payenia province in the backarc of the Transitional SVZ. One new 40Ar-39Ar age determination confirms the Late Pleistocene age of this most northerly part of the province. All analysed rocks have typical subduction zone type incompatible element enrichment, and the rocks of the Northern Segment, together with the neighbouring Nevado Volcanic Field, have isotopic compositions intermediate between adjacent Transitional SVZ arc rocks and southern Payenia OIB-type basaltic rocks. Modelling the Ba-Th-Sm variation we demonstrate that fluids as well as 1-2% melts of upper continental crust (UCC) enriched their mantle sources, and La-Nb-Sm variations additionally indicate that the pre-metasomatic sources ranged from strongly depleted to undepleted mantle. Low Eu/Eu* and Sr/Nd also show evidence for a UCC component in the source. The contribution of Chile Trench sediments to the magmas seems insignificant. The Zr/Sm and Hf/Sm ratios are relatively low in many of the Northern Segment rocks, ranging down to 17 and 0.45, respectively, which, together with relatively high Th/U, is argued to indicate that the metasomatizing crustal melts were derived by partial melting of subducted UCC that had residual zircon, in contrast to the UCC melts added to Transitional SVZ arc magmas. Mixing between depleted and undepleted mantle, enriched by UCC and fluids, is suggested by Sr, Nd and Pb isotopes of the Northern Segment and Nevado magmas. The metasomatized undepleted mantle south of the Northern Segment is suggested to be part of upwelling OIB-type mantle, whereas the pre-metasomatically depleted mantle also can be found as a component in some arc

  19. Silica-enriched mantle sources of subalkaline picrite-boninite-andesite island arc magmas (United States)

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


    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

  20. Geochemistry of abyssal peridotites from the super slow-spreading Southwest Indian Ridge near 65°E: Implications for magma source and seawater alteration

    Indian Academy of Sciences (India)

    Zhigang Zeng; Qiaoyun Wang; Xiaomei Wang; Shuai Chen; Xuebo Yin; Zhaoxue Li


    The geochemical characteristics of abyssal peridotite samples from one dredge station (27° 49.74′S, 65° 02.14′E, water depth 4473 m) on the super slow-spreading Southwest Indian Ridge (SWIR) near 65°E were investigated. Abyssal peridotites recovered from this site were comprised mainly of lizardite, chlorite, carbonate and magnetite with minor amounts of talc, pyroxene phenocrysts and sparse olivines. Serpentinites exhibit talc veins and major serpentine derived from serpentinization with relict olivine granuloblasts. Olivine grains in serpentinites display exsolution lamellae, indicating the occurrence of talc reduction or decompression during seawater–rock interaction. Pyroxene shows clear cleavage in two directions, with clinopyroxene or orthopyroxene exsolution lamellae. By contrast, bulk rock trace element patterns of serpentinites reveal depletion in most incompatible elements, similarly to the depleted midocean ridge basalt mantle composition, indicating that the SWIR peridotites originated from a depleted mantle source magma and have experienced partial melting. Meanwhile, Rb, Ba, U, Pb, Sr, Li anomalies and the Ce/Pb ratio suggest that these serpentinites have been strongly altered by seawater.

  1. Evolution of the depleted mantle and growth of the continental crust: improving on the imperfect detrital zircon record (United States)

    Vervoort, J. D.; Kemp, A. I. S.; Patchett, P. J.


    One of the basic tenets of terrestrial geochemistry is that the continental crust has been extracted from the mantle leaving the latter depleted in incompatible elements. Nd and Hf isotopes have long shown that this process has been an essential feature of the Earth throughout its history. There is wide agreement on the general nature of this process, but the details of the isotopic record—and their implications for the depletion of the mantle and the extraction of continental crust—remain debated. Recently, much attention has been given to detrital zircons in both modern and ancient sediments. An advantage of this approach is the integration of the crystallization history of the zircon from the U-Pb chronometer with its Hf isotopic composition, which can provide important information on whether the zircons have been derived from juvenile or reworked crust. One essential requirement in this approach, however, is to unambiguously determine the crystallization ages of the zircons. We suggest that this represents an important—but generally ignored—source of uncertainty in the Hf isotopic record from detrital zircons. The quality filter most often used to assess the integrity of zircon U-Pb systematics is concordance; if a zircon is concordant, it is assumed that the U-Pb age is accurate. A concordance filter is less effective in old zircons, however, because ancient Pb loss, viewed today, parallels concordia. Without the benefit from the geological context of the host rock to the zircons, it is impossible to unambiguously determine it true crystallization age. Ancient Pb loss in zircons produces an apparent age less than the true magmatic age. The initial Hf isotopic composition of these zircons, as a result, will be calculated at the wrong age and will be anomalously low (by ~2.2 epsilon Hf units per 0.1 Ga). Hf model ages, calculated from these parameters, will be artificially old and spurious. The combination of unradiogenic Hf and Hf model ages > U-Pb ages

  2. Major Element Geochemistry of Peridotites from Santa Elena Ophiolite Complex, NW Costa Rica and Their Tectonic Implications (United States)

    Wright, S.; Snow, J. E.; Gazel, E.; Sisson, V.


    The Santa Elena Ophiolite Complex (SEOC) is located on the west coast of Northern Costa Rica, near the Nicaraguan border. It consists primarily of preserved oceanic crustal rocks and underlying upper mantle thrust onto an accretionary complex. The petrogenesis and tectonic origin of this complex have widely been interpreted to be either a preserved mantle portion of the Caribbean Large Igneous Province (CLIP) as it drifted between North and South America from the Galapagos hotpot into the present day Caribbean Ocean around 80 Ma or as the mantle section to the nearby Nicoya complex. Previous structural work suggests that SEOC is a supra-subduction complex, not related to the CLIP or Nicoya. Our preliminary results agree. Mantle peridotites collected from the Santa Elena Ophiolite Complex consist primarily of spinel lherzolite (61 %) with minor amounts of harzburgite and dunite (22 % and 16 % respectively). Spinel Cr# [molar Cr / (Cr+Al)*100] is widely accepted to constrain mantle partial melting and lithospheric melt stagnation. Cr# of spinels within Santa Elena lherzolites fall between 12 and 35, suggesting an extent of 3 % to 13 % partial melting. Cr# of harzburgites range from 35 to 39, suggesting 13 % to 14 % partial melting. This range of partial melting suggests only modest depletion of this exposed portion of the ancient uppermost mantle. TiO2 concentrations of the lherzolite and harzburgite range from 0.004% to 0.128%, with the exception of one sample, SE10 - 17 (0.258%), and fall within the normal melting trend for mantle peridotites. The presence of dunite indicates that melt flow and associated melt - rock reaction with the surrounding peridotite took place within this portion of the mantle. A Cr# of 84.5 from one of these dunite samples indicate that significant melt rock reaction with refractory melts took place. Such results are rarely found in mid-ocean ridge abyssal peridotite settings, and are currently found primarily in forearc tectonic settings

  3. Petrological significance of the abyssal peridotites from the ophiolite belt of Indo-Myanmar Range, northeastern India (United States)

    Maibam, B.; Foley, S.


    The ophiolite belt of the Indo-Myanmar Range of northeastern India is regarded as the continental extension of the Indonesia island arc. The ophiolite sequence forms a belt extending about 200 km from Pukhpur (Nagaland state) in the north to Moreh (Manipur state) in the south. Here we report preliminary mineralogical and phase geochemical data of peridotites collected from the ophiolite sequences of the Ukhrul district in Manipur. Primary minerals are clinopyroxene, orthopyroxene and spinel with scarce relicts of olivine. Andraditic garnet present in a few samples is of secondary origin. The clinopyroxene and orthopyroxene show limited compositional range. Well's (1977) single clinopyroxene thermometry yielded variable equilibrium temperature between 780-1250oC. The lower temperature value suggests post-crystallisation re-equilibration. Equilibrium pressure estimates based on Nimis and Ulmer's single clinopyroxene (1998) and Köhler and Brey's (1990) olivine-spinel geobarometries range from 19 to 25 kbars. Oxygen fugacity expressed in terms of quartz-fayalite-magnetite (Delta QFM) value is calculated as -1.32. Spinel Cr# (Cr/(Cr+Al)) of the peridotites reflects mantle residues after low degree of melting (F = 0.5 - 11%) if derived from primitive mantle. Mineral composition tectonomagmatic discrimination diagrams confirm the studied samples to be abyssal peridotites. Laser Ablation ICPMS analyses of the rare earth element patterns of clinopyroxene in the peridotites show depleted light rare earth element with flat middle and heavy rare earth element patterns, similar to the reported abyssal peridotites. References Wells P.R.A. (1977) CMP, 62, 129-139. Nimis P., Ulmer P. (1998) CMP, 133, 122-135. Köhler T., Brey G.P. (1990) GCA, 54, 2375-2388.

  4. Magnesium stable isotope composition of Earth's upper mantle (United States)

    Handler, Monica R.; Baker, Joel A.; Schiller, Martin; Bennett, Vickie C.; Yaxley, Gregory M.


    The mantle is Earth's largest reservoir of Mg containing > 99% of Earth's Mg inventory. However, no consensus exists on the stable Mg isotope composition of the Earth's mantle or how variable it is and, in particular, whether the mantle has the same stable Mg isotope composition as chondrite meteorites. We have determined the Mg isotope composition of olivine from 22 mantle peridotites from eastern Australia, west Antarctica, Jordan, Yemen and southwest Greenland by pseudo-high-resolution MC-ICP-MS on Mg purified to > 99%. The samples include fertile lherzolites, depleted harzburgites and dunites, cryptically metasomatised ('dry') peridotites and modally metasomatised apatite ± amphibole-bearing harzburgites and wehrlites. Olivine from these samples of early Archaean through to Permian lithospheric mantle have δ25Mg DSM-3 = - 0.22 to - 0.08‰. These data indicate the bulk upper mantle as represented by peridotite olivine is homogeneous within current analytical uncertainties (external reproducibility ≤ ± 0.07‰ [2 sd]). We find no systematic δ25Mg variations with location, lithospheric age, peridotite fertility, or degree or nature of mantle metasomatism. Although pyroxene may have slightly heavier δ25Mg than coexisting olivine, any fractionation between mantle pyroxene and olivine is also within current analytical uncertainties with a mean Δ25Mg pyr-ol = +0.06 ± 0.10‰ (2 sd; n = 5). Our average mantle olivine δ25Mg DSM-3 = - 0.14 ± 0.07‰ and δ26Mg DSM-3 = - 0.27 ± 0.14‰ (2 sd) are indistinguishable from the average of data previously reported for terrestrial basalts, confirming that basalts have stable Mg isotope compositions representative of the mantle. Olivine from five pallasite meteorites have δ25Mg DSM-3 = - 0.16 to - 0.11‰ that are identical to terrestrial olivine and indistinguishable from the average δ25Mg previously reported for chondrites. These data provide no evidence for measurable heterogeneity in the stable Mg isotope

  5. Formation and modification of chromitites in the mantle (United States)

    Arai, Shoji; Miura, Makoto


    Podiform chromitites have long supplied us with unrivaled information on various mantle processes, including the peridotite-magma reaction, deep-seated magmatic evolution, and mantle dynamics. The recent discovery of ultrahigh-pressure (UHP) chromitites not only sheds light on a different aspect of podiform chromitites, but also changes our understanding of the whole picture of podiform chromitite genesis. In addition, new evidence was recently presented for hydrothermal modification/formation chromite/chromitite in the mantle, which is a classical but innovative issue. In this context, we present here an urgently needed comprehensive review of podiform chromitites in the upper mantle. Wall-rock control on podiform chromitite genesis demonstrates that the peridotite-magma reaction at the upper mantle condition is an indispensable process. We may need a large system in the mantle, far larger than the size of outcrops or mining areas, to fulfill the Cr budget requirement for podiform chromitite genesis. The peridotite-magma reaction over a large area may form a melt enriched with Na and other incompatible elements, which mixes with a less evolved magma supplied from the depth to create chromite-oversaturated magma. The incompatible-element-rich magma trapped by the chromite mainly precipitates pargasite and aspidolite (Na analogue of phlogopite), which are stable under upper mantle conditions. Moderately depleted harzburgites, which contain chromite with a moderate Cr# (0.4-0.6) and a small amount of clinopyroxene, are the best reactants for the chromitite-forming reaction, and are the best hosts for podiform chromitites. Arc-type chromitites are dominant in ophiolites, but some are of the mid-ocean ridge type; chromitites may be common beneath the ocean floor, although it has not yet been explored for chromitite. The low-pressure (upper mantle) igneous chromitites were conveyed through mantle convection or subduction down to the mantle transition zone to form

  6. Heterogeneity of Water Concentrations in the Mantle Lithosphere Beneath Hawaii (United States)

    Bizimis, M.; Peslier, A. H.; Clague, D.


    The amount and distribution of water in the oceanic mantle lithosphere has implications on its strength and of the role of volatiles during plume/lithosphere interaction. The latter plays a role in the Earth's deep water cycle as water-rich plume lavas could re-enrich an oceanic lithosphere depleted in water at the ridge, and when this heterogeneous lithosphere gets recycled back into the deep mantle. The main host of water in mantle lithologies are nominally anhydrous minerals like olivine, pyroxene and garnet, where hydrogen (H) is incorporated in mineral defects by bonding to structural oxygen. Here, we report water concentrations by Fourier transform infrared spectrometry (FTIR) on olivine, clino- and orthopyroxenes (Cpx & Opx) from spinel peridotites from the Pali vent and garnet pyroxenite xenoliths from Aliamanu vent, both part of the rejuvenated volcanism at Oahu (Hawaii). Pyroxenes from the Aliamanu pyroxenites have high water concentrations, similar to the adjacent Salt Lake Crater (SLC) pyroxenites (Cpx 400-500 ppm H2O, Opx 200 ppm H2O). This confirms that pyroxenite cumulates form water-rich lithologies within the oceanic lithosphere. In contrast, the Pali peridotites have much lower water concentrations than the SLC ones (10% modal Cpx and low spinel Cr# (0.09-0.10). The contrast between the two peridotite suites is also evident in their trace elements and radiogenic isotopes. The Pali Cpx are depleted in light REE, consistent with minimal metasomatism. Those of SLC have enriched light REE patterns and Nd and Hf isotopes consistent with metasomatism by alkaline melts. These observations are consistent with heterogeneous water distribution in the oceanic lithosphere that may be related to metasomatism, as well as relatively dry peridotites cross-cut by narrow (?) water-rich melt reaction zones.

  7. Age of amphibolites associated with alpine peridotites in the Dinaride ophiolite zone, Yugoslavia (United States)

    Lanphere, M.A.; Coleman, R.G.; Karamata, S.; Pamic, J.


    Amphibolites associated with alpine peridotites in the Central Ophiolite zone in Yugoslavia have K-Ar ages of 160-170 m.y. These amphibolites and associated peridotites underwent deep-seated metamorphism prior to tectonic emplacement into the sedimentary-volcanic assemblage of the Dinarides. The alpine peridotites and associated local rocks of the ophiolite suite are interpreted as Jurassic oceanic crust and upper mantle. ?? 1975.

  8. Highly siderophile element systematics of abyssal peridotites from intermediate and fast spreading ridges (United States)

    Brown, D. B.; Day, J. M.; Waters, C. L.


    Abyssal peridotites are residues of both modern and ancient partial melt extraction at oceanic ridges and can be used to examine melting processes and mantle heterogeneity. The highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re, and the 187Re-187Os system embedded within them), are useful for investigating these issues, as they are generally strongly compatible. To date, limited data on HSE and Os isotopes has been obtained on abyssal peridotites from fast spreading centers. Here, we report new HSE abundance and 187Os/188Os data for Pacific Antarctic Ridge (PAR) and East Pacific Rise (EPR) abyssal peridotites. Samples from the PAR were dredged from two separate localities along the Udintsev Fracture Zone, and EPR samples were taken from Hess Deep. The PAR full spreading rate ranges from 54-83mm/year [1,2] and is 75 mm/year [2] at the Udintsev Fracture Zone. These spreading rates characterize the PAR as an intermediate spreading ridge, whereas the fast spreading EPR has a full rate ranging from 128-157 mm/year [3]. The 187Os/188Os ratios for whole-rocks from the PAR range from 0.114 to 0.134, with Re depletion ages (TRD) varying from 1 Ga to present. Despite the large variation in 187Os/188Os, HSE patterns are primitive mantle-like [4], with Ru/Ir ratios ranging from 1.5-2.1. Depletions in Re and Pd are present, as is expected in partial melt residues, and the samples have undergone 4-15% partial melting based on the rare earth elements (REE). The EPR exhibits higher levels of melt depletion ranging from 18-24%. New results show Hess Deep samples have 187Os/188Os ratios of 0.123 and 0.125 for whole-rocks. These findings indicate that PAR and EPR Os isotopic data overlap with the global record of abyssal peridotites from slower ridges and that Os isotopic heterogeneities are preserved across a wide range of spreading rates and degrees of melt extraction. [1] Géli, L., et al. (1997), Science, 278, 1281-1284; [2] Castillo, P.R., et al. (1998) EPSL, 154

  9. Petrology and geochemistry of Abyssal Peridotites from the Manipur Ophiolite Complex, Indo-Myanmar Orogenic Belt, Northeast India: Implication for melt generation in mid-oceanic ridge environment (United States)

    Krishnakanta Singh, A.


    The Manipur Ophiolite Complex (MOC) located in the Indo-Myanmar Orogenic Belt (IMOB) of Northeast India forms a section of the Tethyan Ophiolite Belt of the Alpine-Himalayan orogenic system. Whole rock compositions and mineral chemistry of mantle peridotites from the MOC show an affinity to the abyssal peridotites, characterized by high contents of Al2O3 (1.28-3.30 anhydrous wt.%); low Cr# of Cr-spinel (0.11-0.27); low Mg# of olivine (˜Fo90) and high Al2O3 in pyroxenes (3.71-6.35 wt.%). They have very low REE concentrations (∑REE = 0.48-2.14 ppb). Lherzolites display LREE-depleted patterns (LaN/SmN = 0.14-0.45) with a flat to slightly fractionated HREE segments (SmN/YbN = 0.30-0.65) whereas Cpx-harburgites have flat to upward-inflected LREE patterns (LaN/SmN = 0.13-1.23) with more fractionated HREE patterns (SmN/YbN = 0.13-0.65) than the lherzolite samples. Their platinum group elements (PGE) contents (<50 ppb) and distinct mantle-normalised PGE patterns with the Pd/Ir values (1.8-11.9) and Pt/Pt* values (0.2-1.1) show an affinity to the characteristic of the residual mantle material. Evaluation of mineralogical and petrological characteristics of these peridotites suggests that they represent the residues remaining after low degree of partial melting (˜2-12%) in the spinel stability field of a mid-oceanic ridge environment. The well-preserved mid-oceanic ridge characteristics of these peridotites further suggest that the mantle section was subsequently trapped in the forearc region of the subduction zone without undergoing significant modification in their chemistry by later subduction-related tectonic and petrological processes before its emplacement to the present crustal level.

  10. Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth's upper mantle (United States)

    Wang, Ze-Zhou; Liu, Sheng-Ao; Liu, Jingao; Huang, Jian; Xiao, Yan; Chu, Zhu-Yin; Zhao, Xin-Miao; Tang, Limei


    compositions of MORB. Therefore, preferential melting of spinel in the peridotites may account for the Zn isotopic difference between spinel peridotites and basalts. By contrast, the absence of Zn isotope fractionation between silicate minerals suggests that Zn isotopes are not significantly fractionated during partial melting of spinel-free garnet-facies mantle. If the studied non-metasomatized peridotites represent the refractory upper mantle, mass balance calculation shows that the depleted MORB mantle (DMM) has a δ66Zn value of +0.20 ± 0.05‰ (2SD), which is lighter than the primitive upper mantle (PUM) estimated in previous studies (+0.28 ± 0.05‰, 2SD, Chen et al., 2013b; +0.30 ± 0.07‰, 2SD, Doucet et al., 2016). This indicates that the Earth's upper mantle has a heterogeneous Zn isotopic composition vertically, which is probably due to shallow mantle melting processes.

  11. Cratonic lithospheric mantle: Is anything subducted?

    Institute of Scientific and Technical Information of China (English)

    William L. Griffin; Suzanne Y. O'ReiUy


    @@ If the subcontinental lithospheric mantle (SCLM) formed through the repeated underthrusting of oceanic slabs, peridotitic SCLM should resemble oceanic peridotites, and mafic rocks (eclogites, s.l.) should be distributed throughout the SCLM. However, cratonic peridotites (both exposed massifs and xenoliths) differ markedly from oceanic and ophiolitic peridotites in their Fe-Cr-Al relationships and abundances of trace elements (Li and B) diagnostic of subduction. "Typical"cratonic peridotites have experienced extensive metaso matism; modelling of their refractory protoliths indicates high-degree melting at high P, perhaps a uniquely Archean process.

  12. Fertility of the Mantle beneath the Ocean Basins: Harzburgite, Lherzolite, and Eclogite in Depleted to Enriched Sources of Abyssal Tholeiites, Ocean Islands, and LIPs (United States)

    Natland, J. H.; Anderson, D. L.


    Current models for the origin of MORB and OIB invoke different degrees of partial melting of a homogeneous lherzolitic source, and a heterogeneous deep mantle source, respectively. In the ocean basins, MORBs are only part of a spectrum of geochemically diverse depleted to enriched basalts that erupt at or near ridges, off-axis seamounts and large igneous provinces. Even at ridges, mantle is locally enriched (e.g. E-MORB). The gradation in compositions from MORB to slightly less depleted tholeiites at LIPS, to variably enriched tholeiitic and alkalic basalts, basanites and olivine nephelinites of many ocean islands requires only differences in depth and degree of partial melting of shallow mantle lherzolite upon which trace-element and isotopic heterogeneity are superimposed. Alkalic basalts and differentiates in the oceans occur along nearly every seamount ridge rising >2000 m above the seafloor, a distribution too extensive to be explained by any number of plumes; this makes a plume origin for similar lavas on linear island chains questionable. Tapping along fractures of a shallow asthenospheric layer of variably enriched and fertile mantle that develops beneath the lithosphere through time is more likely. The long-term differentiation of the Earth, magmatism, recycling, continental rifting, and subduction insure that the upper mantle cannot be well mixed and homogeneous, a common but fallacious assumption in much petrogenetic theory. Mantle major-element and isotopic heterogeneity and variable temperature is a consequence of plate tectonics. Every association of ultramafic rocks in the ocean crust, ophiolites, and xenolith suites demonstrates significant bulk heterogeneity that survives partial melting. Thus sources of modern abyssal tholeiites must be variably fertile with respect to a basaltic melt fraction, and range from average harzburgite to fertile lherzolite, on both local and regional scales. In addition, subduction guarantees that most abyssal basalt

  13. Significance of the whole rock Re-Os ages in cryptically and modally metasomatised cratonic peridotites: Constraints from HSE-Se-Te systematics (United States)

    Luguet, Ambre; Behrens, Melanie; Pearson, D. Graham; König, Stephan; Herwartz, Daniel


    The Re-Os isotopic system is the geochronometer of choice to constrain the timing of lithospheric mantle root formation and reconstruct the evolution of Earth's dynamics from the "mantle" perspective. In order to constrain the effects of metasomatic processes on the Re-Os isotopic system, eleven peridotites from the Letlhakane kimberlite pipe were investigated for whole rock major and trace elements, highly siderophile elements (HSE), Se, Te and 187Os/188Os signatures. These spinel peridotites (SP), garnet peridotites (GP), garnet-phlogopite peridotites (GPP) and phlogopite peridotites (PP) experienced cryptic metasomatism and the GP-GPP-PP additionally constitute a sequence of increasing modal metasomatism. The cryptically metasomatised SP appear devoid of base metal sulphides (BMS) and show suprachondritic Se/Te ratios (15-40) and extremely Pd- and Pt-depleted HSE patterns. These features are characteristic of high-degree partial melting residues. Their 187Os/188Os signatures are thus considered to be inherited from the partial melting event. This implies a Neoarchean (2.5-2.8 Ga, TRD eruption) stabilisation of the Letlhakane mantle root and supports the Letlhakane mantle root being a westerly extension of the Zimbabwe cratonic root. The modally metasomatised peridotites contain BMS whose abundance significantly increases from the GPP to the GP and PP. The BMS-poor GPP are only slightly richer in Pt and Pd than the BMS-free SP but have similarly high Se/Te ratios. The BMS-rich GP and PP exhibit significant enrichments in Pt, Pd, Se, Te resulting in HSE-Se-Te signatures similar to that of the Primitive Upper Mantle (PUM). Addition of 0.001-0.05 wt.% metasomatic BMS ± PGM (platinum group minerals, i.e., Pt-tellurides) to highly refractory residues, such as the Letlhakane SP, reproduce well the HSE-Se-Te systematics observed in the BMS-poor and BMS-rich modally metasomatised peridotites. In the GPP, the negligible addition of metasomatic BMS ± PGM did not disturb

  14. Peridotites of the Izu-Bonin-Mariana forearc and the Eastern Mirdita ophiolite (Albania) : implications for igneous activity during subduction initiation (United States)

    Morishita, T.; Tani, K.; Dilek, Y.


    There have been few studies of the mantle evolution related to igneous activity in the earliest stages of subduction initiation. We examined peridotites recovered from an exhumed crust/mantle section exposed along the landward slopes of the northern Izu-Bonin Trench (Morishita et al., Geology, 2011) and peridotite bodies in the Eastern Mirdita ophiolite, Albania (Morishita et al., Lithos, 2011). Based on the Cr# (=Cr/(Cr+Al) atomic ratio) of spinel in the IBM, two distinctive groups, (1) High-Cr# (> 0.8) dunite and (2) Medium-Cr# (0.4-0.6) dunite, occur close to each other and are associated with refractory harzburgite. Two distinctive melts were in equilibrium with these dunites. In the case of the Eastern Mirdita ophiolite, cpx porphyroclast-bearing harzburgite (Cpx-harzburgite) occurs structurally in the lower parts of the peridotite massifs, whereas harzburgite and dunite are more abundant towards the upper parts. The Cpx-harzburgite was formed as the residue of less-flux partial melting, which are similar to those in abyssal peridotites from MOR systems. On the other hand, harzburgite was produced as a result of enhanced partial melting of depleted peridotites due to infiltration of hydrous LREE-enriched fluids/melts. The wide range of variation in dunites from the IBM forearc and the uppermost section of the EMO probably reflects changing melt compositions from MORB-like melts to boninitic melts in the forearc setting due to an increase of slab-derived hydrous fluids/melts during subduction initiation. This scenario is consistent with the temporal and spatial variation of volcanic rocks in the Izu-Bonin-Mariana (IBM) arc (Reagan et al., G-cubed, 2010). If the "MORB-like" FAB is a ubiquitous phenomenon during the initiation of subduction, we should reconsider our interpretation of the ophiolites.

  15. An integrated petrological, geochemical and Re-Os isotope study of peridotite xenoliths from the Argyle lamproite, Western Australia and implications for cratonic diamond occurrences (United States)

    Luguet, A.; Jaques, A. L.; Pearson, D. G.; Smith, C. B.; Bulanova, G. P.; Roffey, S. L.; Rayner, M. J.; Lorand, J.-P.


    An integrated study of the petrology and Re-Os geochemistry of a suite of peridotite xenoliths, some carrying abundant diamonds, from the richly diamondiferous Argyle AK1 lamproite pipe provides definitive evidence for a depleted lithospheric root of Neoarchean age (T RD eruption ˜ 2.2-3.1 Ga) beneath the Proterozoic Halls Creek Orogen at the margin of the Kimberley Craton, Western Australia. The microdiamonds from the peridotitic xenoliths are similar in their properties to the minor population of small, commercial sized, peridotitic diamonds from Argyle, both formed in the Archean from isotopically mantle-like carbon. The major element bulk chemistry and mineral chemistry of the Argyle peridotites are slightly less depleted than Archean cratonic peridotites as a whole but similar to those reported from Neoarchean-Paleoproterozoic cratonic provinces. The Argyle peridotite xenoliths were derived from within the diamond stability field (1050-1300 °C and 4.9-5.9 GPa) near the base of the lithosphere (typically 160-200 km depth) with a geothermal gradient of 41.5 mW/m 2. This thick diamondiferous lithosphere, estimated at up to 225 km thick from present day seismic S-wave tomography, appears to have persisted since the time of eruption of the Argyle lamproite (˜ 1180 Ma). The existence of late Archean age lithosphere beneath the Argyle diamond pipe, in a region where no crustal rocks of Archean age are known, suggests a decoupling of the crust and mantle in the region of the Halls Creek Orogen, perhaps as a consequence of Paleoproterozoic (˜ 1.85 Ga) reworking and/or subduction at the margin of the Kimberley Craton. The confirmation of an Archean lithospheric root beneath the Argyle pipe at the margin of the Kimberley Craton seemingly conforms with "Clifford's Rule", regarding the restriction of economic diamond deposits to those underlain by Archean cratons. However, Argyle owes its rich diamond grades not to its Neoarchean mantle roots but to the presence of

  16. Mantle hydrocarbons: abiotic or biotic? (United States)

    Sugisaki, R; Mimura, K


    Analyses of 227 rocks from fifty localities throughout the world showed that mantle derived rocks such as tectonized peridotites in ophiolite sequences (tectonites) arid peridotite xenoliths in alkali basalts contain heavier hydrocarbons (n-alkanes), whereas igneous rocks produced by magmas such as gabbro arid granite lack them. The occurrence of hydrocarbons indicates that they were not derived either from laboratory contamination or from held contamination; these compounds found in the mantle-derived rocks are called here "mantle hydrocarbons." The existence of hydrocarbons correlates with petrogenesis. For example, peridotite cumulates produced by magmatic differentiation lack hydrocarbons whereas peridotite xenoliths derived from the mantle contain them. Gas chromatographic-mass spectrometric records of the mantle hydrocarbons resemble those of aliphatics in meteorites and in petroleum. Features of the hydrocarbons are that (a) the mantle hydrocarbons reside mainly along grain boundaries and in fluid inclusions of minerals; (b) heavier isoprenoids such as pristane and phytane are present; and (c) delta 13C of the mantle hydrocarbons is uniform (about -27%). Possible origins for the mantle hydrocarbons are as follows. (1) They were in organically synthesized by Fischer-Tropsch type reaction in the mantle. (2) They were delivered by meteorites and comets to the early Earth. (3) They were recycled by subduction. The mantle hydrocarbons in the cases of (1) and (2) are abiogenic and those in (3) are mainly biogenic. It appears that hydrocarbons may survive high pressures and temperatures in the mantle, but they are decomposed into lighter hydrocarbon gases such as CH4 at lower pressures when magmas intrude into the crust; consequently, peridotite cumulates do not contain heavier hydrocarbons but possess hydrocarbon gases up to C4H10.

  17. Mineralogy of the mid-ocean-ridge basalt source from neodymium isotopic composition of abyssal peridotites. (United States)

    Salters, Vincent J M; Dick, Henry J B


    Inferring the melting process at mid-ocean ridges, and the physical conditions under which melting takes place, usually relies on the assumption of compositional similarity between all mid-ocean-ridge basalt sources. Models of mantle melting therefore tend to be restricted to those that consider the presence of only one lithology in the mantle, peridotite. Evidence from xenoliths and peridotite massifs show that after peridotite, pyroxenite and eclogite are the most abundant rock types in the mantle. But at mid-ocean ridges, where most of the melting takes place, and in ophiolites, pyroxenite is rarely found. Here we present neodymium isotopic compositions of abyssal peridotites to investigate whether peridotite can indeed be the sole source for mid-ocean-ridge basalts. By comparing the isotopic compositions of basalts and peridotites at two segments of the southwest Indian ridge, we show that a component other than peridotite is required to explain the low end of the (143)Nd/(144)Nd variations of the basalts. This component is likely to have a lower melting temperature than peridotite, such as pyroxenite or eclogite, which could explain why it is not observed at mid-ocean ridges.

  18. Lithosphere destabilization by melt percolation during pre-oceanic rifting: Evidence from Alpine-Apennine ophiolitic peridotites (United States)

    Piccardo, Giovanni; Ranalli, Giorgio


    Orogenic peridotites from Alpine-Apennine ophiolite Massifs (Lanzo, Voltri, External and Internal Ligurides, - NW Italy, and Mt. Maggiore - Corsica) derive from the mantle lithosphere of the Ligurian Tethys. Field/structural and petrologic/geochemical studies provide constraints on the evolution of the lithospheric mantle during pre-oceanic passive rifting of the late Jurassic Ligurian Tethys ocean. Continental rifting by far-field tectonic forces induced extension of the lithosphere by means of km-scale extensional shear zones that developed before infiltration of melts from the asthenosphere (Piccardo and Vissers, 2007). After significant thinning of the lithosphere, the passively upwelling asthenosphere underwent spinel-facies decompression melting along the axial zone of the extensional system. Silica-undersaturated melt fractions percolated through the lithospheric mantle via diffuse/focused porous flow and interacted with the host peridotite through pyroxenes-dissolving/olivine-precipitating melt/rock reactions. Pyroxene dissolution and olivine precipitation modified the composition of the primary silica-undersaturated melts into derivative silica-saturated melts, while the host lithospheric spinel lherzolites were transformed into pyroxene-depleted/olivine-enriched reactive spinel harzburgites and dunites. The derivative liquids interacted through olivine-dissolving/orthopyroxene+plagioclase-crystallizing reactions with the host peridotites that were impregnated and refertilized (Piccardo et al., 2015). The saturated melts stagnated and crystallized in the shallow mantle lithosphere (as testified by diffuse interstitial crystallization of euhedral orthopyroxene and anhedral plagioclase) and locally ponded, forming orthopyroxene-rich/olivine-free gabbro-norite pods (Piccardo and Guarnieri, 2011). Reactive and impregnated peridotites are characterized by high equilibration temperatures (up to 1250 °C) even at low pressure, plagioclase-peridotite facies

  19. Compositions and processes of lithospheric mantle beneath the west Cathaysia block, southeast China (United States)

    Zhang, Hui; Zheng, Jianping; Pan, Shaokui; Lu, Jianggu; Li, Yihe; Xiang, Lu; Lin, Abing


    Knowledge about the nature and history of the lithospheric mantle beneath the west Cathaysia block (South China) is still sparse. The major- and trace-element compositions and H2O contents of minerals from peridotite xenoliths entrained in the Cenozoic lamprophyres of the Anyuan area (SE China), were conducted to investigate the nature and evolution of the lithospheric mantle, as well as the factors controlling the distribution of water. The xenoliths including spinel harzburgites and lherzolites are moderately refractory (Mg# Olivine = 90.2-91.2) with minor fertile lherzolites (Mg# Olivine = 89.1-89.9). Clinopyroxenes in lherzolites show variable REE patterns from LREE-depleted to LREE-enriched patterns, and commonly exhibit negative anomalies of U, Pb and Ti. The mantle represented by the xenoliths mostly experienced 1-10% partial melting and obvious subsequent silicate metasomatism. H2O contents of clinopyroxene, orthopyroxene, and olivine in the peridotites are 320-404 ppm, 138-200 ppm, and 11-33 ppm, respectively. The whole-rock H2O contents range from 63 to 120 ppm, similar to that estimated for the MORB source. The negative correlations of H2O contents with Mg# in olivine and lack of correlation correlations with (La/Yb)N in clinopyroxene suggest that the H2O contents are mainly controlled by the partial melting process rather than mantle metasomatism. The fertile and moderately refractory peridotite xenoliths have similar equilibrium temperatures, implying that the lithospheric mantle is not compositionally stratified. Integrated with published data, we suggest that the widespread fertile and moderately refractory lithospheric mantle beneath the studied area (west Cathaysia), even the whole South China, might be eroded or melt-rock reacted by upwelling asthenospheric materials. Finally, the cooling of the upwelled asthenospheric materials resulted in the formation of the accreted lithospheric mantle mixed with pre-existing moderately refractory volumes.

  20. Coupled extremely light Ca and Fe isotopes in peridotites (United States)

    Zhao, Xinmiao; Zhang, Zhaofeng; Huang, Shichun; Liu, Yufei; Li, Xin; Zhang, Hongfu


    Large metal stable isotopic variations have been observed in both extraterrestrial and terrestrial samples. For example, Ca exhibits large mass-dependent isotopic variation in terrestrial igneous rocks and mantle minerals (on the order of ∼2‰ variation in 44Ca/40Ca). A thorough assessment and understanding of such isotopic variations in peridotites provides important constraints on the evolution and compositon of the Earth's mantle. In order to better understand the Ca and Fe isotopic variations in terrestrial silicate rocks, we report Ca isotopic compositions in a set of peridotitic xenoliths from North China Craton (NCC), which have been studied for Fe isotopes. These NCC peridotites have large Ca and Fe isotopic variations, with δ44/40Ca ranging from -0.08 to 0.92 (delta value relative to SRM915a) and δ57/54Fe (delta value relative to IRMM-014) ranging from -0.61 to 0.16, and these isotopic variations are correlated with large Mg# (100 × Mg/(Mg + Fe) molar ratio) variation, ranging from 80 to 90. Importantly, NCC Fe-rich peridotites have the lowest 44Ca/40Ca and 57Fe/54Fe ratios in all terrestrial silicate rocks. In contrast, although ureilites, mantle rocks from a now broken differentiated asteroid(s), have large Mg# variation, from 70 to 92, they have very limited δ57Fe/54Fe variation (0.03-0.21, delta value relative to IRMM-014). Our model calculations show that the coupled extremely light Ca-Fe isotopic signatures in NCC Fe-rich peridotites most likely reflect kinetic isotopic fractionation during melt-peridotite reaction on a timescale of several to 104 years. In addition, our new data and compiled literature data show a possible compositional effect on the inter-mineral Ca isotopic fractionation between co-existing clinopyroxene and orthopyroxene pairs.

  1. Effects of mother lode-type gold mineralization on 187Os/188Os and platinum group element concentrations in peridotite: Alleghany District, California (United States)

    Walker, R.J.; Böhlke, J.K.; McDonough, W.F.; Li, J.


    Osmium isotope compositions and concentrations of Re, platinum group elements (PGE), and Au were determined for host peridotites (serpentinites and barzburgites) and hydrothermally altered ultramafic wall rocks associated with Mother Lode-type hydrothermal gold-quartz vein mineralization in the Alleghany district, California. The host peridotites have Os isotope compositions and Re, PGE, and Au abundances typical of the upper mantle at their presumed formation age during the late Proterozoic or early Paleozoic. The hydrothermally altered rocks have highly variable initial Os isotope compositions with ??os, values (% deviation of 187OS/188OS from the chondritic average calculated for the approx. 120 Ma time of mineralization) ranging from -1.4 to -8.3. The lowest Os isotope compositions are consistent with Re depletion of a chondritic source (e.g., the upper mantle) at ca. 1.6 Ga. Most of the altered samples are enriched in Au and have depleted and fractionated abundances of Re and PGE relative to their precursor peridotites. Geoehemical characteristics of the altered samples suggest that Re and some PGE were variably removed from the ultramafic rocks during the mineralization event. In addition to Re, the Pt and Pd abundances of the most intensely altered rocks appear to have been most affected by mineralization. The 187Os-depleted isotopic compositions of some altered rocks are interpreted to be a result of preferential 187Os loss via destruction of Re-rich phases during the event. For these rocks, Os evidently is not a useful tracer of the mineralizing fluids. The results do, however, provide evidence for differential mobility of these elements, and mobility of 187Os relative to the initial bulk Os isotope composition during hydrothermal metasomatic alteration of ultramafic rocks. ?? 2007 Society of Economic Geologists, Inc.

  2. Constraints From the Rock Record, and Complementary Speculation, on Subduction and the Evolution of the Mantle (United States)

    Kelemen, P.


    As time permits, I will cover some of the following: (1) What crustal components are subducted for long term recycling? (a) Revisiting an old approach, relatively new constraints on the "subduction component" in arc magmas can be derived from comparison of primitive MORB with a compilation of primitive arc lavas (Kelemen et al., ToG 03). These provide quantitative estimates for the composition of the "arc residue" in subducting oceanic crust and sediment. (b) It may be that substantial recycling in subduction settings is from the hanging wall, via subdution erosion (von Huene & Scholl, Rev Geophys 91) and "delamination" (Herzberg et al CMP 83; Kay & Kay GCA 88; Arndt & Goldstein T'phys 89). (c) Subducting sediment may migrate into the mantle wedge via buoyancy (Kelemen et al., ToG 03; Gerya & Yuen EPSL 03). (d) New ICPMS data are available on trace element characteristics of arc (Kelemen et al., ToG 03; Greene et al J Pet 06) and oceanic lower crust (not published, sorry!). I will illustrate evolution of radiogenic parent-daughter ratios as constrained by these data. (2) Where do residual peridotites go during subduction? I see three interesting possibilities. (a) Highly depleted cratonic mantle peridotites formed as relatively shallow residues (Bernstein et al EPSL 98), were carried to greater depth to form metamorphic garnet (Kelemen et al EPSL 98), and then were imbricated or rose buoyantly to become a long-lasting part of the cratonic mantle (Oxburgh & Parmentier, JGSL 77), where they have been affected by Arc(hean) processes (Kelemen et al EPSL 98). High light REE contents as well as measured high H2O in cratonic mantle indicate that it is not dry and viscous, so its long term stability is not well understood. There is insufficient work on how compositional buoyancy of highly depleted residues affects cratonic mantle stability. (b) Perhaps some cold, dry residues remain for long periods in the lower mantle. Trace element data suggest that some ubiquitous

  3. Mantle Xenoliths of Cerro Mercedes, Costa Rica, Central America (United States)

    Lindsay, F. N.; Carr, M. J.; Herzberg, C. T.; Feigenson, M. D.


    Mantle peridotite occurs as xenoliths in lavas and bombs at Cerro Mercedes, a Plio-Quaternary potassic alkaline basalt volcano approximately 70 km behind the volcanic front of northern Costa Rica (Tournon and Alvarado, 1997). Mineral exploration led to the first discovery of abundant mantle xenoliths in Central America (Vargas and Alfaro, 1992). The compositions of 71 xenoliths recovered in January 2003 include dunite, harzburgite, lherzolite and olivine websterite. Twenty xenoliths have a diameter of at least 3 cm. The nodules are abundant in basalt outcrops and the rare bombs. In spite of substantial soil development in a rain forest environment, both xenoliths and host lava remain well preserved. Olivine, pyroxenes and spinel are common, plagioclase is present and garnet appears to be absent. There is no obvious shearing or deformation and several pyroxenes are as much as 1 cm in diameter. The mineralogy suggests a relatively shallow upper mantle source, within either the lithosphere or possibly the uppermost asthenosphere. Cerro Mercedes, at latitude 10° 58' N and longitude 82° 21' W, lies along the Rio San Juan, which is locally the border between Nicaragua and Costa Rica, Central America. This location approximately coincides with a boundary between dominantly depleted mantle to the northwest and OIB or Galapagos-like mantle to the southeast. We will use mineralogical data to better define the likely depths and oxidation states of representative nodules and isotopic data to define the type of mantle source.

  4. Water contents in pyroxenes of intraplate lithospheric mantle (United States)

    Bonadiman, C.; Hao, Y.-T.; Coltorti, M.; Dallai, L.; Faccini, B.; Hu, H.; Qunke, X.


    Water contents of clinopyroxene and orthopyroxene in mantle peridotites from various xenolith occurrences in intraplate settings (both oceanic and continental) were determined by Fourier Transform Infrared Spectroscopy (FTIR). The localities are as follow: Sal Island (Cape Verde Archipelago); Baker Rocks and Greene Point (Northern Victoria Land, Antarctica); Panshishan and Lianshan (Subei Basin, Eastern China). They represent well-known localities where detailed petrographical and geochemical studies have already been carried out or areas which are currently under investigation. The water incorporated in these pyroxenes is low (cpx, 37-399ppm; opx: 9-166ppm)(or very low as in Greene Point, Antarctica; cpx, 5-16ppm; opx: 9-16ppm) and, among each population, no clear correlation with melting parameters (MgO contents) in single mineral is evident. Results are compared with the available literature data on water contents in mantle pyroxene which includes peridotites from on-craton (hosted by kimberlitic-type magmas) and off-craton (hosted by alkaline basic magmas), as well as subarc mantle settings. The "relatively dry" (cpx: 140-528 ppm; opx: 38-280 ppm) sub-arc mantle xenoliths (Peslier et al., 2002) are shown to be wetter than the intraplate (off-craton) xenoliths. Cratonic mantle pyroxenes are only represented by a few determinations on garnet peridotites and eclogite from Kaapvaal and Colorado Plateau. They record the highest water contents (cpx: 342-1012 ppm; opx: 180-491 ppm) so far measured in mantle pyroxenes from various tectonic settings. Despite the limited data set, the indication that the cratonic mantle is strongly hydrated is compelling. Rehydration for the Colorado Plateau craton may be due to the Farallon plate subduction (Li et al., 2008), while for Kaapvaal Craton it might be related to young (<100Ma) metasomatic enrichments (Griffin et al., 2003a; Kobussen et al., 2008). If this is the case then the Archean mantle water content needs to be

  5. Geochemical characteristics of aluminum depleted and undepleted komatiites and HREE-enriched low-Ti tholeiites, western Abitibi greenstone belt: A heterogeneous mantle plume-convergent margin environment (United States)

    Fan, J.; Kerrich, R.


    A compositionally diverse suite of komatiites, komatiitic basalts, and basalts coexist in the Tisdale volcanic assemblage of the late-Archean (˜2.7 Ga) Abitibi greenstone belt. The komatiites are characterized by a spectrum of REE patterns, from low total REE contents (9 ppm) and pronounced convex-up patterns to greater total REE (18 ppm) and approximately flat-distributions. Thorium and niobium are codepleted with LREE. Komatiites with the most convex-up patterns have low Al 2O 3 (4.7 wt%) contents and Al 2O 3/TiO 2(12) ratios; they are interpreted to be the Al-depleted variety of komatiite derived from a depleted mantle source. Those komatiites and komatiitic basalts with flatter REE patterns are characterized by greater Al 2O 3 (7.0 wt%) and near chondritic Al 2O 3/TiO 2 (20) ratios; they are interpreted to be Al-undepleted komatiites generated from trace element undepleted mantle. For the komatiites and komatiitic basalts collectively, Gd/Ybn ratios are negatively correlated with La/Smn, but positively with MgO and Ni. The spectrum of patterns is interpreted as mixing between Al, HREE, Y-depleted, and Sc-depleted komatiites and Al-undepleted komatiites in a heterogeneous mantle plume. Auminum-depleted komatiites are characterized by negative Zr and Hf anomalies, consistent with majorite garnet-liquid D's for HFSE and REEs, signifying melt segregation at depths of >400 km. Tisdale Al-undepleted komatiites and komatiitic basalts have small negative to zero Zr(Hf)/MREE fractionation, signifying melt segregation in or above the garnet stability field. Collectively, the komatiites have correlations of Zr/Zr∗ and Hf/Hf ∗ with Gd/Ybn, and hence the Zr(Hf)/MREE fractionations are unlikely to have stemmed from alteration or crustal contamination. Two types of basalts are present. Type I basalts are Mg-tholeiites with near flat REE and primitive mantle normalized patterns, compositionally similar to abundant Mg-tholeiites associated with both Al-undepleted and Al-depleted

  6. Ultramafic xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for multiple metasomatic events in the lithospheric mantle beneath the Wyoming craton (United States)

    Downes, H.; Macdonald, R.; Upton, B.G.J.; Cox, K.G.; Bodinier, J.-L.; Mason, P.R.D.; James, D.; Hill, P.G.; Hearn, B.C.


    Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountains volcanic field (Montana, USA), derived from the lower lithosphere of the Wyoming craton, can be divided based on textural criteria into tectonite and cumulate groups. The tectonites consist of strongly depleted spinel lherzolites, harzbugites and dunites. Although their mineralogical compositions are generally similar to those of spinel peridotites in off-craton settings, some contain pyroxenes and spinels that have unusually low Al2O3 contents more akin to those found in cratonic spinel peridotites. Furthermore, the tectonite peridotites have whole-rock major element compositions that tend to be significantly more depleted than non-cratonic mantle spinel peridotites (high MgO, low CaO, Al2O3 and TiO2) and resemble those of cratonic mantle. These compositions could have been generated by up to 30% partial melting of an undepleted mantle source. Petrographic evidence suggests that the mantle beneath the Wyoming craton was re-enriched in three ways: (1) by silicate melts that formed mica websterite and clinopyroxenite veins; (2) by growth of phlogopite from K-rich hydrous fluids; (3) by interaction with aqueous fluids to form orthopyroxene porphyroblasts and orthopyroxenite veins. In contrast to their depleted major element compositions, the tectonite peridotites are mostly light rare earth element (LREE)-enriched and show enrichment in fluid-mobile elements such as Cs, Rb, U and Pb on mantle-normalized diagrams. Lack of enrichment in high field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf) suggests that the tectonite peridotites have been metasomatized by a subduction-related fluid. Clinopyroxenes from the tectonite peridotites have distinct U-shaped REE patterns with strong LREE enrichment. They have 143Nd/144Nd values that range from 0??5121 (close to the host minette values) to 0??5107, similar to those of xenoliths from the nearby Highwood Mountains. Foliated mica websterites also have low 143Nd

  7. Contamination of the Convecting Mantle in Eastern Tethyan 'Subduction Factories' (United States)

    Flower, M. F.; Nguyen, T. H.


    As subduction gives way to collision at the end of a Wilson Cycle the associated magmatic activity becomes increasingly enriched in potassium and other large-ion lithophile elements. This is usually attributed to the addition of continental crust-derived material to the convecting mantle wedge. Corresponding depletions in high-field strength elements (Ti and Nb) are more commonly explained in terms of accessory phase buffering or protracted reaction of melts with mantle wallrock. It is increasingly apparent that mantle wedge magmatic sources range from 'fertile' (lherzolitic) to 'refractory' (harzburgitic) although the extent to which this corresponds to the LILE and HFSE variation is unclear. Mantle wedge mass balances clearly hold clues to enrichment-depletion histories of the convecting asthenosphere with respect to both the overriding and subducting plates. With a view to better understanding these effects we have used the MELTS algorithm to calculate hypothetical partial melt compositions as a function of source fertility and H2O content, in the pressure range, 0-1.0 GPa as a basis comparison for natural partial melts. Primitive magmas characterizing the Mariana (western Pacific) and Sunda-Banda (Indonesia) arcs, and the northeastern syntaxis of the India-Asia collision suture (Yunnan) appear to resemble calculated equilibrium melts of refractory (basalt-depleted) peridotite, variably enriched in lithophile and light rare earth elements. These comparisons lead to three observations. 1) HFSE and Fe abundances in primitive MORB, calcalkaline, and boninite magmas, and their respective high-potassium variants are consistent with those implied by phase equilibria associated with partial melting and fractionation, suggesting accessory phases, wall-rock reaction, and slab contamination are probably not important as causes of HFSE depletions. 2) Magmatic sources at convergent and colliding margins are typically refractory (basalt-depleted) compared to those yielding

  8. Water-rock interactions, orthopyroxene growth, and Si-enrichment in the mantle: evidence in xenoliths from the Colorado Plateau, southwestern United States (United States)

    Smith, Douglas; Alexis Riter, J. C.; Mertzman, Stanley A.


    Water-rock interactions and consequent orthopyroxene growth are documented by mantle xenoliths from opposite margins of the Colorado Plateau province. The interactions are inferred from a distinctive texture plus composition of orthopyroxene in spinel peridotite, in which porphyroblasts of orthopyroxene with inclusions of resorbed olivine are zoned to interiors exceptionally low in Al 2O 3 (Bandera Crater, New Mexico, in the southeastern margin of the Colorado Plateau, extending the known distribution from the northwestern margin (Grand Canyon field). Evidence that Si-enrichment locally accompanied movement of aqueous fluid is provided by an orthopyroxenite xenolith that is about 95% enstatite. The enstatite occurs in curved laths to 25 mm long, and the texture and composition (Al 2O 3 1 to 2 wt%, Mg/(Mg + Fe) 0.92) are attributed to growth during subsolidus interaction between peridotite and hydrous fluid. Modal orthopyroxene calculated from 4 bulk rock analyses of peridotite xenoliths from the Grand Canyon field ranges from 26 to 29%, more than in comparably depleted oceanic mantle. The mantle root of the Colorado Plateau may have formed from accreted ocean lithosphere and subsequently been enriched in Si by aqueous metasomatism at widely distributed sites. Similar fluid-rock interaction may have contributed to the orthopyroxene-enrichment characteristic of some mantle xenoliths from roots of Archaean cratons.

  9. Lithospheric Mantle heterogeneities beneath northern Santa Cruz province, Argentina (United States)

    Mundl, Andrea; Ntaflos, Theodoros; Bjerg, Ernesto


    Mantle xenoliths from Don Camilo, an area located on the North margin of the Deseado Masiff in Patagonia, comprise spinel bearing lherzolites, harburgites and dunites, wehrlites, clinopyroxenites and gabbros. The most common rock type in our collection is spinel-lherzolite followed by dunites. Harzurgites, wehrlites and gabbros are less widespread. Spinel-lherzolites and harzburgites have protogranular textures whereas dunites have equigranular to equigranular tabular textures. There are two kinds of dunites: mantle dunites and cumulate dunites. The olivine mg# in the mantle dunites vary within a narrow range, from 90.5 to 91.5 and the NiO content from 0.39 to 0.42 wt%, whereas in the cumulate dunites the mg# ranges from 87 to 90.5 and the NiO content from 0.22 to 0.40 wt%. Both types of dunites contain fine grained interstitial diopside. Hydrous phases, besides one sample that contains amphibole, were so far not found. The spinel peridotites have whole rock REE abundances depleted in LREE [(La/Yb)N=0.34-0.85)] and the dunitesare LREE enriched [(La/Yb)N=3.49]. LA-ICP-MS analyses of cpx show that a number of the studied spinel peridotite xenoliths experienced cryptic metasomatism. Three groups of xenoliths have been recognized according to REE and other incompatible trace element patterns in cpx: group I has depleted LREE abundances, group II is highly enriched in LREE (La=20-30 x C1) and group 3 has moderate LREE enrichments. The core of some clinopyroxenes in group II has depleted LREE similar to those in group I, apparently representing relictic cores not affected by metasomatism. In addition the metasomatized clinopyroxenes are significantly enriched in Sr, Th and U. Evidently, the metasomatic agent was a H2O-rich fluid (high LREE, Sr, Th and U). Clinopyroxene Sr and Nd isotopic ratios vary largely from 0.702671 to 0.705788 and from 0.51229 to 0.513251 respectively. Mantle and cumulate dunites have experienced modal metasomatism. In both types of dunites the

  10. Iron isotope tracing of mantle heterogeneity within the source regions of oceanic basalts (United States)

    Williams, Helen M.; Bizimis, Michael


    Mineralogical variations in the Earth's mantle and the relative proportions of peridotitic versus enriched and potentially crustally-derived pyroxenitic domains within the mantle have important implications for mantle dynamics, magma generation, and the recycling of surface material back into the mantle. Here we present iron (Fe) stable isotope data (δ57Fe, deviation in 57Fe/54Fe from the IRMM-014 standard in parts per thousand) for peridotite and garnet-pyroxenite xenoliths from Oahu, Hawaii and explore Fe isotopes as tracer of both peridotitic and pyroxenitic components in the source regions of oceanic basalts. The pyroxenites have δ57Fe values that are heavy (0.10 to 0.27‰) relative to values for mid-ocean ridge and ocean island basalts (MORB; OIB; δFe57∼0.16‰) and the primitive mantle (PM; δFe57∼0.04‰). Pyroxenite δ57Fe values are positively correlated with bulk pyroxenite titanium and heavy rare earth element (REE) abundances, which can be interpreted in terms of stable isotope fractionation during magmatic differentiation and pyroxene cumulate formation. In contrast, the peridotites have light δ57Fe values (-0.34 to 0.14‰) that correlate negatively with degree of melt depletion and radiogenic hafnium isotopes, with the most depleted samples possessing the most radiogenic Hf isotope compositions and lightest δ57Fe values. While these correlations are broadly consistent with a scenario of Fe isotope fractionation during partial melting, where isotopically heavy Fe is extracted into the melt phase, leaving behind low-δ57Fe peridotite residues, the extent of isotopic variation is far greater than predicted by partial melting models. One possibility is derivation of the samples from a heterogeneous source containing both light-δ57Fe (relative to PM) and heavy-δ57Fe components. While pyroxenite is a viable explanation for the heavy-δ57Fe component, the origin of the depleted light-δ57Fe component is more difficult to explain, as melting

  11. Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone (United States)

    Shervais, John W.; Jean, Marlon M.


    Enrichment of the mantle wedge above subduction zones with fluid mobile elements is thought to represent a fundamental process in the origin of arc magmas. This "subduction factory" is typically modeled as a mass balance of inputs (from the subducted slab) and outputs (arc volcanics). We present here a new method to model fluid mobile elements, based on the composition of peridotites associated with supra-subduction ophiolites, which form by melt extraction and fluid enrichment in the mantle wedge above nascent subduction zones. The Coast Range ophiolite (CRO), California, is a Jurassic supra-subduction zone ophiolite that preserves mantle lithologies formed in response to hydrous melting. We use high-precision laser ablation ICP-MS analyses of relic pyroxenes from these peridotites to document fluid-mobile element (FME) concentrations, along with a suite of non-fluid mobile elements that includes rare earth and high-field strength elements. In the CRO, fluid-mobile elements are enriched by factors of up to 100× DMM, whereas fluid immobile elements are progressively depleted by melt extraction. The high concentrations of fluid mobile elements in supra-subduction peridotite pyroxene can be attributed to a flux of aqueous fluid or fluid-rich melt phase derived from the subducting slab. To model this enrichment, we derive a new algorithm that calculates the concentration of fluid mobile elements added to the source: C=[C/[[D/(D-PF)]∗[1-(PF/D)

  12. Chemical interactions in the subduction factory: New insights from an in situ trace element and hydrogen study of the Ichinomegata and Oki-Dogo mantle xenoliths (Japan) (United States)

    Satsukawa, Takako; Godard, Marguerite; Demouchy, Sylvie; Michibayashi, Katsuyoshi; Ildefonse, Benoit


    The uppermost mantle in back arc regions is the site of complex interactions between partial melting, melt percolation, and fluid migration. To constrain these interactions and evaluate their consequences on geochemical cycles, we carried out an in situ trace element and water study of a suite of spinel peridotite xenoliths from two regions of the Japan back arc system, Ichinomegata (NE Japan) and Oki-Dogo (SW Japan), using LA-ICPMS and FTIR spectrometry, respectively. This study provides the first full dataset of trace element and hydrogen compositions in peridotites including analyses of all their main constitutive silicate minerals: olivine, orthopyroxene and clinopyroxene. The Ichinomegata peridotites sample a LREE-depleted refractory mantle (Mg# olivine = 0.90; Cr# spinel = 0.07-0.23; Yb clinopyroxene = 7.8-13.3 × C1-chondrite, and La/Yb clinopyroxene = 0.003-0.086 × C1-chondrite), characterized by Th-U positive anomalies and constant values of Nb/Ta. The composition of the studied Ichinomegata samples is consistent with that of an oceanic mantle lithosphere affected by cryptic metasomatic interactions with hydrous/aqueous fluids (crypto-hydrous metasomatism). In contrast, the Oki-Dogo peridotites have low Mg# olivine (0.86-0.93) and a broad range of compositions with clinopyroxene showing ;spoon-shaped; to flat, and LREE-enriched patterns. They are also characterized by their homogeneous compositions in the most incompatible LILE (e.g., Rb clinopyroxene = 0.01-0.05 × primitive mantle) and HFSE (e.g., Nb clinopyroxene = 0.01-2.16 × primitive mantle). This characteristic is interpreted as resulting from various degrees of melting and extensive melt-rock interactions. FTIR spectroscopy shows that olivine in both Ichinomegata and Oki-Dogo samples has low water contents ranging from 2 to 7 ppm wt. H2O. In contrast, the water contents of pyroxenes from Ichinomegata peridotites (113-271 ppm wt. H2O for orthopyroxene, and 292-347 ppm wt. H2O for clinopyroxene

  13. Constraints on mantle evolution from 187Os/ 188Os isotopic compositions of Archean ultramafic rocks from southern West Greenland (3.8 Ga) and Western Australia (3.46 Ga) (United States)

    Bennett, Victoria C.; Nutman, Allen P.; Esat, Tezer M.


    of plume materials, reflecting the development of variable Re/Os mantle sources early in Earth history. The lower average 187Os/ 188Os = 0.1247 for abyssal peridotites ( Snow and Reisberg, 1995) indicate that not all regions of the modern mantle have evolved with the same Re/Os ratio. The relative sizes of the various reservoirs are unknown, although mass balance considerations can provide some general constraints. For example, if the unradiogenic 187Os/ 188Os modern abyssal peridotite compositions reflect the prevalent upper mantle composition, then the complementary high Re/Os basaltic reservoir must represent 20 to 40% by mass of the upper mantle (taken here as 50% of the entire mantle), depending on the mean storage age. The difficulties associated with efficient long-term storage of such large volumes of subducted basalt suggest that the majority of the upper mantle is not significantly Re-depleted. Rather, abyssal peridotites sample anomalous mantle regions. The existence of 3.8-Ga mantle peridotites with chondritic 187Os/ 188Os compositions and with Os concentrations similar to the mean abundances measured in modern peridotites places an upper limit on the timing of a late accretionary veneer. These observations require that any highly siderophile element -rich component must have been added to the Earth and transported into and grossly homogenized within the mantle by 3.8 Ga. Either large-scale mixing of impact materials occurred on very short (0-100 myr) timescales or (the interpretation preferred here) the late veneer of highly siderophile elements is unrelated to the lunar terminal cataclysm estimated to have occurred at ˜3.8 to 3.9 Ga.

  14. Melt- versus fluid-induced metasomatism in spinel to garnet wedge peridotites (Ulten Zone, Eastern Italian Alps): clues from trace element and Li abundances (United States)

    Scambelluri, M.; Hermann, J.; Morten, L.; Rampone, E.


    The peridotite bodies of the Ulten Zone (Upper Austroalpine, Italian Eastern Alps) are enclosed in Variscan migmatites and derive from a mantle wedge environment. They display the progressive transformation of porphyroclastic spinel peridotites ( T=1,200°C; P=1.5 GPa) into fine-grained garnet-amphibole peridotites ( T=850°C; P=3 GPa). Detailed bulk-rock and mineral trace element analyses of a sample suite documenting the entire metamorphic evolution of the peridotites revealed several stages of metasomatism. The spinel peridotites derive from a depleted mantle that became enriched in some large ion lithophile element (LILE) and light rare earth elements (LREE). The same signature pertains to clinopyroxene and orthopyroxene, indicating that this metasomatic signature was acquired at the recorded temperature of 1,200°C. Such a temperature is considerably above the wet peridotite solidus and hence the metasomatic agent must have been a hydrous melt. Moreover, the Li-enrichment of the spinel-facies pyroxenes (up to 24 ppm Li) reflects disequilibrium distribution after exchange with a presumably mafic melt. cpx/opx D Li=3-7 and cpx/ol D Li=2.7-8 indicate that the spinel-facies clinopyroxene hosts higher Li amounts than the coexisting minerals. LREE fractionation, variable LREE enrichment, LILE enrichment with respect to HFSE (average clinopyroxene Pb N /Nb N =16-90) in spinel lherzolites can be related to chromatographic effects of porous melt flow. The significant enrichment of pyroxenes from the spinel lherzolites in Pb, U and Li indicates that the metasomatic melt was subduction-related. All these features suggest that the spinel lherzolites formed a mantle wedge layer percolated by melts carrying recycled crustal components and rising from a deeper source of subduction magmas. The garnet + amphibole peridotites equilibrated at temperatures well below the wet solidus in the presence of an aqueous fluid. Bulk-rock trace element patterns display pronounced positive

  15. Layering of the lithospheric mantle beneath the Siberian Craton: Modeling using thermobarometry of mantle xenolith and xenocrysts (United States)

    Ashchepkov, I. V.; Vladykin, N. N.; Ntaflos, T.; Kostrovitsky, S. I.; Prokopiev, S. A.; Downes, H.; Smelov, A. P.; Agashev, A. M.; Logvinova, A. M.; Kuligin, S. S.; Tychkov, N. S.; Salikhov, R. F.; Stegnitsky, Yu. B.; Alymova, N. V.; Vavilov, M. A.; Minin, V. A.; Babushkina, S. A.; Ovchinnikov, Yu. I.; Karpenko, M. A.; Tolstov, A. V.; Shmarov, G. P.


    Single-grain thermobarometric studies of xenocrysts were used to compile local SCLM transects through the major regions of kimberlite magmatism in Siberia and longer transects through the subcontinental mantle lithosphere (SCLM) beneath the Siberian craton. The mantle structure was obtained using P-Fe#, Ca in garnets, oxygen fugacity values fO2 and calculated temperatures T°C. The most detail transect obtained for the Daldyn field on the Udachnaya-Zarnitsa reveals layering showing an inclination of > 35° to Udachnaya. Mantle layering beneath the Alakit field determined from the Krasnopresnenskaya-Sytykanskaya transect shows a moderate inclination from N to S. The inflection near Yubileinaya-Aykhal is also supported by the extreme depletion in peridotites with low-Fe sub-Ca garnets. Beneath the Malo-Botuobinsky field the sharply layered mantle section starts from 5.5 GPa and reveals step-like P-Fe#Ol trends for garnets and ilmenites. The deeper part of SCLM in this field was originally highly depleted but has been regenerated by percolation of protokimberlites and hybrid melts especially beneath Internationalnaya pipe. The three global transects reveal flat layering in granite-greenstone terranes and fluctuations in the granulite-orthogneiss Daldyn collision terranes. The mantle layering beneath the Daldyn - Alakite region may have been created by marginal accretion. Most of southern fields including the Malo-Botuobinsky field reveal flat layering. The primary subduction layering is smoothed beneath the Alakit field. Lower Jurassic kimberlites from the Kharamai-Anabar kimberlite fields reveal a small decrease of the thickness of the SCLM and heating of its base. The Jurassic Kuoyka field shows an uneven base of the SCLM inclined from west to east. SCLM sequences sampled at this time started mainly from depths of 130 km, but some pipes still showed mantle roots to 250 km. The garnet series demonstrates an inclined straight line pyroxenite P-Fe# trend due to

  16. How inheritance, geochemical and geophysical properties of the lithospheric mantle influence rift development and subsequent collision (United States)

    Picazo, Suzanne; Chenin, Pauline; Müntener, Othmar; Manatschal, Gianreto; Karner, Garry; Johnson, Christopher


    In magma-poor rifted margins, the rift structures, width of necking zones and overall geometry are controlled by the heterogeneities of geochemical and geophysical properties of the crust and mantle. In this presentation we focus on the mantle heterogeneities and their major implications on the closure of a hyper-extended rifted system. In our study, we review the clinopyroxene and spinel major element composition from the Liguria-Piemonte domain, the Pyrenean domain, the Dinarides and Hellenides ophiolites and the Iberia-Newfoundland rifted margins (Picazo et al, 2016). It would seem that during an extensional cycle i.e., from post-orogenic collapse to polyphase rifting to seafloor spreading, the mineral compositions of mantle peridotites are systematically modified. The initially heterogeneous subcontinental mantle cpx (inherited mantle type 1) equilibrated in the spinel peridotite field and is too enriched in Na2O and Al2O3 to be a residue of syn-rift melting. The heterogeneous inherited subcontinental mantle becomes progressively homogenized due to impregnation by MORB-type melts (refertilized mantle-type 2) during extensional thinning of the lithosphere. At this stage, cpx equilibrate with plagioclase and display lower Na2O and Al2O3 and high Cr2O3 contents. The system might evolve into breakup and oceanization (mantle type 3) i.e., self-sustained steady-state seafloor spreading. The different mantle-types are present in various reconstructed sections of magma-poor margins and display a systematic spatial distribution from mantle type 1 to 3 going oceanwards in Western and Central Europe. We estimated the density of the three identified mantle types using idealized modal peridotite compositions using the algorithm by Hacker et al, (2003). The density of the refertilized plagioclase peridotite is predicted to be lower than that of inherited subcontinental and depleted oceanic mantle. This has some interesting consequences on the reactivation of rifted margins

  17. Abyssal peridotites reveal the near-chondritic Fe isotopic composition of the Earth (United States)

    Craddock, Paul R.; Warren, Jessica M.; Dauphas, Nicolas


    Terrestrial oceanic and continental basalts are enriched by approximately +0.1‰ in 56Fe/54Fe ratio relative to primitive, undifferentiated meteorites (chondrites). The δ56Fe values of terrestrial basalts are also distinct from those of basalts from Mars and asteroid Vesta, which have chondritic Fe isotopic compositions. The processes responsible for the isotopic enrichment of terrestrial basalts are debated, in part because the Fe isotopic composition of the mantle source of terrestrial basalts is unknown. Here we report Fe isotopic measurements of abyssal peridotites, which are the residues of limited melting at oceanic ridges and are thus the best proxies for the composition of the convective portion of the mantle. Our data show that abyssal peridotites have a mean δ56Fe value of +0.010±0.007‰ (relative to IRMM-014), which is indistinguishable from chondrites. After correcting this data for seafloor weathering and mantle melting, we estimate the average Fe isotopic composition of the terrestrial mantle to be δ56Fe=+0.025±0.025‰, which is also indistinguishable from chondrites, within current analytical precision. We determine that the maximum shift in δ56Fe for peridotite residues during partial mantle melting is 0.01‰. Our results argue against isotopic fractionation during core-mantle differentiation or iron vaporization during the Moon-forming giant impact, because both processes would yield a bulk mantle δ56Fe value that is non-chondritic. In addition, our results suggest that disproportionation of mantle Fe2+-Fe3+ in perovskite and Fe0 metal and segregation of metal to the core could not have been a driver for Fe isotopic fractionation in the silicate mantle. Instead, the different iron isotopic compositions of abyssal peridotites and MORBs support mounting evidence for iron isotopic fractionation of melts but not residues during the formation of oceanic and continental crust.

  18. Eclogite-melt/peridotite reaction: Experimental constrains on the destruction mechanism of the North China Craton

    Institute of Scientific and Technical Information of China (English)


    To study the mechanism of melt-peridotite reaction pertinent to the destruction of the North China Craton (NCC) lithosphere, a series of experiments were performed at a pressure of 2.0 GPa and temperatures from 1250 to 1400°C using Bixiling eclogite and Damaping peridotite as starting materials. The experimental results show that the reaction between eclogite melt and peridotite causes dissolution of olivine and orthopyroxene and precipitation of clinopyroxene in the melt. The experimental run products, characterized by a lherzolite/pyroxenite/garnet-pyroxenite sequence, are consistent with the mantle xenoliths in the Neogene Hannuoba basalt of the NCC found by Liu et al. (2005). It suggests that the mafic lower continental crust was probably recycled into the mantle during the Mesozoic Era. In the experiments conducted at 1300 and 1350°C, the resulting melts have a high Mg# andesite signature, indicating that the melt-peridotite reaction may have played a major role in the generation of high Mg# andesite. Our experimental results support the hypothesis that melts derived from foundered eclogite in the asthenosphere will consume the lithospheric peridotites. Therefore, melt-peridotite reaction is an important mechanism for the destruction/thinning of the lithosphere.

  19. 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.


    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 (Californi

  20. Metasomatic processes within the fertile lithospheric Mantle beneath Don Camilo, Santa Cruz, Argentina (United States)

    Ntaflos, Th.; Mundl, A.; Bjerg, E. A.; Tschegg, C.; Kosler, J.


    Mantle xenoliths in alkalibasalts from Don Camilo, an area located at the North margin of the Deseado Massif in Patagonia, comprise mainly spinel-bearing lherzolites and dunites as well as less common harburgites, wehrlites, clinopyroxenites and gabbros. Spinel-lherzolites and harzburgites have protogranular textures whereas dunites have equigranular to equigranular tabular textures. There are two kinds of dunites: mantle and cumulate dunites. The olivine mg# in the mantle dunites vary within a narrow range from 90.5 to 91.5 and the NiO content from, 0.39 to 0.42 wt. %, whereas in the cumulate dunites the mg# ranges from 87 to 90.5 and the NiO content from 0.22 to 0.40 wt. %. Both types of dunite contain fine grained interstitial diopside. Hydrous phases, besides one sample that contains amphibole, were so far not found. The spinel peridotites have Primitive Mantle normalized (PM) whole rock REE abundances depleted in LREE [(La/Yb)N=0.34-0.85)] and the dunites are LREE enriched [(La/Yb)N=3.49]. LA-ICP-MS analyses of cpx show that a number of the studied spinel peridotite xenoliths experienced cryptic metasomatism. Three groups of xenoliths have been recognized according to their PM normalized REE and other incompatible trace element patterns in cpx: group I has depleted LREE abundances and group II is highly enriched in LREE (20-30 x PM) and group III has moderate LREE enrichments. Cores of some clinopyroxenes in group II have depleted LREE similar to those in group I, apparently representing relictic cores not affected by metasomatism. In addition, the metasomatized clinopyroxenes are significantly enriched in Sr, Th and U. Evidently, the metasomatic agent was a H2O-rich fluid (high LREE, Sr, Th and U). Mantle and cumulate dunites have experienced modal metasomatism. In both types of dunite interstitial clinopyroxene appear to be of metasomatic origin. The clinopyroxene from cumulate dunites depleted LREE abundances and low HREE indicating that they have been

  1. Nd-isotopes in selected mantle-derived rocks and minerals and their implications for mantle evolution (United States)

    Basu, A.R.; Tatsumoto, M.


    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.

  2. Comparison between the magnetic and petrological characteristics of the peridotites from the gorringe ridge and the peridotites from the mid-ocean ridges (United States)

    Popov, K. V.; Bazylev, B. A.; Shcherbakov, V. P.; Gapeev, A. K.


    The purpose of our work was to obtain the most possible detailed information about the composition, concentration, and structural features of the magnetic minerals contained in the rock to reveal the differences in the magnetic properties of the peridotites under various circumstances of the mantle magmatism and different conditions of metamorphism. To do this, we examined and analyzed the magnetic and petrographic characteristics of four collections of oceanic and alpinotype spinel peridotites. The main object for comparing the magnetic characteristics was the Gorringe ridge, which lies in the eastern part of the Atlantic Ocean. The peridotite samples from the Gorringe ridge differ from the other collections in many magnetic parameters: I n , χ, Q, I rs / I s , H c , H cr , and H m . The principal question of our work was to clarify the nature of the Earth's crust where the Gorringe ridge formed. This subject was studied many times in the literature, but the researchers did not reach a common opinion. In accordance with our data, the spinel peridotites from the Gorringe ridge represent a subcontinental lithosphere mantle of the Iberian continental margin. During the metamorphism, the formation of magnetite occurred in the peridotites of the Gorringe ridge in several stages and had a regressive character. Our investigations explain the results of the analysis of the anomalous magnetic field over the Gorringe ridge, which is characterized by sharp roughness and high intensity of the local signchanging anomalies.

  3. Mixing the mantle marble-cake: timescale constraints from Os isotopes (United States)

    Parman, S.; Pearson, G.; Nowell, G.; van Hunen, J.


    In their seminal paper, Allegre and Turcotte (1986) presented a model in which the upper mantle is a mixture of depleted, harzburgitic mantle and subducted basalt that has been mechanically mixed together, the mantle marble-cake. Since their publication, most studies of mantle heterogeneity have focused on the enriched components, which are equated with subducted basalt and/or sediments, and successfully explain OIB Sr-Nd-Pb isotopic systematics. In this talk, we will focus on a different part of the marble-cake, depleted (harzburgitic) heterogeneities. Though abundant in abyssal peridotites and ophiolites, these have been difficult to study geochemically because they have very low concentrations of typical trace elements and radiogenic isotopes, and are overprinted by any mixing with enriched mantle or melts. However, Os is compatible during mantle melting, is enriched in depleted mantle and thus is robust with respect to mixing with enriched components or metasomatism. Somewhat surprisingly, Os isotope studies of the convecting mantle show clear evidence for depleted heterogeities up to 2 billion years old, but the relative paucity of data (less than 100 analyses), makes it difficult to extract meaningful mixing information. Rapid analysis of osmiridium grains by laser-ablation inductively coupled multi-collector plasma mass spectrometry now allows large Os datasets to be acquired (100s of datapoints), which are suitable for statistical analyses (Meibom, 2002). Here we present new and published laser-ablation analyses of osmiridiums from a global collection. The data generally show an exponential decrease in heterogeneities with age, such that over 90% of heterogeneities are destroyed within 2 billion years, though rare heterogeneities as old as 2.7 Ga survive. The exponential decrease in survivorship is generally consistent with the mechanical mixing model of Allegre & Turcotte (1986). Subsequent 2-dimensional mixing models suggest that high-viscosity blobs can

  4. Distribution of trace elements in spinel and garnet peridotites

    Institute of Scientific and Technical Information of China (English)


    The distribution of trace elements in the upper mantle has been discussed on the basis of the trace element abundances in bulk rocks and constituent minerals of two spinel and garnet facies peridotite xenoliths in alkali basalts from eastern China.The data presented are consistent with the suggestion that highly incompatible elements (Rb,Ba,Th,U,Sr,Nb,Ta) mainly reside in intergranular components,and to a lesser extent in fluid inclusions in minerals.The LILE composition in olivine and orthopyroxene can be seriously affected by the presence of fluid inclusions.Consequently the subsolidus partitioning of the LILE cannot be used to infer the olivine-melt and orthopyroxene-melt partition coefficients for these elements.There is a significant difference in (Opx/Cpx)HREE ratios for spinel and garnet peridotites,suggesting a P-T control on equilibrium partition coefficients.

  5. Distribution of trace elements in spinel and garnet peridotites

    Institute of Scientific and Technical Information of China (English)



    The distribution of trace elements in the upper mantle has been discussed on the basis of the trace element abundances in bulk rocks and constituent minerals of two spinel and garnet facies peridotite xenoliths in alkali basalts from eastern China. The data presented are consistent with the suggestion that highly incompatible elements (Rb, Ba, Th, U, Sr, Nb, Ta) mainly reside in intergranular components, and to a lesser extent in fluid inclusions in minerals. The LILE composition in olivine and orthopyroxene can be seriously affected by the presence of fluid inclusions. Consequently the subsolidus partitioning of the LILE cannot be used to infer the olivine-melt and orthopyroxene-melt partition coefficients for these elements. There is a significant difference in (Opx/Cpx)HREE ratios for spinel and garnet peridotites, suggesting a P-T control on equilibrium partition coefficients.

  6. Modeling of Mauritius as a Heterogeneous Mantle Plume (United States)

    Moore, J. C.; White, W. M.; Paul, D.; Duncan, R. A.


    Mauritius Island (20°20' S, 57°30' E) is located in the western Indian Ocean and is the penultimate volcanic island of the Réunion mantle plume. Mauritius has a well-established history of episodic volcanism and erosional hiatus, traditionally characterized by three chemically and temporally distinct eruptive phases: 1) the voluminous shield-building lavas of the Older Series (8.4-5.5 Ma), 2) the Intermediate Series (3.5-1.9 Ma), and 3) the Younger Series (1.0-0.00 Ma; Duncan, unpub. data). Recent collaboration with the Mauritian Water Resource Unit has permitted the study of a series of newly available drill cores, facilitating an advanced subsurface investigation into the evolution of the island. Radiometric dating of deep lava units from these cores has identified the earliest known sample from Mauritius (B18-1; 8.4 Ma) and demonstrated the existence of Intermediate and Younger Series lavas at previously unanticipated depths, some greater than 150 meters. Calculated volumes for the combined post- erosional lavas exceed 35 km3, closely resembling new results for Hawaiian analogues (20-60 km3; Garcia, pers. comm.). While these two post-erosional series remain temporally distinct (a 0.9 M.y. hiatus remains despite new dates), they are indistinguishable in major, trace, and isotopic composition. The shield building Older Series lavas are enriched in incompatible trace elements relative to the post-erosional lavas, an inverse relationship to that observed at both Hawaii (Maui, Oahu, and Kauai) and Tahaa (Societies). In contrast isotope systematics are consistent, with shield building lavas having more enriched isotopic signatures than post-erosional lavas. The observed differences cannot be explained solely by variations in the extent of partial melting and require distinct and heterogeneous sources for the shield and post-erosional lavas. Two magma generation scenarios for a heterogeneous mantle plume with enriched (eclogitic) and depleted (peridotitic

  7. Trace-element geochemistry of mantle olivine and application to mantle petrogenesis and geothermobarometry


    De Hoog, Cees-Jan; Gall, Louise; Cornell, David H.


    Trace-element compositions of olivine from 75 mantle rocks of diverse origin, including xenoliths from kimberlites, basaltic lavas and orogenic peridotites, were determined by laser ablation ICP-MS to study systematic variations between mantle lithologies, partitioning mechanisms in olivine and their potential for geothermobarometry and unravelling mantle processes. Samples were selected to cover a wide range of forsterite contents (89.1-93.4), equilibration temperatures and pressures (750-14...

  8. Nanoscale variations in 187Os isotopic composition and HSE systematics in a Bultfontein peridotite (United States)

    Wainwright, A. N.; Luguet, A.; Schreiber, A.; Fonseca, R. O. C.; Nowell, G. M.; Lorand, J.-P.; Wirth, R.; Janney, P. E.


    Understanding the mineralogical controls on radiogenic chronometers is a fundamental aspect of all geochronological tools. As with other common dating tools, it has become increasingly clear that the Re-Os system can be impacted by multiple mineral formation events. The accessory and micrometric nature of the Re-Os-bearing minerals has made assessing this influence complex. This is especially evident in cratonic peridotites, where long residence times and multiple metasomatic events have created a complex melting and re-enrichment history. Here we investigate a harzburgitic peridotite from the Bultfontein kimberlite (South Africa) which contains sub-micron Pt-Fe-alloy inclusions within base metal sulphides (BMS). Through the combination of the focused ion beam lift-out technique and low blank mass spectrometry we were able to remove and analyse the Pt-Fe-alloy inclusions for their Re-Os composition and highly siderophile element (HSE) systematics. Six repeats of the whole-rock yield 187Os/188Os compositions of 0.10893-0.10965, which correspond to Re depletion model ages (TRD) of 2.69-2.79 Ga. The Os, Ir and Pt concentrations are slightly variable across the different digestions, whilst Pd and Re remain constant. The resulting HSE pattern is typical of cratonic peridotites displaying depleted Pt and Pd. The Pt-Fe-alloys have PUM-like 187Os/188Os compositions of 0.1294 ± 24 (2-s.d.) and 0.1342 ± 38, and exhibit a saw-tooth HSE pattern with enriched Re and Pt. In contrast, their BMS hosts have unradiogenic 187Os/188Os of 0.1084 ± 6 and 0.1066 ± 3, with TRD ages of 2.86 and 3.09 Ga, similar to the whole-rock systematics. The metasomatic origin of the BMS is supported by (i) the highly depleted nature of the mantle peridotite and (ii) their Ni-rich sulphide assemblage. Occurrence of Pt-Fe-alloys as inclusions within BMS grains demonstrates the genetic link between the BMS and Pt-Fe-alloys and argues for formation during a single but continuous event of silicate melt

  9. Mantle olivine xenocrysts entrained in Mesozoic basalts from the North China craton:Implication for replacement process of lithospheric mantle

    Institute of Scientific and Technical Information of China (English)

    ZHANG Hongfu; YING Jifeng; XU Ping; MA Yuguang


    Mesoxzoic(125 Ma) Fangcheng basalts from Shandong Province contain clearly zoned olivines that are rare in terrestrial samples and provide first evidence for the replacement of lithospheric mantle from high-Mg peridotites to low-Mg peridotites through peridotite-melt reaction. Zoned olivines have compostions in the core(Mg#=87.2-90.7)similar to those olivines from the mantle peridotitic xenoliths entrained in Cenozoic basalts from the North China craton and in the rim (Mg#=76.8-83.9)close to olivine phenocrysts of the host basalts (75.7-79.0).These compositional features as well as rounded crystal shapes and smaller grain sizes (300-800μm)demonstrate that these zoned olivines are mantle xenocrysts , important type of the replacement of lithospheric mantle.The reaction resulted in the transformation of the Paleozoic refractory (high-Mg)peridotites to the late Mesozoic fertile (low-Mg) and radiogenic isotope-enriched peridotites,leading to the loss of old lithospheric mantle.

  10. Iron geochemistry of the mantle (United States)

    Humayun, M.; Campbell, T. J.; Brandon, A. D.; Davis, F. A.; Hirschmann, M. M.


    The Fe/Mg ratio is an important constraint on the compositionally controlled density of the mantle. However, this ratio cannot be inferred from erupted lavas from OIB or MORB sources, but must be determined directly from mantle peridotites. Recently, the Fe/Mn ratio of erupted lavas has been used as an indicator of potential Fe variability in the mantle driven by core-mantle interaction, recycled oceanic crust, or even variations in the temperature of mantle melting. The classic compilation of McDonough & Sun (1995) provided the currently accepted Fe/Mn ratio of the upper mantle, 60±10. The uncertainty on this ratio allows for 15-30% variability in mantle iron abundances, which is equivalent to a density variation larger than observed by seismic tomography in the mantle. To better understand the relationship between mantle peridotites and erupted lavas, and to search for real variability in the Fe/Mn ratio of mantle peridotites, we report precise new ICP-MS measurements of the transition element geochemistry of suites of mantle xenoliths that have known Fe/Mg ratios. For 12 Kilbourne Hole xenoliths, we observe a clear correlation between Fe/Mn and MgO (or Fe/Mg) over an Fe/Mn range of 59-72. Extrapolation of this trend to a Primitive Mantle (PM) MgO content of 37.8 yields an Fe/Mn of 59±1 for the PM. Our new analyses of KLB-1 powder and fused glass beads yield an Fe/Mn of 61.4 for both samples, which plots on the Kilbourne Hole Fe/Mn vs. MgO trend. A set of ten xenoliths from San Carlos yield a wide range of Fe/Mn (56-65) not correlated with MgO content. The San Carlos xenoliths may have experienced a metasomatic effect that imprinted variable Fe/Mn. A clinopyroxene-rich lithology from San Carlos yields an Fe/Mn of 38, which plots on an extension of the Kilbourne Hole Fe/Mn vs. MgO trend. These new results, and those from other xenolith localities being measured in our lab, provide new constraints on the compositional variability of the Earth's upper mantle. Mc

  11. Large Calcium Isotopic Variation in Peridotitic Xenoliths from North China Craton (United States)

    Huang, S.; Zhao, X.; Zhang, Z.


    Calcium is the fifth most abundant element in the Earth. The Ca isotopic composition of the Earth is important in many aspects, ranging from tracing the Ca cycle on the Earth to comparing the Earth to other terrestrial planets. There is large mass-dependent Ca isotopic variation, measured as δ44/40Ca relative to a standard sample, in terrestrial igneous rocks: about 2 per mil in silicate rocks, compared to 3 per mil in carbonates. Therefore, a good understanding of the Ca isotopic variation in igneous rocks is necessary. Here we report Ca isotopic data on a series of peridotitic xenoliths from North China Craton (NCC). There is about 1 per mil δ44/40Ca variation in these NCC peridotites: The highest δ44/40Ca is close to typical mantle values, and the lowest δ44/40Ca is found in an Fe-rich peridotite, -1.13 relative to normal mantle (or -0.08 on the SRM 915a scale). This represents the lowest δ44/40Ca value ever reported for igneous rocks. Combined with published Fe isotopic data on the same samples, our data show a positive linear correlation between δ44/40Ca and δ57/54Fe in NCC peridotites. This trend is inconsistent with mixing a low-δ44/40Ca and -δ57/54Fe sedimentary component with a normal mantle component. Rather, it is best explained as the result of kinetic isotopic effect caused by melt-peridotite reaction on a time scale of several hundreds of years. In detail, basaltic melt reacts with peridotite, replaces orthopyroxene with clinopyroxene, and increases the Fo number of olivine. Consistent with this interpretation, our on-going Mg isotopic study shows that low-δ44/40Ca and -δ57/54Fe NCC peridotites also have heavier Mg isotopes compared to normal mantle. Our study shows that mantle metasomatism plays an important role generating stable isotopic variations within the Earth's mantle.

  12. Scales of preservation and root causes of heterogeneities in the convecting upper mantle of the Iapetan Ocean (United States)

    O'Driscoll, B.; Walker, R. J.; Day, J. M.; Daly, J. S.


    Ophiolites are particularly valuable resources for assessing the timing, causes and extent of mantle heterogeneity. This is because appropriate examples allow field-based observations to be coupled with geochemical investigations of otherwise inaccessible mantle. The Shetland (UK; ~492 Ma) and Leka (Norway; ~497 Ma) ophiolites each comprise a portion of early-Paleozoic (Iapetan) oceanic lithosphere with well-exposed mantle and lower crustal sections, as well as petrologic Mohorovicic discontinuities. The mantle sections of both ophiolites are highly heterogeneous at the cm-to-m scales, most noticeably manifested by the formation of dunite lenses and layers in dominantly harzburgitic host rock. In the Leka and Shetland rocks, dunite bodies often host podiform chromitite (≥ 60 vol.% Cr-spinel) seams. At both localities, chromitite seams are 0.01-2 m thick and at the outcrop scale may appear complexly deformed, forming stock-work veins and patches. Shetland peridotites are pervasively serpentinized, so that primary silicate minerals (olivine ± orthopyroxene) are rarely preserved. Despite this alteration, comparisons with abyssal peridotites suggest that Shetland peridotites and chromitites generally retain their primary magmatic Re-Os isotope signatures and highly siderophile element (HSE) abundances. Harzburgites preserve evidence for a Mesoproterozoic depletion event, but are dominated by contemporary chondritic, ambient upper mantle compositions (γOs492Ma of -5.3 to +2.6). Dunite bodies have more variable and more radiogenic Os isotope compositions (γOs492Ma of -3.3 to +12.4) than harzburgites; a feature imparted as a consequence of high degrees of melt-rock interaction during supra-subduction zone (SSZ)-type melt extraction. Sub-m scale mantle heterogeneities with respect to Os isotopes are common. One Shetland harzburgite-dunite pair sampled <1 m apart has γOs492Ma values of -5.3 and +1.3, respectively. Individual chromitite seams, separated by hundreds of

  13. Ocean Basalt Simulator version 1 (OBS1): Trace element mass balance in adiabatic melting of a pyroxenite-bearing peridotite (United States)

    Kimura, Jun-Ichi; Kawabata, Hiroshi


    present a new numerical trace element mass balance model for adiabatic melting of a pyroxenite-bearing peridotite for estimating mantle potential temperature, depth of melting column, and pyroxenite fraction in the source mantle for a primary ocean basalt/picrite. The Ocean Basalt Simulator version 1 (OBS1) uses a thermodynamic model of adiabatic melting of a pyroxenite-bearing peridotite with experimentally/thermodynamically parameterized liquidus-solidus intervals and source mineralogy. OBS1 can be used to calculate a sequence of adiabatic melting with two melting models, including (1) melting of peridotite and pyroxenite sources with simple mixing of their fractional melts (melt-melt mixing model), and (2) pyroxenite melting, melt metasomatism in the host peridotite, and melting of the metasomatized peridotite (source-metasomatism model). OBS1 can be used to explore (1) the fractions of peridotite and pyroxenite, (2) mantle potential temperature, (3) pressure of termination of melting, (4) degree of melting, and (5) residual mode of the sources. In order to constrain these parameters, the model calculates a mass balance for 26 incompatible trace elements in the sources and in the generated basalt/picrite. OBS1 is coded in an Excel spreadsheet and runs with VBA macros. Using OBS1, we examine the source compositions and conditions of the mid-oceanic ridge basalts, Loihi-Koolau basalts in the Hawaiian hot spot, and Jurassic Shatsky Rise and Mikabu oceanic plateau basalts and picrites. The OBS1 model shows the physical conditions, chemical mass balance, and amount of pyroxenite in the source peridotite, which are keys to global mantle recycling.

  14. Water Content of the Oceanic Lithosphere at Hawaii from FTIR Analysis of Peridotite Xenoliths (United States)

    Peslier, Anne H.; Bizmis, Michael


    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


    Directory of Open Access Journals (Sweden)

    A. I. Kiselev


    . Density inversion occurs if one assumes the presence of the asthenosphereic material in the composition similar to that of the primitive mantle or lherzolite KH in amounts no less than 1.40 and 0.83 wt. %, respectively, of the conventionally neutral fluid. This amount of the fluid seems to be overestimated and thus does not fully correlate with the current estimates of the fluid content in the mantle. Therefore, the most appropriate material for delamination of the thickened lithosphere is only the fluid-bearing asthenosphere which composition corresponds to that of the depleted mantle of middle-ocean ridges (DMM being the reservoir existing from the Precambrian. In our model, abyssal peridotite is most similar to DMM as compared with other more fertile compositions of the lithosphere. Heat advection due to uplift of fluid-bearing plumes that occurred much time after collisional events may initiate repeated delamination of gravitationally instable parts of the orogenic and cratonic lithosphere. 

  16. Organic Analysis of Peridotite Rocks from the MAR (United States)

    Bassez, M.; Takano, Y.; Ohkouchi, N.


    At the bottom of the terrestrial oceans, where tectonic forces separate the lithospheric plates along mid-ocean ridges, the ultramafic rocks of the upper-mantle, the peridotites, are exposed to circulating seawater and the hydrolysis of their silicate constituents, the olivine and pyroxenes minerals, into serpentine, occurs at various degrees. The H2, released during the serpentinization process, could react with the CO2 embedded inside the rock, to form methane and saturated hydrocarbons, in the context of catalytic reactions involving hydrothermal CO2. As a consequence, the simple molecules H2O, H2, CO2, CH4, could be present inside the rock and, with the N2 of the environment and an activation source, they could lead to simple organic molecules of biological relevance. Serpentinized peridotite rocks located on hydrothermal sites could consequently contain organic molecules, including amino acids. Some similarities with the Murchison meteorite can be noticed and are described. In order to verify this hypothesis, we conducted preliminary experimental analyses of the organic content of two serpentinized peridotite rocks of the Ashadze (12° 58'N, 4,080 m) and Logatchev (14° 43'N, 2,970 m) hydrothermal sites in the Mid-Atlantic Ridge. The samples have been dredged on the seafloor in march 2007, during the French-Russian Ifremer Serpentine cruise. Polar and non polar fractions of the grounded samples have been analysed by gas chromatoraphy/mass spectrometry techniques. We identify a wide variety of amino acids including protein and non-protein amino acids. Our analyses show that the identified amino acids are L-form predominent: for the Ashadze peridotite rock, for instance, the D/L ratio is 0.18. The results also show similarities with the values on D/L ratios found for sedimentary amino acids. We conclude in a biological origin for the identified amino acid peaks. However some peaks remain unidentified. We also detect long-chain n-alkane compounds (pdf

  17. Relative strength of lithospheric mantle compared to granulite lower crust in orogenic roots: insight from field laboratory. (United States)

    Kusbach, V.; Ulrich, S.; Schulmann, K.


    The continental lithosphere is composed by strong lithospheric mantle and weak lower crust for average and hot geotherms. However, some experiments and seismic studies show that the strength contrast between mantle and crust can vary in order of several magnitudes. The internal zone of the European Variscan orogen (Bohemian Massif, Czech Republic) contains large complexes of Ky - K-feldspar granulites with incorporated spinel and garnet peridotites that can respond to question of mantle-lower crust strength contrast from the field perspective. The studied spinel-garnet harzburgite body (the Mohelno peridotite) represents probably a fragment of strongly depleted oceanic lithosphere showing peak conditions of 22,4-27,6 kbar and 1120-1275°C. The peridotite forms large folded sheet with steep hinge and vertical axial plane. It exhibits presence of spinel along the outer arc and the internal part of the fold and garnet along inner arc, both related to coarse-grained orthopyroxene - olivine microstructure. This coarse microstructure is dynamically recrystallized forming fine-grained matrix (~10 - 20 microns) and the EBSD measurements show presence of axial [100] LPO olivine pattern dominantly along the outer arc of the fold and in spinel harzburgite, while the inner arc of the fold and partly also garnet harzburgite reveals presence of axial [010] LPO pattern. Steep foliation and sub-horizontal to moderately plunging lineation determined from olivine EBSD data defines the shape of the megafold. Host rocks exhibit transposed mylonitic fabric S1-2 revealing peak conditions of 18 kbar, 800°C and heterogeneous D3 retrogression at about 10 - 7 kbar, 650°C. The foliation S2-3 is fully concordant with limbs of peridotite megafold, but close to the outer arc it is affected by asymmetrical folds with axial planar leucosomes coherent with the shape of the megafold hinge zone. In contrast, the S2 in the internal part of the megafold is affected by sinistral and dextral melt

  18. Explosion of Ultrahigh Pressure Minerals in Mantle

    Institute of Scientific and Technical Information of China (English)

    BAI Wenji; YANG Jingsui; FANG Qingsong; YAN Binggang; ZHANG Zhongming


    @@ The microexplosion stucture of ultrahigh pressure minerals was found for the first time in podform chromitites within the mantle peridotite facies of Luobusa ophiolite along the Yarlung Zangbo suture zone.The explosion stuctures of high-energy silicate inclusions are commonly seen in thin sections (see figure).

  19. Phase transition in aluminous silica in the lowermost mantle (United States)

    Tronnes, R. G.; Andrault, D.; Konopkova, Z.; Morgenroth, W.; Liermann, H.


    Lower mantle basaltic lithologies contain 35-40% Mg-perovskite, 20-30% Ca-perovskite, 15-25% Al-rich phases (NAL and Ca-ferrite phases) and 15-20% silica-dominated phases. The Fe-rich Mg-perovskite makes basaltic material denser than peridotite throughout the lower mantle below 720 km depth, with important implications for mantle dynamics. Partial separation of subducted basaltic crust from depleted lithosphere might occur within the strongly heterogeneous D" zone. Further details on phase transitions and equation of states for the various minerals, however, are needed for more complete insights. The silica-dominated phases have considerable solubility of alumina [1]. We investigated silica with 4 and 6 wt% alumina to 120 GPa, using LH-DAC at the Extreme Conditions Beamline (P02.2) at PETRA-III, DESY. Powdered glass mixed with 10-15 wt% Pt-powder was compressed and heated in NaCl pressure media in Re-gaskets. The transition from the CaCl2-structured phase to seifertite (alpha-PbO2-structure) occurs at about 116 GPa at 2500 K. This is intermediate between the transition pressures of about 122 GPa and 100-113 GPa reported for similar temperatures for pure SiO2 [2] and a basalt composition [1], respectively. The CaCl2-structured silica phase crystallized along with seifertite, consistent with a binary phase loop trending towards lower pressure with increasing Al-content. The presence of an Al-rich Ca-ferrite phase (near the MgAl2O4-NaAlSiO4-join) in basaltic material indicates that the Al-solubility limits for the silica-dominated phases in basaltic compositions may be similar to those in the binary system SiO2-AlO1.5. Based on the X-ray pattern refinement, our samples show no significant volume change across the transition. Even so, the transition could be associated with a significant density change if the Al substitution mechanisms are different in CaCl2-structured phase and seifertite. The most likely situation is that Al-substitution occurs via O-vacancies in the

  20. Rare earth element characteristics of pyrope garnets from the Kaavi-Kuopio kimberlites – implications for mantle metasomatism

    Directory of Open Access Journals (Sweden)

    Marjaleena Lehtonen


    Full Text Available Peridotitic garnet xenocrysts from five kimberlite pipes in the Kaavi-Kuopio area of eastern Finland have been studied using major and trace element geochemistry to obtain information on the stratigraphy, compositional variability and evolutionary history of the underlyinglithospheric mantle. Ni thermometry on garnet xenocrysts gives 650–1350°C and, when extrapolated to the geotherm determined using mantle xenoliths, indicates a sampling interval of c. 80–230 km. Three distinct mantle layers are recognized based on the xenolith/xenocryst record: (1 A shallow, 180 km, composed largely of fertile material. The chondrite-normalized REE profiles of subcalcic harzburgitic garnet xenocrysts originating from layer 2 bear evidence of an extensive ancient melt extraction event, similar to that observed in lithosphere underlying Archean cratons elsewhere. Memory of this eventhas possibly also been preserved in the REEN signatures of rare depleted garnets from layer 3 and in the CCGE pyropes from layer 1 despite their saturation in Ca. The lherzolitic and megacryst garnet varieties exhibit LREEN depletion relative to MREEN and HREEN, withthe steady enrichment from SmN to YbN typical of Ca-saturated mantle garnets. The enrichment of MREE and HREE probably derives from a metasomatic event caused by silicate melts close in composition to megacryst magma, which also imprinted a Ti-metasomatic overprint on many pyrope garnets. Harzburgitic and rare lherzolitic garnets, however, appear to have escaped this metasomatism.

  1. Persistence of fertile and hydrous lithospheric mantle beneath the northwestern Ethiopian plateau: Evidence from modal, trace element and Sr-Nd-Hf isotopic compositions of amphibole-bearing mantle xenoliths (United States)

    Alemayehu, Melesse; Zhang, Hong-Fu; Aulbach, Sonja


    We present new trace element compositions of amphiboles, Sr-Nd-Hf isotope compositions of clinopyroxenes and mineral modes for spinel peridotite xenoliths that were entrained in a Miocene alkali basalt (Gundeweyn, northwestern Ethiopian plateau), in order to understand the geochemical evolution and variation occurring within the continental lithospheric mantle (CLM) in close proximity to the East African Rift system, and its dynamic implications. With the exception of a single amphibole-bearing sample that is depleted in LREE (La/YbN = 0.45 × Cl), amphiboles in lherzolites and in one harzburgite show variable degrees of LREE enrichment (La/YbN = 2.5-12.1 × Cl) with flat HREE (Dy/YbN = 1.5-2.1 × Cl). Lherzolitic clinoyroxenes have 87Sr/86Sr (0.70227 to 0.70357), 143Nd/144Nd (0.51285 to 0.51346), and 176Hf/177Hf (0.28297 to 0.28360) ranging between depleted lithosphere and enriched mantle. LREE-enriched clinopyroxenes generally have more enriched isotope compositions than depleted ones. While lherzolites with isotope compositions similar to those of the Afar plume result from the most recent metasomatic overprint, isotope compositions more depleted than present-day MORB can be explained by an older melt extraction and/or isotopic rehomogenisation event, possibly related to the Pan-African orogeny. Several generations of amphibole are recognized in accord with this multi-stage evolution. Texturally unequilibrated amphibole occurring within the peridotite matrix and in melt pockets attest to continued hydration and refertilization of the lithospheric mantle subsequent to Oligocene flood basalt magmatism, during which an earlier-emplaced inventory of amphibole was likely largely consumed. However, a single harzburgite contains amphibole with the highest Mg# and lowest TiO2 content, which is interpreted as sampling a volumetrically subordinate mantle region beneath the Ethiopian plateau that was not tapped during flood basalt magmatism. Strikingly, both trace

  2. 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;


    .4 and eHf 0 +5 to +8) indistinguishable from the host low-silica basalts and, except for 207Pb/204Pb, overlapping with the HIMU mantle reservoir. Laser line scans across grain boundaries in the xenoliths show, however, that the host magma contribution is restricted to minor degrees of melt infiltration...... along grain boundaries during ascent, with the distinctive peridotite isotopic compositions having been imparted earlier by mantle metasomatism. Two mantle metasomatic styles are distinguished from pyroxene trace element concentrations (in particular, rare earth elements, Ti, Zr and Hf......) and are interpreted to be the result of reaction of peridotite with CO2- bearing magmas. The occurrence of two subtly chemically different but isotopically indistinguishable styles of metasomatism in rocks with the same equilibrium temperatures within the same mantle column may be due to separate volatile-rich melts...

  3. Interaction between protokimberlite melts and mantle lithosphere: Evidence from mantle xenoliths from the Dalnyaya kimberlite pipe, Yakutia (Russia

    Directory of Open Access Journals (Sweden)

    I.V. Ashchepkov


    Full Text Available The Dalnyaya kimberlite pipe (Yakutia, Russia contains mantle peridotite xenoliths (mostly lherzolites and harzburgites that show both sheared porphyroclastic (deformed and coarse granular textures, together with ilmenite and clinopyroxene megacrysts. Deformed peridotites contain high-temperature Fe-rich clinopyroxenes, sometimes associated with picroilmenites, which are products of interaction of the lithospheric mantle with protokimberlite related melts. The orthopyroxene-derived geotherm for the lithospheric mantle beneath Dalnyaya is stepped similar to that beneath the Udachnaya pipe. Coarse granular xenoliths fall on a geotherm of 35 mWm−2 whereas deformed varieties yield a 45 mWm−2 geotherm in the 2–7.5 GPa pressure interval. The chemistry of the constituent minerals including garnet, olivine and clinopyroxene shows trends of increasing Fe# (=Fe/(Fe + Mg with decreasing pressure. This may suggest that the interaction with fractionating protokimberlite melts occurred at different levels. Two major mantle lithologies are distinguished by the trace element patterns of their constituent minerals, determined by LA-ICP-MS. Orthopyroxenes, some clinopyroxenes and rare garnets are depleted in Ba, Sr, HFSE and MREE and represent relic lithospheric mantle. Re-fertilized garnet and clinopyroxene are more enriched. The distribution of trace elements between garnet and clinopyroxene shows that the garnets dissolved primary orthopyroxene and clinopyroxene. Later high temperature clinopyroxenes related to the protokimberlite melts partially dissolved these garnets. Olivines show decreases in Ni and increases in Al, Ca and Ti from Mg-rich varieties to the more Fe-rich, deformed and refertilized ones. Minerals showing higher Fe# (0.11–0.15 are found within intergrowths of low-Cr ilmenite-clinopyroxene-garnet related to the crystallization of protokimberlite melts in feeder channels. In P-f(O2 diagrams, garnets and Cr-rich clinopyroxenes

  4. Concentrates and mantle xenocrysts from the Lao river Guinea and reconstructions of the mantle structure (United States)

    Afanasiev, Valentin; Ashchepkov, Igor; Nikolenko, Eeugeny


    the interaction protokimberlite magma with subducted material. The calculated PTXFO2 diagrams with monominreral methods (Ashchepkov et al ., 2010 -2014) the layered structure and presence of thick pyroxenite enriched lens in the middle part of mantle section which also contain harzburgite associations. He interval 4.5-5.5 GPa contains the Crromite bearing peridotites together with Ga- harburghites. Cr- low pyroxenites probablly are associated with the ilmenites while some of them are more Fe rich and reflects the de -eclogitization. The typical deformed peridotite association were not detected but heating for the garnets is found along the magma feeder traced by ilmenites. The TRE geochemistry show that most of Cr clinopyroxenes are typical for Ga- bearing peridotites with concave upward REE patterns. They are also are rich in Nb - Ba- U and extremely depleted in Zr suggesting probably the preceding depletion with the H2O rich melts . Zircon is abundant in concentrates what corresponds to large scale H2O bearing metasomatism. Garnets demonstrates semi round HREE - high patterns with MREE humper typical for pyroxenites and small LREE- enrichment. RFBR grants 05-05-64718, 03-05-64146; 11 -05-00060a 11-05-91060-PICSа

  5. Garnet peridotite and associated high-grade rocks from Sulawesi, Indonesia (United States)

    Helmers, H.; Maaskant, P.; Hartel, T. H. D.


    The effects of collision between three major plates define the geological development of eastern Indonesia. Garnet peridotite and associated granulite-facies contact rocks are described from two sites within the valley of the active Palu-Koro left-lateral, strike-slip fault crossing central Sulawesi. Disrupted parts of a medium- to low-grade metamorphic complex intruded by Neogene granite occur on both sides of the fault. Thermobarometry on minerals and fluids in the garnet peridotite reveals a re-equilibration path from a depth of 60 km upward. Chemistry points to metasomatic effects — isolated trace-element enrichment — by a CO 2-rich liquid and fluid in a peridotite of oceanic affiliation. The granulite shows an increase in T and incipient melting at the arrival of peridotite. The sequence of fluid inclusions of an evolving CO 2CH 4(N 2)H 2O-bearing fluid defines a concave decompression path suggesting rapid uplift. Trace element chemistry of granulite with basaltic to peraluminious rhyolitic composition indicates island-arc affinity. The described history may well reflect the processes beneath a mantled gneiss dome, present as a coeval metamorphic aureole around the garnet peridotite outcrops.

  6. Crystal preferred orientation in peridotite ultramylonites deformed by grain size sensitive creep, étang de Lers, Pyrenees, France

    NARCIS (Netherlands)

    Drury, M.R.; Avé Lallemant, H.G.; Pennock, G.M.; Palasse, L.N.


    In naturally deformed upper mantle rocks a strong olivine crystallographic preferred orientation (CPO) occurs in rocks with grain sizes larger than about 15 mm. Finer grained peridotites tend to have weak to random olivine CPO. The different types of olivine CPO are usually interpreted in relation t

  7. Zn isotopic heterogeneity in the mantle: A melting control? (United States)

    Doucet, Luc S.; Mattielli, Nadine; Ionov, Dmitri A.; Debouge, Wendy; Golovin, Alexander V.


    We present new Zn elemental and isotope data on seventeen fertile and refractory mantle peridotite xenoliths. Eleven fertile peridotites are garnet and spinel lherzolites from Vitim and Tariat (Siberia and Mongolia) and represent some of the most pristine fertile peridotites available. Six refractory peridotites are spinel harzburgites from the Udachnaya kimberlite (Siberian craton) that are nearly pristine residues of high-degree polybaric melting at high pressure (7-4 GPa). Geochemical data suggest that Zn isotopic compositions in the peridotites have not been affected by post-melting processes such as metasomatism, contamination by the host-magmas or alteration. The fertile peridotites have uniform Zn concentrations (59 ± 2 ppm) and Zn isotopic compositions with δ66Zn (relative to JMC-Lyon-03-0749l) = +0.30 ± 0.03‰ consistent with the Bulk Silicate Earth estimates of δ66Zn = +0.28 ± 0.05‰ (Chen et al., 2013). The refractory peridotites have Zn concentrations ranging from 30 to 48 ppm and δ66Zn from + 0.10 ± 0.01 ‰ to + 0.18 ± 0.01 ‰ with an average of + 0.14 ± 0.03 ‰. Our data suggest that the lithospheric mantle has a heterogeneous Zn isotopic composition. Modeling of Zn isotope partitioning during partial melting of fertile mantle suggests that high degrees of melt extraction (>30%) may significantly fractionate Zn isotopes (up to 0.16‰) and that during mantle melting, Zn concentrations and isotopic compositions are mainly controlled by the stability of clinopyroxene and garnet within the melting residue. Because the stability of clinopyroxene and garnet is mainly pressure dependent we suggest that both the depth and the degrees of melt extraction may control Zn isotope fractionation during mantle melting.

  8. Platinum group and chalcophile element systematics of serpentinized peridotites from the St. Elena ophiolite in Costa Rica (United States)

    Holm, J.; Bizimis, M.; Schwarzenbach, E. M.; Foustoukos, D.; Frisby, C. P.; Brandon, A. D.; Gazel, E.


    We present in situ LA-ICPMS data on platinum group element (PGE) and chalcophile element (namely Cu, Ag, Te, Au, Pb) systematics in sulfides from partially serpentinized peridotites of the St. Elena ophiolite, Costa Rica. PGE are strong indicators of primary mantle processes, though their behavior during low temperature alteration processes such as serpentinization is not well understood. St. Elena sulfides are dominantly pentlandites that coexist with Fe-Ni alloys and native Cu. This indicates extremely low fO2 and fS2 conditions likely established during the early stages of serpentinization. We observe extremely variable PGE-Re concentrations in the sulfides, (e.g. [Os] = 2 - 100,000 times primitive mantle, PM). Low [Os] sulfides have high Pd/Os, which in turn correlates positively with Cu concentrations, suggesting Pd enrichment through Cu-rich fluids (e.g. Schwarzenbach et al., 2014, CMP) as opposed to melt-rock reaction. Sulfide PM-normalized PGE-Re patterns are dominated by strong Pt depletions (e.g., Pt/Pd = 0.80-0.0009). Occasional Pt enrichments over Pd and Ru (or Rh) in a PM-normalized pattern (~5% of the sulfide population) often correlate with Te and/or Au enrichments. Pt enrichment was also observed in a composite pentlandite-awaruite, suggesting possible exsolution of Pt from sulfides under extremely low fS2 conditions. Pb concentrations do not correlate with other chalcophiles or PGE. Pb ranges from 0.01-31.64 ppm with the majority of sulfides <5 ppm, and an average concentration of 2.77 ppm (n=64). Assuming that this Pb concentration is representative of mantle sulfides, this implies that Pb is not dominantly held in sulfides in the upper mantle. Combination of in situ and bulk rock PGE analyses will be used to distinguish the effects of primary magmatic signatures (e.g., melt depletion, melt-rock interaction) and secondary processes such as serpentinization on the PGE-Re and chalcophile element systematics of these sulfides.

  9. Organic analysis of peridotite rocks from the Ashadze and Logatchev hydrothermal sites. (United States)

    Bassez, Marie-Paule; Takano, Yoshinori; Ohkouchi, Naohiko


    This article presents an experimental analysis of the organic content of two serpentinized peridotite rocks of the terrestrial upper mantle. The samples have been dredged on the floor of the Ashadze and Logatchev hydrothermal sites on the Mid-Atlantic Ridge. In this preliminary analysis, amino acids and long chain n-alkanes are identified. They are most probably of biological/microbial origin. Some peaks remain unidentified.

  10. Early Eocene clinoenstatite boninite and boninite-series dikes of the ophiolite of New Caledonia; a witness of slab-derived enrichment of the mantle wedge in a nascent volcanic arc (United States)

    Cluzel, Dominique; Ulrich, Marc; Jourdan, Fred; Meffre, Sebastien; Paquette, Jean-Louis; Audet, Marc-Antoine; Secchiari, Arianna; Maurizot, Pierre


    Clinoenstatite-bearing boninites (CE-boninite) from the serpentinite sole of the Cenozoic ophiolite of New Caledonia near Nepoui have been dated by the 40Ar/39Ar method, yielding two plateau ages of 47.4 ± 0.9 Ma and 50.4 ± 1.3 Ma. Coarser grained, geochemically similar boninite-series felsic dikes consistently yielded U-Pb zircon ages of ca. 54 Ma. Nepoui CE-boninites display whole rock geochemical features similar to that of Cape Vogel boninites (Papua-New Guinea). They similarly have been generated by low degree hydrous melting of depleted peridotite. High contents in LILE and LREE, and some elemental ratios suggest source enrichment by subduction-derived fluids and melts. However, unlike the Cape Vogel boninite, moderately depleted MORB-like isotopic signatures (εNd50 = 7.9) rule out the role of OIB-like, or E-MORB component that might account for the relatively high LREE and LILE contents measured in the rocks. Nd isotopic ratios and positive anomalies in Zr and Hf are closely similar to that of the slightly older felsic dikes (55-50 Ma) that crosscut the peridotite from the ophiolite in New Caledonia. Most of these magmas have been generated by slab melting during the early stages of intra-oceanic subduction. The Early Eocene subduction started at or near the "oceanic" ridge and involved young and hot lithosphere; therefore, slab-derived melts may have reacted locally with hot depleted peridotites. Finally, water influx into the mantle wedge during the subduction of slightly older (cooler and hydrated) lithosphere initiated a low degree partial melting event in the mantle wedge and generated the CE-boninite magma. Geochemical modeling of hydrous melting of a depleted mantle re-enriched by slab melts suggest that the additional slab melt component was derived from the partial melting of a BABB-like barroisite-bearing eclogite, similar to some elements of the Eocene HP-LT Pouebo terrane. This potential magma source is similar to the BABB-like HT amphibolites

  11. The Oceanic Lithosphere as Reactive Filter: Implications for MORB and Abyssal Peridotite Compositions (United States)

    Luffi, P. I.; Lee, C.; Antoshechkina, P. M.


    Melt-rock reaction in the lithosphere is, as suggested by textural observations and compositional data, a ubiquitous phenomenon capable of generating locally diverse peridotite series, such as those observed at oceanic spreading centers and transform faults, and may represent an important mechanism of creating compositional diversity in MORBs [1]. Whereas our understanding of the principles governing reactive melt transport is supported by basic theories and models, studies that attempt to quantify the physical conditions and mechanisms creating heterogeneities in the oceanic lithosphere are still limited in number [e.g. 2]. Using Adiabat_1ph 3.0 [3] in combination with the pMELTS algorithm [4], we have previously shown that reactive percolation of basaltic melts through depleted harzburgites can generate the dunite-(wehrlite)-harzburgite-lherzolite spectrum observed in the abyssal mantle and ophiolites, and that the amplitude of transformations is a function of thermal boundary layer thickness and amount of available melt [5]. To gain further insight into how melt-rock reactions shape the oceanic lithosphere, here we extend our study to show that the major and trace element variability in the oceanic mantle and rising melts are also significantly influenced by the mechanism of melt transport. If associated with cooling, distributed porous melt percolation (simulated by incremental addition of the same amount of melt) more efficiently converts harzburgites into fertile lherzolites and creates more pronounced compositional gradients in the abyssal mantle than imparted during channelized melt influx (simulated as batch addition of large amounts of melt) under otherwise identical circumstances. To remain within the tholeiitic trend observed in MORB, reacted melts must be released before clinopyroxene precipitation peaks. Further reaction with harzburgite causes liquids to evolve toward boninite-like compositions. As reaction progresses with decreasing temperature, the

  12. Monomineral universal clinopyroxene and garnet barometers for peridotitic, eclogitic and basaltic systems

    Directory of Open Access Journals (Sweden)

    I.V. Ashchepkov


    Full Text Available New versions of the universal Jd-Di exchange clinopyroxene barometer for peridotites, pyroxenites and eclogites, and also garnet barometer for eclogites and peridotites were developed. They were checked using large experimental data sets for eclogitic (∼530 and peridotitic systems (>650. The precision of the universal Cpx barometer for peridotites based on Jd-Di exchange is close to Cr-Tschermakite method produced by Nimis and Taylor (2000. Cpx barometer was transformed by the substitution of major multiplier for KD by the equations dependent from Al-Na-Fe. Obtained equation in combination with the thermometer of Nimis and Taylor (2000 allow to reconstruct position of the magma feeder systems of the alkali basaltic magma within the mantle diapirs in modern platforms like in Vitim plateau and other Southern Siberia localities and several localities worldwide showing good agreement of pressure ranges for black and green suites. These equations allow construct PTX diagrams for the kimberlite localities in Siberia and worldwide calculating simultaneously the PT parameters for different groups of mantle rocks. They give very good results for the concentrates from kimberlite lamproites and placers with mantle minerals. They are useful for PT estimates for diamond inclusions. The positions of eclogite groups in mantle sections are similar to those determined with new Gar–Cpx barometer produced by C. Beyer et al. (2015. The Fe rich eclogites commonly trace the boundary between the lower upper parts of subcontinental lithospheric mantle (SCLM at 3–4 GPa marking pyroxenite eclogites layer. Ca-rich eclogites and especially grospydites in SCLM beneath Precambrian kimberlites occurs near pyroxenite layer but in younger mantle sections they became common in the lower parts. The diamondiferous Mg Cr-less group eclogites referring to the ancient island arc complexes are also common in the middle part of mantle sections and near 5–6 GPa. Commonly

  13. Carbon elemental and isotopic composition in mantle xenoliths from Spain: Insights on sources and petrogenetic processes (United States)

    Bianchini, G.; Natali, C.


    The carbon elemental concentration (C wt%) and isotopic (δ13C ‰) composition of mantle xenoliths from the Tallante and Calatrava volcanic occurrences (in South-East and Central Spain, respectively) have been investigated to identify carbon sources and processes occurring in distinct geodynamic settings of the Iberian Peninsula. The peridotitic mantle xenoliths from Calatrava show elemental C ranging from 0.11 to 2.87 wt% which is coupled with a continuous isotopic variation from very negative values (δ13C - 26.1‰) to typical mantle values (δ13C - 5.9‰). On the other hand, the Tallante mantle xenolith suite displays lower C contents (0.06-0.15 wt%) showing a tighter variation with 13C-depleted values ranging between - 20.1 and - 23.7‰; higher elemental C up to 0.41 wt% displaying distinctly less negative isotopic values (δ13C between - 13.8 and - 11.9‰) have been recorded in veins crosscutting Tallante peridotites, plausibly representing the product of metasomatic reactions. The data from the two investigated xenolith suites invariably display a good correlation between elemental and isotopic composition, suggesting a mantle origin for carbon and Rayleigh-type fractionation as the process responsible for the observed C-δ13C variation. However, the correlation between the carbon isotopic data with other isotopic tracers (e.g. 87Sr/86Sr, 3He/4He) used to identify distinct mantle components and metasomatic reactions, indicates systematic differences between the two xenolith suites suggesting that beneath the Betic Cordillera (where Tallante is located) the deep C-cycle involves recycling, via subduction preceding/accompanying continental collision, of crustal components back in the mantle. Coherently, geochemical trends observed in the Tallante xenoliths seem to be influenced by metasomatic agents generated by melting of crustal lithologies that according to the analysis of a metasedimentary xenolith can contain C up to 1.2 wt% having δ13C of ca. - 18

  14. Melting the lithosphere: Metasomes as a source for mantle-derived magmas (United States)

    Rooney, Tyrone O.; Nelson, Wendy R.; Ayalew, Dereje; Hanan, Barry; Yirgu, Gezahegn; Kappelman, John


    Peridotite constitutes most of the Earth's upper mantle, and it is therefore unsurprising that most mantle-derived magmas exhibit evidence of past equilibrium with an olivine-dominated source. Although there is mounting evidence for the role of pyroxenite in magma generation within upwelling mantle plumes, a less documented non-peridotite source of melts are metasomatic veins (metasomes) within the lithospheric mantle. Here we present major and trace element analyses of 66 lavas erupted from a small Miocene shield volcano located within the Ethiopian flood basalt province. Erupted lavas are intercalated with lahars and pyroclastic horizons that are overlain by a later stage of activity manifested in small cinder cones and flows. The lavas form two distinctive petrographic and geochemical groups: (A) an olivine-phyric, low Ti group (1.7-2.7 wt.% TiO2; 4.0-13.6 wt.% MgO), which geochemically resembles most of the basalts in the region. These low Ti lavas are the only geochemical units identified in the later cinder cones and associated lava flows; (B) a clinopyroxene-phyric high Ti group (3.1-6.5 wt.% TiO2; 2.8-9.2 wt.% MgO), which resembles the Oligocene HT-2 flood basalts. This unit is found intercalated with low Ti lavas within the Miocene shield. In comparison to the low Ti group, the high Ti lavas exhibit a profound depletion in Ni, Cr, Al, and Si, and significant enrichment in Ca, Fe, V, and the most incompatible trace elements. A characteristic negative K anomaly in primitive-mantle normalized diagrams, and Na2O > K2O, suggests a source rich in amphibole, devoid of olivine, and perhaps containing some carbonate and magnetite. While melt generation during rift development in Ethiopia is strongly correlated with the thermo-chemical anomalies associated with the African Superplume, thermobaric destabilization and melting of mantle metasomes may also contribute to lithospheric thinning. In regions impacted by mantle plumes, such melts may be critical to weakening

  15. Europium and strontium anomalies in the MORB source mantle (United States)

    Tang, Ming; McDonough, William F.; Ash, Richard D.


    Lower crustal recycling depletes the continental crust of Eu and Sr and returns Eu and Sr enriched materials into the mantle (e.g., Tang et al., 2015, Geology). To test the hypothesis that the MORB source mantle balances the Eu and Sr deficits in the continental crust, we carried out high precision Eu/Eu∗ and Sr/Sr∗ measurement for 72 MORB glasses with MgO >8.5% from the Pacific, Indian, and Atlantic mid-ocean ridges. MORB glasses with MgO ⩾ 9 wt.% have a mean Eu/Eu∗ of 1.025 ± 0.025 (2 σm, n = 46) and Sr/Sr∗ of 1.242 ± 0.093 (2 σm, n = 41) and these ratios are positively correlated. These samples show both positive and negative Eu and Sr anomalies, with no correlations between Eu/Eu∗ vs. MgO or Sr/Sr∗ vs. MgO, suggesting that the anomalies are not produced by plagioclase fractionation at MgO >9 wt.% and, thus, other processes must be responsible for generating the anomalies. We term these MORB samples primitive MORBs, as they record the melt Eu/Eu∗ and Sr/Sr∗ before plagioclase fractionation. Consequently, the mean oceanic crust, including cumulates, has a bulk Eu/Eu∗ of ∼1 and 20% Sr excess. Considering that divalent Sr and Eu(II) diffuse faster than trivalent Pr, Nd, Sm, and Gd, we evaluated this kinetic effect on Sm-Eu-Gd and Pr-Sr-Nd fractionations during spinel peridotite partial melting in the MORB source mantle. Our modeling shows that the correlated Eu and Sr anomalies seen in primitive MORBs may result from disequilibrium mantle melting. Melt fractions produced during early- and late-stage melting may carry positive and negative Eu and Sr anomalies, respectively, that overlap with the ranges documented in primitive MORBs. Because the net effect of disequilibrium melting is to produce partial melts with bulk positive Eu and Sr anomalies, the MORB source mantle must have Eu/Eu∗ continental crustal materials, which have positive Eu and Sr anomalies, are partially mixed into the upper mantle (i.e., MORB source region), a

  16. The peridotitic laterites of New Caledonia: a possible analogue for the Martian regolith ? (United States)

    Nzokwe, G. Y.; Ferre, E. C.; Fifarek, R.; Frima, C.


    Peridotites are very common in the Solar System where they generally form the mantle of rocky planets. On Earth, large volumes of peridotites are brought to the surface as obduction ophiolites (Oman, Newfoundland, Papua, New Caledonia). Such peridotites interact with various aqueous fluids throughout their geologic history. High temperature serpentinization (up to 600° C) begins at the ocean ridge with seawater, followed by low temperature serpentinization (below 400° C) away from the ridge. The obduction process generally leads to further alteration by continental aqueous fluids along structural discontinuities. Finally, exposure to the surface results in supergene serpentinization, the formation of weathering rinds, and the development of soils on peridotites that, particularly under hot and humid climates, leads to the formation of peridotitic laterites. The peridotitic laterites of New Caledonia provide excellent opportunities for geoscience investigations because, due to nickel mining, borehole cores through alteration profiles are available. The peridotite nappe, emplaced in the late Eocene, has an estimated thickness up to 3 km and is exposed over an area of 8000 km2. Common lithologies include harzbugite, dunite, wherlite and cumulate gabbro. Regardless of the bedrock lithology, the lateritic profiles display an increase in concentration of Fe oxides and hydroxides towards the top. The alteration paths of distinct bedrock types tend to converge toward the same mineralogy. Low-field and high-field magnetic techniques have been combined to refine the vertical zonation of the lateritic profiles into distinct horizons. Magnetite is the main component in the magnetic susceptibility budget while hematite controls the coercivity. Several distinguishing features are observed in peridotitic laterites: 1) bulbous hematite concretions, 2) reddish fine-grained hematite, 3) gray flaky specular hematite (up to 10 mm in length), 4) ferricretes and manganocretes, and 5

  17. A primitive mantle source for the Neoarchean mafic rocks from the Tanzania Craton

    Institute of Scientific and Technical Information of China (English)

    Y.A. Cook; I.V. Sanislav; J. Hammerli; T.G. Blenkinsop; P.H.G.M. Dirks


    Mafic rocks comprising tholeiitic pillow basalt, dolerite and minor gabbro form the basal stratigraphic unit in the ca. 2.8 to 2.6 Ga Geita Greenstone Belt situated in the NW Tanzania Craton. They outcrop mainly along the southern margin of the belt, and are at least 50 million years older than the supracrustal assemblages against which they have been juxtaposed. Geochemical analyses indicate that parts of the assemblage approach high Mg-tholeiite (more than 8 wt.%MgO). This suite of samples has a restricted compositional range suggesting derivation from a chemically homogenous reservoir. Trace element modeling suggests that the mafic rocks were derived by partial melting within the spinel peridotite field from a source rock with a primitive mantle composition. That is, trace elements maintain primitive mantle ratios (Zr/Hf ¼ 32e35, Ti/Zr ¼ 107e147), producing flat REE and HFSE profiles [(La/Yb)pm ¼ 0.9 e1.3], with abundances of 3e10 times primitive mantle and with minor negative anomalies of Nb [(Nb/La)pm ¼ 0.6e0.8] and Th [(Th/La)pm ¼ 0.6e0.9]. Initial isotope compositions (3Nd) range from 1.6 to 2.9 at 2.8 Ga and plot below the depleted mantle line suggesting derivation from a more enriched source compared to present day MORB mantle. The trace element composition and Nd isotopic ratios are similar to the mafic rocks outcropping w50 km south. The mafic rocks outcropping in the Geita area were erupted through oceanic crust over a short time period, between w2830 and w2820 Ma; are compo-sitionally homogenous, contain little to no associated terrigenous sediments, and their trace element composition and short emplacement time resemble oceanic plateau basalts. They have been interpreted to be derived from a plume head with a primitive mantle composition.

  18. H Diffusion in Olivine and Pyroxene from Peridotite Xenoliths and a Hawaiian Magma Speedometer (United States)

    Peslier, A. H.; Bizimis, M.


    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.

  19. Petrological characteristics of mantle xenoliths from the Azrou-Timahdite quaternary basalts, middle atlas, Morocco: A mineral chemistry perspective (United States)

    Chanouan, Lhoussaine; Ikenne, Moha; Gahlan, Hisham A.; Arai, Shoji; Youbi, Nasrrddine


    Quaternary alkali basalts of the Azrou-Timahdite area contain a wide variety of ultramafic mantle xenoliths (e.g. lherzolites, wehrlites, pyroxenites and amphibolites). A comprehensive mineral chemistry studies using electron probe micro-analysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) allows us to propose a petrogenetic history of these ultramafic rocks. The studied mantle xenoliths contain spinel, which attests for their derivation from depths from 30 to 70 km, i.e. the intermediate zone of continental lithospheric upper mantle. The olivine forsterite (Fo) content is higher in lherzolites than in wehrlite, Fo88-91 and Fo80-83, respectively. The orthopyroxene is enstatite with relatively high Mg number (Mg#) (0.88-0.91). The clinopyroxene displays Mg# (0.88-0.92) similar to or slightly higher than that of olivine and orthopyroxene, indicating equilibrium between these phases. Progressive depletion of this ratio in lherzolite clinopyroxene suggests the generation of peridotitic rocks through a series of partial melting processes at different degrees. LREE enrichment in clinopyroxene combined with decoupling of Ti and Na (Fusible major elements) indicates mantle metasomatic processes, which possibly resulted in recrystallization of clinopyroxene. The wehrlitic lithologies are possibly the end product of the metasomatism. The LaN/YbN and Ti/Eu ratios of spine-bearing lherzolites and non-spongy clinopyroxenes combined with the low Ti contents and the Zr and Nb negative anomalies argue for an alkali-silicate metasomatism. Given the above scenario, we conclude that the mineralogical variations in the upper mantle beneath this part of the Middle Atlas Mountains can be attributed to a combination of mantle processes including partial melting that occurs in response of a mantle upwelling, and an alkali-silicate metasomatism.

  20. Implications of εNd—La/Nb,Ba/Nb,Nb/Th Diagrams to Mantle Heterogeneity—Classification of Island—arc Basalts and Decomposition of EMII Component

    Institute of Scientific and Technical Information of China (English)



    A group of εNd/Nb,Ba/Nb,Nb/Th diagrams are used to study mantle heterogeneity.Island-arc basalts(IAB) are distributed in a triangle of these diagrams. Three end-member components (the MORB-type depleted mantle, the fluid released from subducted oceanic crust and the sediments from the continental crust) of the source of IAB may be displayed in these diagrams. Two types of IAB are identified .They are of the two-component type (with little continental sediments), such as the basalts from Aletians and New Britain ,and the three-compeonent type, such as those from Sunda, Lesser Antilles and Andes. In addition ,the EMII type mantle-derived rocks may also be divided into two groups. One is exemplified by continental flood basalts and some peridotite xenoliths, similar to IAB, with high La/Nb and Ba/Nb and low Nb/Th ratios, The other includes the Samoa-type oceanic island basalts, with low La/Nb and Ba/Nb and high Nb/Th ratios. The corresponding two sub-components of EMII are EMIIM, which is related to the metasomatism of lithosphere mantle by fluids released from the subducted oceanic crust, and EMIISR, related to the intervention of recycling continental sediments into the convective mantle.

  1. Origin of primitive andesites by melt-rock reaction in the sub-arc mantle (Invited) (United States)

    Rapp, R. P.


    The genetic relationship between primitive granitoids, including high-Mg andesites (HMAs) and bajaites, and primary granitoids, or "pristine" adakites, has been vigorously debated since Defant and Drummond (1991; henceforth D&D) first applied the term "adakite" to refer to Cenozoic arc magmas (andesites and dacites) "associated with young subducting lithosphere", with low Y and Yb, low high-field strength elements (HFSEs), high Sr, and high Sr/Y and (La/Yb)N ratios "relative to island arc andesite-dacite-rhyolite". These characteristics were attributed to an origin for adakites by partial melting of basaltic crust within the subducting slab (hence "slab melts"). That such a process can produce melts with the characteristics described by D&D has since been largely confirmed by dehydration melting experiments on hydrous metabasalt at ~1-4 GPa. Attention was also drawn to the geochemical similarities between "adakites" and large-ion lithophile element (LILE)-enriched, high-field strength element (HFSE) depleted magnesian andesites (HMAs) from Adak Island in the western Aleutians, first described by Kay (1978), implying a genetic relationship between primary granitoid (adakites) formed by partial melting of basaltic ocean crust in the subducting slab, transformed to garnet-amphibolite or eclogite, and primitive magnesian andesites (HMAs) with high Mg-numbers (Mg# = molar Mg/(Mg+Fe)x100) and high concentrations of Ni and Cr. What then is the true origin of these enigmatic arc magmas, with both crustal and mantle, derivative and primitive, geochemical signatures? Kay (1978) suggested a "hybrid" model, in which "hydrous melting of eclogite (slab melting) results in a small volume of dacitic melt ("pristine adakite"), which rises into the hotter overlying peridotite wedge and equilibrates with olivine and orthopyroxene, reacting with olivine until it becomes andesitic". In this paper, I will discuss the results of melt-rock reaction experiments modelling this peridotite

  2. Mantle amphibole control on arc and within-plate chemical signatures: Quaternary lavas from Kurdistan Province, Iran (United States)

    Kheirkhah, M.; Allen, M. B.; Neill, I.; Emami, M. H.; McLeod, C.


    New analyses of Quaternary lavas from Kurdistan Province in west Iran shed light on the nature of collision zone magmatism. The rocks are from the Turkish-Iranian plateau within the Arabia-Eurasia collision. Compositions are typically basanite, hawaiite and alkali basalt. Sr-Nd isotope values are close to BSE, which is similar to Quaternary alkali basalts of NW Iran, but distinct from a depleted source melting under Mount Ararat. The chemical signatures suggests variable melting of two distinct sources. One inferred source produced melts with La/Nb from~3.5 to~1.2, which we model as the result of depletion of amphibole during ≤1% melting in the garnet stability field. We infer phlogopite in the source of potassic lavas from Takab. Lithosphere delamination or slab break-off mechanisms for triggering melting are problematic, as the lithosphere is~150-200km thick. It is possible that the negative dT/dP section of the amphibole peridotite solidus was crossed as a result of lithospheric thickening in the collision zone. This explanation is conditional upon the mantle source being weakly hydrated and so only containing a small proportion of amphibole, which can be exhausted during small degrees of partial melting. Our model maybe viable for other magmatic areas within orogenic plateaux, e.g. northern Tibet. Depletion of mantle amphibole may also help explain larger scale transitions from arc to within-plate chemistry in orogens, such as the Palaeogene Arabia-Eurasia system.

  3. The origin of spongy texture in minerals of mantle xenoliths from the Western Qinling, central China (United States)

    Su, Ben-Xun; Zhang, Hong-Fu; Sakyi, Patrick Asamoah; Yang, Yue-Heng; Ying, Ji-Feng; Tang, Yan-Jie; Qin, Ke-Zhang; Xiao, Yan; Zhao, Xin-Miao; Mao, Qian; Ma, Yu-Guang


    Spongy textures are observed in mantle peridotite xenoliths hosted in Cenozoic kamafugites from the Western Qinling, central China. These textures are mainly developed in clinopyroxenes and spinels, and occur as spongy rims consisting of low-Na clinopyroxene, ilmenite, and bubbles, enclosing nonspongy cores. The ilmenites and bubbles exhibit shapes and sizes that vary with the width of the spongy rims. The spongy-textured minerals preserve primary shapes and well-defined grain boundaries and do not show apparent interaction with contact minerals or observed melts except the subsequent melts forming melt pockets. The xenocrysts display reactive zoning textures with host magmas rather than spongy textures. Compositionally, the spongy rims are enriched in Ca, Ti, and most trace elements, have high Cr#, and are depleted in Na, Al, Fe, AlVI, and AlIV/AlVI compared with the cores. These observations suggest that melts/host magmas did not play any significant role in the formation of the spongy textures. We therefore propose that spongy-textured clinopyroxenes and spinels in Western Qinling peridotite xenoliths developed from a decompression-induced partial melting event prior to formation of melt pockets and xenolith entrainment in host magmas.

  4. Mantle rock exposures at oceanic core complexes along mid-ocean ridges

    Directory of Open Access Journals (Sweden)

    Ciazela Jakub


    Full Text Available The mantle is the most voluminous part of the Earth. However, mantle petrologists usually have to rely on indirect geophysical methods or on material found ex situ. In this review paper, we point out the in-situ existence of oceanic core complexes (OCCs, which provide large exposures of mantle and lower crustal rocks on the seafloor on detachment fault footwalls at slow-spreading ridges. OCCs are a common structure in oceanic crust architecture of slow-spreading ridges. At least 172 OCCs have been identified so far and we can expect to discover hundreds of new OCCs as more detailed mapping takes place. Thirty-two of the thirty-nine OCCs that have been sampled to date contain peridotites. Moreover, peridotites dominate in the plutonic footwall of 77% of OCCs. Massive OCC peridotites come from the very top of the melting column beneath ocean ridges. They are typically spinel harzburgites and show 11.3–18.3% partial melting, generally representing a maximum degree of melting along a segment. Another key feature is the lower frequency of plagioclase-bearing peridotites in the mantle rocks and the lower abundance of plagioclase in the plagioclase-bearing peridotites in comparison to transform peridotites. The presence of plagioclase is usually linked to impregnation with late-stage melt. Based on the above, OCC peridotites away from segment ends and transforms can be treated as a new class of abyssal peridotites that differ from transform peridotites by a higher degree of partial melting and lower interaction with subsequent transient melt.

  5. Isla Isabel (Nayarit, México): Quaternary alkalic basalts with mantle xenoliths erupted in the mouth of the Gulf of California (United States)

    Housh, Todd B.; Aranda-Gómez, José Jorge; Luhr, James F.


    Isla Isabel is a small island (~ 1.06 km 2) off the coast of Nayarit where intraplate-type alkali-basaltic volcanic rocks erupted during the Quaternary, and one of two Mexican islands (along with Isla Guadalupe) where it is known that the alkali basalts have transported peridotite xenoliths to the surface. The volume of the island is estimated at approximately 4.2 km 3; the volume above sea level is estimated to be approximately 0.03 km 3. The dominant volcanic rock type exposed above sea level is analcime-cemented basaltic tuff-breccia, the rapidly lithified products of hydrovolcanic eruptions; however, subaerial scoria-fall deposits and lava flows are also present. Ten analyzed Isla Isabel volcanic rocks are relatively homogeneous alkali basalts with 4.5-6.4 wt.% nepheline in the CIPW norm. Compared to other analyzed Mexican intraplate-type volcanic rocks from oceanic and continental localities, at the same MgO content, the Isabel samples are depleted in TiO 2, FeO total, and Na 2O, and moderately incompatible trace elements (e.g., Sm, Eu, Tb, and Zr). On the other hand, they are enriched in CaO and Al 2O 3, and relatively compatible trace elements (e.g., Lu and Sc). It is suggested that relative to other Mexican intraplate alkaline lavas the Isabel lavas were derived by higher degrees of melting at lower pressures of mantle that is characterized by ancient HIMU-like isotope systematics. Isabel olivine phenocrysts and their spinel inclusions range to Mg#s that overlap with the peridotite minerals in the accompanying mantle xenoliths, which has not been reported at other Mexican xenolith localities. Olivine-hosted glass inclusions in rapidly quenched scoria yielded the first estimates for volatile contents in Mexican intraplate-type mafic melts; maximum values are: 1.3 wt.% H 2O, 2241 ppm CO 2, 1500 ppm F, 1300 ppm S, and 1200 ppm Cl. Isabel peridotites (Ol ± Hy ± Di + Sp ± Pl) include dunites, harzburgites, and rare plagioclase-harzburgite, and are depleted in

  6. Compositional Evolution of the Mantle (United States)

    Bennett, V. C.


    The mantle is the Earth's largest chemical reservoir comprising 82% of its total volume and 65% of its mass. The mantle constitutes almost all of the silicate Earth, extending from the base of the crust (which comprises only 0.6% of the silicate mass) to the top of the metallic core at 2,900 km depth. The chemical compositions of direct mantle samples such as abyssal peridotites (Chapter 2.04) and peridotite xenoliths (Chapter 2.05), and of indirect probes of the mantle such as basalts from mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) (Chapter 2.03), and some types of primitive granites, tell us about the compositional state of the modern mantle, with ever increasingly detailed information providing strong evidence for chemical complexity and heterogeneity at all scales (Chapter 2.03). This chemical heterogeneity must reflect the complex physical interplay of a number of distinct long-lived geochemical reservoirs that are identified primarily by their radiogenic isotopic compositions.Many of the chapters in this volume provide detailed images of the current chemical and physical state of the Earth's mantle, whereas other contributions examine the starting composition for the Earth (Chapter 2.01). This chapter attempts to link these two areas by tracking the composition of the mantle through time. The first part of this chapter is a summary of the empirical evidence for secular change in the chemical composition of the mantle from the formation of the Earth at 4.56 Ga throughto the present day. The emphasis is on results from the long-lived radiogenic isotopic systems, in particular 147Sm-143Nd, 176Lu-176Hf, 87Rb-87Sr, and 187Re-187Os systems as these isotopic data provide some of the best constraints on the composition of the mantle in the first half of Earth history, and the timing and extent of chemical differentiation that has affected the mantle over geologic time. Selected trace element data and the "short-lived" 146Sm-142Nd isotopic systems

  7. Genesis of andesitic boninitic magmas at mid-ocean ridges by melting of hydrated peridotites: Geochemical evidence from DSDP Site 334 gabbronorites (United States)

    Nonnotte, Philippe; Ceuleneer, Georges; Benoit, Mathieu


    The gabbronoritic cumulates drilled at DSDP Site 334 (Mid-Atlantic Ridge off the FAMOUS area) are neither crystallization products of the associated basalts, nor from any MORB composition documented along ocean ridges. Their parent melts are richer in SiO 2 than MORB at a given MgO content, as attested by the crystallization sequence starting with an olivine + calcic and sub-calcic pyroxene assemblages. These melts are issued from a source highly depleted in incompatible elements, likely residual peridotite left after MORB extraction. To understand the role of water in the genesis of these lithologies whose occurrence in a mid-ocean ridge setting is rather puzzling, we performed a geochemical study on clinopyroxene separates following an analytical protocol able to remove the effects of water rock interactions post-dating their crystallization. Accordingly, the measured isotopic signatures can be used to trace magma sources. We find that Site 334 clinopyroxenes depart from the global mantle correlation: normal MORB values for the 143Nd / 144Nd ratio (0.51307-0.51315) are associated to highly radiogenic 87Sr / 86Sr (0.7034-0.7067) ratios. This indicates that the parent melts of Site 334 cumulates are issued from a MORB source but that seawater contamination occurred at some stage of their genesis. The extent of contamination, traced by the Sr isotopic signature, is variable within all cumulates but more developed for gabbronorites sensus stricto, suggesting that seawater introduction was a continuous process during all the magmatic evolution of the system, from partial melting to fractional crystallization. Simple masse balance calculations are consistent with a contaminating agent having the characters of a highly hydrated (possibly water saturated) silica-rich melt depleted in almost all incompatible major, minor and trace elements relative to MORB. Mixing in various proportions of contaminated melts similar to the parent melts of Site 334 cumulates with MORB can

  8. Subduction of shallowly formed arc cumulates: Evidence from clinopyroxene compositions of garnet peridotites in the Rio San Juan Complex, northern Dominican Republic (United States)

    Hattori, K.; Tubrett, M.; Saumur, B.-M.; Guillot, S.


    Garnet peridotites are very rare in oceanic subduction complexes, with only two reported occurrences. One is in the Sambagawa metamorphic belt in Shikoku, Japan, and the other example is in the southern part of the Rio Juan Complex, northern Dominican Republic. In both locations, garnet peridotite occurs in close association with eclogites in high metamorphic grade of the terranes. The Rio Juan Complex represents rocks formed during the southwestern subduction of the Proto-Caribbean oceanic plate below the Carribean Plate during late Cretaceous to early Eocene. Garnet peridotites (clinopyroxene[Cpx]-bearing dunite, wehrlite, olivine clinopyroxenite) occur as large (garnet formation under high pressures. The geochemical data suggest that Cpx-rich cumulates formed at a relatively shallow level in the mantle wedge, and subsequently drug towards the subduction plane by mantle flow, followed by metamorphism that formed garnet in the subduction channel

  9. Petrologic, Geochemical and Isotopic Study of 3.1Ga Peridotite-Chromitite Suite from the Western Dharwar Craton, India: Evidence for Recycling of Oceanic Crust in the Mesoarchean (United States)

    Mondal, S. K.; Mukherjee, R.; Rosing, M. T.; Frei, R.; Waight, T.


    Sill-like ultramafic intrusions with massive chromitite bodies are common in Archean greenstone belts such as in the Zimbabwe craton and in the Singhbhum and Dharwar cratons of the Indian shield. In the western Dharwar craton deformed massive chromitites are hosted within dissected peridotitic rocks of the Nuggihulli schist belt and part of early to mid-Archean greenstone belts. The linear ultramafic belts are often associated with gabbroic rocks containing Ti-V-bearing magnetite bands and surrounded by the tonalite-trondhjemite- granodioritic (TTG) suites of rocks. Detailed electron microprobe study reveals high Cr-numbers (0.72- 0.87) and moderate Mg-numbers (0.47-0.59) for chromite and very high Fo content (~ Fo97) for interstitial olivine in massive chromitites from the seams. Our study suggests that the original igneous compositions of chromite grains are preserved in some of the massive chromitites, whereas accessory chromites in serpentinized peridotites are extensively altered to ferritchromit. The primitive composition of chromites along with high Fo content of olivine suggest parental melts produced by high degrees of partial melting of the source mantle. The tectonic discrimination plots plus parental melt calculations indicate derivation from a high-Mg komatiitic magma having a liquid Al2O3 ~ 9.64 wt% and liquid (FeO/MgO)wt% ~ 0.2 which is similar to the compositions recorded for komatiitic rocks within the schist belt. A Pb-isotope study of fourteen samples from this suite including peridotite, chromitite, gabbro and ultramafic-mafic schist rocks define an isochron yielding an age of 3159±180 Ma which is similar to the 207Pb/206Pb zircon age of ~ 3.1 Ga of the surrounding TTG-suite (Bidyananda et al. 2003). Ten samples from this suite have strongly scattering Sm-Nd model ages that range from 2.34 to 3.96 Ga (average 3.1 Ga) with respect to a depleted mantle source. The average initial Nd ratio, calculated at the presumed emplacement age of 3.1 Ga, is

  10. Nature of the mantle roots beneath the North American craton: mantle xenolith evidence from Somerset Island kimberlites (United States)

    Schmidberger, S. S.; Francis, D.


    The recently discovered Nikos kimberlite on Somerset Island, in the Canadian Arctic, hosts an unusually well preserved suite of mantle xenoliths dominated by garnet-peridotite (lherzolite, harzburgite, dunite) showing coarse and porphyroclastic textures, with minor garnet-pyroxenite. The whole rock and mineral data for 54 Nikos xenoliths indicate a highly refractory underlying mantle with high olivine forsterite contents (ave. Fo=92.3) and moderate to high olivine abundances (ave. 80 wt.%). These characteristics are similar to those reported for peridotites from the Archean Kaapvaal and Siberian cratons (ave. olivine Fo=92.5), but are clearly distinct from the trend defined by oceanic peridotites and mantle xenoliths in alkaline basalts and kimberlites from post-Archean continental terranes (ave. olivine Fo=91.0). The Nikos xenoliths yield pressures and temperatures of last equilibration between 20 and 55 kb and 650 and 1300°C, and a number of the peridotite nodules appear to have equilibrated in the diamond stability field. The pressure and temperature data define a conductive paleogeotherm corresponding to a surface heat flow of 44 mW/m 2. Paleogeotherms based on xenolith data from the central Slave province of the Canadian craton require a lower surface heat flow (˜40 mW/m 2) indicating a cooler geothermal regime than that beneath the Canadian Arctic. A large number of kimberlite-hosted peridotites from the Kaapvaal craton in South Africa and parts of the Siberian craton are characterized by high orthopyroxene contents (ave. Kaapvaal 32 wt.%, Siberia 20 wt.%). The calculated modal mineral assemblages for the Nikos peridotites show moderate to low contents of orthopyroxene (ave. 12 wt.%), indicating that the orthopyroxene-rich mineralogy characteristic of the Kaapvaal and Siberian cratons is not a feature of the cratonic upper mantle beneath Somerset Island.

  11. Mantle wedge serpentinization effects on slab dips

    Directory of Open Access Journals (Sweden)

    Eh Tan


    Full Text Available The mechanical coupling between a subducting slab and the overlying mantle wedge is an important factor in controlling the subduction dip angle and the flow in mantel wedge. This paper investigates the role of the amount of mantle serpentinization on the subduction zone evolution. With numerical thermos-mechanical models with elasto-visco-plastic rheology, we vary the thickness and depth extent of mantle serpentinization in the mantle wedge to control the degree of coupling between the slab and mantle wedge. A thin serpentinized mantle layer is required for stable subduction. For models with stable subduction, we find that the slab dip is affected by the down-dip extent and the mantle serpentinization thickness. A critical down-dip extent exists in mantle serpentinization, determined by the thickness of the overriding lithosphere. If the down-dip extent does not exceed the critical depth, the slab is partially coupled to the overriding lithosphere and has a constant dip angle regardless of the mantle serpentinization thickness. However, if the down-dip extent exceeds the critical depth, the slab and the base of the overriding lithosphere would be separated and decoupled by a thick layer of serpentinized peridotite. This allows further slab bending and results in steeper slab dip. Increasing mantle serpentinization thickness will also result in larger slab dip. We also find that with weak mantle wedge, there is no material flowing from the asthenosphere into the serpentinized mantle wedge. All of these results indicate that serpentinization is an important ingredient when studying the subduction dynamics in the mantle wedge.

  12. Structure and emplacement of the Alpine-type peridotites from Beni Bousera, Rif, Morocco: A polyphase tectonic interpretation (United States)

    Reuber, Ingrid; Michard, André; Chalouan, Ahmed; Juteau, Thierry; Jermoumi, Bahija


    The Beni Bousera peridotite antiform, in the southern branch of the Gibraltar orocline exhibits a lherzolitic core, containing thin pyroxenite layers surrounded by harzburgites, then by dunites and garnet-bearing dunite. In the peripheral areas, the amount of pyroxenite greatly increases by the development of garnet-pyroxenite veins. These data indicate a gradient of partial-melting increasing upward. We conclude that a reversed geothermal gradient was temporarily established in the upper part of the peridotite, just under its migmatitic roof (kinzigite aureole, then sillimanite-gneiss). Study of ductile, penetrative structures and discussion of the later "cold" structures show that the foliation of the tectonites was originally flat, with a NW-SE-trending stretching lineation. Orthopyroxene crystals are used as tectonic markers. "Isostrain zones" indicate that simple shear and finite strain increase upward. We conclude that a ductile shear zone acted between peridotite and gneiss. Strain heating along this shear zone probably has been responsible for partial melting of the upper part of the peridotite. In our model, the peridotite emplacement into the mid-continental crust began during crustal extension. Then, as comparison with the Rondà massif suggests, an intracrustal thrusting stage was followed by high-temperature and relatively low-pressure metamorphism and by intrusion of acidic dikes. The uplift of the mantle slab continued by compressive upthrusting and isostatic doming. This complex, polyphase evolution probably extended beyond the Alpine orogeny, as suggested by Kornprobst.

  13. Archean Lithosphere Beneath Arctic Canada: Lu-Hf Isotope Systematics for Kimberlite-Hosted Garnet-Peridotites From Somerset Island (United States)

    Schmidberger, S. S.; Simonetti, A.; Francis, D.; Gariepy, C.


    Knowledge of the age of lithospheric mantle underlying the continents provides valuable constraints for the timing of formation and stabilization of Archean cratons. This study reports Lu-Hf isotopic data for garnet-peridotites, and their constituent garnets, from the Nikos kimberlite (100 Ma) on Somerset Island in the Canadian Arctic obtained using a Micromass IsoProbe multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) at GEOTOP-UQAM. The low temperature peridotites (1100 C; 160-190 km) and their 176Hf/177Hf(0.1Ga) isotopic compositions (0.28265-0.28333; \\epsilonHf(0.1Ga)=-2 to +22) are less radiogenic than those of the shallow xenoliths. A Lu-Hf isochron for six peridotites yields a mid Archean age of 3.4\\pm0.3 Ga and an initial 176Hf/177Hf ratio of 0.28101\\pm24. The remaining peridotites (n=9), in contrast, are characterized by extremely high (+35) initial \\epsilonHf(3.4Ga) values, which correlate negatively with their 176Lu/177Hf ratios, suggesting addition of Hf as a result of metasomatic interaction with the host kimberlite. The garnets from the low temperature (3.4 Ga old) peridotites are characterized by high 176Lu/177Hf ratios and define an errorchron age of 1.4\\pm0.2 Ga, which may reflect re-equilibration of Hf during kimberlite magmatism.

  14. New perspectives on primary magmas and related mantle sources from Mt. Etna (Sicily, Italy) (United States)

    Alesci, Giuseppe; Giacomoni, Pier Paolo; Coltorti, Massimo; Ferlito, Carmelo


    Mt. Etna is one of the most active volcano in the world and the understanding of its eruptive dynamics is fundamental to reduce volcanic risk related to the frequent volcanic events. In this respect the complete evolution of Mt. Etna magmatismand the reconstruction of its mantle source was considered performing a detailed sampling from Tholeiitic to Recent Mongibello periods. Backward mass balance fractionation models, starting from real and primitive analyzed magmas, allowed to reconstruct the "primary" and un-fractionated melt compositions.The Tholeiitic suite did not necessitate a backward reconstruction for the presence of real primary un-fractionated compositions. The least differentiated products for each alkaline period were identified and about 17 to 19% of a solid assemblage made up of Ol (87 to 100%) and Cpx (up to 13%) was added to re-equilibrate the basalts with mantle olivine (Fo87). A further subtraction of an average 23.3% of a solid assemblage constituted by Ol (7-18%), Cpx (26-55%) and Plag (21-48%) is needed to get to the most differentiated erupted lavas. To reach the most evolved terms ishypothesized the involvement of 27% Ol, 14% Cpx, 29% Opx, 15% Plg and 15% Cr-sp for a total amount of 53.1% of Gabbroic material.On average an estimation of about 40% of material should be considered beneath the volcanic edifice taking into account the entire volume of the products emitted by the volcano. Thus if according to [1] the whole volcano edifice amounts to about 370 km3, about 150 km3 of magma should be left behind from the magma on its way from the mantle to the surface. Although Etnean alkaline lavas appear enriched in K, Rb, Th and U and depleted in Ti, Y and Yb with respect to the Iblean magmas, their overall composition appear quite similar.Thus partial melting models were developed for each Etnean magmatic period, based on a mantle composition similar to that of the nearbyIblean lithosphere. Results indicate twodifferent sources for Tholeiitic

  15. Serpentinization: Getting water into a low permeability peridotite (United States)

    Ulven, Ole Ivar


    , we are able to improve the understanding of how water is able to penetrate deep into the crust to pervasively serpentinize the initially low permeability peridotite. Jamtveit, B., Austrheim, H., and Putnis, A., ``Disequilibrium metamorphism of stressed lithosphere'', Earth-Sci. Rev. 154, 2016, pp. 1 - 13. Plümper, O., Røyne, A., Magraso, A., and Jamtveit, B., ``The interface-scale mechanism of reaction-induced fracturing during upper mantle serpentinization'', Geology 40, 2012, pp. 1103 - 1106. Rudge, J. F., Kelemen, P. B., and Spiegelman, M., ``A simple model of reaction induced cracking applied to serpentinization and carbonation of peridotite'', Earth Planet. Sc. Lett. 291, 2010, Issues 1-4, pp. 215 - 227. Ulven, O. I., Storheim, H., Austrheim, H., and Malthe-Sørenssen, A., ``Fracture Initiation During Volume Increasing Reactions in Rocks and Applications for CO2 Sequestration'', Earth Planet. Sc. Lett. 389C, 2014a, pp. 132 - 142, doi:10.1016/j.epsl.2013.12.039. Ulven, O. I., Jamtveit, B., and Malthe-Sørenssen, A., ``Reaction-driven fracturing of porous rock'', J. Geophys. Res. Solid Earth 119, 2014b, doi:10.1002/2014JB011102. Ulven, O. I., and Sun, W.C., ``Borehole breakdown studied using a two-way coupling dual-graph lattice model for fluid-driven fracture'', under review.

  16. Negative CO2 emissions via subsurface mineral carbonation in fractured peridotite (United States)

    Kelemen, P. B.; Matter, J.


    Uptake of CO2 from surface water via mineral carbonation in peridotite can be engineered to achieve negative CO2 emissions. Reaction with peridotite, e.g., CO2 + olivine (A), serpentine (B) and brucite (C), forms inert, non-toxic, solid carbonates such as magnesite. Experimental studies show that A can be 80% complete in a few hours with 30 micron powders and elevated P(CO2) [1,2,3]. B is slower, but in natural systems the rate of B+C is significant [4]. Methods for capture of dilute CO2 via mineral carbonation [4,5,6,7] are not well known, though CO2 storage via mineral carbonation has been discussed for decades [8,9]. Where crushed peridotite is available, as in mine tailings, increased air or water flow could enhance CO2 uptake at a reasonable cost [4,5]. Here we focus on enhancing subsurface CO2 uptake from surface water flowing in fractured peridotite, in systems driven by thermal convection such as geothermal power plants. Return of depleted water to the surface would draw down CO2 from the air [6,7]. CO2 uptake from water, rate limited by flow in input and output wells, could exceed 1000 tons CO2/yr [7]. If well costs minus power sales were 0.1M to 1M and each system lasts 10 years this costs < 10 to 100 per ton CO2. As for other CCS methods, upscaling requires infrastructure resembling the oil industry. Uptake of 1 Gt CO2/yr at 1000 t/well/yr requires 1M wells, comparable to the number of producing oil and gas wells in the USA. Subsurface CO2 uptake could first be applied in coastal, sub-seafloor peridotite with onshore drilling. Sub-seafloor peridotite is extensive off Oman, New Caledonia and Papua New Guinea, with smaller amounts off Spain, Morocco, USA, etc. This would be a regional contribution, used in parallel with other methods elsewhere. To achieve larger scale is conceivable. There is a giant mass of seafloor peridotite along slow-spreading mid-ocean ridges. Could robotic drills enhance CO2 uptake at a reasonable cost, while fabric chimneys

  17. Melting the hydrous, subarc mantle: the origin of primitive andesites (United States)

    Mitchell, Alexandra L.; Grove, Timothy L.


    of mantle lherzolite under anhydrous conditions. This study shows that experimental melts of a FM peridotite plus the addition of alkalis reproduce the compositions of natural PAs in SiO2, Al2O3, TiO2, Cr2O3, MgO, and Na2O at 1.0-1.2 GPa and H2O contents of 0-7 wt%. Our results also suggest that PAs form under a maximum range of extents of melting from F = 0.2-0.3. The CaO contents of the melts produced are 1-5 wt% higher than the natural samples. This is not a result of a depleted source composition or of extremely high extents of melt but is potentially caused by a very low CaO content contribution from deeper in the mantle wedge.

  18. Nickel isotopic composition of the mantle (United States)

    Gall, Louise; Williams, Helen M.; Halliday, Alex N.; Kerr, Andrew C.


    This paper presents a detailed high-precision study of Ni isotope variations in mantle peridotites and their minerals, komatiites as well as chondritic and iron meteorites. Ultramafic rocks display a relatively large range in δ60 Ni (permil deviation in 60 Ni /58 Ni relative to the NIST SRM 986 Ni isotope standard) for this environment, from 0.15 ± 0.07‰ to 0.36 ± 0.08‰, with olivine-rich rocks such as dunite and olivine cumulates showing lighter isotope compositions than komatiite, lherzolite and pyroxenite samples. The data for the mineral separates shed light on the origin of these variations. Olivine and orthopyroxene display light δ60 Ni whereas clinopyroxene and garnet are isotopically heavy. This indicates that peridotite whole-rock δ60 Ni may be controlled by variations in modal mineralogy, with the prediction that mantle melts will display variable δ60 Ni values due to variations in residual mantle and cumulate mineralogy. Based on fertile peridotite xenoliths and Phanerozoic komatiite samples it is concluded that the upper mantle has a relatively homogeneous Ni isotope composition, with the best estimate of δ60Nimantle being 0.23 ± 0.06‰ (2 s.d.). Given that >99% of the Ni in the silicate Earth is located in the mantle, this also defines the Ni isotope composition of the Bulk Silicate Earth (BSE). This value is nearly identical to the results obtained for a suite of chondrites and iron meteorites (mean δ60 Ni 0.26 ± 0.12‰ and 0.29 ± 0.10‰, respectively) showing that the BSE is chondritic with respect to its Ni isotope composition, with little to no Ni mass-dependent isotope fractionation resulting from core formation.

  19. Water in the Cratonic Mantle Lithosphere (United States)

    Peslier, A. H.


    The fact that Archean and Proterozoic cratons are underlain by the thickest (>200 km) lithosphere on Earth has always puzzled scientists because the dynamic convection of the surrounding asthenosphere would be expected to delaminate and erode these mantle lithospheric "keels" over time. Although density and temperature of the cratonic lithosphere certainly play a role in its strength and longevity, the role of water has only been recently addressed with data on actual mantle samples. Water in mantle lithologies (primarily peridotites and pyroxenites) is mainly stored in nominally anhydrous minerals (olivine, pyroxene, garnet) where it is incorporated as hydrogen bonded to structural oxygen in lattice defects. The property of hydrolytic weakening of olivine [4] has generated the hypothesis that olivine, the main mineral of the upper mantle, may be dehydrated in cratonic mantle lithospheres, contributing to its strength. This presentation will review the distribution of water concentrations in four cratonic lithospheres. The distribution of water contents in olivine from peridotite xenoliths found in kimberlites is different in each craton (Figure 1). The range of water contents of olivine, pyroxene and garnet at each xenolith location appears linked to local metasomatic events, some of which occurred later then the Archean and Proterozoic when these peridotites initially formed via melting. Although the low olivine water contents ( 6 GPa at the base of the Kaapvaal cratonic lithosphere may contribute to its strength, and prevent its delamination, the wide range of those from Siberian xenoliths is not compatible with providing a high enough viscosity contrast with the asthenophere. The water content in olivine inclusions from Siberian diamonds, on the other hand, have systematically low water contents (water contents. The olivine inclusions, however, may have been protected from metasomatism by their host diamond and record the overall low olivine water content of

  20. Geochemistry of the Dongdegou Peridotites in Southwestern Tianshan Mountains and Its Geological Significance%西南天山东德沟橄榄岩的地球化学特征及其地质意义

    Institute of Scientific and Technical Information of China (English)

    孔凡梅; 吴苏; 李旭平; 李守军


    The Dongdegou peridotites are located at the northern of orogenic belt between the Tarim Plate and the Yili-Central Tianshan Plate, southwestern Tianshan Mountains, China, have a lot of amphibole, phlogopite and other hydrous minerals, which shows various degrees of metamorphism. Major elements of the peridotites change in a wider range and toward Al-rich or Ca-rich tendency during the late alteration, their Nb/Ta and Zr/Hf ratios display that they have the characteristics of refractory mantle residuals. The chondrite-normalized REE patterns of the peridotites are types of LREE-enriched patterns, based on the obvious fractionation between LREE and HREE, and also enriched in LILE of Rb, Ba, Sr and the radioactive element Th. However, the major elements are refractory, when the rare earth elements or incompatible trace elements are enriched in the same samples. Thus, the decoupling phenomenon indicates the major elements and the trace elements have their own evolutionary characteristics, respectively. Therefore, the Dongdegou peridotites are the depleted wedge mantle residuals after partial melting, and the characteristics of trace elements indicate they might have been metasomatized by the melts or fluids from a subducted slab. Geochemical studies of the peridotites illustrate also that they most likely come from a SSZ Mantle wedge, just as the result from the research of their mineralogy and petrology.%东德沟橄榄岩出露于中国新疆伊犁—中天山板块和塔里木板块之间西南天山造山带的北缘,含大量角闪石、金云母等含水矿物,显示全部遭受了不同程度的交代变质作用.岩石主量元素含量变化较大,其后期蚀变基本向富铝和富钙两个方向变化,其Nb/Ta和Zr/Hf值显示难熔地幔残余的特征.该区橄榄岩轻重稀土分馏较明显,稀土元素配分曲线总体为LREE富集的右倾型,且富集大离子亲石元素Rb、Ba、Sr和放射性元素Th,但岩石呈现不一致的解耦现象,即

  1. Role of the transition zone and 660 km discontinuity in mantle dynamics (United States)

    Ringwood, A. E.


    Recent seismic evidence suggests that subducted slabs experience resistance to further descent when they encounter the 660 km seismic discontinuity. Several possible causes of this resistance are evaluated. It is concluded that the chemical composition of the lower mantle is similar to that of the upper mantle, and that compositional change is therefore unlikely to be the cause of resistance to slab penetration. The proposal that a large increase of viscosity at the 660 km discontinuity impedes descending slabs is also rejected. However, three other factors are identified, each of which is capable of causing substantial resistance to descending slabs: (1) the negative slope of the transformation of silicate spinel to Mg-perovskite+magnesiowuestite; (2) differentiation of oceanic lithosphere into basaltic and depleted peridotitic layers, causing the slab to be buoyant compared with surrounding mantle pyrolite between depths of 660-800 km; (3) the accumulation of former oceanic crust to produce a gravitationally stable layer of garnetite (about 50 km thick) on top of the 660 km discontinuity. The combined effects of these sources of resistance provide a filter for subducted slabs. Those slabs with seismic zones extending below 600 km may possess sufficient negative buoyancy and strength to overcome the barriers and penetrate into the lower mantle. However, the resistance causes strong buckling and plastic thickening of these slabs, which accumulate to form huge blobs or 'megaliths' underneath the 660 km discontinuity. In contrast, slabs with seismic zones extending no deeper than 300 km possess much smaller degrees of negative buoyancy and strength and hence are unable to penetrate the 660 km discontinuity. Slabs of this type are recycled within the transition zone and upper mantle. Mixing and petrological homogenization processes are less efficient in the transition zone than in the upper mantle (above 400 km). The transition zone is composed mainly of ancient slabs

  2. Study of seismic tomography in Panxi paleorift area of southwestern China--Structural features of crust and mantle and their evolution

    Institute of Scientific and Technical Information of China (English)


    Structural features of the typical continental paleorift in Panxiarea are revealed by seismic tomography. (1) In the profile along the minor axis of Panxi paleorift, we found alternating high and low-velocity strips existing at different depths in the crust, presenting itself as a "sandwich" structure. The existence of these high and low-velocity anomaly strips is related to the basal lithology in the rift area. (2) An addition layer with velocity values of 7.1-7.5 km/s and 7.8 km/s exists from the base of lower crust to uppermost mantle and its thickness is about 20 km. Some study results indicate that the addition layer results from the invasion of mantle material. (3) A lens-shaped high-velocity body surrounded by relatively low-velocity material is observed at depths of 110-160 km between Huaping and Huidong in the axis of the paleorift. This is the first time to discover it in the upper mantle of the paleorift. Based on the results of geology, petrology and geochemistry, we infer that the formation of the addition layer and the lens-shaped high-velocity body in the upper mantle are related to the deep geodynamic process of generation, development and termination of the rift. On the one hand, the upwelling of asthenosphere mantle caused partial melting, and then the basaltic magma from the partial melted material further resulted in underplating and formed the crustal addition layer. On the other hand, the high-density content of mineral facies was increased in the residual melted mass of intensely depleted upper mantle, formed by basalt withdrawing. The solid-melt medium in the depleted upper mantle was mainly an accumulation of garnet and peridotite because the heating effect of lithosphere was relatively weakened in the later riftogenesis, so that a lens-shaped high-density and high-velocity zone was produced in the upper mantle. The results indicate that the energy and material exchange between asthenosphere and lithosphere and remarkable underplating would

  3. Melt-peridotite reactions in upwelling EM1-type eclogite bodies

    DEFF Research Database (Denmark)

    Søager, Nina; Holm, Paul Martin


    the Payenia volcanic province (34–38 °S) in Argentina, for which Sr, Nd and double-spike Pb isotope ratios are presented, and from other north Patagonian volcanic fields may provide details of the eclogite melt–peridotite reactions taking place in the melting column of an upwelling OIB-type mantle....... The isotopic composition of the uncontaminated lavas is highly restricted but the Payenia basalts fall in two distinct trace element groups termed the high and low Nb/U groups, which both have EM1-type trace element patterns but with subtle differences that cannot be explained by contamination, fractionation...

  4. Estimating Upper Mantle Hydration from In Situ Electrical Conductivity (United States)

    Behrens, J.; Constable, S.; Heinson, G.; Everett, M.; Weiss, C.; Key, K.


    The electrical conductivity of 35-40 Ma Pacific plate has been measured in situ; one robust result is the presence of bulk anisotropy in the lithospheric upper mantle. We interpret this anisotropy to be a result of hydrothermal circulation into the upper mantle along spreading-ridge-parallel normal faults: the associated zones of serpentinized peridotite provide the pathways of enhanced electrical conductivity required by the data. Our modeling bounds the range of possible anisotropic ratios, which are then used to estimate the amount of water required to serpentinize the requisite amounts of peridotite. These data sets, however, do not indicate anisotropy in the bulk conductivity of the crust, nor in the asthenospheric mantle. This second point is significant, as recent measurements of sub-continental asthenospheric conductivity have been interpreted to indicate anisotropy aligned with present plate motion, with the diffusion of hydrogen through olivine advanced as an explanation.

  5. Petrology, Mineralogy and Geochemistry of the Emeishan Continental Flood Basalts, SW China:Evidence for Activity of Mantle Plumes

    Institute of Scientific and Technical Information of China (English)

    ZHANG Zhaochong; HAO Yanli; WANG Fusheng; John J. MAHONEY


    Electronic microprobe analyses for olivine, clinopyroxene and Cr-spinel in picrites, which we have discovered recently in the Emeishan continental flood basalt province (ECFBP), show that the olivine is rich in Mg, and that Cr-spinel is rich in Cr. Based on the olivine-melt equilibrium, the primary parental melt compositions are calculated. The high-Mg olivine-hosted picrite can be regarded as parental melt. Thus, the melting temperature and pressure are estimated:T=1600℃ and P=4.5 GPa. It suggests that the picrites are connected with the activity of mantle plumes. Their major element composition is comparable to many other CFBs by their high Fe8, (CaO/Al2O3)8 and low Nas, indicating a high pressure. All rocks display a similar chondrite-normalized REE patterns, i.e., enrichment of LREE, relative depletion of HFSE and absence of negative Nb and Ta but depletion in P and K. Some incompatible element ratios, such as La/Ta, La/Sm, (La/Nb)pM, (Th/Ta)pM, are in a limited range, show that they were derived from the mantle plume, and there was no or little crustal contamination during magma ascent en route to the surface. They were generated by 7% partial melting of garnet peridotite. The axis of the plume might be located beneath Lijiang Town, Yunnan province.

  6. Subduction-zone cycling of nitrogen in serpentinized mantle rocks (United States)

    Halama, R.; Bebout, G. E.; John, T.; Scambelluri, M.


    Nitrogen (N) has shown great potential as a geochemical tracer of volatiles recycling, in part because of large differences in the N isotope composition of the various Earth reservoirs. The subduction flux of N in serpentinized oceanic mantle could be as important as N input flux in oceanic crust and even sediment because, although its N concentrations are lower, its volume is potentially far greater than that of the crust/sediment. However, recycling of oceanic mantle rocks is still poorly constrained for the N cycle, and N isotope data for subduction-related ultramafic rocks are scarce [1]. The primary goal of this study is to characterize the subduction flux of N in subducting altered oceanic mantle by documenting concentrations and isotopic compositions of N in mantle rocks that reflect different stages of the metamorphic subduction zone cycle. The results are crucial to assess the composition of N recycled into the mantle, to determine the extent to which N can be retained in subducted mantle rocks to depths approaching those beneath arcs, and to balance N subduction-zone inputs with outputs in arc volcanic gases. Moreover, information has been gained regarding the redistribution and isotope fractionation of N via ultramafic dehydration and metamorphic fluid-rock interaction. The samples analyzed in this study are ultramafic rocks from shallow oceanic environments to increasing P-T conditions up to depths of ~70 km. Three distinct metamorphic grades, reflecting seafloor fluid uptake, water release due to brucite breakdown and the final antigorite breakdown, were investigated: 1. Pre-subduction serpentinized mantle peridotite from non-subducted ophiolite sequences from the Northern Apennines, Italy (Monte Nero). 2. Eclogite-facies antigorite serpentinites from the Ligurian Alps, Italy (Erro Tobbio). 3. Eclogite-facies chlorite harzburgites derived from dehydration of serpentinites from the Betic Cordillera, Spain (Cerro de Almirez). The pre

  7. Geochemical, mineralogical and Re-Os isotopic constraints on the origin of Tethyan oceanic mantle and crustal rocks from the Central Pontides, northern Turkey (United States)

    Çelik, Ömer Faruk; Marzoli, Andrea; Marschik, Robert; Chiaradia, Massimo; Mathur, Ryan


    Chromite, ultramafic and mafic rocks from Eldivan, Yapraklı, Ayli Dağ, Küre, Elekdağ and Kızılırmak in northern Turkey have been studied to determine their mineral and whole-rock geochemical, and Re-Os isotope geochemical characteristics. Most of the studied peridotites display depleted but commonly V-shaped chondrite-normalized rare-earth element (REE) patterns while some peridotites as well as pyroxenites from all areas exhibit light REE depleted patterns. Olivine (forsterite 82 to 92 mol%) and spinel (chromium number 13 to 63) in the studied peridotites exhibit a wide range of compositions. Compositions of spinels suggest that peridotites from Eldivan, Ayli Dağ and Küre experienced relatively large degrees of partial melting ( 15 and 19 wt%), whereas those of the Kızılırmak area most likely reflect lower melting degrees ( 4-6 wt%). Whole-rock and mineral chemical data indicate that the ultramafic rocks are similar to abyssal and supra-subduction zone peridotites. The ultramafic rocks of the investigated areas exhibit a wide range of 187Re/188Os (0.12 to 6.6) and measured 187Os/188Os (0.122-1.14), while the basaltic rocks from Küre, Eldivan and Kızılırmak areas have high 187Re/188Os (128-562) and measured 187Os/188Os (0.724-1.943). On the other hand, chromite from Eldivan, Elekdağ and Kızılırmak show high Os contents (21.81-44.04 ppb) and low 187Re/188Os (0.015-0.818) and 187Os/188Os (0.122-0.133). Re-Os model ages (TChur) for all analyzed samples yielded scattered ages ranging from Jurassic to Proterozoic. Overall, geochemical data are interpreted to reflect different degrees of partial melting, melt - rock interactions and metasomatic effects that produced a heterogeneous mantle in a supra-subduction setting.

  8. Ultra-refractory mantle domains in the Luqu ophiolite (Tibet): Petrology and tectonic setting (United States)

    Zhang, Chang; Liu, Chuan-Zhou; Wu, Fu-Yuan; Ji, Wen-Bin; Liu, Tong; Xu, Yang


    Fresh mantle peridotites occur in the Luqu ophiolite, which is located at the central segment of the Yarlung-Tsangpo Suture Zone in the Tibetan Plateau. This study presents major and trace element compositions of clinopyroxene from harzburgites and dunites of the Luqu ophiolite, with the aim to study melt depletion and late-stage melt-rock interaction processes. The studied harzburgites have refractory to ultra-refractory compositions, as indicated by low whole-rock Al2O3 and high MgO contents, as well as high spinel Cr# values. Clinopyroxenes in the Luqu harzburgites are variably depleted in light rare earth elements (LREEs) relative to the middle and heavy rare earth elements (MREEs and HREEs), with Yb contents of 1.4-3.4 times of CI chondrites. Modeling results suggest that the Luqu harzburgites have experienced up to 8% garnet-facies melting followed by 10-18% fractional melting in the stability field of spinel. In both cases melting occurred under anhydrous conditions. This suggests that the Luqu ophiolite originated from a mid-ocean ridge setting rather than a supra-subduction zone environment. Nevertheless, the high degrees of partial melting are inconsistent with the thin oceanic crustal sequence preserved in the Luqu ophiolite, implying that the Luqu peridotites might have experienced melt extraction prior to entering into the spreading centers of the Neo-Tethyan Ocean during the Early Cretaceous. Clinopyroxenes in the dunites are interstitial among olivine grains. They display slightly LREE-enriched patterns and have higher REE contents than those in the harzburgites. They also show markedly positive anomalies in Sr, Zr and Hf. These features suggest that the dunites were generated through reaction between percolating melts and the harzburgites, during which the interstitial clinopyroxenes formed.

  9. Abiotic Methane in Land-Based Serpentinized Peridotites: New Discoveries and Isotope Surprises (United States)

    Whiticar, M. J.; Etiope, G.


    Until 2008, abiotic methane in land-based serpentinized ultramafic rocks was documented (including gas and C- and H- isotope compositions) only at sites in Oman, Philippines, New Zealand and Turkey. Methane emanates from seeps and/or hyperalkaline water springs along faults and is associated with molecular hydrogen. These were considered to be very unusual and rare occurrences of gas. Now, methane is documented for peridotite-based springs or seeps (in ophiolites, orogenic massifs or intrusions) in US, Canada, Costa Rica, Greece, Italy, Japan, New Caledonia, Portugal, Spain and United Arab Emirates. Gas flux measurements are indicating that methane can also flux as invisible microseepages from the ground, through fractured peridotites, even far removed from seeps and springs. Methane C-isotope ratios range from -6 to -37 permil (VPDB) for dominantly abiotic methane. The more 13-C depleted values, e.g., California, are likely mixed with biotic gas (microbial and thermogenic gas). Methane H-isotope ratios cover a wide range from -118 to -333 permil (VSMOW). The combination of C- and H-isotopes clearly distinguish biotic from abiotic methane. Radiocarbon (14-C) analysis from bubbling seeps in Italian peridotites indicate that the methane is fossil (pMC temperatures of land-based peridotites (generally methane production. We discuss some hypotheses concerning gas generation in water vs. dry systems, i.e., in deeper, older, waters or in unsaturated rocks). We also discuss low vs. high temperatures, i.e., at the present-day low T conditions or at higher temperatures eventually occurring in the early stages of peridotite emplacement on land.

  10. The amount of recycled crust in sources of mantle-derived melts. (United States)

    Sobolev, Alexander V; Hofmann, Albrecht W; Kuzmin, Dmitry V; Yaxley, Gregory M; Arndt, Nicholas T; Chung, Sun-Lin; Danyushevsky, Leonid V; Elliott, Tim; Frey, Frederick A; Garcia, Michael O; Gurenko, Andrey A; Kamenetsky, Vadim S; Kerr, Andrew C; Krivolutskaya, Nadezhda A; Matvienkov, Vladimir V; Nikogosian, Igor K; Rocholl, Alexander; Sigurdsson, Ingvar A; Sushchevskaya, Nadezhda M; Teklay, Mengist


    Plate tectonic processes introduce basaltic crust (as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts (10 to 30%), ocean island and continental basalts (many >60%), and komatiites (20 to 30%). These results imply involvement of 2 to 20% (up to 28%) of recycled crust in mantle melting.

  11. Electrical conductivity anisotropy of partially molten peridotite under shear deformation (United States)

    Zhang, B.; Yoshino, T.; Yamazaki, D.; Manthilake, G. M.; Katsura, T.


    Recent ocean bottom magnetotelluric investigations have revealed a high-conductivity layer (HCL) with high anisotropy characterized by higher conductivity values in the direction parallel to the plate motion beneath the southern East Pacific Rise (Evans et al., 2005) and beneath the edge of the Cocos plate at the Middle America trench offshore of Nicaragua (Naif et al., 2013). These geophysical observations have been attributed to either hydration (water) of mantle minerals or the presence of partial melt. Currently, aligned partial melt has been regarded as the most preferable candidate for explaining the conductivity anisotropy because of the implausibility of proton conduction (Yoshino et al., 2006). In this study, we report development of the conductivity anisotropy between parallel and normal to shear direction on the shear plane in partial molten peridotite as a function of time and shear strain. Starting samples were pre-synthesized partial molten peridotite, showing homogeneous melt distribution. The partially molten peridotite samples were deformed in simple shear geometry at 1 GPa and 1723 K in a DIA-type apparatus with uniaxial deformation facility. Conductivity difference between parallel and normal to shear direction reached one order, which is equivalent to that observed beneath asthenosphere. In contrast, such anisotropic behavior was not found in the melt-free samples, suggesting that development of the conductivity anisotropy was generated under shear stress. Microstructure of the deformed partial molten peridotite shows partial melt tends to preferentially locate grain boundaries parallel to shear direction, and forms continuously thin melt layer sub-parallel to the shear direction, whereas apparently isolated distribution was observed on the section perpendicular to the shear direction. The resultant melt morphology can be approximated by tube like geometry parallel to the shear direction. This observation suggests that the development of

  12. Calcium isotopic composition of mantle xenoliths and minerals from Eastern China (United States)

    Kang, Jin-Ting; Zhu, Hong-Li; Liu, Yu-Fei; Liu, Fang; Wu, Fei; Hao, Yan-Tao; Zhi, Xia-Chen; Zhang, Zhao-Feng; Huang, Fang


    This study presents calcium isotope data for co-existing clinopyroxenes (cpx), orthopyroxenes (opx), and olivine (ol) in mantle xenoliths to investigate Ca isotopic fractionation in the upper mantle. δ44/40Ca (δ44/40Ca (‰) = (44Ca/40Ca)SAMPLE/(44Ca/40Ca)SRM915a - 1) in opx varies from 0.95 ± 0.05‰ to 1.82 ± 0.01‰ and cpx from 0.71 ± 0.06‰ to 1.03 ± 0.12‰ (2se). δ44/40Ca in ol (P-15) is 1.16 ± 0.08‰, identical to δ44/40Ca of the co-existing opx (1.12 ± 0.09‰, 2se). The Δ44/40Caopx-cpx (Δ44/40Caopx-cpx = δ44/40Caopx-δ44/40Cacpx) shows a large variation ranging from -0.01‰ to 1.11‰ and it dramatically increases with decreasing of Ca/Mg (atomic ratio) in opx. These observations may reflect the effect of opx composition on the inter-mineral equilibrium fractionation of Ca isotopes, consistent with the theoretical prediction by first-principles theory calculations (Feng et al., 2014). Furthermore, Δ44/40Caopx-cpx decreases when temperature slightly increases from 1196 to 1267 K. However, the magnitude of such inter-mineral isotopic fractionation (1.12‰) is not consistent with the value calculated by the well-known correlation between inter-mineral isotope fractionation factors and 1/T2 (Urey, 1947). Instead, it may reflect the temperature control on crystal chemistry of opx (i.e., Ca content), which further affects Δ44/40Caopx-cpx. The calculated δ44/40Ca of bulk peridotites and pyroxenites range from 0.76 ± 0.06‰ to 1.04 ± 0.12‰ (2se). Notably, δ44/40Ca of bulk peridotites are positively correlated with CaO and negatively with MgO content. Such correlations can be explained by mixing between a fertile mantle end-member and a depleted one with low δ44/40Ca, indicating that Ca isotopes could be a useful tool in studying mantle evolution.

  13. Mantle xenoliths from Szentbékálla, Balaton: Geochemical and petrological constraints on the evolution of the lithospheric mantle underneath Pannonian Basin, Hungary (United States)

    Ntaflos, Theo; Bizimis, Michael; Abart, Rainer


    Pliocene alkali basalts from the Bakony-Balaton Highland Volcanic Field (BBHVF) in the western Pannonian Basin carry mantle xenoliths comprising hydrous and anhydrous spinel peridotites. The studied mantle xenoliths from Szentbékálla, near Lake Balaton, Hungary, are fine- and coarse-grained fertile to depleted spinel lherzolites, spinel harzburgites and dunites, with protogranular, porphyroclastic, and secondary protogranular and mosaic equigranular textures. Melt pockets, with shapes resembling amphibole, are common in a number of samples, whereas other samples have thin films of intergranular glass. Bulk-rock major element abundances show that the mantle lithosphere beneath this area experienced variable degrees (up to 20%) of partial melting. The clinopyroxene trace elements systematics retain a record of melt depletion and metasomatic processes attributed to subduction-related melt/fluids or to the infiltration of percolating undersaturated melts in the Pannonian lithospheric mantle. The radiogenic isotopes of Sr, Nd and Hf in clinopyroxene suggest that this metasomatism was a relatively recent event. Textural evidence suggests that the calcite filling up the vesicles in the melt pockets and in veinlets cross-cutting the constituent minerals is of epigenetic nature and not due to carbonatite metasomatism. The non-metasomatized primitive mantle normalized clinopyroxene REE abundances mimic those, but at higher values, of their bulk-rock REE patterns. Bulk-rock and clinopyroxene REE with upward and downward LREE respectively, indicate up to 2.4% host basalt infiltration. The calculated bulk composition of the melt pockets is identical to small amphibole relics found as inclusions in second generation clinopyroxene within the melt pockets, suggesting incongruent melting of amphibole, without the need for additional metasomatic melt/fluids to initiate the amphibole breakdown. The heat for the temperature increase necessary for amphibole breakdown was derived from

  14. Sound velocity measurements of CaSiO3 perovskite to 133 GPa and implications for lowermost mantle seismic anomalies (United States)

    Kudo, Yuki; Hirose, Kei; Murakami, Motohiko; Asahara, Yuki; Ozawa, Haruka; Ohishi, Yasuo; Hirao, Naohisa


    We report the measurements of aggregate shear velocity (VS) of CaSiO3 perovskite (CaPv) at high pressure (P) between 32 and 133 GPa and room temperature (T) on the basis of Brillouin spectroscopy. The sample had a tetragonal perovskite structure throughout the experiments. The measured P-VS data show the shear modulus and its pressure derivative at ambient condition to be G0=115.8 GPa and G'=1.20, respectively. The zero-pressure shear velocity is determined to be VS0=5.23 km/s, in good agreement with the previous estimate inferred from the ultrasonic measurements on Ca(Si,Ti)O3 perovskite at 1 bar. Our experimental results are broadly consistent with the earlier calculations on tetragonal CaPv but exhibit lower velocity at equivalent pressure. Such tetragonal CaPv is present in cold subducting slabs and possibly in wide areas of the lowermost mantle. While primitive mantle includes certain amount of CaPv, a depleted peridotite (former harzburgite) layer in subducted oceanic lithosphere is deficient in CaPv and enriched in ferropericlase in the lower mantle. Such harzburgite exhibits 0.9% faster VS and 0.7% slower bulk sound velocity (VΦ) at the lowermost mantle P-T conditions if CaPv is present in the tetragonal form in the surrounding mantle. The observed fast VS and slow VΦ anomalies in the D" layer underneath the circum-Pacific region might be attributed in large part in the presence of subducted harzburgitic materials.

  15. Nature and Evolution of the lithospheric mantle beneath the Hoggar swell (Algeria): a record from mantle xenoliths. (United States)

    Kourim, Fatna; Bodinier, Jean-Louis; Alard, Olivier; Bendaoud, Abderrahmane; Vauchez, Alain; Dautria, Jean-Marie


    The mantle xenoliths sampled by the Quaternary alkaline volcanics from the Tahalgha district (Central Hoggar) represent the subcontinental lithospheric mantle beneath the boundary between two major structural domains of the Tuareg Shield: the "Polycyclic Central Hoggar" to the East and the "Western Hoggar", or "Pharusian Belt", to the West. Samples were collected from volcanic centres located on both sides of the 4°10, a major lithospheric shear zone separating these two domains. Although showing substantial variations in their deformation microstructures, equilibrium temperatures, and modal and chemical compositions, the studied samples do not display systematic variations of these features across the 4°10. The observed variations rather record small-scale heterogeneities distributed throughout the whole studied area and mostly related to the asthenosphere-lithosphere interaction events associated with the evolution of the Hoggar swell, in the Cenozoic. These features include partial annealing of pre-existing deformation microstructures, post-deformation metasomatic reactions, and trace-element enrichment, coupled with heating from 750-900°C (low-temperature lherzolites) to 900-1150°C (intermediate-T lherzolites and high-T harzburgites and wehrlites). Trace element modelling confirms that the whole range of REE fractionation observed in the Tahalgha xenoliths may be accounted for by reactive porous flow involving a single stage of basaltic melt infiltration into a LREE-depleted protolith. The striking correlations between equilibrium temperatures and trace-element enrichments favor a scenario whereby the high-temperature peridotites record advective heat transport along melt conduits while the intermediate- and low-temperature lherzolites would represent more conductive heating of the host Mechanical Boundary Layer. This indicates that the lithosphere did not reach thermal equilibrium, suggesting that the inferred heating event was transient and rapidly erased

  16. Deformation history of Pinatubo peridotite xenoliths: constraints from microstructural observation and determination of olivine slip systems (United States)

    Yamamoto, Takafumi; Ando, Jun-ichi; Tomioka, Naotaka; Kobayashi, Tetsuo


    The deformation history of the Pinatubo peridotite xenoliths was estimated on the basis of the microstructural observations and the determination of olivine slip systems. The latter was performed by using three methods: lattice-preferred orientation (LPO), crystallographic analysis of subgrain boundaries, and direct characterization of dislocations. The Pinatubo peridotites are composed of coarse olivine grains containing numerous fluid inclusions and some fine aggregates of orthopyroxene and amphibole grains, which implies intense fluid-rock interaction. The development of euhedral fine recrystallized olivine grains along the healed cracks within the coarse olivine grains suggests that the strain-free grains were nucleated and grew during static recovery. The LPO patterns and the analyses of subgrain boundaries indicate the activation of a [100]{0kl} slip system that developed under high temperature, low pressure, and dry deformation conditions. Although dislocations showing the [100]{0kl} slip system are dominantly observed, the other slip systems which could be formed by the deformation under moderate-high water content and lower-temperature conditions are also developed. The discrepancy between the results of dislocation characterization and the other two methods might have been caused by fulfilling the von Mises criterion or overprinting dislocation microstructures. Either way, the possible deformation history of the Pinatubo peridotites can be explained by the following scenario. The peridotites plastically moved from the back-arc to the fore-arc adjacent region, where CO2-rich saline fluid was trapped, by the corner flow of a mantle wedge. They were then annealed and metasomatized during entrapment of the upwelling magma.

  17. Deformation history of Pinatubo peridotite xenoliths: constraints from microstructural observation and determination of olivine slip systems (United States)

    Yamamoto, Takafumi; Ando, Jun-ichi; Tomioka, Naotaka; Kobayashi, Tetsuo


    The deformation history of the Pinatubo peridotite xenoliths was estimated on the basis of the microstructural observations and the determination of olivine slip systems. The latter was performed by using three methods: lattice-preferred orientation (LPO), crystallographic analysis of subgrain boundaries, and direct characterization of dislocations. The Pinatubo peridotites are composed of coarse olivine grains containing numerous fluid inclusions and some fine aggregates of orthopyroxene and amphibole grains, which implies intense fluid-rock interaction. The development of euhedral fine recrystallized olivine grains along the healed cracks within the coarse olivine grains suggests that the strain-free grains were nucleated and grew during static recovery. The LPO patterns and the analyses of subgrain boundaries indicate the activation of a [100]{0 kl} slip system that developed under high temperature, low pressure, and dry deformation conditions. Although dislocations showing the [100]{0 kl} slip system are dominantly observed, the other slip systems which could be formed by the deformation under moderate-high water content and lower-temperature conditions are also developed. The discrepancy between the results of dislocation characterization and the other two methods might have been caused by fulfilling the von Mises criterion or overprinting dislocation microstructures. Either way, the possible deformation history of the Pinatubo peridotites can be explained by the following scenario. The peridotites plastically moved from the back-arc to the fore-arc adjacent region, where CO2-rich saline fluid was trapped, by the corner flow of a mantle wedge. They were then annealed and metasomatized during entrapment of the upwelling magma.

  18. Chemical and petrological heterogenity of lithospheric mantle beneath N Patagonia (Argentina) - case study of Cerro Chenque xenoliths. (United States)

    Kozdrowska, Dominika; Matusiak-Małek, Magdalena; Ntaflos, Theodoros; Puziewicz, Jacek; Bjerg, Ernesto


    Mantle-xenoliths-bearing, back-arc Pliocene - Quaternary alkali basalts occur in N Patagonia, Argentina (Bjerg et al., 2005, J. of S. Am. Sci.). The Cerro Chenque (Rio Negro province) trachybasaltic lavas carry small (up to 10 cm in diameter) xenoliths of anhydrous, spinel bearing harzburgites, dunites and less abundant clino- , orthopyroxenites, websterites. The xenolith suite comprises also gabbros and norites, which are not discussed in this study. All the phases forming xenoliths are rich in Mg (Fo=90.5-93.5%; mg#Opx=0.90-0.94; mg#Cpx=0.91-0.95). Composition of spinel is extremely variable (mg#=0.65-0.85; cr#=0.00-0.70). Three types (A, B, C) of REE patterns occur in clinopyroxene and orthopyroxene: (1) type A (harzburgites and orthopyroxenites) is U-shaped in both the pyroxenes, REE contents vary significantly (e.g. La=~0.5 primitive mantle values (PM), ~5 PM, and ~50PM); (2) type B (dunites and harzburgites) clinopyroxene has flat HREE and is continuously enriched in LREE up to 10x PM, orthopyroxene is U-shaped; (3) type C (harzburgites) clinopyroxene is convex upward, La=5-7PM), orthopyroxene is continuously depleted in LREE. Clinopyroxene of all the types is poor in Ti, while normalized content of other trace elements is strongly variable. Trace element compositions of Cerro Chenque xenolithc clinopyroxene cover whole compositional range of mantle-derived clinopyroxene from Rio Negro province presented by Bjerg et al., 2005 (op.cit.). In xenoliths where clino- and orthopyroxene are in equilibrium, the calculated temperatures are always around 1000°C (Brey and Köhler,1990, JoP). No spinel-clinopyroxene symplectites suggesting peridotite provenance from garnet stability field were observed. Strong variations in rock-type and chemical composition of minerals forming Cerro Chenque xenoliths suggest complicated structure of upper mantle beneath N Patagonia. At present stage of study we suggest that Earth's lithospheric mantle in this region: - suffered from

  19. Petrology of a Neoproterozoic Alaskan-type complex from the Eastern Desert of Egypt: Implications for mantle heterogeneity (United States)

    Khedr, Mohamed Zaki; Arai, Shoji


    This paper details petrological and geochemical studies of an ultramafic-mafic intrusion in the Southern Eastern Desert of Egypt. The Dahanib complex shows a concentric zonation, from dunites at the core, through chromitites, clinopyroxene-rich dunites, wehrlites, harzburgites, gabbronorites and layered gabbros, to hornblende gabbros/diorites at the rim, similar to other Alaskan-type complexes. These lithologies typically feature cumulate textures and layering. Their pyroxenes (Mg#s, 0.54-0.94) evidence Fe, Mn and Na enrichment, but Al, Cr, Mg and Ti are depleted with differentiation. Their chromian spinels have a wide range of Cr# (0.31-0.61), along with high Ti and Fe, as a result of their origin through crystal accumulation and reaction with interstitial liquids. The clinopyroxenes (Cpxs) in peridotites and gabbroic rocks, which are high in REE concentration (2-100 times chondrite), are depleted in LREE relative to HREE and are similar to Cpx crystallized from asthenospheric melts. The mineral inclusions in spinel, the chemistry of Cpx in peridotites (rich in Al, Cr, Na, Ti and ΣREE = 13.7), and the melts in equilibrium with Cpx suggest that the Neoproterozoic lithosphere were partially refertilized by trace asthenospheric melts. The early magmas were possibly enriched by Mg, Cr, Ni, Ti, V and Sr, while the evolved types were rich in Fe, Mn, Na, Li, Zr, Co and REE via crystal accumulation and the interaction with interstitial liquids. The Neoproterozoic sub-arc mantle in Egypt is chemically heterogeneous and generally low in Nb, Ta, Zr and K, due to the low solubility of HFSE in slab-derived fluids and no other external addition of these elements. The large variations in lithology and chemistry, as well as the occurrence of scattered chromitite clots in the Dahanib peridotites, are related to a continuous supply of primitive magmas and/or the reaction between interstitial liquids and early cumulus crystals during multistage fractional crystallization. The

  20. Experimental melting of phlogopite-peridotite in the garnet stability field (United States)

    Condamine, Pierre; Médard, Etienne; Devidal, Jean-Luc


    Melting experiments have been performed at 3 GPa, between 1150 and 1450 °C, on a phlogopite-peridotite source in the garnet stability field. We succeeded to extract and determine the melt compositions of both phlogopite-bearing lherzolite and harzburgite from low to high degrees of melting (ϕ = 0.008-0.256). Accounting for the presence of small amounts of F in the mantle, we determined that phlogopite coexists with melt >150 °C above the solidus position (1150-1200 °C). Fluorine content of phlogopite continuously increases during partial melting from 0.2 to 0.9 wt% between 1000 and 1150 °C and 0.5 to 0.6 wt% between 1150 and 1300 °C at 1 and 3 GPa, respectively. The phlogopite continuous breakdown in the lherzolite follows the reaction: 0.59 phlogopite + 0.52 clinopyroxene + 0.18 garnet = 0.06 olivine + 0.23 orthopyroxene + 1.00 melt. In the phlogopite-harzburgite, the reaction is: 0.93 phlogopite + 0.46 garnet = 0.25 olivine + 0.14 orthopyroxene + 1.00 melt. Melts from phlogopite-peridotite sources at 3 GPa are silica-undersaturated and are foiditic to trachybasaltic in composition from very low (0.8 wt%) to high (25.6 wt%) degrees of melting. As observed at 1 GPa, the potassium content of primary mantle melts is buffered by the presence of phlogopite, but the buffering values are higher, from 6.0 to 8.0 wt% depending on the source fertility. We finally show that phlogopite garnet-peridotite melts are very close to the composition of the most primitive post-collisional lavas described worldwide.

  1. Sheared peridotite xenolith from the V. Grib kimberlite pipe, Arkhangelsk Diamond Province, Russia: Texture, composition, and origin

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    Alexey Vladimirovich Kargin


    Full Text Available The petrography and mineral composition of a mantle-derived garnet peridotite xenolith from the V. Grib kimberlite pipe (Arkhangelsk Diamond Province, Russia was studied. Based on petrographic characteristics, the peridotite xenolith reflects a sheared peridotite. The sheared peridotite experienced a complex evolution with formation of three main mineral assemblages: (1 a relict harzburgite assemblage consist of olivine and orthopyroxene porphyroclasts and cores of garnet grains (Gar1 with sinusoidal rare earth elements (REE chondrite C1 normalized patterns; (2 a neoblastic olivine and orthopyroxene assemblage; (3 the last assemblage associated with the formation of clinopyroxene and garnet marginal zones (Gar2. Major and trace element compositions of olivine, orthopyroxene, clinopyroxene and garnet indicate that both the neoblast and clinopyroxene-Gar2 mineral assemblages were in equilibrium with a high Fe-Ti carbonate-silicate metasomatic agent. The nature of the metasomatic agent was estimated based on high field strength elements (HFSE composition of olivine neoblasts, the garnet-clinopyroxene equilibrium condition and calculated by REE-composition of Gar2 and clinopyroxene. All these evidences indicate that the agent was a high temperature carbonate-silicate melt that is geochemically linked to the formation of the protokimberlite melt.

  2. Multi-stage metasomatism revealed by trace element and Li isotope distributions in minerals of peridotite xenoliths from Allègre volcano (French Massif Central) (United States)

    Gu, Xiaoyan; Deloule, Etienne; France, Lydéric; Ingrin, Jannick


    The modal, chemical, and isotopic compositions of mantle peridotite are largely modified by metasomatic processes, which may affect them repeatedly. Xenoliths are commonly used to characterize those metasomatic processes along with the structure, and chemical and isotopic compositions of mantle domains. Nevertheless, the original mantle signatures born by mantle xenoliths are potentially obscured by the interactions occurring between the host magma and the xenolith itself. Here we attempt to identify to which degree the original Li content and isotopic composition, as well as other trace element contents of mantle xenoliths, can be modified by interaction with the host magma. Peridotite xenoliths that have suffered extensive exchange with the entraining magma were sampled in the solidified lava lake of Allègre, Southern French Massif Central, in order to decipher the signature related to peridotite-melt interaction, and to further unravel the evolution of the sub-continental lithospheric mantle. In-situ trace element analyses of clinopyroxene (Cpx) were performed via LA-ICP-MS, and the Li content and isotopic composition of pyroxene and olivine (Ol) via SIMS. Negative HFSE anomalies (Ti/Eu ratios as low as 437) and markedly high LREE/HREE ratios ((La/Yb)N as high as 79) are characteristic of mantle metasomatism at depth. Lithium isotope systematics indicates that at least two different metasomatic events affected the peridotite. Exceptionally high Li contents in Cpx (up to 50 ppm) and slight Li enrichment of Ol rims are ascribed to diffusive Li influx with a positive δ7Li value (+ 3.2‰) from the host magma after entrainment. Conversely, Ol cores preserve extremely light Li isotopic compositions (δ7Li as low as - 25‰) with high Li contents (up to 4.4 ppm) compared to normal mantle, indicating a metasomatic event that occurred before xenolith entrainment. The negative δ7Li signature of this early metasomatism may be related to subduction-related fluids

  3. Experimental study of carbonate formation in oceanic peridotite (United States)

    Grozeva, Niya G.; Klein, Frieder; Seewald, Jeffrey S.; Sylva, Sean P.


    Interactions of CO2-rich aqueous fluids with mantle peridotite have major implications for geochemical budgets and microbial life in the shallow oceanic lithosphere through the formation of carbonate minerals and reduced carbon species. However, the underlying mechanisms controlling the transformation of CO2 to carbonates in ultramafic-hosted hydrothermal systems remain incompletely understood. A long-term laboratory experiment was conducted at 300 °C and 35 MPa to investigate serpentinization and carbonate formation pathways during hydrothermal alteration of peridotite. Powdered harzburgite was initially reacted with a Ca-rich aqueous fluid for 14,592 h (608 days) and changes in fluid composition were monitored with time. Once the system reached a steady state, a CO2(aq)-rich fluid was injected and allowed to react with the system for 5907 h (246 days). Fluid speciation and mineral analyses suggest that serpentinization of harzburgite in the CO2-poor system led to the precipitation of serpentine, brucite, magnetite, and minor calcite, in addition to other minor phases including chlorite and sulfur-poor Ni sulfides. The addition of the CO2(aq)-rich fluid caused dolomite, Ca-rich dolomite, and high-Mg calcite to form at the expense of olivine, calcite, and brucite, while serpentine remained unreactive. Replacement textures and mineral assemblages mimic those documented in carbonate-altered seafloor serpentinites, particularly those from the Mid-Atlantic Ridge and the Iberia Margin. In contrast to thermodynamic predictions, magnesite did not form in the experiment because the dissolution of clinopyroxene, in combination with the lack of serpentine reactivity, maintained low Mg/Ca ratios in solution. Clinopyroxene dissolution and unreactive serpentine may similarly maintain low Mg/Ca ratios in submarine serpentinization systems and limit magnesite formation in subseafloor environments. Results of this study suggest that the formation of Ca-Mg carbonates by mineral

  4. Magnesium isotopic composition of the oceanic mantle and oceanic Mg cycling (United States)

    Liu, Ping-Ping; Teng, Fang-Zhen; Dick, Henry J. B.; Zhou, Mei-Fu; Chung, Sun-Lin


    To constrain the Mg isotopic composition of the oceanic mantle, investigate Mg isotope fractionation of abyssal peridotites during seafloor alteration, and assess Mg budget in the oceans, a suite of 32 abyssal peridotite samples from the Gakkel Ridge and Southwest Indian Ridge (SWIR) was, for the first time, selected for high-precision Mg isotope analyses. Although most of these samples are extensively altered, largely by serpentinization and weathering, primary olivine, diopside and enstatite grains are preserved in some samples. Olivine grains from the least altered samples have δ26Mg varying from -0.30 to -0.12‰ (n = 7), whereas enstatite and diopside have δ26Mg varying from -0.27 to -0.16‰ (n = 7), and from -0.23 to -0.09‰ (n = 6), respectively. Whole-rock δ26Mg values range from -0.24 to 0.03‰ with an average of -0.12 ± 0.13‰ (2SD, n = 32). Strongly serpentinized peridotites have lower average δ26Mg values (δ26Mg = -0.19 ± 0.07‰, 2SD, n = 7) than weathering-dominated ones (δ26Mg = -0.10 ± 0.12‰, 2SD, n = 25). Calculated Mg isotopic compositions of fresh mantle peridotites vary from -0.29 to -0.13‰, beyond the previously reported range of the subcontinental lithospheric mantle (-0.25 ± 0.04‰) and the analytical uncertainty (±0.07‰, 2SD). Our study therefore indicates that the oceanic mantle may have similar but slightly heterogeneous Mg isotopic compositions to that of subcontinental lithospheric mantle. Secondary serpentinization does not fractionate Mg isotopes of abyssal peridotites, whereas low-T weathering and formation of clay can result in the enrichment of heavy Mg isotopes in abyssal peridotites. This study also demonstrates that fluid-rock interaction does not necessarily produce rocks with intermediate Mg isotopic compositions. Magnesium isotopes of the rocks thereafter are dependent on the secondary minerals formed. We also conclude that the release of light Mg isotopes into the ocean during alteration of abyssal

  5. Major element composition of an Early Enriched Reservoir: constraints from 142Nd/144Nd isotope systematics in the early Earth and high-pressure melting experiments of a primitive peridotite (United States)

    Kondo, Nozomi; Yoshino, Takashi; Matsukage, Kyoko N.; Kogiso, Tetsu


    The Accessible Silicate Earth (ASE) has a higher 142Nd/144Nd ratio than most chondrites. Thus, if the Earth is assumed to have formed from these chondrites, a complement low-142Nd/144Nd reservoir is needed. Such a low-142Nd/144Nd reservoir is believed to have been derived from a melt in the early Earth and is called the Early Enriched Reservoir (EER). Although the major element composition of the EER is crucial for estimating its chemical and physical properties (e.g., density) and is also essential for understanding the origin and fate of the EER, which are both major factors that determine the present composition of the Earth, it has not yet been robustly established. In order to determine the major element composition of the EER, we estimated the age and pressure-temperature conditions to form the EER that would best explain its Nd isotopic characteristics, based on Sm-Nd partitioning and its dependence on pressure, temperature, and melting phase relations. Our estimate indicates that the EER formed within 33.5 Myr of Solar System formation and at near-solidus temperatures and shallow upper-mantle pressures. We then performed high-pressure melting experiments on primitive peridotite to determine the major element composition of the EER at estimated temperature at 7 GPa and calculated the density of the EER. The result of our experiments indicates that the near-solidus melt is iron-rich komatiite. The estimated density of the near-solidus melt is lower than that of the primitive peridotite, suggesting that the EER melt would have ascended in the mantle to form an early crust. Given that high mantle potential temperatures are assumed to have existed in the Hadean, it follows that the EER melt was generated at high pressure and, therefore, its composition would have been picritic to komatiitic. As the formation age of the EER estimated in our study precedes the last giant, lunar-forming impact, the picritic to komatiitic crust (EER) would most likely have been

  6. Slab melting beneath the Cascades Arc driven by dehydration of altered oceanic peridotite (United States)

    Walowski, Kristina J; Wallace, Paul J.; Hauri, E.H.; Wada, I.; Clynne, Michael A.


    Water is returned to Earth’s interior at subduction zones. However, the processes and pathways by which water leaves the subducting plate and causes melting beneath volcanic arcs are complex; the source of the water—subducting sediment, altered oceanic crust, or hydrated mantle in the downgoing plate—is debated; and the role of slab temperature is unclear. Here we analyse the hydrogen-isotope and trace-element signature of melt inclusions in ash samples from the Cascade Arc, where young, hot lithosphere subducts. Comparing these data with published analyses, we find that fluids in the Cascade magmas are sourced from deeper parts of the subducting slab—hydrated mantle peridotite in the slab interior—compared with fluids in magmas from the Marianas Arc, where older, colder lithosphere subducts. We use geodynamic modelling to show that, in the hotter subduction zone, the upper crust of the subducting slab rapidly dehydrates at shallow depths. With continued subduction, fluids released from the deeper plate interior migrate into the dehydrated parts, causing those to melt. These melts in turn migrate into the overlying mantle wedge, where they trigger further melting. Our results provide a physical model to explain melting of the subducted plate and mass transfer from the slab to the mantle beneath arcs where relatively young oceanic lithosphere is subducted.

  7. Carbonatite metasomatized peridotite xenoliths from southern Patagonia: implications for lithospheric processes and Neogene plateau magmatism (United States)

    Gorring, Matthew L.; Kay, Suzanne M.

    The mineral chemistry, major and trace element, and Sr-Nd isotopic composition of Cr-diopside, spinel peridotite xenoliths from the Estancia Lote 17 locality in southern Patagonia document a strong carbonatitic metasomatism of the backarc continental lithosphere. The Lote 17 peridotite xenolith suite consists of hydrous spinel lherzolite, wehrlite, and olivine websterite, and anhydrous harzburgite and lherzolite. Two-pyroxene thermometry indicates equilibration temperatures ranging from 870 to 1015°C and the lack of plagioclase or garnet suggests the xenoliths originated from between 40 and 60km depth. All of the xenoliths are LILE- and LREE-enriched, but have relatively low 87Sr/86Sr (0.70294 to 0.70342) and high ɛNd (+3.0 to +6.6), indicating recent trace element enrichment ( 25Ma, based on the low 87Sr/86Sr and high Rb concentrations of phlogopite separates) in the long-term, melt-depleted Patagonian lithosphere. Lote 17 peridotite xenoliths are divided into two basic groups. Group 1 xenoliths consist of fertile peridotites that contain hydrous phases (amphibole+/-phlogopite+/-apatite). Group 1 xenoliths are further subdivided into three groups (a, b, and c) based on distinctive textures and whole-rock chemistry. Group 1 xenolith mineralogy and chemistry are consistent with a complex metasomatic history involving variable extents of recent carbonatite metasomatism (high Ca/Al, Nb/La, Zr/Hf, low Ti/Eu) that has overprinted earlier metasomatic events. Group 2 xenoliths consist of infertile, anhydrous harzburgites and record cryptic metasomatism that is attributed to CO2-rich fluids liberated from Group 1 carbonatite metasomatic reactions. Extremely variable incompatible trace element ratios and depleted Sr-Nd isotopic compositions of Lote 17 peridotite xenoliths indicate that the continental lithosphere was neither the primary source nor an enriched lithospheric contaminant for Neogene Patagonian plateau lavas. Neogene plateau magmatism associated with formation

  8. Two-component mantle melting-mixing model for the generation of mid-ocean ridge basalts: Implications for the volatile content of the Pacific upper mantle (United States)

    Shimizu, Kei; Saal, Alberto E.; Myers, Corinne E.; Nagle, Ashley N.; Hauri, Erik H.; Forsyth, Donald W.; Kamenetsky, Vadim S.; Niu, Yaoling


    We report major, trace, and volatile element (CO2, H2O, F, Cl, S) contents and Sr, Nd, and Pb isotopes of mid-ocean ridge basalt (MORB) glasses from the Northern East Pacific Rise (NEPR) off-axis seamounts, the Quebrada-Discovery-GoFar (QDG) transform fault system, and the Macquarie Island. The incompatible trace element (ITE) contents of the samples range from highly depleted (DMORB, Th/La ⩽ 0.035) to enriched (EMORB, Th/La ⩾ 0.07), and the isotopic composition spans the entire range observed in EPR MORB. Our data suggest that at the time of melt generation, the source that generated the EMORB was essentially peridotitic, and that the composition of NMORB might not represent melting of a single upper mantle source (DMM), but rather mixing of melts from a two-component mantle (depleted and enriched DMM or D-DMM and E-DMM, respectively). After filtering the volatile element data for secondary processes (degassing, sulfide saturation, assimilation of seawater-derived component, and fractional crystallization), we use the volatiles to ITE ratios of our samples and a two-component mantle melting-mixing model to estimate the volatile content of the D-DMM (CO2 = 22 ppm, H2O = 59 ppm, F = 8 ppm, Cl = 0.4 ppm, and S = 100 ppm) and the E-DMM (CO2 = 990 ppm, H2O = 660 ppm, F = 31 ppm, Cl = 22 ppm, and S = 165 ppm). Our two-component mantle melting-mixing model reproduces the kernel density estimates (KDE) of Th/La and 143Nd/144Nd ratios for our samples and for EPR axial MORB compiled from the literature. This model suggests that: (1) 78% of the Pacific upper mantle is highly depleted (D-DMM) while 22% is enriched (E-DMM) in volatile and refractory ITE, (2) the melts produced during variable degrees of melting of the E-DMM controls most of the MORB geochemical variation, and (3) a fraction (∼65% to 80%) of the low degree EMORB melts (produced by ∼1.3% melting) may escape melt aggregation by freezing at the base of the oceanic lithosphere, significantly enriching it in

  9. Numerical modeling of mantle wedge processes and exhumation of UHP mantle in subduction zones (United States)

    Gorczyk, W.; Gerya, T. V.; Guillot, S.; Connolly, J. A.; Yuen, D.


    The upwelling of subduction generated partially molten rocks is potentially a mechanism for the exhumation of UHP rocks through the mantle wedge. We investigated this processes using a 2-D coupled petrological- thermomechanical model that incorporates slab dehydration and water transport as well as partial melting of mantle and crustal rocks. This approach allows us to study the dynamics of mantle wedge processes including evolution of partially molten plumes and their interaction with surrounding dry mantle. To study the internal structure of the plumes we used ultra-high resolution numerical simulations with 10 billion active markers to detail the internal structure of natural plumes originating from the slab. The plumes consist of partially molten hydrated peridotite, dry solid mantle and subducted oceanic crust, which may comprise up to 12 volume % of the plume. As the plumes grow and mature these materials mix chaotically resulting in attenuation and duplication of the original layering on scales of 1-1000 m. Comparison of numerical results with geological observations from the Horoman ultramafic complex in Japan suggests that mixing and differentiation processes related to development of partially molten plumes above slabs may be responsible for strongly layered lithologically mixed (marble cake) structure of asthenospheric mantle wedges. The recent discovery of garnet bearing peridotites in the subduction zone of the Great Antilles in Hispaniola has raised questions about the process that leads to their exhumation. To evaluate whether upwelling plumes are a plausible exhumation mechanism we investigated the dynamics of subduction of slow spreading ridges. The results show that subduction of strongly serpentinized oceanic plate causes strong dehydration of the slab and leads to a rheological weakening of the interface between subducting and overriding plate. This weakening triggers trench retreat and massive asthenospheric upwelling into the gap between the

  10. Mantle Volatiles - Distribution and Consequences (United States)

    Luth, R. W.


    O and 10-44% of the CO2 that is subducted is returned to the surface in arc magmatism. He emphasized that the "missing" volatiles may have multiple fates, including incorporation into the mantle wedge, large-scale fluid flow up along the interface between the subducting slab and overlying mantle, and transport into the deeper mantle.Because of the hydrous nature of arc magmatism, a common hypothesis is that there is a hydrous phase that breaks down at subarc conditions to trigger melting in the overlying mantle wedge to produce arc magmas. A key research goal has been to identify this phase, or phases. For example, serpentine in peridotite will break down during subduction to produce olivine+orthopyroxene+fluid or, in cooler slabs, a progression of DHMSs, the last of which may survive into the transition zone.At some point, however, because of the limited thermal or pressure stability of the hydrous phases, water will be liberated from the slab into the surrounding mantle. At this point, the water will either exist as a fluid, a melt - or something intermediate if we are above the second critical end point in the relevant system (Wyllie and Ryabchikov, 2000) - or it may dissolve into nominally anhydrous phases.The understanding of the relevant phase relations for the other volatiles is not as advanced. For carbon, we have a reasonable understanding of its phase stability in the mantle, but there is still no good understanding of the relative importance of carbonates, elemental carbon, and other forms as hosts for carbon in the mantle. In the upper mantle, sulfur resides primarily in sulfides; their behavior during partial melting will play a major role in the geochemical cycling of sulfur as well as of chalcophile elements. The halogens are rare (and rarely studied) in mantle-derived samples; more insight into their behavior is currently coming from the study of mantle-derived magmas.This review will first consider the evidence from mantle-derived magmas pertaining

  11. Metasomatic control of water contents in the Kaapvaal cratonic mantle (United States)

    Peslier, A. H.; Woodland, A. B.; Bell, D. R.; Lazarov, M.; Lapen, T. J.


    Water and trace element contents were measured by FTIR and laser ablation-ICPMS on minerals from peridotite xenoliths in kimberlites of the Kaapvaal craton from Finsch, Kimberley, Jagersfontein (South Africa), Letseng-La-Terae, and Liqhobong (Lesotho) mines. The peridotites record a wide range of pressure, temperature, oxygen fugacity, and metasomatic events. Correlations between water content or OH vibration bands with major, minor and trace elements in pyroxene and garnet precludes disturbance during xenolith entrainment by the host kimberlite magma and indicate preservation of mantle water contents. Clinopyroxene water contents (150-400 ppm H2O, by weight) correlate with those of orthopyroxene (40-250 ppm). Olivines (Peslier et al., 2008, 2010) and garnets have 0-86 and 0-20 ppm H2O, respectively. Relations in individual xenolith suites between the amount of water and that of incompatible elements Ti, Na, Fe3+ and rare earths in minerals suggests that metasomatism by oxidizing melts controls the water content of olivine, pyroxene and garnet. At pressures ⩽5.5 GPa, hydrous, alkaline, siliceous fluids or melts metasomatized Liqhobong and Kimberley peridotites, producing high water contents in their olivine, pyroxenes and garnet. At higher pressures, the percolation of ultramafic melts reacting with peridotite resulted in co-variation of Ca, Ti and water at the edge of garnets at Jagersfontein, and the overall crystallization of garnet with lower water contents than those in the original peridotites. The upward migration of these ultramafic melts through the lithospheric mantle also increased the water content of olivines with decreasing pressure at Finsch Mine. H2O/Ce ratios of melts in equilibrium with Kaapvaal peridotites range from 100 to 20,000 and the larger values may indicate metasomatism in subduction zone settings. Metasomatic events in Kaapvaal peridotites are thought to have occurred from the Archean to the Mesozoic. However, circumstantial evidence

  12. Distribution and Size of Pyroxenite Bodies in the Mantle (United States)

    Herzberg, C.


    Understanding the rock type that melted to yield magmas at large igneous provinces is a difficult petrological problem because most major element oxide abundances are consistent with either peridotite or pyroxenite sources. However, CaO can often be used as an indicator because accumulated fractional melts of mantle peridotite have about 10% within the garnet lherzolite stability field, an abundance that does not change significantly over a wide range of initial and final melting pressures. In contrast, many primary magmas formed by melting of various pyroxene-garnet lithologies have less than 10% CaO. A lithological source analysis is now possible for many but not all primitive LIP lava compositions that had experienced only olivine addition and subtraction. Results show that peridotite is the dominant source for magmatism at the Azores, Cape Verde, Ascension, St. Helena, and Tristan da Cunha. It can be inferred that mantle below the central and south Atlantic hemisphere is mostly free of subducted crust. The Cook-Austral chain is similar, but all other Pacific OIB exhibit both peridotite and variable pyroxenite source melting, indicating a much greater role for subducted crust in the Pacific hemisphere. Hawaii is distinguished from all other OIB in containing huge pyroxenite bodies at about the 10 kilometer scale. In the Indian Ocean, pyroxenite melting is major for Kerguelen, and difficult to evaluate for Reunion. Pyroxenite melting plays a major role for all continental flood basalts. In all cases, compositions of pyroxenite sources and their partial melts are highly variable. Some sources can form in a second stage by reaction of peridotite with partial melts of subducted oceanic crust (Sobolev et al., 2005), but others can form as bimineralic eclogite and clinopyroxenite residues of partial melts of subducted oceanic crust. Within any volcano that displays both peridotite and pyroxenite source lithologies, lithophile trace element abundances are usually much

  13. Deformation and melt in natural mantle rocks: The Hilti Massif (Oman) and the Othris Massif (Greece)

    NARCIS (Netherlands)

    Dijkstra, A.H.


    For a full understanding of plate tectonics, one of the central paradigms in Earth Sciences, it is critical to know the mechanical properties of the material of which the earth's upper mantle consists, i.e., peridotite. The cold outer shell of the Earth, the lithosphere, is broken up into strong and

  14. Major element composition of the lithospheric mantle under the North Atlantic craton

    DEFF Research Database (Denmark)

    Bizzarro, Martin; Stevenson, R.K.


    The composition and thermal state of the lithospheric mantle under the North Atlantic craton was investigated using a suite of peridotite xenoliths from the diamond-bearing Sarfartoq kimberlite dike swarm of southwestern Greenland. Elevated olivine and whole-rock Mg# (>0.9) attest to the refracto...

  15. Geochemical and Textural Constraints on Wehrlite Formation by Melt-rock Reaction in the Shallow Subcontinental Lithospheric Mantle (Oran, Tell Atlas, N-Algeria) (United States)

    Hidas, Károly; Garrido, Carlos J.; Marchesi, Claudio; Bodinier, Jean-Louis; Louni-Hacini, Amina; Azzouni-Sekkal, Abla; Konc, Zoltán; Dautria, Jean-Marie; Varas-Reus, Maria Isabel


    As a result of the Miocene collision between the Alborán domain and the south Iberian and Maghrebian passive margins, the Betic and the Rif-Tell mountains form an arc-shaped orogenic belt in the westernmost Mediterranean (e.g. [1]). This belt is characterized by the presence of subcontinental lithospheric mantle exhumed as orogenic peridotites [2-4], and entrained by basaltic magmatism. Mantle xenoliths entrained in Plio-Pleistocene alkali basalts in the innermost Betics in South Spain provided invaluable data to study the structure and composition of the subcontinental lithospheric mantle beneath the northern limb of this mountain belt [5-7]. In contrast, information from the southern limb is scarce, even though alkali basalts of the same age (Algeria) contain large amounts of plagioclase to spinel facies peridotite mantle xenoliths with lherzolitic, harzburgitic and wehrlitic modal compositions [6]. Here we report detailed geochemical and textural study of metasomatized mantle xenoliths from this area. The studied spinel-facies mantle xenoliths normally have coarse granular and porphyroclastic textures, whereas in the plagioclase-bearing lithologies fine-grained equigranular fabric becomes abundant. Olivine and orthopyroxene of the coarse-grained lherzolites and harzburgites reflect usual major element geochemical compositions with Mg# in the range of 90-93. Clinopyroxene in these rocks have an overall depleted LREE pattern with slight variation in the most incompatible elements indicating cryptic metasomatism. The Crystal Preferred Orientation (CPO) of olivine shows an axial-[100] pattern characterized by a strong alignment of [100]-axes near or parallel to the peridotite lineation. Wehrlitic lithologies show more variable major element compositions and an important enrichment in LREE in clinopyroxene yet with MREE/HREE ratios comparable to those in harzburgite and lherzolite. Modal enrichment in clinopyroxene and development of fine-grained equigranular

  16. Melt/rock reaction at oceanic peridotite/gabbro transition as revealed by trace element chemistry of olivine (United States)

    Rampone, Elisabetta; Borghini, Giulio; Godard, Marguerite; Ildefonse, Benoit; Crispini, Laura; Fumagalli, Patrizia


    Several recent studies have documented that reactions between melt and crystal mush in primitive gabbroic rocks (via reactive porous flow) have an important control in the formation of the lower oceanic crust and the evolution of MORBs. In this context, olivine-rich rocks can form either by fractional crystallization of primitive melts or by open system reactive percolation of pre-existing (possibly mantle-derived) olivine matrix. To address this question, we performed in-situ trace element analyses (by LA-ICP-MS) of olivine from the Erro-Tobbio ophiolite Unit (Ligurian Alps), where mantle peridotites show gradational contacts with an hectometer-scale body of troctolites and plagioclase wehrlites, and both are cut by later decameter-wide lenses and dykes of olivine gabbros. Previous studies inferred that troctolites and olivine gabbros represent variably differentiated crystallization products from primitive MORB-type melts. Olivines in the three rock types (mantle peridotites, troctolites, olivine gabbros) exhibit distinct geochemical signature and well-defined elemental correlations. As expected, compatible elements (e.g. Ni) show the highest concentrations in peridotites (2580-2730 ppm), intermediate in troctolites (2050-2230 ppm) and lowest in gabbros (1355-1420 ppm), whereas moderate incompatible elements (e.g. Mn, Zn) show the opposite behaviour. By contrast, highly incompatible elements like Zr, Hf, Ti, HREE are variably enriched in olivines of troctolites, and the enrichment in absolute concentrations is coupled to development of significant HFSE/REE fractionation (ZrN/NdN up to 80). AFC modelling shows that such large ZrN/NdN ratios in olivines are consistent with a process of olivine assimilation and plagioclase crystallization at decreasing melt mass, in agreement with textural observations. In-situ trace element geochemistry of olivine, combined with microstructural investigations, thus appears a powerful tool to investigate reactive percolation and the

  17. Striped iron zoning of olivine induced by dislocation creep in deformed peridotites. (United States)

    Ando, J; Shibata, Y; Okajima, Y; Kanagawa, K; Furusho, M; Tomioka, N

    Deformation of solid materials affects not only their microstructures, but also their microchemistries. Although chemical unmixing of initially homogeneous multicomponent solids is known to occur during deformation by diffusion creep, there has been no report on their chemical zoning due to deformation by dislocation creep, in either natural samples or laboratory experiments. Here we report striped iron zoning of olivine ((Mg,Fe)2SiO4) in deformed peridotites, where the iron concentration increases at subgrain boundaries composed of edge dislocations. We infer that this zoning is probably formed by alignment of edge dislocations dragging a so-called Cottrell 'atmosphere' of solute atoms (iron in this case) into subgrain boundaries during deformation of the olivine by dislocation creep. We have found that the iron zoning does not develop in laboratory experiments of high strain rates where dislocations move too fast to drag the Cottrell atmosphere. This phenomenon might have important implications for the generation of deep-focus earthquakes, as transformation of olivine to high-pressure phases preferentially occurs in high-iron regions, and therefore along subgrain boundaries which would be preferentially aligned in plastically deformed mantle peridotites.

  18. Oxygen isotope heterogeneity of the mantle beneath the Canary Islands: insights from olivine phenocrysts (United States)

    Gurenko, Andrey A.; Bindeman, Ilya N.; Chaussidon, Marc


    peridotite (HIMU-type component inherited its radiogenic isotope composition from ancient, ~1 to 2 Ga, recycled ocean crust) and pyroxenite (young, <1 Ga, recycled oceanic crust preserved as eclogite with depleted MORB-type isotopic signature) components of the Canary plume. The model calculations yield 5.2 and 5.9 ± 0.3‰ for peridotite- and pyroxenite-derived melts, respectively, which appeared to correspond closely to the worldwide HIMU-type OIB and upper limit N-MORB δ 18O values. This difference together with the broad range of δ 18O variations found in the Canarian olivines cannot be explained by thermodynamic effects of oxygen isotopic fractionation and are believed to represent true variations in the mantle, due to oceanic crust and continental lithosphere recycling.

  19. Major and trace element whole rock and mineral chemistry of Southern Patagonian Mantle Xenoliths (United States)

    Mundl, Andrea; Ntaflos, Theodoros; Bjerg, Ernesto; Hauzenberger, Christoph


    from 2.06 to 6.11 wt% and 0.77 to 1.95 wt%, respectively. Opx compositions are in the range En88-91Fs10-7Wo1.5-1.2 with mg# varying between 0.89 and 0.92. Primary Sp shows variable cr# from 0.17 in lherzolites to 0.50 in dunite. Gt is a Cr-rich pyrope with Cr2O3 between 1 and 1.5 wt%. PAVF peridotites have Primitive Mantle normalized whole rock REE abundances enriched in LREE [(La/Yb)N=1.4-13.8]. LA-ICP-MS analyses of Cpx show that most of the PAVF Sp-peridotites experienced cryptic metasomatism [(La/Yb)N=8.4-9.1], only Cpx from 4 peridotites are depleted in LREE [(La/Yb)N=0.3-0.7]. Their enriched whole rock LREE abundances suggest host basalt infiltration. Equilibrium P-T estimates yield temperatures in the range of 975 to 1140 °C for Salsa samples with pressures from 18 to 21 kbar. El Ruido peridotites equilibrated in the range of 950 to 1175°C and pressures of 18.7 to 19.7 kbar for Gt-peridotites. Two El Ruido samples show lower equilibration temperatures of ~750°C indicating subsolidus cooling. Potrok Aike samples yield equilibration temperatures of 950 to 1000°C with pressures of ~20 kbar. Sp inclusions in Gt indicate initial P-T conditions within the Sp-stability field and subsequent passing through the Gt-stability field.

  20. Xenoliths From Isla Isabel, Nayarit, Mexico: The Nature of the Upper Mantle Underneath the Western Part of the Mexican Volcanic Belt (United States)

    Housh, T. B.; Aranda-Gomez, J. J.; Luhr, J. F.


    Isla Isabel is located ~65 km NW of San Blas (Nayarit), off the Pacific coast of central Mexico. The island is a Quaternary (Ar/Ar < 0.7 Ma) volcanic complex built atop attenuated continental crust. Isabel lies on the east side of the mouth of the Gulf of California, near the area previously occupied (early Pliocene) by Los Cabos Block. Southeast of Isabel, on the mainland, is the NW-trending Tepic-Zacoalco rift, a major volcano-tectonic structure in the western part of the Mexican Volcanic Belt. On land, the rift is the boundary between the Jalisco and Sierra Madre Occidental blocks, and Isabel lies along its projection. Immediately S of Isabel is the San Blas Trough, a swale that trends NW-SE, co-linear with a gravity lineation parallel to the Tamayo and San Blas fault zones, which are the transform boundaries between the northern Rivera and North American plates. Plio-Quaternary alkaline and calc-alkaline lavas have erupted contemporaneously in the Tepic-Zacoalco rift, but so far no mantle xenoliths have been reported in them. Isabel's rocks are intra-plate type alkaline basalts to trachybasalts, with 5-6%\\ normative Ne. Primary paragenesis in the lavas is: Ol + Pl + Cpx + TMt. Small (< 5 cm) peridotite xenoliths, and xenocrysts derived from them, are ubiquitous in the rocks. Eleven xenoliths were studied comprising 3 dunites, 7 harzburgites (one Pl-bearing), and 1 gabbro. Compared to other Mexican xenolith localities N of the MVB, they are refractory as they are depleted in, or lack, Cpx. Ol crystals in xenoliths are homogenous and their Mg#\\ s range as follows: peridotites (92-88), gabbro (84), and Pl-harzburgite (80). Cores of primary Ol phenocrysts (90.5-86.5) in Isabel's volcanic rocks are significantly higher in Mg#\\ s than in Ol from other Mexican xenolith localities (max. 86) and overlap with the associated peridotite xenoliths. Such overlap has not been reported for other Mexican xenolith localities. Xenolith equilibration temperatures for 5

  1. Diamond growth in mantle fluids (United States)

    Bureau, Hélène; Frost, Daniel J.; Bolfan-Casanova, Nathalie; Leroy, Clémence; Esteve, Imène; Cordier, Patrick


    In the upper mantle, diamonds can potentially grow from various forms of media (solid, gas, fluid) with a range of compositions (e.g. graphite, C-O-H fluids, silicate or carbonate melts). Inclusions trapped in diamonds are one of the few diagnostic tools that can constrain diamond growth conditions in the Earth's mantle. In this study, inclusion-bearing diamonds have been synthesized to understand the growth conditions of natural diamonds in the upper mantle. Diamonds containing syngenetic inclusions were synthesized in multi-anvil presses employing starting mixtures of carbonates, and silicate compositions in the presence of pure water and saline fluids (H2O-NaCl). Experiments were performed at conditions compatible with the Earth's geotherm (7 GPa, 1300-1400 °C). Results show that within the timescale of the experiments (6 to 30 h) diamond growth occurs if water and carbonates are present in the fluid phase. Water promotes faster diamond growth (up to 14 mm/year at 1400 °C, 7 GPa, 10 g/l NaCl), which is favorable to the inclusion trapping process. At 7 GPa, temperature and fluid composition are the main factors controlling diamond growth. In these experiments, diamonds grew in the presence of two fluids: an aqueous fluid and a hydrous silicate melt. The carbon source for diamond growth must be carbonate (CO32) dissolved in the melt or carbon dioxide species in the aqueous fluid (CO2aq). The presence of NaCl affects the growth kinetics but is not a prerequisite for inclusion-bearing diamond formation. The presence of small discrete or isolated volumes of water-rich fluids is necessary to grow inclusion-bearing peridotitic, eclogitic, fibrous, cloudy and coated diamonds, and may also be involved in the growth of ultradeep, ultrahigh-pressure metamorphic diamonds.

  2. Physico-chemical transition from peridotite assemblage to the eclogite one (experimental data at 7.0 GPa). (United States)

    Butvina, Valentina; Litvin, Yurii


    Peridotites and eclogites, including diamond-bearing ones, are the basic ultra-basic and basic rocks of the upper mantle (Ringwood, 1969, 1975; Sobolev, 1974; Marakushev, 1985; Taylor & Anand, 2004). These rocks are presented in the assemblage of mantle xenolyths in kimberlites, but the basic minerals of peridotite paragenesis, olivine, orthopyroxene, garnet and clinopyroxene as well as of an eclogite paragenesis, garnet and omphacite are wide-spread synthetic inclusions in diamonds. The cases of finding minerals and peridotite and eclogite parageneses in diamond are described. It implies that these parageneses can have a single mantle source. However, the formation of peridotite and eclogite mineral parageneses at differentiation of the primary ultrabasite melt during physico-chemical single process is possible only at overcoming the 'eclogite' thermal barrier (O'Hara, 1968; Litvin, 1991). Eclogite genesis is one of the most difficult and discussional problems of modern petrology. Among investigators there is an opinion about eclogite heterogeneity not only on conditions of formation (crust, mantle), but also by composition of the initial rocks (para-, orthoeclogites) as well as by the way of their formation (magmatic, metamorphic, metasomatic). In literature diamond-bearing eclogite nodules of kimberlite pipes are often considered as metamorphic, which are formed at subduction of the Archean or of the Proterozoic oceanic crust (MacGregor & Manton, 1986; McCandless & Gurney, 1986, 1997 et al.). Only the presence of Na2O in garnet and K2O in clinopyroxene is a criterion of their participation in mantle magmatic processes. Together with the hypotheses considered on eclogite origin there exists a version suggested in papers (Kushiro, 1972; Kushiro & Yoder, 1974), according to which mantle eclogites could be formed due to peridotite substance in the processes of fractional crystallization of ultrabasite magmas. The present paper is devoted to the experimental study of

  3. Chlorine and fluorine partition coefficients and abundances in sub-arc mantle xenoliths (Kamchatka, Russia): Implications for melt generation and volatile recycling processes in subduction zones (United States)

    Bénard, A.; Koga, K. T.; Shimizu, N.; Kendrick, M. A.; Ionov, D. A.; Nebel, O.; Arculus, R. J.


    We report chlorine (Cl) and fluorine (F) abundances in minerals, interstitial glasses, and melt inclusions in 12 andesite-hosted, spinel harzburgite xenoliths and crosscutting pyroxenite veins exhumed from the sub-arc lithospheric mantle beneath Avacha volcano in the Kamchatka Arc (NE Russia). The data are used to calculate equilibrium mineral-melt partition coefficients (D mineral / melt) for Cl and F relevant to subduction-zone processes and unravel the history of volatile depletion and enrichment mechanisms in an arc setting. Chlorine is ∼100 times more incompatible in pyroxenes (DClmineral/melt = 0.005-0.008 [±0.002-0.003]) than F (DFmineral/melt = 0.50-0.57 [±0.21-0.24]), which indicates that partial melting of mantle sources leads to strong depletions in Cl relative to F in the residues. The data set in this study suggests a strong control of melt composition on DCl,Fpyroxene/melt, in particular H2O contents and Al/(Al + Si), which is in line with recent experiments. Fluorine is compatible in Ca-amphibole in the 'wet' sub-arc mantle (DFamphibole/melt = 3.5-3.7 [±1.5]) but not Cl (DClamphibole/melt = 0.03-0.05 [±0.01-0.03]), indicating that amphibole may fractionate F from Cl in the mantle wedge. The inter-mineral partition coefficients for Cl and F in this study are consistent amongst different harzburgite samples, whether they contain glass or not. In particular, disseminated amphibole hosts much of the Cl and F bulk rock budgets of spinel harzburgites (DClamphibole/pyroxene up to 14 and DFamphibole/pyroxene up to 40). Chlorine and fluorine are variably enriched (up to 1500 ppm Cl and 750 ppm F) in the parental arc picrite and boninite melts of primitive pyroxenite veins (and related melt inclusions) crosscutting spinel harzburgites. Based on the data in this study, the main inferences on the behaviour of Cl and F during melting and metasomatic processes in the sub-arc mantle are as follow: (i) Melting models show that most depleted mantle protoliths

  4. Dikes, joints, and faults in the upper mantle (United States)

    Wilshire, H.G.; Kirby, S.H.


    Three different types of macroscopic fractures are recognized in upper-mantle and lower-crustal xenoliths in volcanic rocks from around the world: 1. (1) joints that are tensile fractures not occupied by crystallized magma products 2. (2) dikes that are tensile fractures occupied by mafic magmas crystallized to pyroxenites, gabbros or hydrous-mineral-rich rocks, 3. (3) faults that are unfilled shear fractures with surface markings indicative of shear displacement. In addition to intra-xenolith fractures, xenoliths commonly have polygonal or faceted shapes that represent fractures exploited during incorporation of the xenoliths into the host magma that brought them to the surface. The various types of fractures are considered to have formed in response to the pressures associated with magmatic fluids and to the ambient tectonic stress field. The presence of fracture sets and crosscutting relations indicate that both magma-filled and unfilled fractures can be contemporaneous and that the local stress field can change with time, leading to repeated episodes of fracture. These observations give insight into the nature of deep fracture processes and the importance of fluid-peridotite interactions in the mantle. We suggest that unfilled fractures were opened by volatile fluids exsolved from ascending magmas to the tops of growing dikes. These volatile fluids are important because they are of low viscosity and can rapidly transmit fluid pressure to dike and fault tips and because they lower the energy and tectonic stresses required to extend macroscopic cracks and to allow sliding on pre-existing fractures. Mantle seismicity at depths of 20-65 km beneath active volcanic centers in Hawaii corresponds to the depth interval where CO2-rich fluids are expected to be liberated from ascending basaltic magmas, suggesting that such fluids play an important role in facilitating earthquake instabilities in the presence of tectonic stresses. Other phenomena related to the fractures

  5. Origin and Distribution of Water Contents in Continental and Oceanic Lithospheric Mantle (United States)

    Peslier, Anne H.


    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.

  6. The capacity of hydrous fluids to transport and fractionate incompatible elements and metals within the Earth's mantle (United States)

    Adam, John; Locmelis, Marek; Afonso, Juan Carlos; Rushmer, Tracy; Fiorentini, Marco L.


    silicate melts and aqueous fluids are thought to play critical roles in the chemical differentiation of the Earth's crust and mantle. Yet their relative effects are poorly constrained. We have addressed this issue by measuring partition coefficients for 50 trace and minor elements in experimentally produced aqueous fluids, coexisting basanite melts, and peridotite minerals. The experiments were conducted at 1.0-4.0 GPa and 950-1200°C in single capsules containing (either 40 or 50 wt %) H2O and trace element-enriched basanite glass. This allowed run products to be easily identified and analyzed by a combination of electron microprobe and LAM-ICP-MS. Fluid and melt compositions were reconstructed from mass balances and published solubility data for H2O in silicate melts. Relative to the basanite melt, the solutes from H2O-fluids are enriched in SiO2, alkalis, Ba, and Pb, but depleted in FeO, MgO, CaO, and REE. With increasing pressure, the mutual solubility of fluids and melts increases rapidly with complete miscibility between H2O and basanitic melts occurring between 3.0 and 4.0 GPa at 1100°C. Although LREE are favored over HREE in the fluid phase, they are less soluble than the HFSE (Nb, Ta, Zr, Hf, and Ti). Thus, the relative depletions of HFSE that are characteristic of arc magmas must be due to a residual phase that concentrates HFSE (e.g., rutile). Otherwise, H2O-fluids have the capacity to impart many of the geochemical characteristics that distinguish some rocks and melts from the deep mantle lithosphere (e.g., MARID and lamproites).

  7. The role of eclogite in the mantle heterogeneity at Cape Verde

    DEFF Research Database (Denmark)

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


    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...

  8. Formation of Garnet Orthopyroxenites and Mobility of Siderophile and Chalcophile Elements in the Subcontinental Lithospheric Mantle During Metasomatism by Asthenospheric Mantle- derived Melt Below the Southern South America (United States)

    Wang, J.; Hattori, K. H.; Stern, C.


    Garnet-bearing orthopyroxenite is common as discrete mantle xenoliths and veinlets in peridotite xenoliths brought to the surface by the Quaternary Pali Aike basalts, the southernmost Patagonian plateau basalt field in South America. Orthopyroxenites commonly contain Ti-rich minerals and relict grains of Ol or rare Cpx as inclusions in secondary Opx (>85 % vol). The secondary Opx contains high TiO2 (0.20-0.59 wt%), moderate Al2O3 (2.8-5.1 wt%) and low Mg# (0.84-0.89) compared with Opx in garnet-bearing peridotites. This suggests that secondary Opx formed at the expense of Ol during metasomatism by a Ti-rich evolved melt. Secondary Opx formed from Ol through reaction with slab-melt or fluid has been documented in sub-arc mantle peridotites. In contrast with such Opx in subarc mantle samples, secondary Opx in the Pali Aike orthopyroxenites contains high Ti and Al and low Mg. High Ti and low Mg in our samples reflect the evolved nature of the metasomatizing melt that originated from the underlying asthenospheric mantle. This type of orthopyroxenite may be common elsewhere in the SCLM affected by asthenospheric upwelling. The orthopyroxenites contain similar bulk-rock concentrations of Cr, Ni and PGE as do the peridotites, suggesting that these metals were essentially immobile during this type of metasomatism, and that the metasomatizing melt did not introduce these elements into the mantle. Instead, the metasomatizing melt contributed alkalis, Ti, Si, Cu, and S to the orthopyroxenites. The evolved metasomatizing melt was saturated with S and introduced immiscible sulphide liquid containing Cu and S to the orthopyroxenites. The contents of PGE are independent of S and they are most likely present in alloys, silicate and oxide minerals.

  9. Diverse mantle and crustal components in lavas of the NW Cerros del Rio volcanic field, Rio Grande Rift, New Mexico (United States)

    Duncker, K. E.; Wolff, J. A.; Harmon, R. S.; Leat, P. T.; Dickin, A. P.; Thompson, R. N.


    Products of Pliocene (2 4 Ma) mafic to intermediate volcanism in the northwestern Cerros del Rio, a dominantly mafic volcanic field in the Española Basin of the Rio Grande Rift (RGR), range from 49% to 63% SiO2 and exhibit diversity in silica saturation, trace-element patterns, and isotopic compositions. Tholeiites, which are largely confined to west of the Rio Grande, have trace-element abundances that resemble those of oceanic basalts, but with mild depletions in Nb and Ta, and high 87Sr/86Sr, low 143Nd/144Nd, and high δ18O compared to typical OIB. They are regarded as asthenospherically-derived magmas contaminated with continental crust. Alkali basalts and hawaiites erupted from vents east of the Rio Grande are geochemically distinct, having generally higher overall incompatible-element abundances, but with pronounced depletions in K, Rb, Nb and Ta with respect to Th and LREE. Spatially-associated benmoreites, mugearites and latites (collectively termed “evolved” lavas) have similar trace-element characteristics to the mafic mildly-alkaline compositions, but are typically not as depleted in K. Hawaiites and evolved lavas exhibit a good negative correlation of 143Nd/144Nd with SiO2, due to interaction with lower continental crust. The most silicic “evolved” lavas carry the highest proportions of crustal material, and consequently have higher K/Th than the related hawaiites. Several (mostly mafic) lavas contain abundant crustally-derived resorbed quartz xenocrysts in O-isotope disequilibrium with the host magma. The δ18O values of xenocrystic quartz range over 4‰, indicating a variety of quartz-bearing crustal contaminants beneath the Española Basin. The hawaiites, with their unusual combination of trace-element enrichments and depletions, cannot be generated by any process of fractionation or crustal contamination superposed on a common mantle source type (oceanic or arc-source). It is a regional mantle source type, inasmuch as it was also present

  10. Mars Accreted a Volatile Element-Depleted Late Veneer Indicating Early Delivery of Martian Volatiles (United States)

    Becker, H.; Wang, Z.


    Chalcophile elements in SNC meteorites are used to constrain abundances in the Martian mantle. Strong depletion of Te relative to highly siderophile elements suggests a volatile element-depleted late veneer, requiring that volatiles arrived earlier.

  11. Metasomatizing effects of serpentinization-related hydrothermal fluids in abyssal peridotites: new contributions from Hyblean peridotite xenoliths (southeastern Sicily) (United States)

    Manuella, Fabio Carmelo; Ottolini, Luisa; Carbone, Serafina; Scavo, Lidia


    We studied a partially serpentinized peridotite xenolith, found in the diatreme tuff-breccia deposit at Valle Guffari (Hyblean Plateau, southeastern Sicily, Italy), which is representative of the Hyblean peridotite xenolith suite. We also considered all published (21) whole-rock analyses of Hyblean peridotites, to investigate the metasomatizing effects of seawater-related hydrothermal fluids in the Hyblean basement, an in-situ remnant of the ultraslow-spreading Permian Tethys. In detail, we analyzed the serpentine veins by different techniques (scanning electron microscopy-SEM, electron-probe microanalysis-EPMA, micro-Raman spectroscopy, X-ray powder diffraction-XRPD) to determine the crystal-chemical composition and the structure of the veins. In addition, secondary ion mass spectrometry (SIMS) was applied to measure the abundance of trace elements. Serpentine veins are made up of two Fe-rich polytypes, chrysotile 2Mc1 and lizardite 1T. The chondrite-normalized rare earth element compositions of both serpentine polytypes are lower than 1, except for a modest light rare earth element (LREE) enrichment, and also in some fluid-mobile elements (FME: B, Rb, Sr, U). Conversely, the whole-rock composition of the studied peridotite xenolith is enriched with LREE and other trace elements (B, Sr, P, Th, U, Pb), like most Hyblean peridotites. The REE and multi-element patterns of Hyblean peridotites are akin to those of hydrothermal sediments from the Mid-Atlantic Ridge and St. Demetrio hill (northern Hyblean Plateau), and abyssal peridotites (serpentinites) whose trace element abundance is generally ascribed to melt-rock interaction. The integrated interpretation of the data and the documentation of hydrothermal minerals [(Na,S)-rich apatite, carbonates] in serpentine veins indicate that serpentinization-related hydrothermal fluids do have a primary role in metasomatism (mainly for the abundance of LREE and high field strength elements-HFSE) of ancient (Permian Tethys) and

  12. Water Distribution in the Continental and Oceanic Upper Mantle (United States)

    Peslier, Anne H.


    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).

  13. An experimental study of Li partitioning between olivine and diopside at mantle conditions (United States)

    Yakob, J. L.; Feineman, M. D.; Penniston-Dorland, S. C.; Eggler, D. H.


    Measured 7Li/6Li of mineral separates from mantle xenoliths from diverse localities show unexpectedly large differences between olivines and pyroxenes, often with lighter Li found in the pyroxenes (Jeffcoate et al., 2007; Rudnick and Ionov, 2007; Ionov and Seitz, 2008). Although changes in isotopic fractionation with temperature could explain the differences, a kinetic isotope effect is as likely. Because 6Li diffuses faster than 7Li, bulk lithium exchange between two phases could result in dynamic isotopic fractionation, with the receiving phase becoming lighter and the donating phase becoming heavier. Thus if Li becomes more compatible in cpx upon cooling, that is, if DLiol/cpx is temperature-dependent, the diffusive exchange of Li will generate temporary 6Li enrichment in cpx and depletion in olivine. Experiments were conducted using a piston cylinder apparatus at 1100°C and 1.4 GPa (1-5 days) to determine DLiol/cpx. San Carlos olivine and Dekalb diopside were finely ground for starting materials. A mixture of olivine (52 wt%), diopside (34 wt%), albite (7 wt%), and quartz (7 wt%) powders (0.0145 g) was loaded into a Pt capsule inside of a Ni crucible. Milli-Q water with 100 ppm Li and 500 ppm Ba (obtained through dilution of stock solutions) was added (0.1100 g) to serve as the lithium source. Lithium concentrations in olivine and diopside from experiments held for 1, 3, and 5 days were determined by laser ablation ICP-MS. Partition coefficients DLiol/cpx from runs at 3 and 5 days are, within error, the same, 1.9 (0.3). These fall in the lower portion of the range, D = 2-7, of limited previous measurements (Brenan et al., 1998b, Blundy and Dalton, 2000; Caciagli-Warman 2010). Partitioning experiments at 700 and 900°C are ongoing. References Blundy, J. and Dalton, J. (2000) Experimental comparison of trace element partitioning between clinopyroxene and melt in carbonate and silicate systems, and implications for mantle metasomatism. Contrib. Mineral. Petrol

  14. Upper mantle geotherm for eastern China and its geological implications

    Institute of Scientific and Technical Information of China (English)

    徐义刚; 林传勇; 史兰斌; J-C.C.Mercier; J.V.Ross


    The equilibrium temperature and pressure of both spinel and garnet peridotite xenolithsfrom eastern China have been estimated by using different geothermobarometers that are currentlyconsidered to be reliable.Based on these data,the upper mantle geotherm for eastern China has beenconstructed.The obtained geotherm is higher than that for old craton regions,but is similar to that for southeasternAustralia.The most prominent feature of the geotherm is that there is a slight gradient at about 60 kmdepth,and the inflection of the geotherm just passes through the transition line between spinel and garnetfacies.It is likely that the inflection represents the botmdary between the lithosphere and asthenosphere.Thestructures of the upper mantle beneath eastern China have also been discussed by combining the results ofpetrologic,rheological and dynamical studies of the upper mantle.

  15. An olivine-free mantle source of Hawaiian shield basalts. (United States)

    Sobolev, Alexander V; Hofmann, Albrecht W; Sobolev, Stephan V; Nikogosian, Igor K


    More than 50 per cent of the Earth's upper mantle consists of olivine and it is generally thought that mantle-derived melts are generated in equilibrium with this mineral. Here, however, we show that the unusually high nickel and silicon contents of most parental Hawaiian magmas are inconsistent with a deep olivine-bearing source, because this mineral together with pyroxene buffers both nickel and silicon at lower levels. This can be resolved if the olivine of the mantle peridotite is consumed by reaction with melts derived from recycled oceanic crust, to form a secondary pyroxenitic source. Our modelling shows that more than half of Hawaiian magmas formed during the past 1 Myr came from this source. In addition, we estimate that the proportion of recycled (oceanic) crust varies from 30 per cent near the plume centre to insignificant levels at the plume edge. These results are also consistent with volcano volumes, magma volume flux and seismological observations.

  16. K-Rich and Carbonatic Melt Metasomatism in Depleted Upper Mantle: Geochemical Evidences from Peridotites in Pre-Pilot Hole of Chinese Continental Scientific Drilling Project%亏损上地幔中的富钾熔体和碳酸盐交代作用:来自CCSD预先导孔橄榄岩的地球化学证据

    Institute of Scientific and Technical Information of China (English)

    李天福; 杨经绥; 张儒媛


    中国大陆科学钻探工程预先导孔(CCSD-PP1)打在苏鲁超高压变质带芝麻坊超镁铁岩体上,钻孔穿透超镁铁岩体115m.超镁铁岩体由二辉橄榄岩、方辉橄榄岩和少量单辉橄榄岩和纯橄岩组成,与上下围岩接触的橄榄岩被强烈蛇纹石化.多数橄榄岩含有石榴石或其假象,普遍含有金云母和菱镁矿,少量样品中有钛斜硅镁石.在化学成分上,橄榄岩的Mg#指数变化于90.3~92.6之间,MgO含量(36.61%~49.15%,平均45.17%)与Na2O(0.01%~0.25%)、Al2O3(0.07%~3.71%,多数<2.0%,平均1.46%)和CaO(0.12%~2.53%,一个高达3.30%,平均1.00%)呈负相关关系.与主量易熔元素相对亏损的特点相反,橄榄岩中显示了稀土元素富集、分馏和配分曲线显示近于平行和相似的特点,(La/Lu)N比值为3.18~33.05;此外,多数样品具有高Ba(最高比原始地幔高100多倍)含量,在蛛网图上显示Rb、Nb、Ta、Zr、Hf和Sr的负异常,Ti/Eu比值均低于1300.岩相学特征和难熔主量元素与不相容元素之间的无相关性表明橄榄岩至少受到了形成金云母和菱镁矿的2次交代作用.富含金云母的橄榄岩(例如C25-143-61,C32-149-71)具有富钾趋势,并且显示K2O与Rb、Ba和Th等大离子亲石元素的正相关关系,未见K2O和稀土元素、Sr和Ca之间的相关关系.这些特征表明橄榄岩被含水、硅铝质碱性熔体交代,之后又被高Ba低Rb和高场强元素的镁质(菱镁矿)碳酸岩熔体交代,并且强烈地改变了Ba的丰度和显示了特定地幔碳酸盐的稀土元素配分型式.全岩具有不均一的高放射性Sr(87 Sr/86Sr=0.708 4~0.720 1)和低放射性Nd(εNd(t)=-1.14~-8.55)组成,结合已有的氧同位素研究资料,表明预先导孔PP1中的橄榄岩所代表的地幔可能在早期就遭受了来自深部的介质的交代作用.

  17. Role of the subduction filter in mantle recycling (United States)

    Kimura, J. I.; Skora, S. E.; Gill, J.; Van Keken, P. E.


    Subduction modifies the descending basaltic and sedimentary oceanic crust and generates felsic arc materials and continental crust. Studies of element mass balances in the subduction zone therefore reveal the evolution of the Earth's two major geochemical reservoirs: the continent crust and mantle. We use the Arc Basalt Simulator ver.4 (ABS4) to model the geochemical mass balance during dehydration by prograde metamorphism and melting of the slab followed by subsequent flux melting of the wedge mantle caused by the addition of slab-derived liquids. The geochemistry of high-Mg andesite or adakite formed in a hot subduction zone is akin to the present-day bulk continental crust and to the Archean (>2 Ga) Tonalite-Trondjhemite-Granodiorite composition. Therefore, the residual slab and the metasomatized mantle wedge at hot subduction zones should be the most plausible sources for materials recycled back into the deep mantle. Model calculations of isotopic growth in the residual slab and mantle formed in hot subduction zones reproduce fairly well the EM1-FOZO-HIMU isotope arrays found in ocean island basalts (OIBs) of deep mantle plume origin, although FOZO with high 3He/4He is not generated by this slab recycling process. The recycled materials are bulk igneous ocean crust for HIMU and metasomatized mantle wedge peridotite for EM1. In contrast, the EM2-FOZO array can be generated in a cold subduction zone with igneous oceanic crust for FOZO and sediment for EM2 sources. Necessary residence time are ~2 Ga to form HIMU-FOZO-EM1 and ~1 Ga to form EM2-FOZO. The subducted oceanic crust (forming HIMU) and mantle wedge peridotite (forming EM1) may have travelled in the mantle together. They then melted together in an upwelling mantle plume to form the EM1-FOZO-HIMU isotopic variations found frequently in OIBs. In contrast, the less frequent EM2-FOZO array suggests a separate source and recycling path. These recycling ages are consistent with the change in the mantle potential

  18. Melting and Crystallization at Core Mantle Boundary (United States)

    Fiquet, G.; Pradhan, G. K.; Siebert, J.; Auzende, A. L.; Morard, G.; Antonangeli, D.; Garbarino, G.


    Early crystallization of magma oceans may generate original compositional heterogeneities in the mantle. Dense basal melts may also be trapped in the lowermost mantle and explain mantle regions with ultralow seismic velocities (ULVZs) near the core-mantle boundary [1]. To test this hypothesis, we first constructed the solidus curve of a natural peridotite between 36 and 140 gigapascals using laser-heated diamond anvil cells. In our experiments, melting at core-mantle boundary pressures occurs around 4100 ± 150 K, which is a value that can match estimated mantle geotherms. Similar results were found for a chondritic mantle [2] whereas much lower pyrolitic melting temperatures were recently proposed from textural and chemical characterizations of quenched samples [3]. We also investigated the melting properties of natural mid ocean ridge basalt (MORB) up to core-mantle boundary (CMB) pressures. At CMB pressure (135 GPa), we obtain a MORB solidus temperature of 3950 ±150 K. If our solidus temperatures are in good agreement with recent results proposed for a similar composition [4], the textural and chemical characterizations of our recovered samples made by analytical transmission electron microscope indicate that CaSiO3 perovskite (CaPv) is the liquidus phase in the entire pressure range up to CMB. The partial melt composition is enriched in FeO, which suggests that such partial melts could be gravitationnally stable at the core mantle boundary. Our observations are tested against calculations made using a self-consistent thermodynamic database for the MgO-FeO-SiO2 system from 20 GPa to 140 GPa [5]. These observations and calculations provide a first step towards a consistent thermodynamic modelling of the crystallization sequence of the magma ocean, which shows that the existence of a dense iron rich and fusible layer above the CMB at the end of the crystallization is plausible [5], which is in contradiction with the conclusions drawn in [4]. [1] Williams

  19. Subduction initiation, recycling of Alboran lower crust, and intracrustal emplacement of subcontinental lithospheric mantle in the Westernmost Mediterranean (United States)

    Varas-Reus, María Isabel; Garrido, Carlos J.; Bosch, Delphine; Marchesi, Claudio; Hidas, Károly; Booth-Rea, Guillermo; Acosta-Vigil, Antonio


    Unraveling the tectonic settings and processes involved in the annihilation of subcontinental mantle lithosphere is of paramount importance for our understanding of the endurance of continents through Earth history. Unlike ophiolites -- their oceanic mantle lithosphere counterparts -- the mechanisms of emplacement of the subcontinental mantle lithosphere in orogens is still poorly known. The emplacement of subcontinental lithospheric mantle peridotites is often attributed to extension in rifted passive margins or continental backarc basins, accretionary processes in subduction zones, or some combination of these processes. One of the most prominent features of the westernmost Mediterranean Alpine orogenic arcs is the presence of the largest outcrops worldwide of diamond facies, subcontinental mantle peridotite massifs; unveiling the mechanisms of emplacement of these massifs may provide important clues on processes involved in the destruction of continents. The western Mediterranean underwent a complex Alpine evolution of subduction initiation, slab fragmentation, and rollback within a context of slow convergence of Africa and Europe In the westernmost Mediterranean, the alpine orogeny ends in the Gibraltar tight arc, which is bounded by the Betic, Rif and Tell belts that surround the Alboran and Algero-Balearic basins. The internal units of these belts are mostly constituted of an allochthonous lithospheric domain that collided and overthrusted Mesozoic and Tertiary sedimentary rocks of the Mesozoic-Paleogene, South Iberian and Maghrebian rifted continental paleomargins. Subcontinental lithospheric peridotite massifs are intercalated between polymetamorphic internal units of the Betic (Ronda, Ojen and Carratraca massifs), Rif (Beni Bousera), and Tell belts. In the Betic chain, the internal zones of the allochthonous Alboran domain include, from bottom to top, polymetamorphic rock of the Alpujarride and Malaguide complexes. The Ronda peridotite massif -- the

  20. Mantle differentiation and chemical cycling in the Archean (Invited) (United States)

    Lee, C.


    Differentiation of Earth’s silicate mantle is largely controlled by solid-state convection. Today, upwelling mantle leads to decompression melting. Melts, being of low density, rise to form the continental and oceanic crusts. Because many trace elements, such as heat-producing U, Th and K, as well as the noble gases, preferentially partition into melts, melt extraction concentrates these elements into the crust or atmosphere. However, one by-product of whole-mantle convection is that melting during the Earth’s first billion years was likely deep and hot. Such high pressure melts may have been dense, allowing them to stall, crystallize and later founder back into the lower mantle. These sunken lithologies would have ‘primordial’ chemical signatures despite a non-primordial origin. As the Earth cools, the proportion of upwards melt segregation relative to downwards melt segregation increases, removing volatiles and other incompatible elements to the surface. Recycling of these elements back into the Earth’s interior occurs by subduction, but because of chemical weathering, hydrothermal alteration and photosynthetic reactions occurring in the Earth’s exosphere, these recycled materials may re-enter the mantle already chemically transformed. In particular, photosynthetic production of oxygen and, especially, the progressive oxygenation of the Earth’s atmosphere require removal of reduced carbon from the Earth’s surface. If such removal occurred by subduction, the mantle would have become progressively reduced. During the Archean and early Proterozoic, much of this material may have contributed to making cratonic mantle, and if so, cratonic mantle may have been assembled by reduced building blocks, perhaps explaining the origin of diamonds with organic carbon isotopic signatures. The origin of peridotitic diamonds in cratonic mantle could then be explained if the underlying convecting mantle was in fact more oxidizing such that carbonatitic liquids

  1. Garnet peridotites from Pohorje: Petrography, geothermobarometry and metamorphic evolution

    Directory of Open Access Journals (Sweden)

    Mirijam Vrabec


    Full Text Available Ultrahigh-pressure (UHP metamorphism has been recorded in Eo-Alpine garnet peridotites from the PohorjeMts., Slovenia, belonging to the Eastern Alps. The garnet peridotite bodies are found within serpentinized metaultrabasitesin the SE edge of Pohorje and are closely associated with UHP kyanite eclogites. These rocks belongto the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriaticfault system.Garnet peridotites show signs of a complex four-stage metamorphic history. The protolith stage is represented bya low-P high-T assemblage of olivine + Al-rich orthopyroxene + Al-rich clinopyroxene + Cr-spinel. Due to metamorphism,primary clinopyroxene shows exsolutions of garnet, orthopyroxene, amphibole, Cr-spinel and ilmenite. TheUHP metamorphic stage is defined by the assemblage garnet + olivine + Al-poor orthopyroxene + clinopyroxene +Cr-spinel. Subsequent decompression and final retrogression stage resulted in formation of kelyphitic rims aroundgarnet and crystallization of tremolite, chlorite, serpentine and talc.Pressure and temperature estimates indicate that garnet peridotites reached the peak of metamorphism at 4 GPaand 900 °C, that is well within the UHP stability field. Garnet peridotites in the Pohorje Mountains experiencedUHP metamorphism during the Cretaceous orogeny and thus record the highest-pressure conditions of all Eo-Alpinemetamorphism in the Alps.

  2. Lateral variation of differential stress in the uppermost mantle across the island arc of southwest Japan (United States)

    Toriumi, Mitsuhiro


    The dislocation density and the subgrain size of olivine in peridotite xenoliths in southwest Japan were investigated in order to draw out the lateral variation of the differential stress in the upper mantle of the island arc. Alkali basaltic and andesitic dykes including peridotite xenoliths of Dogo, Kikuma, and Shingu are situated about 200 km behind the Nankai Trough, and those of Oki-Dogo and Takashima located at the portions 400-500 km apart from the trough. The mean dislocation densities of olivine are 2 × 10 6 cm -2 for Oki-Dogo, 8 × 10 6 cm -2 for Takashima, 1 × 10 7 cm -2 for Hamada, 5 × 10 7 cm -2 for Aratoyama, 4 × 10 7 cm -2 for Kikuma, 3 × 10 7 cm -2 for Dogo, and 5 × 10 6 cm -2 for Shingu peridotites. It is concluded that the differential stress is high in the uppermost mantle beneath the island arc and low in the back-arc and the mantle wedge behind the plate boundary. The lateral variation of stress may be due to the diapiric upwelling of upper mantle materials under the island arc. The size of the diapir is suggested to be 200 km in width and 60-150 km in depth.

  3. Magma underplating and Hannuoba present crust-mantle transitional zone composition: Xenolith petrological and geochemical evidence

    Institute of Scientific and Technical Information of China (English)

    FAN Qicheng; ZHANG Hongfu; SUI Jianli; ZHAI Mingguo; SUN Qian; LI Ni


    On the basis of mineral assemblage, mineralogy, petrology, and major, trace elemental and isotopic geochemistry of the underplated granulite- and eclogite-facies accumulate, peridotite and pyroxenite xenoliths entrained in Hannuoba Cenozoic basalts, this work constrained the petrological constituents for the crust-mantle transitional zone, which is supported by the results of high-temperature and pressure velocity experiments on rocks and geophysics deep survey. Present lower part of lower crust is mainly composed of granulite-facies mafic accumulates (dominantly plagioclase websterite) and crust-mantle transitional zone dominantly composed of eclogite-facies pyroxenites with or without garnet and spinel lherzolites; Archaean terrain granulite is only nominally early lower crust. Magma underplating in the crust-mantle boundary led to the crustal vertical accretion and the formation of the crust-mantle transitional zone, which is a significant mechanism for the chemical adjustment of the crust-mantle boundary since the Phanerozoic.

  4. Re-Os and Lu-Hf isotopic constraints on the formation and age of mantle pyroxenites from the Bohemian Massif (United States)

    Ackerman, Lukáš; Bizimis, Michael; Haluzová, Eva; Sláma, Jiří; Svojtka, Martin; Hirajima, Takao; Erban, Vojtěch


    We report on the Lu-Hf and Re-Os isotope systematics of a well-characterized suite of spinel and garnet pyroxenites from the Gföhl Unit of the Bohemian Massif (Czech Republic, Austria). Lu-Hf mineral isochrons of three pyroxenites yield undistinguishable values in the range of 336-338 Ma. Similarly, the slope of Re-Os regression for most samples yields an age of 327 ± 31 Ma. These values overlap previously reported Sm-Nd ages on pyroxenites, eclogites and associated peridotites from the Gföhl Unit, suggesting contemporaneous evolution of all these HT-HP rocks. The whole-rock Hf isotopic compositions are highly variable with initial εHf values ranging from - 6.4 to + 66. Most samples show a negative correlation between bulk rock Sm/Hf and εHf and, when taking into account other characteristics (e.g., high 87Sr/86Sr), this may be explained by the presence of recycled oceanic sediments in the source of the pyroxenite parental melts. A pyroxenite from Horní Kounice has decoupled Hf-Nd systematics with highly radiogenic initial εHf of + 66 for a given εNd of + 7.8. This decoupling is consistent with the presence of a melt derived from a depleted mantle component with high Lu/Hf. Finally, one sample from Bečváry plots close to the MORB field in Hf-Nd isotope space consistent with its previously proposed origin as metamorphosed oceanic gabbro. Some of the websterites and thin-layered pyroxenites have variable, but high Os concentrations paralleled by low initial γOs. This reflects the interaction of the parental pyroxenitic melts with a depleted peridotite wall rock. In turn, the radiogenic Os isotope compositions observed in most pyroxenite samples is best explained by mixing between unradiogenic Os derived from peridotites and a low-Os sedimentary precursor with highly radiogenic 187Os/188Os. Steep increase of 187Os/188Os at nearly uniform 187Re/188Os found in a few pyroxenites may be connected with the absence of primary sulfides, but the presence of minor

  5. Hydrothermal processes in partially serpentinized peridotites from Costa Rica: evidence from native copper and complex sulfide assemblages (United States)

    Schwarzenbach, Esther M.; Gazel, Esteban; Caddick, Mark J.


    Native metals and metal alloys are common in serpentinized ultramafic rocks, generally representing the redox and sulfur conditions during serpentinization. Variably serpentinized peridotites from the Santa Elena Ophiolite in Costa Rica contain an unusual assemblage of Cu-bearing sulfides and native copper. The opaque mineral assemblage consists of pentlandite, magnetite, awaruite, pyrrhotite, heazlewoodite, violarite, smythite and copper-bearing sulfides (Cu-pentlandite, sugakiite [Cu(Fe,Ni)8S8], samaniite [Cu2(Fe,Ni)7S8], chalcopyrite, chalcocite, bornite and cubanite), native copper and copper-iron-nickel alloys. Using detailed mineralogical examination, electron microprobe analyses, bulk rock major and trace element geochemistry, and thermodynamic calculations, we discuss two models to explain the formation of the Cu-bearing mineral assemblages: (1) they formed through desulfurization of primary sulfides due to highly reducing and sulfur-depleted conditions during serpentinization or (2) they formed through interaction with a Cu-bearing, higher temperature fluid (350-400 °C) postdating serpentinization, similar to processes in active high-temperature peridotite-hosted hydrothermal systems such as Rainbow and Logatchev. As mass balance calculations cannot entirely explain the extent of the native copper by desulfurization of primary sulfides, we propose that the native copper and Cu sulfides formed by local addition of a hydrothermal fluid that likely interacted with adjacent mafic sequences. We suggest that the peridotites today exposed on Santa Elena preserve the lower section of an ancient hydrothermal system, where conditions were highly reducing and water-rock ratios very low. Thus, the preserved mineral textures and assemblages give a unique insight into hydrothermal processes occurring at depth in peridotite-hosted hydrothermal systems.

  6. Multi stage peridotite-melt reaction evidenced by xenolith suite from Wilcza Góra basanite (SW Poland) (United States)

    Matusiak-Małek, Magdalena; Puziewicz, Jacek; Ntaflos, Theodoros; Grégoire, Michel; Wojtulek, Piotr


    Presence of hydrous phases in Earth's lithospheric mantle is a typical manifestation of modal metasomatism. Mantle xenoliths occurring in Northern margin of the Bohemian Massif usually contain no hydrous phases (Puziewicz et al., 2015). The latter, if present, form small and scarce crystals of pargasitic amphibole (e.g. Nowak et al., 2010, EGU abstracts; Matusiak-Małek et al., 2010, Lithos). The exception are xenoliths from Wilcza Góra basanite (SW Poland). The xenoliths have composition of harzburgite, scarce dunitic and wehrlitic ones occur as well. Pyroxenites of cumulative textures occur also in the Wilcza Góra.Pargasitic amphibole occurs in all the types of the xenoliths. Amphibole forms: 1) large (up to 1 cm) subhedral crystals, 2) exsolution lamellae in pyroxenes, and 3) cores of secondary clinopyroxenes in integranular aggregates. Composition of amphibole varies in wide ranges between xenoliths (e.g. mg# 0.73-0.95). The mineral is in equilibrium with coexisting clinopyroxene (mg#=0.79-0.93) in terms of major and trace elements. Variations in Mg and Fe contents in olivine and orthopyroxene are significant and continuous (Fo=77-92 and mg#=0.85-0.92, respectively). Enrichment of peridotites in Fe may result from: (1) cumulative origin, or (2) metasomatic enrichment due to reaction with mafic silicate melt. But, as was discussed in Matusiak-Małek et al. (2014), features like: lack of cumulative textures, high NiO in olivine (0.25-0.43 wt.%) promotes the second possibility. As clinopyroxene and spinel record no melting (e.g. lack of Al-Na positive and cr#-mg# negative correlations, respectively), they cannot be a primary phase and must have been introduced into peridotite by metasomatic reactions ("stealth" metasomatism; O'Reillly and Griffin, 2013, Springer). Trace element composition of clinopyroxene and amphibole and their equilibrium suggest their crystallization from the fractionating mafic silicate melt, possibly the same which caused enrichment of

  7. Dihedral angle of carbonatite melts in mantle residue near the upper mantle and transition zone (United States)

    Ghosh, S. K.; Rohrbach, A.; Schmidt, M. W.


    Carbonate melts are thought to be ideal metasomatic agents in the deep upper mantle (Green & Wallace, 1988) and these melts are low in viscosities (10-1-10-3 Pa·s) compared to primitive basalt (101-102 Pa·s), furthermore the ability to form an interconnected grain-edge melt network at low melt fractions (3 GPa (Dasgupta et al. 2006, Ghosh et al., 2009), dissolve a number of geochemically incompatible elements much better than silicate melts (Blundy and Dalton, 2000). Previous studies of carbonate melt dihedral angles in olivine-dominated matrices yielded 25-30oat 1-3 GPa, relatively independent of melt composition (Watson et al., 1990) and temperature (Hunter and McKenzie, 1989). Dihedral angles of carbonate melts in contact with deep mantle silicate phases (e.g. garnet, wadsleyite, and ringwoodite) which constitute more than 70 % of the deep upper mantle and transition zone have not been studied yet. We have performed multi-anvil experiments on carbonate-bearing peridotites with 5.0 wt% CO2 from 13.5 to 20 GPa 1550 oC to investigate the dihedral angle of magnesio-carbonatite melts in equilibrium with garnet, olivine (and its high-pressure polymorphs), and clinoenstatite. The dihedral angle of carbonate melts in the deep upper mantle and transition zone is ~30° for majorite garnet and olivine (and its polymorphs) dominated matrices. It does not change with increasing pressure in the range 13.5-20 GPa. Our results suggest that very low melt fractions of carbonatite melt forming in the deep upper mantle and transition zone are interconnected at melt fractions less than 0.01. Consistent with geophysical observations, this could possibly explain low velocity regions in the deep mantle and transition zone.

  8. New model of the mantle lithosphere beneath Kuoyka kimberlite field Yakutia. (United States)

    Ashchepkov, Igor; Kostrovitsky, Sergey; Ovchinnikov, Yury; Tychkov, Nikolai; Khmelnikova, Olga; Palessky, Stanislav


    New data for the 11 pipes from Kuoyka field show that high Cr2O3 garnets to 10- 12% as well as high Cr chromites (to 64%Cr2O3) are found in several more pipes Zaozernaya, Seraya, Slyudyanka, Vodorasdelnaya, Titan, Lusya in addition to Djanga pipe. All garnets belong o lherzolite field and not less than 1/3 are TiO rich. The TiO2 rich chromites are dominating in the Cr- rich population. Metasomatic Cr2O3- rich (to 6%) ilmenites pre in the MgO and TiO2- part of the variation diagrams. The Cr- diopside variations show high variations of Fe and Na content to 4 % suggesting the hybridic origin similar to the Cr- pyroxeneis from Obnazhennaya pyroxenites (Taylor et al ., 2003). Omphicites (to 7 % Na2O) are rare. Cr-amphiboles (pargasites and hornblendes) are common in the upper part of the SCLM as well as in the Anabar and Kharamai region. Reconstructions of the mantle sections show the deep lithospheric roots beneath the Zosernaya pipe (7.5 GPa) traced by the PT conditions for Opx, Cpx, Gar, Cr and Ilm. SCLM is divided in to 4 sections and Ilm trace tow intervals in lower and upper part form 4 GPa. Th HT branch is sporadically found from 7 GPa to the Moho. In other pipes ilmenite and garnet PT estimates are more common in the lower part o mantle section while the Cpx trace mainly middle part of SCLM similar to the Obnazhennaya pip. It seems that kimberlites captured mainly the walls of feeders traced by Cr- low garnets and ilmenites in the lower part of SCLM while peridotitic mantle column was captured starting from the middle part of SCLM. The NS transsect of the Kuoyka field show more fertile mantle sections in the NNW part of the field. The TRE determined for the minerals from Kuoyka field show rather rounded patterns for REE of garnets with high variations in HREE part and small elevation in LREE . The depleted compositions reval the inflection in Eu TRE spidergrams well as relatively small Sr minima. Many of them show Ta peak, relatively small Pb elevation and Th

  9. Multiple magmatism in an evolving suprasubduction zone mantle wedge: The case of the composite mafic-ultramafic complex of Gaositai, North China Craton (United States)

    Yang, Fan; Santosh, M.; Tsunogae, T.; Tang, Li; Teng, Xueming


    The suprasubduction zone mantle wedge of active convergent margins is impregnated by melts and fluids leading to the formation of a variety of magmatic and metasomatic rock suites. Here we investigate a composite mafic-ultramafic intrusion at Gaositai, in the northern margin of the North China Craton (NCC). The hornblende gabbro-serpentinite-dunite-pyroxenite-gabbro-diorite suite surrounded by hornblendites of this complex has long been considered to represent an ;Alaskan-type; zoned pluton. We present petrologic, mineral chemical, geochemical and zircon U-Pb and Lu-Hf data from the various rock types from Gaositai including hornblende gabbro, serpentinite, dunite, pyroxenite, diorite and the basement hornblendite which reveal the case of multiple melt generation and melt-peridotite interaction. Our new mineral chemical data from the mafic-ultramafic suite exclude an ;Alaskan-type; affinity, and the bulk geochemical features are consistent with subduction-related magmatism with enrichment of LILE (K, Rb, and Ba) and LREE (La and Ce), and depletion of HFSE (Nb, Ta, Zr, and Hf) and HREE. Zircon U-Pb geochronology reveals that the hornblendites surrounding the Gaositai complex are nearly 2 billion years older than the intrusive complex and yield early Paleoproterozoic emplacement ages (2433-2460 Ma), followed by late Paleoproterozoic metamorphism (1897 Ma). The serpentinites trace the history of a long-lived and replenished ancient sub-continental lithospheric mantle with the oldest zircon population dated as 2479 Ma and 1896 Ma, closely corresponding with the ages obtained from the basement rock, followed by Neoproterozoic and Phanerozoic zircon growth. The oldest member in the Gaositai composite intrusion is the dunite that yields emplacement age of 755 Ma, followed by pyroxenite formed through the interaction of slab melt and wedge mantle peridotite at 401 Ma. All the rock suites also carry multiple population of younger zircons ranging in age from Paleozoic to

  10. P-V-T Equation of State of (Al,Fe)-bearing Mantle Perovskite and its Implications for Mantle Models (United States)

    Fei, Y.; Ricolleau, A.; Litasov, K.; Prakapenka, V.


    We have made significant progress on accurate measurements of P-V-T equations-of-state of mantle minerals that are of fundamental importance for developing compositional and mineralogical models of the Earth's mantle. In this study, we report new compression data on (Al,Fe)-bearing mantle perovskite up to simultaneous pressure and temperature of 113 GPa and 2120 K. The mantle perovskite was synthesized in the multi-anvil apparatus at 27 GPa and 2073 K, with chemical compositions expected in a peridotitic mantle. It contains 5.86 wt% FeO and 3.84 wt% Al2O3. The pre-synthesized perovsite mixed with Au powder was compressed in neon pressure medium in a symmetric diamond anvil cell. The sample was heated with a double-sided laser-heating system at the GSECARS 13-ID-D beamline (Advanced Photon Source). We performed 8 heating cycles in the pressure range of 30-113 GPa and temperatures up to 2560 K. In-situ synchrotron X-ray diffraction data were collected within a uniformly heated area, using a MAR-CCD area detector. The diffraction pattern contains peaks of orthorhombic perovskite, internal standard Au, and pressure medium Ne. The triplet (020, 112, and 200 diffraction peaks) of the orthorhombic perovskite is well resolve. The present dataset covers the entire P-T range of the lower mantle and requires no extrapolation to compare the mantle density profile derived from seismic observations. In light of the new P-V-T data on the (Al,Fe)-bearing mantle perovskite combined with our recent density data and spin transition of ferropericlase, we finally discuss the compositional and mineralogical models of the lower mantle.

  11. Compositional variations and heterogeneity in fertile lithospheric mantle: peridotite xenoliths in basalts from Tariat, Mongolia (United States)

    Prosperi, B.; Delay, G.; Bazile, R.; Helie, J.; Nuglish, H. J.


    The gas entrainment in a hollow cone spray submitted to variable density is studied experimentally in order to better understand the effect on mixture formation. Particle image velocimetry on fluorescent tracers, associated with a specific processing of the instantaneous velocity fields have been applied to obtain measurement in the close vicinity of the spray edge. In the “quasi-steady” region of the spray, important effect of the ambient density on the mass flow rate of entrained gas (ifmmodeexpandafterdotelseexpandafter\\.fi{m}_{{text{e}}} ) have been pointed out. The axial evolution of ifmmodeexpandafterdotelseexpandafter\\.fi{m}_{{text{e}}} is in good agreement with an integral model that takes the momentum exchange between phases into account.

  12. Mineralogy and petrology of leucite ankaratrites with affinities to kamafugites and carbonatites from the Kayıköy area, Isparta, SW Anatolia, Turkey: Implications for the influences of carbonatite metasomatism into the parental mantle sources of silica-undersaturated potassic magmas (United States)

    Caran, Şemsettin


    In the Kayıköy area of Isparta-Gölcük district, Inner Isparta Angle, SW Anatolia, Turkey, a small volume of newly discovered K-rich mafic potassic magma was emplaced in the form of dome in the vicinity of graben structures under Pliocene (3.68 ± 0.5 Ma) extensional tectonics. Kayıköy leucite ankaratrites are made up of abundant diopside, barian phlogopite and leucite, and lesser olivine, that rarely contains Cr-spinel, nepheline and haüyne, with abundant magnetite. They have low SiO2 (44.00-46.04 wt.%) and Al2O3 (12.10-12.64 wt.%) with high K2O (4.00-4.42 wt.%), CaO (13.50-14.40 wt.%) and MgO (8.52-9.36 wt.%), with high Cr (397-547 ppm) and moderate Ni (57-74 ppm) contents. They represent the less evolved silica-undersaturated mafic potassic magmas within the Isparta-Gölcük volcanic province, and may be considered another parental source to the wide spectrum of the K-rich rocks. They are highly enriched in most of the incompatible elements (e.g., Ba, 2761 to > 10,000 ppm; Sr, 3700-4074 ppm; Th, 33.60-36.99 ppm; Zr, 274-321 ppm) with high LREEs, low HREEs and elevated LREEs/HREEs ratios [(La/Yb)N, 73-80] and are comparable with kamafugite and carbonatites. Trace element patterns have negative P, Ti and Nb-Ta anomalies in common with the Italian kamafugite province and lack of a Eu anomaly, in contrast to the negative Eu anomaly of the Italian province. Their Sr87/86-Nd143/144 (0.703877-0.512765) isotopic compositions, together with those of other potassic volcanics from the Inner Isparta Angle, coincide with the West Quinling (China) kamafugites with highly depleted mantle signatures, and young East African carbonatites. Olivine-Cr-spinel pairs, high Mg# (0.69-0.73) numbers and Cr values, and high incompatible and LREE contents in Kayıköy leucite ankaratritic magma are consistent with near-primary magmas equilibrated with enriched and heterogeneous (peridotitic/pyroxenitic) mantle sources. On the basis of (i) their geochemical signatures [low Ti

  13. The 5 GPa mantle solidus revisited (United States)

    Lesher, C. E.; Pickering-Witter, J.; Baxter, G.; Walter, M. J.


    We critically examine the recent claim that interlaboratory discrepancies in the solidus temperatures of nominally dry, fertile mantle peridotite between 4 and 6 GPa are related to encapsulating material (Re- foil capsules [1] versus graphite-lined Pt capsules [2]). Herzberg et al. [1] argue that reaction between silicate and graphite produces CO2, which causes a substantial freezing point depression of the peridotite solidus (>50\\deg~C at 5 GPa). We report results of a direct experimental comparison of peridotite melting in Re-foil and graphite-lined Pt capsules. We locate both capsules, containing fertile peridotite KR4003 [2], into the central portion of a stepped graphite heater, where thermal gradients are modest and well-characterized. Our experiments are performed in a MA6/8 multianvil, using octahedra of Ceramacast 584 cast with fins to fit 10-mm truncated edge length carbide anvils and calibrated by [3]. We insert axial W-Re (D-type) thermocouples into both ends of the furnace; butting thermocouple leads against the metal capsules to complete the circuit. This arrangement closely matches the thermocouple configurations used in the studies of [1] and [2]. Thermocouple voltage, power consumption, and load are recorded continuously during the experiment. Temperature is controlled by the thermocouple connected through the Pt capsule. We find that the voltage from the second thermocouple drifts significantly above 1650\\deg~C, and gives false temperatures after only 10 minutes at the set point temperature. We also find that the phase assemblages in both capsules of a given experiment are identical down to conditions with trace amounts of quenched melt. At 1670\\deg~C, based on the control thermocouple, KR4003 was partially melted to ~35% and has a residual mineralogy of olivine, orthopyroxene, and garnet. This run temperature is 15\\deg~C below the 5 GPa solidus of [1]. Extrapolation of melt fraction versus temperature data gives a solidus temperature for KR

  14. Deformation of fine-grained synthetic peridotite under wet conditions

    NARCIS (Netherlands)

    McDonnell, R.D.


    Fine-grained hydrated peridotite mylonites have been proposed to play an important role in controlling the strength of the continental lithosphere during rifting. For this reason, the deformation behaviour of wet fine-grained forsterite and forsterite-enstatite materials and the underlying deformati

  15. Mantle melting factors and amagmatic crustal accretion of the Gakkel ridge, Arctic Ocean

    Institute of Scientific and Technical Information of China (English)

    ZHANG Tao; GAO Jinyao; CHEN Mei; YANG Chunguo; SHEN Zhongyan; ZHOU Zhiyuan; WU Zhaocai; SUN Yunfan


    Spreading rate is a primary factor of mantle melting and tectonic behavior of the global mid-ocean ridges. The spreading rate of the Gakkel ridge decreases gradually from west to east. However, the Gakkel ridge can be divided into four thick-and-thin zones with varying crustal thicknesses along ridge axis. This phenomenon indicates that mantle melting of the Gakkel ridge is not a simple function of spreading rate. Mantle temperature, water content, mantle composition, and other factors are important in crustal accretion processes. Based on gravity-derived crustal thickness and wet melting model, we estimate that the mantle potential temperatures of the four zones are 1 270, 1 220, 1 280, and 1 280°C (assuming that mantle water content equals to global average value), with corresponding mantle water contents of 210, 0, 340, and 280 mg/kg (assuming that mantle potential temperature is 1 260°C), respectivly. The western thinned crust zone is best modeled with low mantle temperature, whereas the other zones are mainly controlled by the enhanced conduction caused by the slower spreading rate. Along the Gakkel ridge, the crustal thickness is consistent with rock samples types. Predominated serpentinized peridotite and basalt are found in the area with crustal thickness 2.5 km, respectively. The rock samples are including from basalt to peridotite in the area with crustal thickness between 1.5 and 2.5 km. Based on this consistency, the traditional magmatic accretion zone accounted for only 44% and amagmatic accretion accounted for 29% of the Gakkel ridge. The amagmatic accretion is a significant characteristic of the ultra-slow spreading ridge.

  16. A great thermal divergence in the mantle beginning 2.5 Ga: Geochemical constraints from greenstone basalts and komatiites

    Directory of Open Access Journals (Sweden)

    Kent C. Condie


    Full Text Available Greenstone basalts and komatiites provide a means to track both mantle composition and magma generation temperature with time. Four types of mantle are characterized from incompatible element distributions in basalts and komatiites: depleted, hydrated, enriched and mantle from which komatiites are derived. Our most important observation is the recognition for the first time of what we refer to as a Great Thermal Divergence within the mantle beginning near the end of the Archean, which we ascribe to thermal and convective evolution. Prior to 2.5 Ga, depleted and enriched mantle have indistinguishable thermal histories, whereas at 2.5–2.0 Ga a divergence in mantle magma generation temperature begins between these two types of mantle. Major and incompatible element distributions and calculated magma generation temperatures suggest that Archean enriched mantle did not come from mantle plumes, but was part of an undifferentiated or well-mixed mantle similar in composition to calculated primitive mantle. During this time, however, high-temperature mantle plumes from dominantly depleted sources gave rise to komatiites and associated basalts. Recycling of oceanic crust into the deep mantle after the Archean may have contributed to enrichment of Ti, Al, Ca and Na in basalts derived from enriched mantle sources. After 2.5 Ga, increases in Mg# in basalts from depleted mantle and decreases in Fe and Mn reflect some combination of growing depletion and cooling of depleted mantle with time. A delay in cooling of depleted mantle until after the Archean probably reflects a combination of greater radiogenic heat sources in the Archean mantle and the propagation of plate tectonics after 3 Ga.

  17. A perovskitic lower mantle inferred from high-pressure, high-temperature sound velocity data. (United States)

    Murakami, Motohiko; Ohishi, Yasuo; Hirao, Naohisa; Hirose, Kei


    The determination of the chemical composition of Earth's lower mantle is a long-standing challenge in earth science. Accurate knowledge of sound velocities in the lower-mantle minerals under relevant high-pressure, high-temperature conditions is essential in constraining the mineralogy and chemical composition using seismological observations, but previous acoustic measurements were limited to a range of low pressures and temperatures. Here we determine the shear-wave velocities for silicate perovskite and ferropericlase under the pressure and temperature conditions of the deep lower mantle using Brillouin scattering spectroscopy. The mineralogical model that provides the best fit to a global seismic velocity profile indicates that perovskite constitutes more than 93 per cent by volume of the lower mantle, which is a much higher proportion than that predicted by the conventional peridotitic mantle model. It suggests that the lower mantle is enriched in silicon relative to the upper mantle, which is consistent with the chondritic Earth model. Such chemical stratification implies layered-mantle convection with limited mass transport between the upper and the lower mantle.

  18. Seismic anisotropy of upper mantle in Sichuan and adjacent regions

    Institute of Scientific and Technical Information of China (English)

    CHANG LiJun; WANG ChunYong; DING ZhiFeng


    Based on the polarization analysis of teleseismic SKS waveform data recorded at 94 broadband seismic stations in Sichuan and adjacent regions, the SKS fast-wave direction and the delay time between the fast and slow shear waves were determined at each station using the grid searching method of minimum transverse energy and the stacking analysis method, and the image of upper mantle anisotropy was acquired. The fast-wave polarization directions are mainly NW-SE in the study area,NWW-SEE to its northeast and NS to its west. The delay time falls into the interval [0.47 s, 1.68 s]. The spatial variation of the fast-wave directions is similar to the variation of GPS velocity directions. The anisotropic image indicates that the regional tectonic stress field has resulted in deformation and flow of upper mantle material, and made the alignment of upper mantle peridotite lattice parallel to the direction of material deformation. The crust-upper mantle deformation in Sichuan and adjacent regions accords with the mode of vertically coherent deformation. In the eastern Tibetan Plateau, the crustal material was extruded to east or southeast clue to SE traction force of the upper mantle material. The extrusion might be obstructed by a rigid block under the Sichuan Basin and the crust has been deformed. After a long-term accumulation of tectonic strain energy, the accumulative energy suddenly released in Yingxiu town of the Longmenshan region, and Wenchuan Ms8.0 earthquake occurred.

  19. Abnormal lithium isotope composition from the ancient lithospheric mantle beneath the North China Craton. (United States)

    Tang, Yan-Jie; Zhang, Hong-Fu; Deloule, Etienne; Su, Ben-Xun; Ying, Ji-Feng; Santosh, M; Xiao, Yan


    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.

  20. K-rich lamellar exsolution in clinopyroxene:Constraint on the depth of peridotite source at Zhimafang

    Institute of Scientific and Technical Information of China (English)

    ZHAO Wenxia; HU Yuxian; XIA Feng; WANG Ximei; LIAO Chengzhu; JIANG Dan


    The oriented lamellar K-rich exsolutions within clinopyroxene host were recognized in garnet peridotite massif at Zhimafang in the Sulu ultrahigh pressure metamorphic terrane, eastern China. EMP and TEM analyses revealed that the composition and the index of electron diffraction for this exsolution are corresponding with phlogopite, and the shape is the lamellae with 1цm in width and 5 to 30цm in length. Moreover, the topotaxy is that the (001) of K-rich exsolution parallels to (100) of the clinopyroxene host. The electron diffraction analyzing shows the exsolution is of a 1M polymorphism. The high-resolution lattice fringe image displays the stack features along [001], which only a layer mineral shows. We conclude that the clinopyroxene with 1M and K-rich exsolution was probably derived from mantle depths over 240 km, thus resulting from decompressional exsolution in the initial stage of slab exhumation.

  1. On the nature of the transition between the mantle and crustal units in the Finero Complex (Ivrea-Verbano Zone, Southern Alps (United States)

    Zanetti, Alberto; Langone, Antonio; Tommasi, Andréa; Vauchez, Alain; Padrón-Navarta, Josè Alberto; Giovanardi, Tommaso; Mazzucchelli, Maurizio


    A well-exposed contact between mantle and crustal rocks is present in the Finero Complex (northern Ivrea-Verbano Zone; Southern Alps). The core of the Complex is composed by the Finero Phlogopite Peridotite mantle unit (FPP), which is wrapped out by an intercalation of mafic-ultramafic rocks interpreted as intrusive crustal bodies. The first crustal unit, placed in contact with the FPP, is the Layered Internal Zone (LIZ), which is overlaid by the Amphibole Peridotite and the External Gabbro units. With the aim of characterising the nature of such transition, a detailed investigation has been done on the outcrop at the confluence between Rio Cannobino and Rio Creves. In the transition area, no apparent melt injection (i.e. veins or dykes) from the LIZ is observed into the FPP. A few meters far from the contact, the mantle rocks are similar to those forming the typical FPP sequence. They are coarse-granular phlogopite-amphibole-bearing harzburgite showing a foliation parallel to the contact. The amphibole chemistry is characterised by large Mg# and Cr, Th and U contents, large and linearly-fractionated LREE/PM values, and low Nb, Ta and HREE. Towards the LIZ, the olivine grain-size decreases and the peridotite becomes richer in orthopyroxene, phlogopite and amphibole. At the contact with the LIZ, the harzburgite is replaced by a layer, up to 1-m-thick, of weakly-deformed coarse-granular amphibole-biotite-bearing orthopyroxenite. Besides, approaching the contact, the minerals have larger Fe and Al, and lower Cr. Amphiboles are still enriched in Th, U, and LREE, and depleted in HREE, but with greater absolute values than in the harzburgite farther from the contact. The LIZ starts with dm-thick hornblendites, followed by amphibole gabbro layers containing garnet and clinopyroxene. Both hornblendites and gabbros preserve magmatic textures, with modest deformation and subsolidus recrystallisation. Hornblendites are made by titanian pargasites, definitely richer in Fe, Al

  2. The Paradox of a Wet (High H2O) and Dry (Low H2O/Ce) Mantle: High Water Concentrations in Mantle Garnet Pyroxenites from Hawaii (United States)

    Peslier, Anne H.; Bizimis, Michael


    Water dissolved as trace amounts in anhydrous minerals has a large influence on the melting behavior and physical properties of the mantle. The water concentration of the oceanic mantle is inferred from the analyses of Mid-Ocean Ridge Basalt (MORB) and Oceanic Island Basalt (OIB). but there is little data from actual mantle samples. Moreover, enriched mineralogies (pyroxenites, eclogites) are thought as important sources of heterogeneity in the mantle, but their water concentrations and their effect on the water budget and cycling in the mantle are virtually unknown. Here, we analyzed by FTIR water in garnet clinopyroxenite xenoliths from Salt Lake Crater, Oahu, Hawaii. These pyroxenites are high-pressure (>20kb) crystal fractionates from alkalic melts. The clinopyroxenes (cpx) have 260 to 576 ppm wt H2O, with the least differentiated samples (Mg#>0.8) in the 400-500 ppm range. Orthopyroxene (opx) contain 117-265 ppm H2O, about half of that of cpx, consistent with other natural sample studies, but lower than cpx/opx equilibrium from experimental data. The pyroxenite cpx and opx H2O concentrations are at the high-end of on-and off-craton peridotite xenolith concentrations and those of Hawaiian spinel peridotites. In contrast, garnet has extremely low water contents (Ce is 69 +/-35, lower than estimates of the MORB source (approx 150) or FOZO, C (200-250) mantle component, but consistent with "dry" EM sources (Ce, and may contribute to EM-type OIB sources, like that of Samoa basalts. Therefore, a low H2O/Ce mantle source may not necessarily be "dry".

  3. Construction of mantle sequence beneath Yubileynaya kimberlite pipe. (United States)

    Ashchepkov, I. V.; Vladykin, N. V.; Logvinova, A. M.; Zinchuk, N. N.; Khmelnikova, O. S.; Palessky, S. V.; Nikolaeva, I. V.


    Concentrate from Yubileinaya kimberlite pipe (>1000 analyses), Alakite field, Russia, was used for thermobarometry (Nimis, Taylor, 2000; Ryan et al,1997; Ashchepkov, 2002). Cpx geotherm reveal divide at 35 kbars. Three lherzolitic units below this boundary correspond to 40mv/m2 geotherm at the upper part and are close to 35 mv/m2 at the lower part. Upper geotherm part is more dispersed irregularly deviating to LT conditions. The inflection from harzburgite layer low in Cpx to hot ‘asthenospheric’ branch at 1350oC 65 kbar was determined Sequence upper 35 kbars is represented by Ca-Ti- enriched peridotites- pyroxenites judging from Garnet and CPx compositions. Lower unites are represented by NaCr(3-4% each)- rich CPx and Ca-Fe enriched garnets. Garnet reveal also presence of harzburgites in each three units and relic unmetasomitised lherzolite fragments rare occurring in CPx concentrate. Amphiboles in upper part (divided into 8 groups are TiO2 -enriched in the lower part of section and in upper part. Different scheme of isomorphism for- Cr-rich chromites in deep seated units suggest difference in oxygen fugacity and reactied agents. Ilmenite trend consisting from two units: >49% TiO2 Ilm slightly enriched in Al2O3 (~ 0.7 %) while more fractionated Fe- rich ilmenites are Al-poor. Seven ilmenite groups coinciding with amount of spinel groups are likely corresponding to mantle columns layers. Ilmenite megacrystalline suit trace feeding vein system of fractionating pre -eruption kimberlite- carbonatite melts. Accompanied Cr enrichment and Ti rise in spinels and other peridotite minerals in lower units is caused by interaction of melts evolving in chambers and rising in pre-eruption vein system with wall rock peridotites. Division into two parts at 35-40kbar boundary where slab’s PT path crosses dehydration peridotite line (van Keken et al, 2002) probably appeared during lithospheric keel growth accompanied by slab melting and percolation of subduction-related melts

  4. Constraints from Naturally Deformed Peridotites on Controls on Olivine Lattice Preferred Orientation (United States)

    Bernard, R. E.; Behr, W. M.


    Seismic anisotropy in the upper mantle is produced primarily by lattice preferred orientations (LPO) in olivine formed during viscous deformation. Because seismic anisotropy is one of the principal means of characterizing upper mantle flow directions, it is critical to understand how LPO is affected by deformation conditions. Laboratory experiments suggest that water content and stress magnitude each play key roles in the development of LPO in olivine under experimental conditions, but it is unclear to what extent these results apply to natural conditions. We use peridotite xenoliths from a wide range of tectonic settings (Lunar Craters, Geronimo, and San Carlos volcanic fields in the Basin and Range; Cima and Deadman Lake volcanic fields in the Mojave; the Navajo Volcanic field in the Colorado Plateau; and the Potrillo volcanic field in the Rio Grande Rift region) to investigate correlations between water content, stress, and olivine LPO in natural rocks. Water contents were measured using Secondary Ion Mass Spectrometry, stress magnitudes using paleopiezometry, and LPOs using electron backscatter diffraction. The samples examined exhibit a range of fabric types, including A-, B-, C-, and E-type LPOs. Mojave xenoliths show no difference in water content between A- and E-type LPO; instead, differences in fabric type appear to reflect variations in strain magnitude. Samples from the Navajo volcanic field do show a correlation between water influx and stress magnitude as they exhibit abundant hydrous minerals and high water contents, stress magnitudes greater than 250 MPa and B-type olivine LPOs. Additional results from other xenolith suites will be presented at the meeting.

  5. Experimental constraints on melting temperatures in the MgO-SiO2 system at lower mantle pressures (United States)

    Baron, Marzena A.; Lord, Oliver T.; Myhill, Robert; Thomson, Andrew R.; Wang, Weiwei; Trønnes, Reidar G.; Walter, Michael J.


    Eutectic melting curves in the system MgO-SiO2 have been experimentally determined at lower mantle pressures using laser-heated diamond anvil cell (LH-DAC) techniques. We investigated eutectic melting of bridgmanite plus periclase in the MgO-MgSiO3 binary, and melting of bridgmanite plus stishovite in the MgSiO3-SiO2 binary, as analogues for natural peridotite and basalt, respectively. The melting curve of model basalt occurs at lower temperatures, has a shallower dT / dP slope and slightly less curvature than the model peridotitic melting curve. Overall, melting temperatures detected in this study are in good agreement with previous experiments and ab initio simulations at ∼25 GPa (Liebske and Frost, 2012; de Koker et al., 2013). However, at higher pressures the measured eutectic melting curves are systematically lower in temperature than curves extrapolated on the basis of thermodynamic modelling of low-pressure experimental data, and those calculated from atomistic simulations. We find that our data are inconsistent with previously computed melting temperatures and melt thermodynamic properties of the SiO2 endmember, and indicate a maximum in short-range ordering in MgO-SiO2 melts close to Mg2SiO4 composition. The curvature of the model peridotite eutectic relative to an MgSiO3 melt adiabat indicates that crystallization in a global magma ocean would begin at ∼100 GPa rather than at the bottom of the mantle, allowing for an early basal melt layer. The model peridotite melting curve lies ∼ 500 K above the mantle geotherm at the core-mantle boundary, indicating that it will not be molten unless the addition of other components reduces the solidus sufficiently. The model basalt melting curve intersects the geotherm at the base of the mantle, and partial melting of subducted oceanic crust is expected.

  6. Discriminating between pyroxenite and peridotite sources for continental flood basalts (CFB) in southern Africa using olivine chemistry (United States)

    Howarth, Geoffrey H.; Harris, Chris


    Continental Flood Basalts (CFB) result from voluminous outpourings of magma that often precede continental break-up. Notwithstanding the petrogenetic importance of CFBs, the nature of the mantle source for such magmas is contentious, particularly with regard to picrites with Ni-rich olivine phenocrysts. Previous studies have suggested that Ni-rich olivines associated with plume volcanism in regions of thickened (>90 km) lithosphere are related to either source mineralogy differences (peridotite versus pyroxenite) or change in olivine-melt partitioning due to pressure increase. In order to evaluate these two hypotheses, we present trace element data for olivines from the Karoo CFB Tuli and Mwenezi picrites and the Etendeka CFB Horingbaai/LTZ-L type picrites, all of which erupted in regions of thickened (>90 km) lithosphere in southern Africa. Karoo picrite olivines are Ni-rich, Ca- and Mn-poor, and have low (1.4) 100*Mn/Fe, which is more consistent with high temperature melting of a dominantly peridotitic source. We also show that the Karoo and Etendeka olivines are characterized by distinct Mn/Zn ratios of 15, respectively. In addition, bulk rock geochemical data compilations and previously reported olivine δ18O for Karoo and Etendeka CFBs are discussed in order to further constrain source components based on previously described pyroxenite melt geochemical indices such as MgO-CaO systematics, FeO/MnO, Zn/Fe, and FC3MS (FeO/CaO-3*MgO/SiO2). These geochemical indices suggest a pyroxenite-dominated source for Karoo CFBs as well as for Etendeka ferropicrites whereas a peridotite-dominated source is indicated for Etendeka Horingbaai/LTZ-L type picrites analyzed in this study. Based on our data, Ni-enrichment of olivine in plume-related magmas in regions of thickened lithosphere in southern Africa is not ubiquitous. We therefore suggest that mineralogical variation of the source is a more likely major control of olivine chemistry and parent melt variations for Karoo

  7. Melting and subsolidus phase relations in peridotite and eclogite systems with reduced Csbnd Osbnd H fluid at 3-16 GPa (United States)

    Litasov, Konstantin D.; Shatskiy, Anton; Ohtani, Eiji


    diamonds. The oxygen fugacity in the Earth's mantle decreases with pressure from about fayalite-magnetite-quartz at shallow depths of 20-50 km to about iron-wustite at 250-300 km according to fO2 estimations from cratonic peridotite. We show significant increase of solidus temperatures in peridotite and eclogite coexisting with reduced CH4-H2O fluid relative to the systems with oxidized H2O-CO2 fluid. We emphasize that redox melting by change of oxidation state across a mantle section, a phase transition, or the lithosphere-asthenosphere boundary can be the dominant melting process in the deep Earth's interior.

  8. Sulfur and carbon geochemistry of the Santa Elena peridotites: Comparing oceanic and continental processes during peridotite alteration (United States)

    Schwarzenbach, Esther M.; Gill, Benjamin C.; Gazel, Esteban; Madrigal, Pilar


    Ultramafic rocks exposed on the continent serve as a window into oceanic and continental processes of water-peridotite interaction, so called serpentinization. In both environments there are active carbon and sulfur cycles that contain abiogenic and biogenic processes, which are eventually imprinted in the geochemical signatures of the basement rocks and the calcite and magnesite deposits associated with fluids that issue from these systems. Here, we present the carbon and sulfur geochemistry of ultramafic rocks and carbonate deposits from the Santa Elena ophiolite in Costa Rica. The aim of this study is to leverage the geochemistry of the ultramafic sequence and associated deposits to distinguish between processes that were dominant during ocean floor alteration and those dominant during low-temperature, continental water-peridotite interaction. The peridotites are variably serpentinized with total sulfur concentrations up to 877 ppm that is typically dominated by sulfide over sulfate. With the exception of one sample the ultramafic rocks are characterized by positive δ34Ssulfide (up to + 23.1‰) and δ34Ssulfate values (up to + 35.0‰). Carbon contents in the peridotites are low and are isotopically distinct from typical oceanic serpentinites. In particular, δ13C of the inorganic carbon suggests that the carbon is not derived from seawater, but rather the product of the interaction of meteoric water with the ultramafic rocks. In contrast, the sulfur isotope data from sulfide minerals in the peridotites preserve evidence for interaction with a hydrothermal fluid. Specifically, they indicate closed system abiogenic sulfate reduction suggesting that oceanic serpentinization occurred with limited input of seawater. Overall, the geochemical signatures preserve evidence for both oceanic and continental water-rock interaction with the majority of carbon (and possibly sulfate) being incorporated during continental water-rock interaction. Furthermore, there is

  9. Spinel-olivine-pryoxene equilibrium iron isotopic fractionation and applications to natural peridotites

    Energy Technology Data Exchange (ETDEWEB)

    Roskosz, Mathieu; Sio, Corliss K. I.; Dauphas, Nicolas; Bi, Wenli; Tissot, Francois L. H.; Hu, Michael Y.; Zhao, Jiyong; Alp, Esen E.


    Eight spinel-group minerals were synthesized by a flux-growth method producing spinels with varying composition and Fe3+/Fe-tot ratios. The mean force constants of iron bonds in these minerals were determined by synchrotron nuclear resonant inelastic X-ray scattering (NRIXS) in order to determine the reduced isotopic partition function ratios (beta-factors) of these spinels. The mean force constants are strongly dependent on the Fe3+/Fe-tot of the spinel but are independent, or weakly dependent on other structural and compositional parameters. From our spectroscopic data, it is found that a single redox-dependent calibration line accounts for the effects of Fe3+/Fe-tot on the beta-factors of spinels. This calibration successfully describes the equilibrium Fe isotopes fractionation factors between spinels and silicates (olivine and pyroxenes). Our predictions are in excellent agreement with independent determinations for the equilibrium Fe isotopic fractionations for the magnetite- fayalite and the magnetite-hedenbergite couples. Our calibration applies to the entire range of Fe3+/Fe-tot ratios found in natural spinels and provides a basis for interpreting iron isotopic variations documented in mantle peridotites. Except for a few exceptions, most of the samples measured so far are in isotopic disequilibrium, reflecting metasomatism and partial melting processes.

  10. Continental hyperextension, mantle exhumation, and thin oceanic crust at the continent-ocean transition, West Iberia: New insights from wide-angle seismic (United States)

    Davy, R. G.; Minshull, T. A.; Bayrakci, G.; Bull, J. M.; Klaeschen, D.; Papenberg, C.; Reston, T. J.; Sawyer, D. S.; Zelt, C. A.


    Hyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has been accommodated by the rotation of continental fault blocks, which are underlain by the S reflector, an interpreted detachment fault, along which exhumed and serpentinized mantle peridotite is observed. West of these features, the enigmatic Peridotite Ridge has been inferred to delimit the western extent of the continent-ocean transition. An outstanding question at this margin is where oceanic crust begins, with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies. Here we present results from a 160 km long wide-angle seismic profile (Western Extension 1). Travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks separated from the underlying mantle by the S reflector. The S reflector correlates with the 6.0-7.0 km s-1 velocity contours, corresponding to peridotite serpentinization of 60-30%, respectively. West of the Peridotite Ridge, shallow and sparse Moho reflections indicate the earliest formation of an anomalously thin oceanic crustal layer, which increases in thickness from ~0.5 km at ~20 km west of the Peridotite Ridge to ~1.5 km, 35 km further west. P wave velocities increase smoothly and rapidly below top basement, to a depth of 2.8-3.5 km, with an average velocity gradient of 1.0 s-1. Below this, velocities slowly increase toward typical mantle velocities. Such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle rock with depth.

  11. Recycling of ancient subduction-modified mantle domains in the Purang ophiolite (southwestern Tibet) (United States)

    Gong, Xiao-Han; Shi, Ren-Deng; Griffin, W. L.; Huang, Qi-Shuai; Xiong, Qing; Chen, Sheng-Sheng; Zhang, Ming; O'Reilly, Suzanne Y.


    Ophiolites in the Indus-Yarlung Zangbo (IYZ) suture (southern Tibet) have been interpreted as remnants of the Neo-Tethyan lithosphere. However, the discovery of diamonds and super-reducing, ultra-high pressure (SuR-UHP) mineral assemblages (e.g., coesite after stishovite, olivine after wadsleyite, native metals, alloys, and moissanite) in some of these massifs and associated chromitites requires a re-evaluation of their origin and evolution. A new petrological and geochemical study of the Purang ophiolite in the western IYZ suture sheds new lights on these issues. The depleted harzburgites of the Purang massif have low modal contents of clinopyroxene ( 40 ~ 70) and pyroxenes (> 16 in orthopyroxene, and > 20 in clinopyroxene), suggesting high degrees of melt extraction (> 20%). These features are not consistent with formation in a (ultra-) slow-spreading mid-ocean ridge. These peridotites have high modal contents of orthopyroxene; this, and the extremely high Cr# of spinels in these peridotites, suggests modification in a subduction zone. The clinopyroxene-rich harzburgites and lherzolites contain rare spinel-pyroxene symplectites after garnet. Their clinopyroxenes have low MREE-to-HREE ratios ((Sm/Yb)N recycling model for the presence of SuR-UHP phases in the IYZ ophiolites. The infiltration of MORB melts through these ancient depleted peridotites during their final exhumation in a (ultra-) slow-spreading center may have refertilized them to produce the clinopyroxene-rich peridotites.

  12. The composition of mantle plumes and the deep Earth (United States)

    Hastie, Alan R.; Fitton, J. Godfrey; Kerr, Andrew C.; McDonald, Iain; Schwindrofska, Antje; Hoernle, Kaj


    Determining the composition and geochemical diversity of Earth's deep mantle and subsequent ascending mantle plumes is vital so that we can better understand how the Earth's primitive mantle reservoirs initially formed and how they have evolved over the last 4.6 billion years. Further data on the composition of mantle plumes, which generate voluminous eruptions on the planet's surface, are also essential to fully understand the evolution of the Earth's hydrosphere and atmosphere with links to surface environmental changes that may have led to mass extinction events. Here we present new major and trace element and Sr-Nd-Pb-Hf isotope data on basalts from Curacao, part of the Caribbean large igneous province. From these and literature data, we calculate combined major and trace element compositions for the mantle plumes that generated the Caribbean and Ontong Java large igneous provinces and use mass balance to determine the composition of the Earth's lower mantle. Incompatible element and isotope results indicate that mantle plumes have broadly distinctive depleted and enriched compositions that, in addition to the numerous mantle reservoirs already proposed in the literature, represent large planetary-scale geochemical heterogeneity in the Earth's deep mantle that are similar to non-chondritic Bulk Silicate Earth compositions.

  13. Enriched and depleted source components for tholeiitic and alkaline lavas from Zuni-Bandera, New Mexico: Inferences about intraplate processes and stratified lithosphere (United States)

    Menzies, Martin A.; Kyle, Philip R.; Jones, Michael; Ingram, Gerry


    During the Pleistocene and Holocene, alkaline and tholeiitic magmas were erupted in the Zuni-Bandera volcanic field (ZBVF) on the western flanks of the Rio Grande Rift, New Mexico. While most of the alkaline basalts are magnesian (i.e., >8.0% MgO), the tholeiitic basalts show evidence of fractionation of olivine and clinopyroxene. The ZBVF lavas have intraplate chemistry similar to that of alkaline and tholeiitic basalts in other continental and oceanic environments, and Sr and Nd isotopes range from depleted compositions similar to mid-ocean ridge basalts to enriched compositions similar to ocean islands like Kerguelen. Slightly higher Th/Ta and Ba/Nb ratios in some of the ZBVF tholeiitic basalts correlate with isotopic ratios, and this may indicate minor involvement of a Th rich component (i.e., crust). Alternatively, such characteristics may merely reflect intraplate processes if one considers that the isotopic variation observed in the ZBVF is not that different from what is observed in Hawaiian basalts. Systematic changes in elemental and isotopic ratios with the degree of partial melting (Zr/Y) reveal that the geochemistry of the ZBVF magmas is dominated by two components: (1) a depleted mantle component which produced an enriched alkaline magma as a small degree melt (e.g., Ta/Yb=6.0; {La/Yb}N=60; Zr/Y=19; 87Sr/86Sr=0.703); and (2) an enriched mantle component which produced tholeiitic magmas as a larger degree melt (e.g., Ta/Yb <0.5{La/Yb}N < 15.0; Zr/Y=3-4; 87Sr/86Sr = 0.706). While the depleted component is interpreted to be MORB asthenosphere the enriched component is more problematical and may reside in plume-contaminated asthenosphere or stratified lithosphere. The lower lithosphere beneath the Proterozoic crust of the western USA appears to be chemically stratified and comprises a depleted peridotite protolith stabilized in the Proterozoic that may have been subsequently enriched due to interaction with fluids related to (1) suprasubduction processes

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

    DEFF Research Database (Denmark)

    Schutt, Derek; Lesher, Charles


    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...... 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......) and VP/VS. In contrast, melt depletion has little effect on VP, but leads to an increase in VS and a decrease in ρbulk and VP/VS. Garnet (gt) and clinopyroxene (cpx) enrichment cause an increase in ρbulk, VP, VS, and VP/VS. The isolation of the major contributions to xenolith compositional variations...

  15. Two cpx and opx generation in the southern Patagonian lithospheric mantle (United States)

    Melchiorre, M.; Benoit, M.; Coltorti, M.; Gregoire, M.


    similar patterns with cpx2 often showing more enriched values. Spinel has the smallest dimension rounded or elongated in shape. They are characterized by variable Al2O3 and MgO enrichment, most of them plotting on the normal mantle array, with mg# and cr# (Cr2O3/ Cr2O3+Al2O3, at%) varying between 53.14 and 60.83 and 43.45 and 59.14 respectively. Isolated spinels (not surrounded by cpx) present the highest cr# (and lowest mg#), while spinels surrounded by clinopyroxenes present higher Al2O3 content. The above reported textural and geochemical features indicate that i) opx2 was added to the system after the harzburgites was formed and ii) cpx growing around spinel is not an isochemical process, due to a simple deformation and recrystallization event, but, at least Al2O3 and REE, should have been added to the system. Time relationships between opx and cpx formation are not constrained, but assuming that the two phenomena are linked together, their formation point toward a SiO2-, Al2O3- and REE-rich metasomatizing agent affecting an already moderately to strongly depleted peridotite.

  16. Oxo-amphiboles in mantle xenoliths: evidence for H2O-rich melt interacting with the lithospheric mantle of Harrow Peaks (Northern Victoria Land, Antarctica) (United States)

    Gentili, S.; Bonadiman, C.; Biagioni, C.; Comodi, P.; Coltorti, M.; Zucchini, A.; Ottolini, L.


    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

  17. Sapphirine and fluid inclusions in Tel Thanoun mantle xenoliths,Syria (United States)

    Bilal, Ahmad


    Volcanoes along the Syrian rift, which extend a distance of about 1000 km, brought to the surface mantle xenoliths within erupted basalts, during multiples periods of volcanic activity. Xenoliths in early Cretaceous volcanoes originate in the garnet peridotite field of the subcontinental mantle, whereas those in recent Cenozoic volcanoes, the prime object of this study, are issued from shallower levels (spinel peridotite field). The recent discovery of sapphirine-bearing websterite in Tel Thanoun, a small volcanic diatreme inside the larger Quaternary volcanic field (Djebel Al Arab), allows us to estimate the P-T evolution and fluid-rock interaction at the volcanic source. Harzburgites and lherzolites are equilibrated at a temperature of about 1000 °C at a depth of 35-40 km. Sapphirine appears to have formed during cooling, at depth at a temperature of about 900 °C, at a time where spinel exsolution occurred in harzburgite and lherzolite pyroxenes. This occurred in the presence of a high-density pure CO2 fluid phase, still present in primary fluid inclusions. The highly-aluminous sapphirine-bearing protolith might be former garnet websterite (possibly uplifted during cretaceous magmatism), which resided and cooled in the spinel peridotite stability field, and was then dragged and brought to the surface by quaternary basalts.

  18. Metasomatic origin of garnet orthopyroxenites in the subcontinental lithospheric mantle underlying Pali Aike volcanic field, southern South America (United States)

    Wang, Jian; Hattori, Keiko H.; Stern, Charles R.


    Garnet-bearing orthopyroxenites occur as both discrete xenoliths and as veinlets in peridotite xenoliths that were brought to the surface by Quaternary Pali Aike alkali basalts in the southernmost of the Patagonian plateau lava fields in southern South America. Orthopyroxenites commonly contain Ti-rich minerals and also relict grains of olivine (Ol) and clinopyroxene (Cpx) occurring as inclusions in secondary orthopyroxene (Opx). The secondary Opx contains high TiO2 (0.20-0.59 wt.%), moderate Al2O3 (2.87-5.10 wt.%) and low magnesium numbers (Mg#, 84.5-89.2) compared with Opx in garnet-bearing peridotites. This suggests that secondary Opx formed at the expense of Ol and Cpx during metasomatism by an evolved Ti-rich melt. The orthopyroxenites contain bulk-rock concentrations of Cr, Ni and platinum group elements similar to peridotites, suggesting that these metals were essentially immobile during metasomatism, and that the metasomatizing melt did not introduce these elements. Instead, the metasomatizing melt contributed alkalis, Ti, Si, Al, and S to the orthopyroxenites based on increased concentrations of these elements and mineralogy. Secondary Opx has also been documented in mantle peridotite xenoliths derived from beneath both active arcs and ancient cratons. In comparison with such Opx in these mantle samples, the secondary Opx in the Pali Aike xenoliths contains relatively high Ti and Al and low Mg. High Ti and low Mg in these samples reflect the evolved nature of the metasomatizing melt that originated from the underlying asthenospheric mantle. The type of secondary Opx, typified by these samples, may be common elsewhere in subcontinental mantle lithosphere (SCLM) affected by asthenospheric upwelling.

  19. Slab-derived halogens and noble gases illuminate closed system processes controlling volatile element transport into the mantle wedge (United States)

    Kobayashi, Masahiro; Sumino, Hirochika; Nagao, Keisuke; Ishimaru, Satoko; Arai, Shoji; Yoshikawa, Masako; Kawamoto, Tatsuhiko; Kumagai, Yoshitaka; Kobayashi, Tetsuo; Burgess, Ray; Ballentine, Chris J.


    Halogen and noble gas systematics are powerful tracers of volatile recycling in subduction zones. We present halogen and noble gas compositions of mantle peridotites containing H2O-rich fluid inclusions collected at volcanic fronts from two contrasting subduction zones (the Avacha volcano of Kamchatka arc and the Pinatubo volcano of Luzon arcs) and orogenic peridotites from a peridotite massif (the Horoman massif, Hokkaido, Japan) which represents an exhumed portion of the mantle wedge. The aims are to determine how volatiles are carried into the mantle wedge and how the subducted fluids modify halogen and noble gas compositions in the mantle. The halogen and noble gas signatures in the H2O-rich fluids are similar to those of marine sedimentary pore fluids and forearc and seafloor serpentinites. This suggests that marine pore fluids in deep-sea sediments are carried by serpentine and supplied to the mantle wedge, preserving their original halogen and noble gas compositions. We suggest that the sedimentary pore fluid-derived water is incorporated into serpentine through hydration in a closed system along faults at the outer rise of the oceanic, preserving Cl/H2O and 36Ar/H2O values of sedimentary pore fluids. Dehydration-hydration process within the oceanic lithospheric mantle maintains the closed system until the final stage of serpentine dehydration. The sedimentary pore fluid-like halogen and noble gas signatures in fluids released at the final stage of serpentine dehydration are preserved due to highly channelized flow, whereas the original Cl/H2O and 36Ar/H2O ratios are fractionated by the higher incompatibility of halogens and noble gases in hydrous minerals.

  20. On the iron isotope composition of Mars and volatile depletion in the terrestrial planets (United States)

    Sossi, Paolo A.; Nebel, Oliver; Anand, Mahesh; Poitrasson, Franck


    Iron is the most abundant multivalent element in planetary reservoirs, meaning its isotope composition (expressed as δ57Fe) may record signatures of processes that occurred during the formation and subsequent differentiation of the terrestrial planets. Chondritic meteorites, putative constituents of the planets and remnants of undifferentiated inner solar system bodies, have δ57Fe ≈ 0 ‰; an isotopic signature shared with the Martian Shergottite-Nakhlite-Chassignite (SNC) suite of meteorites. The silicate Earth and Moon, as represented by basaltic rocks, are distinctly heavier, δ57Fe ≈ + 0.1 ‰. However, some authors have recently argued, on the basis of iron isotope measurements of abyssal peridotites, that the composition of the Earth's mantle is δ57Fe = + 0.04 ± 0.04 ‰, indistinguishable from the mean Martian value. To provide a more robust estimate for Mars, we present new high-precision iron isotope data on 17 SNC meteorites and 5 mineral separates. We find that the iron isotope compositions of Martian meteorites reflect igneous processes, with nakhlites and evolved shergottites displaying heavier δ57Fe (+ 0.05 ± 0.03 ‰), whereas MgO-rich rocks are lighter (δ57Fe ≈ - 0.01 ± 0.02 ‰). These systematics are controlled by the fractionation of olivine and pyroxene, attested to by the lighter isotope composition of pyroxene compared to whole rock nakhlites. Extrapolation of the δ57Fe SNC liquid line of descent to a putative Martian mantle yields a δ57Fe value lighter than its terrestrial counterpart, but indistinguishable from chondrites. Iron isotopes in planetary basalts of the inner solar system correlate positively with Fe/Mn and silicon isotopes. While Mars and IV-Vesta are undepleted in iron and accordingly have chondritic δ57Fe, the Earth experienced volatile depletion at low (1300 K) temperatures, likely at an early stage in the solar nebula, whereas additional post-nebular Fe loss is possible for the Moon and angrites.

  1. Stable isotopic (O, H) evidence for hydration of the central Colorado Plateau lithospheric mantle by slab-derived fluids (United States)

    Marshall, E. W.; Barnes, J.; Lassiter, J. C.


    The Colorado Plateau is a tectonically stable, relatively undeformed Proterozoic lithospheric province in the North America Cordillera. Although the stability of the Colorado Plateau suggests that it is rheologically strong, evidence from xenoliths show that the lithospheric mantle is extensively hydrated (e.g., presence of hydrous minerals, 'high' water contents in nominally anhydrous minerals), and therefore weakened. In addition, LREE enrichments in clinopyroxene (cpx) imply that the lithospheric mantle has been metasomatized ([1],[2]). Here we analyze mineral separates from spinel and garnet peridotite xenoliths from the Navajo Volcanic Field (NVF), located in the center of the Plateau, for their oxygen and hydrogen isotope compositions. These compositions are compared to those of xenoliths at the margins of the Plateau: spinel peridotites from the Grand Canyon Volcanic Field (GCVF) in the west and Zuni-Bandera Volcanic Field (ZBVF) in the east. NVF xenoliths are significantly more hydrous than the xenoliths on the margins of the Colorado Plateau based on modal abundances of hydrous minerals and structural water in olivine (e.g. [3]). All hydrous phases have high δD values (antigorite = -71 to -46‰ (n = 6 xenoliths); chlorite = -49 to -31‰ (n=3); amphibole = -47‰ (n=1)) compared to normal mantle (~-80‰), suggesting the addition of a fluid that is enriched in D compared to typical mantle. δ18O values for the same hydrous minerals range from 6.0 to 6.6‰ (n=6). δ18O values of olivine from NVF spinel peridotites have a narrow range, 5.0 to 5.4‰ (n = 4), near mantle olivine values (~5.2‰). Olivines from spinel peridotites from the GCVF and ZBVF also have mantle-like δ18O values (5.1 to 5.2‰ (n=3) and 5.1 to 5.4‰ (n=7), respectively). However, olivines and orthopyroxenes (opx) from NVF garnet peridotites have a slightly larger range and some record 18O enrichment (olivine = 5.1 to 5.6‰ (n = 3); opx = 5.9‰ (n=1)). The high δ18O values of

  2. Effects of volatiles on melt production and reactive flow in the mantle

    CERN Document Server

    Keller, Tobias


    Magmatism in the Earth interior has a significant impact on its dynamic, thermal and compositional evolution. Experimental studies of petrology of mantle melting find that small concentrations of water and carbon dioxide have a significant effect on the solidus temperature and distribution of melting in the upper mantle. However, it has remained unclear what effect small fractions of deep, volatile-rich melts have on melting and melt transport in the shallow asthenosphere. We present a method to simulate the thermochemical evolution of the upper mantle in the presence of volatiles. The method is based on a novel, thermodynamically consistent framework for reactive, disequilibrium, multi-component melting/crystallisation. This is coupled with a system of equations representing conservation of mass, momentum, and energy for a partially molten grain aggregate. Application of this method to upwelling-column models demonstrates that it captures leading-order features of hydrated and carbonated peridotite melting. ...

  3. A Consensus on Mantle Potential Temperatures? (Invited) (United States)

    Putirka, K. D.


    C. All three groups further estimate that MORs exhibit a T range of 100 deg. C (Herzberg & Asimow, 2008; Lee et al., 2009) or 140 deg. C (Putirka et al., 2007). The agreement is especially remarkable given that each group uses different methods and/or equations. These absolute Tp values are furthermore consistent with a broad array of geophysical and experimental observations, including ocean floor bathymetry and heat flow (Stein & Stein, 1996), estimates for the peridotite solidus (Hirschmann, 2000), seismic estimates for depths of melting beneath MORs (MELT seismic Team, 1998), phase transitions at 670 km for a pyrolite mantle (Hirose, 2002), and excess bathymetry at Hawaii (Sleep, 1990). In addition, when pressures are calculated based on Si-activity (and T) for ocean islands (Putirka, 2008; RiMG), P is correlated with other indicators of partial melting depths, such as FeOt, and Na/Ti (Putirka, 2008 RiMG); these cross correlations indicate that inter-ocean island P and T estimates are real, and that thermal differences are an important control on melt composition. Finally, 3He/4He (a possible lower mantle signature; Kurz, 1993) positively correlates with both F and Tp for ocean islands (Putirka, 2008, Geology, p. e176), providing perhaps the most direct evidence in support of Morgan’s (1971) model that thermal plumes are driven by excess heat from the core-mantle boundary.

  4. An introduction of Markov chain Monte Carlo method to geochemical inverse problems: Reading melting parameters from REE abundances in abyssal peridotites (United States)

    Liu, Boda; Liang, Yan


    Markov chain Monte Carlo (MCMC) simulation is a powerful statistical method in solving inverse problems that arise from a wide range of applications. In Earth sciences applications of MCMC simulations are primarily in the field of geophysics. The purpose of this study is to introduce MCMC methods to geochemical inverse problems related to trace element fractionation during mantle melting. MCMC methods have several advantages over least squares methods in deciphering melting processes from trace element abundances in basalts and mantle rocks. Here we use an MCMC method to invert for extent of melting, fraction of melt present during melting, and extent of chemical disequilibrium between the melt and residual solid from REE abundances in clinopyroxene in abyssal peridotites from Mid-Atlantic Ridge, Central Indian Ridge, Southwest Indian Ridge, Lena Trough, and American-Antarctic Ridge. We consider two melting models: one with exact analytical solution and the other without. We solve the latter numerically in a chain of melting models according to the Metropolis-Hastings algorithm. The probability distribution of inverted melting parameters depends on assumptions of the physical model, knowledge of mantle source composition, and constraints from the REE data. Results from MCMC inversion are consistent with and provide more reliable uncertainty estimates than results based on nonlinear least squares inversion. We show that chemical disequilibrium is likely to play an important role in fractionating LREE in residual peridotites during partial melting beneath mid-ocean ridge spreading centers. MCMC simulation is well suited for more complicated but physically more realistic melting problems that do not have analytical solutions.

  5. Low-Ti basalts from the Faroe Islands constrain the early Iceland depleted plume component

    DEFF Research Database (Denmark)

    Søager, Nina; Holm, Paul Martin

    -Toft, J, Kingsley, R., Schilling, J.G., 2000: Depleted Iceland mantle plume geochemical signature: Artifact of multicomponent mixing? Geochemistry, Geophysics, Geosystems vol.1. Thirlwall, M.F., Gee, M.A.M., Taylor, R.N., Murton, B.J., 2004: Mantle components in Iceland and adjecent ridges investigated...

  6. Support for a Uniformitarian Model of Continental Mantle Lithosphere Formation from the "Near-Cratonic" Composition of Proterozoic Southern African Mantle Lithosphere (United States)

    Janney, P. E.


    The transition at the end of the Archean between the generation of cratonic and mobile belt continental lithosphere is regarded as a first-order change in the mode of generation of continental lithosphere. It is widely debated whether this transition represented a fundamental change in the process by which the lithospheric mantle was generated (i.e., as melting residues of deep-seated mantle upwellings to residues of relatively shallow mantle melting at subduction zones), or whether it primarily reflected a more gradual change in the conditions (i.e., temperatures, depths and degrees of melting) of lithosphere generation in a suprasubduction zone setting. The marked contrast, in many cases, between the major element compositions of peridotite xenoliths from Archean cratons and those from adjacent post-Archean mobile belts has accentuated the significance of this transition. Peridotite xenoliths from the post-Archean mobile belt terranes surrounding the Kaapvaal craton in southern Africa are clearly Proterozoic in age from Re-Os isotope constraints, but they are unusual in that they share several key similarities in composition and mineralogy with Archean Kaapvaal peridotites (e.g., low bulk-rock Al2O3, relatively low modal olivine and high modal orthopyroxene). Although they lack the low FeO and high olivine Mg# values of the most extreme Kaapvaal samples, they show a very large degree of overlap (extending to olivine Mg# values of greater than 93 for example). These similarities support a common mode of origin for cratonic and post-cratonic lithosphere in southern Africa (although varying somewhat in the degrees and depths of melt extraction) and a similar history of post-formation modification. A comparison of the conditions of melt extraction for cratonic and post-cratonic lithosphere inferred from compatible and mildly incompatible trace elements will be presented.

  7. Zoned mantle convection. (United States)

    Albarède, Francis; Van Der Hilst, Rob D


    We review the present state of our understanding of mantle convection with respect to geochemical and geophysical evidence and we suggest a model for mantle convection and its evolution over the Earth's history that can reconcile this evidence. Whole-mantle convection, even with material segregated within the D" region just above the core-mantle boundary, is incompatible with the budget of argon and helium and with the inventory of heat sources required by the thermal evolution of the Earth. We show that the deep-mantle composition in lithophilic incompatible elements is inconsistent with the storage of old plates of ordinary oceanic lithosphere, i.e. with the concept of a plate graveyard. Isotopic inventories indicate that the deep-mantle composition is not correctly accounted for by continental debris, primitive material or subducted slabs containing normal oceanic crust. Seismological observations have begun to hint at compositional heterogeneity in the bottom 1000 km or so of the mantle, but there is no compelling evidence in support of an interface between deep and shallow mantle at mid-depth. We suggest that in a system of thermochemical convection, lithospheric plates subduct to a depth that depends - in a complicated fashion - on their composition and thermal structure. The thermal structure of the sinking plates is primarily determined by the direction and rate of convergence, the age of the lithosphere at the trench, the sinking rate and the variation of these parameters over time (i.e. plate-tectonic history) and is not the same for all subduction systems. The sinking rate in the mantle is determined by a combination of thermal (negative) and compositional buoyancy and as regards the latter we consider in particular the effect of the loading of plates with basaltic plateaux produced by plume heads. Barren oceanic plates are relatively buoyant and may be recycled preferentially in the shallow mantle. Oceanic plateau-laden plates have a more pronounced

  8. Sr-Nd-Pb-C-O isotope systematics of carbonated ultramafic xenoliths from Mafu, Taiwan: Evidence for an extremely enriched lithospheric mantle source beneath the extended margin of the South China block (United States)

    Smith, A. D.; Wen, D.; Chung, S.; Wang, K.; Chiang, H.; Tsai, C.


    Deep-seated carbonate melt is widely proposed as an effective agent to metasomatize the lithospheric mantle. However, such carbonate melts may have a great diversity of composition and a mantle or recycled origin remains unclear. Here we present the evidence for unique carbonate metasomatism of the continental lithospheric mantle (CLM) beneath the extended southeast margin of the South China block from severely replaced spinel peridotite xenoliths from Mafu, northwestern Taiwan. The metasomatic calcitic carbonates and whole carbonated xenoliths from Mafu have unusually low trace element abundances (total REE abundance residue (86Sr/87Sr = 0.7041; ɛ Nd = +4.9, one residual sample up to +12; 206Pb/204Pb = 18.3), the Sr-Nd-Pb-C-O isotope systematics of the carbonates (86Sr/87Sr = 0.7044-0.7045; ɛ Nd = -6.9 to -7.7; 206Pb/204Pb = 18.5; δ13C = -4.5 to -5.7; δ18O = +21.8 to +22.9) reveal an extremely enriched and heterogeneous CLM. According to the Nd model age, the enriched component evolved for at least 1 Gyrs after isolation from the depleted CLM, before the Late Miocene entrapment. Coupled with high Sr/Nd, Ba/Th, La/Yb, Zr/Hf, and low Nb/U, Ce/Pb, Th/U, Ti/Eu ratios, this EM1-like metasomatic agent may be one of the most efficient percolating melt to cause disequlibrium interaction or Sr-Nd isotope decoupling. To a broader interpretation, it offers an alternative to account for some enriched signatures in mantle lithosphere, such as the extreme components of the Hawaiian plume. Instead of the "ghost plagioclase" (Sobolev et al., 2000), melting of depleted lithosphere which was metasomatized by this kind of enriched carbonate melt with high Sr, Ba, Pb but low Al and REE contents, may even better explain the geochemical features observed from the melt inclusions in Mauna Loa olivines.

  9. Water Content in the SW USA Mantle Lithosphere: FTIR Analysis of Dish Hill and Kilbourne Hole Pyroxenites (United States)

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


    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.

  10. Mantle Temperature, Mantle Composition, Mantle Heterogeneity, and the Composition of the Upper Mantle: The View from a Global Synthesis of MORB (United States)

    Langmuir, C. H.; Gale, A.; Dalton, C. A.


    A new comprehensive review of global MORB can address outstanding issues such mantle temperature vs. mantle composition in controlling MORB compositions, the mean composition of ocean ridge basalts, the K/U ratio of the MORB reservoir, and the implications for silicate Earth mass balance of the composition of the upper mantle. We created a global catalogue of ridge segments to assign every sample to a segment. We carried out interlaboratory corrections for major elements, and examined data from each segment to ensure appropriate fractionation correction. We included large unpublished data sets from the Langmuir and Schilling laboratories, assembling the most comprehensive data set for MORB. Data averaged by segment permit calculation of averages that include weighting by segment length and spreading rate. The segment-based approach, comprehensive data set, individualized fractionation correction and interlaboratory corrections distinguish these results from earlier efforts. We also carried out bootstrapping statistical tests for meaningful errors on average compositions. The mean composition of the ocean crust is best determined by a segment length and spreading rate weighted arithmetic mean. As with other recent efforts, notably Su (2002) and also Arevalo and McDonough (2009), the mean composition is substantially more enriched than previous MORB estimates. Average MORB implies a MORB mantle Sm/Nd and Nd isotopic composition similar to the 'non-chondritic primitive mantle' composition based on 142Nd. Then continental crust/MORB mantle mass balance is not possible using a non-chondritic (depleted) bulk silicate earth composition, unless there is a large unsampled depleted reservoir. In contrast to Arevalo and McDonough, who suggested a K/U ratio for MORB of 19,000, we find K/U of 12,340±810, in line with earlier estimates. The discrepancy can be understood from contrasts in methodology, as we determine average K/ average U, while they determine average K/U. To

  11. Oxidation state of Paleozoic subcontinental lithospheric mantle below the Pali Aike volcanic field in southernmost Patagonia (United States)

    Wang, Jian; Hattori, Keiko H.; Li, Jianping; Stern, Charles R.


    Mantle xenoliths in the Quaternary Pali Aike alkaline basalts of southernmost Patagonia include lherzolites and harzburgites with and without garnet. The values of fO 2 for all xenoliths range from 0.33 logarithmic unit below the fayalite-magnetite-quartz buffer (FMQ - 0.33) to FMQ + 0.75, which overlap those for abyssal peridotites. The fO 2 data, together with the bulk rock and mineral compositions, suggest that the subcontinental lithospheric mantle (SCLM) below Pali Aike formed through the accretion of oceanic lithosphere. The oceanic accretion likely occurred in mid to late Paleozoic time when the southern Patagonian terrane formed along the southwest margin of Gondwana. Relict spinel inclusions in garnet suggest that garnet-facies peridotites formed from spinel-facies peridotites most likely in response to pressure increase during the accretion. Comparable fO 2 for garnet- and spinel-facies peridotites suggests that this spinel-garnet transformation was not accompanied by changes in fO 2. Metasomatism by asthenosphere-derived melt through a slab window resulted in the formation of Ti-bearing minerals, and lowering of Mg and enrichment of Ti in bulk rocks and minerals. The replacement of olivine by orthopyroxene formed orthopyroxenites in extreme cases. No significant change in fO 2 is associated with this metasomatism since similar oxidation state is observed between un-metasomatized (FMQ - 0.23 to + 0.48) and metasomatized (FMQ - 0.33 to + 0.75) samples. This is explained by the fO 2 of the asthenospheric melt (~ FMQ - 0.50) similar to the SCLM prior to the metasomatism.

  12. Phase E in a water-saturated peridotite system at 9.3 GPa (United States)

    Kawamoto, Tatsuhiko; Leinenweber, Kurt; Hervig, Richard L.


    The stability of hydrous phases in a natural upper mantle system has been investigated at 9.3 GPa using a gel of KLB-1 peridotite composition with brucite which contains 14 wt. percent (30 atom. percent) water. No hydrous mineral was found at 950 (+150 -50) degree C. At 800 degree C, an assemblage of phase A, phase E, enstatite, clinohumite, and garnet is obtained. Although there is a significant thermal gradient over the sample, phase E is found to be surrounded by phase A in the lower temperature part. Electron probe analyses show that phase E has 35.5 SiO2, 4.4 Al2O3, 41.1 MgO and 8.5 wt. percent FeO* (Mg value is 90) with an oxide sum of 89.7 wt. percent, and possesses a stoichiometry similar to that proposed by Kanzaki. CaO and TiO2 are both less than 0.1 wt. percent. Coexisting phase A has 0.5 wt. percent CaO but only 0.4 wt. percent Al2O3 concentration. Phase A coexists with only enstatite in the water-saturated MgO-FeO-SiO2 system at 800 degree C and 9.3 GPa as well as the results in the water-saturated MgO-SiO2 system. Therefore it is suggested that the addition of Al2O3 expands the stability field of phase E to lower than 13 - 17 GPa in the water-saturated MgO-SiO2 system.

  13. Volcanism, Earth Degassing and Replenished Lithosphere Mantle (United States)

    Bailey, D. K.


    Volcanism that pierces plate interiors is characteristically rich in alkalis and volatiles, and its cause and persistence are essentially expressions of the Earth's outgassing. The general balance of mobile elements (such as H, C, F and Cl) rules out recycling of sea floor, hydrosphere, sediments or atmosphere: furthermore, it is not in accord with accepted planet degassing budgets. The typical eruptive mode of volatile-rich magmatism means that the observed regional chemical variations, and even differences between adjacent volcanoes, must largely reflect source heterogeneity. In a broader context, this magmatism is also at odds with a concept of continental crust underlain by strongly depleted (refractory) mantle. Repetition of activity along crustal zones of weakness shows that the lithosphere mantle (a) is structurally complex and (b) still holds continuing (or continual) rich reserves of mobile elements. Unbroken lithosphere muffles the evolutionary escape of volatiles from the deep mantle: any lesion that appears then offers easy escape channels, whereby volatiles are drained from a large mantle region and funnelled through the plate. Horizontal movement of thick continental lithosphere releases volatiles from deep sources, imparting some of the special chemical characteristics of the stable continental magmatism. Present evidence requires consideration of the continental lithosphere as a site of primordial heterogeneity that has been accentuated rather than diminished by geological processes.

  14. An experimental study of the carbonation of serpentinite and partially serpentinised peridotites (United States)

    Lacinska, Alicja M.; Styles, Michael T.; Bateman, Keith; Hall, Matthew; Brown, Paul D.


    In situ sequestration of CO2 in mantle peridotites has been proposed as a method to alleviate the amount of anthropogenic CO2 in the atmosphere. This study presents the results of eight-month long laboratory fluid-rock experiments on representative mantle rocks from the Oman-United Arab Emirates ophiolite to investigate this process. Small core samples (3 cm long) were reacted in wet supercritical CO2 and CO2-saturated brine at 100 bar and 70°C. The extent of carbonate formation, and hence the degree of carbon sequestration, varied greatly depending on rock type, with serpentinite (lizardite-dominated) exhibiting the highest capacity, manifested by the precipitation of magnesite MgCO3 and ferroan magnesite (Mg,Fe)CO3. The carbonate precipitation occurred predominantly on the surface of the core and subordinately within cross-cutting fractures. The extent of the CO2 reactions appeared to be principally controlled by the chemical and mineralogical composition of the rock, as well as the rock texture, with all these factors influencing the extent and rate of mineral dissolution and release of Mg and Fe for subsequent reaction with the CO2. It was calculated that ≈ 0.7 g of CO2 was captured by reacting ≈ 23 g of serpentinite, determined by the mass of magnesite formed. This equates to ≈ 30 kg CO2 per tonne of host rock, equivalent to ≈ 3% carbonation in half a year. However, recycling of carbonate present in veins within the original rock sample could mean that the overall amount is around 2%. The increased reactivity of serpentinite was associated with preferential dissolution of more reactive types of serpentine minerals and brucite, that were mainly present in the cross-cutting veins. The bulk of the serpentinite rock was little affected. This study, using relatively short term experiments, suggests that serpentinite might be a good host rock for CO2 sequestration, although long term experiments might prove that dunite and harzburgite could be an effective

  15. An Experimental Study of the Carbonation of Serpentinite and Partially Serpentinised Peridotites

    Directory of Open Access Journals (Sweden)

    Alicja M. Lacinska


    Full Text Available In situ sequestration of CO2 in mantle peridotites has been proposed as a method to alleviate the amount of anthropogenic CO2 in the atmosphere. This study presents the results of 8-month long laboratory fluid-rock experiments on representative mantle rocks from the Oman-United Arab Emirates ophiolite to investigate this process. Small core samples (3 cm long were reacted in wet supercritical CO2 and CO2-saturated brine at 100 bar and 70°C. The extent of carbonate formation, and hence the degree of carbon sequestration, varied greatly depending on rock type, with serpentinite (lizardite-dominated exhibiting the highest capacity, manifested by the precipitation of magnesite MgCO3 and ferroan magnesite (Mg,FeCO3. The carbonate precipitation occurred predominantly on the surface of the core and subordinately within cross-cutting fractures. The extent of the CO2 reactions appeared to be principally controlled by the chemical and mineralogical composition of the rock, as well as the rock texture, with all these factors influencing the extent and rate of mineral dissolution and release of Mg and Fe for subsequent reaction with the CO2. It was calculated that ≈0.7 g of CO2 was captured by reacting ≈23 g of serpentinite, determined by the mass of magnesite formed. This equates to ≈30 kg CO2 per ton of host rock, equivalent to ≈3% carbonation in half a year. However, recycling of carbonate present in veins within the original rock sample could mean that the overall amount is around 2%. The increased reactivity of serpentinite was associated with preferential dissolution of more reactive types of serpentine minerals and brucite that were mainly present in the cross-cutting veins. The bulk of the serpentinite rock was little affected. This study, using relatively short term experiments, suggests that serpentinite might be a good host rock for CO2 sequestration, although long term experiments might prove that dunite and harzburgite could be as

  16. Seismic anisotropy of upper mantle in eastern China

    Institute of Scientific and Technical Information of China (English)


    Based on the polarization analysis of teleseismic SKS waveform data recorded at 65 seismic stations which respectively involved in the permanent and temporary broadband seismograph networks deployed in eastern China, the SKS fast-wave direction and the delay time between the fast and slow shear waves at each station were determined by use of SC method and the stacking analysis method, and then the image of upper mantle anisotropy in eastern China was acquired. In the study region, from south to north, the fast-wave polarization directions are basically EW in South China, gradually clockwise rotate to NWW-SEE in North China, then to NW-SE in Northeast China. The delay time falls into the interval [0.41 s, 1.52 s]. Anisotropic characteristics in eastern China indicate that the upper mantle anisotropy is possibly caused by both the collision between the Indian and Eurasian Plates and the subduction from the Pacific and Philippine Sea Plates to the Eurasian Plate. The collision between two plates made the crust of western China thickening and uplifting and the material eastwards extruding, and then caused the upper mantle flow eastwards and southeastwards. The subduction of Pacific Plate and Philippine Sea Plate has resulted in the lithosphere and the asthenosphere deformation in eastern China, and made the alignment of upper mantle peridotite lattice parallel to the deformation direction. The fast-wave polarization direction is consistent with the direction of lithosphere extension and the GPS velocity direction, implying that the crust-upper mantle deformation is possibly a vertically coherent deformation.

  17. Seismic anisotropy of upper mantle in eastern China

    Institute of Scientific and Technical Information of China (English)

    CHANG LiJun; WANG ChunYong; Ding ZhiFeng


    Based on the polarization analysis of teleseismic SKS waveform data recorded at 65 seismic stations which respectively involved in the permanent and temporary broadband seismograph networks de-ployed in eastern China,the SKS fast-wave direction and the delay time between the fast and slow shear waves at each station were determined by use of SC method and the stacking analysis method,and then the image of upper mantle anisotropy in eastern China was acquired.In the study region,from south to north,the fast-wave polarization directions are basically EW in South China,gradually clock-wise rotate to NWW-SEE in North China,then to NW-SE in Northeast China.The delay time falls into the Interval [0.41 s,1.52 s].Anisotropic characteristics in eastern China indicate that the upper mantle anisotropy is possibly caused by both the collision between the Indian and Eurasian Plates and the subduction from the Pacific and Philippine Sea Plates to the Eurasian Plate.The collision between two plates made the crust of western China thickening and uplifting and the material eastwards extruding,and then caused the upper mantle flow eastwards and southeastwards.The subduction of Pacific Plate and Philippine Sea Plate has resulted in the lithosphere and the asthenosphere deformation in eastern China,and made the alignment of upper mantle peridotite lattice parallel to the deformation direction.The fast-wave polarization direction is consistent with the direction of lithosphere extension and the GPS velocity direction,implying that the crust-upper mantle deformation is possibly a vertically co-herent deformation.

  18. Growth and mixing dynamics of mantle wedge plumes (United States)

    Gorczyk, Weronika; Gerya, Taras V.; Connolly, James A. D.; Yuen, David A.


    Recent work suggests that hydrated partially molten thermal-chemical plumes that originate from subducted slab as a consequence of Rayleigh-Taylor instability are responsible for the heterogeneous composition of the mantle wedge. We use a two-dimensional ultrahigh-resolution numerical simulation involving 10 × 109 active markers to anticipate the detailed evolution of the internal structure of natural plumes beneath volcanic arcs in intraoceanic subduction settings. The plumes consist of partially molten hydrated peridotite, dry solid mantle, and subducted oceanic crust, which may compose as much as 12% of the plume. As plumes grow and mature these materials mix chaotically, resulting in attenuation and duplication of the original layering on scales of 1-1000 m. Comparison of numerical results with geological observations from the Horoman ultramafic complex in Japan suggests that mixing and differentiation processes related to development of partially molten plumes above slabs may be responsible for the strongly layered lithologically mixed (marble cake) structure of asthenospheric mantle wedges.

  19. Density structure of the cratonic mantle in southern Africa

    DEFF Research Database (Denmark)

    Artemieva, Irina; Vinnik, Lev P.


    . An unusually high topography may be caused by a low density (high depletion) of the cratonic lithospheric mantle and/or by the dynamic support of the mantle with origin below the depth of isostatic compensation (assumed here to be at the lithosphere base). We use free-board constraints to examine the relative...... contributions of the both factors to surface topography in the cratons of southern Africa. Our analysis takes advantage of the SASE seismic experiment which provided high resolution regional models of the crustal thickness.We calculate the model of density structure of the lithospheric mantle in southern Africa...... and show that it has an overall agreement with xenolith-based data for lithospheric terranes of different ages. Density of lithospheric mantle has significant short-wavelength variations in all tectonic blocks of southern Africa and has typical SPT values of ca. 3.37-3.41g/cm3 in the Cape Fold and Namaqua...

  20. Origin of geochemical mantle components: Role of subduction filter (United States)

    Kimura, Jun-Ichi; Gill, James B.; Skora, Susanne; van Keken, Peter E.; Kawabata, Hiroshi


    We quantitatively explore element redistribution at subduction zones using numerical mass balance models to evaluate the roles of the subduction zone filter in the Earth's geochemical cycle. Our models of slab residues after arc magma genesis differ from previous ones by being internally consistent with geodynamic models of modern arcs that successfully explain arc magma genesis and include element fluxes from the dehydration/melting of each underlying slab component. We assume that the mantle potential temperature (Tp) was 1400-1650°C at 3.5-1.7 Ga and gradually decreased to 1300-1350°C today. Hot subduction zones with Tp ˜1650°C have a thermal structure like modern SW Japan where high-Mg andesite is formed which is chemically like continental crust. After 2.5-1.7 Gyr of storage in the mantle, the residual igneous oceanic crust from hot subduction zones can evolve isotopically to the HIMU mantle component, the residual base of the mantle wedge to EMI, the residual sediment becomes an essential part of EMII, and the residual top of the mantle wedge can become the subcontinental lithosphere component. The Common or Focal Zone component is a stable mixture of the first three residues occasionally mixed with early depleted mantle. Slab residues that recycled earlier (˜2.5 Ga) form the DUPAL anomaly in the southern hemisphere, whereas residues of more recent recycling (˜1.7 Ga) underlie the northern hemisphere. These ages correspond to major continental crust forming events. The east-west heterogeneity of the depleted upper mantle involves subcontinental mantle except in the Pacific.

  1. Thermodynamic constraints on mineral carbonation of serpentinized peridotite (United States)

    Klein, Frieder; Garrido, Carlos J.


    Carbonation of serpentinite appears to be a promising process to mitigate rising concentrations of atmospheric CO 2. However, given that chemical affinities of mineral carbonation reactions in serpentinized peridotite change drastically with temperature, protolith and the activities of dissolved constituents, this process is incompletely understood. In this report numerical models are used to investigate the heterogeneous phase equilibria and the speciation of intergranular fluids during mineral carbonation of serpentinite/dunite at 100, 200, 300 and 400 °C. The results suggest that temperature-dependent activity gradients of both aqueous carbon dioxide (aCO 2,aq) and aqueous silica (aSiO 2,aq) determine the carbonation potential of serpentinite. At low to moderate temperatures there is ample thermodynamic driving force for the carbonation of serpentinite, as the gradients in aSiO 2,aq and aCO 2,aq set between talc-quartz-magnesite and serpentine-brucite equilibria are steepest. With increasing temperature these activity gradients become smaller because the stability fields of brucite, serpentine and talc expand toward higher aCO 2,aq and aSiO 2,aq while the stability field of magnesite contracts. In addition, the enhanced solubility of quartz toward higher temperatures shifts the talc-quartz-magnesite equilibrium to very high aCO 2,aq. Field observations of carbonated serpentinite reveal a rather simple sequence of arrested carbonation reactions, which is typically fossilized as spatially distinct metasomatic zones. A zone of serpentinized peridotite is followed by a zone of serpentine + carbonate, then talc + carbonate and finally quartz + carbonate. Our modeling results not only reproduce these observations, they also suggest that complete carbonation is favored at low to moderate temperatures. Since complete carbonation of serpentinite is achieved at relatively small fluid to rock ratios at temperatures ≤ 200 °C, prolonged fluid influx at low temperatures

  2. The LIP-OIB transitional phase in the Galapagos mantle plume (United States)

    Trela, J.; Gazel, E.; Vidito, C. A.; Class, C.; Jicha, B. R.; Bizimis, M.; Herzberg, C. T.; Alvarado-Induni, G.


    Although significant work has been done on LIPS and OIB, no complete record of the evolution of a mantle plume is available at this point. Galapagos-related lavas provide a complete record of the evolution of a mantle plume since the plume's initial stages in the Cretaceous. Our petrological models (PRIMELT2) suggest that the Galapagos plume head that formed the Caribbean Large Igneous Province (CLIP) at ~95 Ma melted at hotter temperatures than the ocean island basalt (OIB) equivalents of the modern archipelago. While this work suggests a significant decrease in mantle potential temperatures (Tp) over time, the exact mechanism responsible for secular cooling of the Galapagos plume remains unclear. One viable explanation is that plumes entraining recycled oceanic crust (pyroxenite) will be cooler than purely peridotite plumes, due to the effect of dense pyroxenite on the plume's buoyancy. High-precision electron microprobe analyses on olivine cores from the ~70 Ma Galapagos-related Quepos terrane in Costa Rica indicate a mixed peridotite-pyroxenite source lithology, not evident during the LIP stage. The appearance of this pyroxenitic component correlates with the first record of an EMII isotopic signature (Northern Galapagos Domain), and significant high-field strength enrichments in the Galapagos plume related lavas. This dense pyroxenite component may explain the marked decrease in Tp observed at ~70 Ma due to its effect on the plume's buoyancy. Otherwise, the pyroxenite component may have been diluted during voluminous basalt production of the CLIP by high peridotite melt fractions. Future research will incorporate further petrological modeling, olivine chemistry, and radiogenic isotope work of accreted Galapagos terranes in Central America to test whether or not a decrease in Tp correlates with increasing pyroxenite content in source melts.

  3. Upper mantle fluids involved in diamond formation and mantle metasomatism (United States)

    Sverjensky, D. A.


    Diamond formation coupled with metasomatic reactions involving the interaction of fluids with silicate host rocks provides important clues about the deep carbon cycle. However, quantitative modeling of diamond formation with silicate rock metasomatism has not been possible. Here the Deep Water (DEW) model [1] was used to generate equilibrium constants for irreversible chemical mass transfer calculations monitoring evolving fluid chemistry during diamond formation and coupled silicate reactions. Conceptual models for diamond formation in two environments were constructed for the purpose of illustrating the role of pH in diamond-forming systems. For cratonic diamonds, fluid at 900°C and 5.0 GPa was derived in equilibrium with a carbonated mafic part of a subducting slab consisting of pure diopside, enstatite, pyrope, phlogopite, magnesite, diamond and pyrite. The fluid was assumed to infiltrate and react at constant T and P with a model metasedimentary eclogite (jadeite, pyrope, kyanite and coesite). Abundant diamond was predicted to precipitate as reactant silicate minerals were destroyed and secondary pyrope, jadeite and kyanite were precipitated, which could represent the solid inclusions in natural diamonds. The final fluid chemistry was extremely enriched in Si and depleted in Ca relative to the initial fluid, consistent with the worldwide fluid inclusion trend from carbonatitic fluid to silicic fluid. The logfO2 changed by only 0.2, whereas pH continuously decreased as reaction with jadeite and kyanite and precipitation of secondary pyrope removed Mg2+ and added H+ to the fluid. Most of the carbon precipitated as diamond was derived from decreasing concentrations of formate and propionate. In the UHPM scenario, fluid at 600°C and 5.0 GPa in carbonated peridotite (forsterite, antigorite, clinochlore, magnesite, and pyrrhotite) in a subducting slab was assumed to infiltrate and react at constant T and P with a different model metasedimentary eclogite (jadeite

  4. Where is mantle's carbon? (United States)

    Oganov, A. R.; Ono, S.; Ma, Y.


    Due to the strongly reducing conditions (the presence of metallic iron was suggested both by experiments [1] and theory [2]), diamond was believed to be the main host of carbon through most of the lower mantle [3]. We showed [4] that cementite Fe3C is another good candidate to be the main host of "reduced" carbon in the mantle, reinforcing an earlier hypothesis [5]. The fate of "oxidised" carbon (in subducted slabs) is of particular importance - if carbonates decompose producing fluid CO2, this would have important implications for the chemistry and rheology of the mantle. Knowledge of crystal structures and phase diagrams of carbonates is crucial here. The high-pressure structures of CaCO3 were predicted [6] and subsequently verified by experiments. For MgCO3, Isshiki et al. [7] found a new phase above 110 GPa, and several attempts were made to solve it [8,9]. Here [4], using an evolutionary algorithm for crystal structure prediction [10], we show that there are two post-magnesite phases at mantle-relevant pressure range, one stable at 82-138 GPa, and the other from 138 GPa to ~160 GPa. Both are based on threefold rings of CO4-tetrahedra and are more favourable than all previously proposed structures. We show that through most of the P-T conditions of the mantle, MgCO3 is the major host of oxidized carbon in the Earth. We predict the possibility of CO2 release at the very bottom of the mantle (in SiO2-rich basaltic part of subducted slabs), which could enhance partial melting of rocks and be related to the geodynamical differences between the Earth and Venus. 1.Frost D.J., Liebske C., Langenhorst F., McCammon C.A., Tronnes R.G., Rubie D.C. (2004). Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle. Nature 428, 409-412. 2.Zhang F., Oganov A.R. (2006). Valence and spin states of iron impurities in mantle-forming silicates. Earth Planet. Sci. Lett. 249, 436-443. 3.Luth R.W. (1999). Carbon and carbonates in the mantle. In: Mantle

  5. Petrogenesis of basaltic volcanic rocks from the Pribilof Islands, Alaska, by melting of metasomatically enriched depleted lithosphere, crystallization differentiation, and magma mixing (United States)

    Chang, J.M.; Feeley, T.C.; Deraps, M.R.


    trace element characteristics are similar to those of ocean island basalts (OIB), including enrichment in alkalis and incompatible trace elements. These characteristics are interpreted to indicate that their mantle source experienced an ancient melt-removal event that is reflected in depleted radiogenic isotopic compositions and was then re-enriched by metasomatism that elevated incompatible trace element contents, but was too young to produce a time-integrated change in radiogenic isotopic ratios. Evidence suggests that the Pribilof Island basalts did not form in either a plume or a back-arc basin tectonic setting. Rather, they were produced by melting of metasomatically hydrated upper mantle peridotite at relatively low temperatures and were able to erupt at the surface through extensional or transtensional faults that served as conduits for the magmas. ?? The Author 2009. Published by Oxford University Press.

  6. Mixing and Progressive Melting of Deep and Shallow Mantle Sources in the NE Atlantic and Arctic

    DEFF Research Database (Denmark)

    Trønnes, Reidar; Debaille, Vincianne; Erambert, M.


    peridotitic, in contrast to the ROC-lithologies, recording a higher proportion of eclogites and hybridized pyroxenites. The combined Hf-Ndisotope systematics also discriminate between these two enriched source components. The high proportion of the SCLM-component in the asthenosphere along the Kolbeinsey......, Mohns, Knipovich and Gakkel Ridges reflects the young, narrow and slow-spreading character of the corresponding oceanic basins. These ridges appear to sample mantle sources with higher proportions of locally derived SCLM-material than other mid-ocean ridges....

  7. Toward a coherent model for the melting behavior of the deep Earth's mantle (United States)

    Andrault, D.; Bolfan-Casanova, N.; Bouhifd, M. A.; Boujibar, A.; Garbarino, G.; Manthilake, G.; Mezouar, M.; Monteux, J.; Parisiades, P.; Pesce, G.


    Knowledge of melting properties is critical to predict the nature and the fate of melts produced in the deep mantle. Early in the Earth's history, melting properties controlled the magma ocean crystallization, which potentially induced chemical segregation in distinct reservoirs. Today, partial melting most probably occurs in the lowermost mantle as well as at mid upper-mantle depths, which control important aspects of mantle dynamics, including some types of volcanism. Unfortunately, despite major experimental and theoretical efforts, major controversies remain about several aspects of mantle melting. For example, the liquidus of the mantle was reported (for peridotitic or chondritic-type composition) with a temperature difference of ∼1000 K at high mantle depths. Also, the Fe partitioning coefficient (DFeBg/melt) between bridgmanite (Bg, the major lower mantle mineral) and a melt was reported between ∼0.1 and ∼0.5, for a mantle depth of ∼2000 km. Until now, these uncertainties had prevented the construction of a coherent picture of the melting behavior of the deep mantle. In this article, we perform a critical review of previous works and develop a coherent, semi-quantitative, model. We first address the melting curve of Bg with the help of original experimental measurements, which yields a constraint on the volume change upon melting (ΔVm). Secondly, we apply a basic thermodynamical approach to discuss the melting behavior of mineralogical assemblages made of fractions of Bg, CaSiO3-perovskite and (Mg,Fe)O-ferropericlase. Our analysis yields quantitative constraints on the SiO2-content in the pseudo-eutectic melt and the degree of partial melting (F) as a function of pressure, temperature and mantle composition; For examples, we find that F could be more than 40% at the solidus temperature, except if the presence of volatile elements induces incipient melting. We then discuss the melt buoyancy in a partial molten lower mantle as a function of pressure

  8. Geochemistry and petrology of spinel lherzolite xenoliths from Xalapasco de La Joya, San Luis Potosi, Mexico: Partial melting and mantle metasomatism (United States)

    Liang, Yan; Elthon, Don


    Spinel Iherzolite xenoliths from Xalapasco de La Joya, San Luis Potosi, Mexico, are divided into two distinct groups according to their major element and trace element characteristics. Group Ia xenoliths are characterized by light rare earth element (LREE) depletion ((La/Lu)N = 0.10-0.77 in clinopyroxene) and linear major and compatible trace element relationships. Group Ib xenoliths are characterized by FeO and Na2O enrichment and higher (La/Lu)N ratios (0.80-4.1 in clinopyroxene) and complex major element relationships. These samples, which have a range of equilibrium temperatures of 910°-1070°C, exhibit protogranular textures and typical orthopyroxene+clinopyroxene+spinel clusters. Modal abundances and chemical compositions of the group Ia xenoliths vary from primitive (15.2% clinopyroxene, 38.5% MgO, 1824 ppm Ni) to moderately depleted (6.4-8.7% clinopyroxene, 43.8-44.1% MgO, 2192 ppm Ni). Systematic variations of major elements and compatible trace elements in the group Ia xenoliths are interpreted to result from various degrees (<25%) of partial melting and melt extraction, followed by subsolidus equilibration and annealing. The extracted melts have a range of compositions similar to picritic basalts. Abundances of moderately incompatible trace elements, Sc and Cr, in the group Ia minerals have been substantially redistributed during subsolidus equilibration. In a few of these xenoliths there appears to be vestiges of incipient metasomatism, but metasomatism has not substantially influenced the group as a whole. Group Ib xenoliths have been substantially influenced by metasomatic processes. The ∑FeO and Na2O contents of the cores of clinopyroxenes in group Ib xenoliths are higher than clinopyroxenes in group Ia samples. The higher La contents and La/Lu ratios in group Ib clinopyroxenes (compared to group Ia), together with this FeO and Na2O enrichment, suggest that equilibration of basanites with residual mantle has been a major process in the evolution

  9. Lunar maria - result of mantle plume activity? (United States)

    Sharkov, E.

    It is generally accepted that lunar maria are the result of catastrophic impact events. However, comparative studying of the Earth's and the Moon's tectonomagmatic evolution could evidence about another way of these specific structures origin. Such studies showed that the both planetary bodies evolved on the close scenario: their geological development began after solidification of global magmatic oceans which led to appearance of their primordial crusts: granitic on the Earth and anorthositic - on the Moon. The further evolution of the both bodies occurred in two stages. For their first stages, lasted ˜2.5 mlrd. years on the Earth and ˜1.5 mlrd. years on the Moon, were typical melts, generated in depleted mantle (Bogatikov et al., 2000). However, at the boundary 2.2-2.0 Ga ago on the Earth and 3.9-3.8 Ga on the Moon another type of magmas appeared: geochemical enriched Fe-Ti picrites and basalts, characteristic for the terrestrial Phanerozoic plume-related situations, and basaltic mare magmatism with high-Ti varieties on the Moon. It suggests that evolution of the Earth's magmatism was linked with ascending of mantle plumes (superplumes) of two generation: (1) generated in the mantle, depleted during solidification of magmatic ocean and Archean magmatic activity, and (2) generated at the core-mantle boundary (CMB). The latter were enriched in the mantle fluid components (Fe, Ti, alkalies, etc); this lighter material could ascend to shallower depths, leading to change of tectonic processes, in particular, to appearance of plate tectonics as the major type of tectonomagmatic activity till now (Bogatikov et al., 2000). By analogy to the Earth, magmatism of the Moon was also linked with ascending of mantle plumes: (1) generated in the depleted mantle (magnesian suite) and (2) generated at the lunar CMB with liquid at that time metallic core (mare basalt and picrites with high-Ti varieties). Like on the Earth, these plumes were lighter than the older plumes, and

  10. Alpe Arami garnet peridotite from depth >300 km: revisited in 15 years (Invited) (United States)

    Dobrzhinetskaya, L.; Lesher, C. E.; Bozhilov, K. N.; Green, H. W.


    In the mid-1990s we recognized that collisional orogenic belts with UHP metamorphic rocks of crustal affinities, might contain mantle peridotites uplifted from depths of >300km (Dobrzhinetsklaya et al., 1996). We proposed that ilmenite rods + chromite flakes in olivine are the result of exsolution, and that they imply high solubility of TiO2 in Ol at high P-T. The conclusion that the Alpe Arami peridotite massif originates from great depth remains controversial despite the subsequent discovery of Cpx inclusions in Grt containing exsolution lamellae of clinoenstatite displaying antiphase domains. The latter observation requires that the originally precipitated pyroxene had a high-pressure C2/c space group structure consistent with an origin from >8 GPa (~250 km) (Bozhilov et al., 1999). We used confocal laser scanning microscopy to obtain quantitative 3D measurements of Ilm abundance in Ol. We find that Ilm rods can reach >1 vol. % in Ol supporting our contention that Ilm rods exsolved from olivine at 9-12 GPa, e.g. >300 km (Bozhilov et al., 2003). Arguments that Ilm rods hosted by Ol are reaction products from the breakdown of Ti-clinohumite (Risold et al., 2003) are unconvincing given that Ti-clinohumite breakdown does not produce Ilm + chromite that are characteristic of Alpe Arami olivine. Likewise, we have confirmed the high solubility of Ti in olivine at high P-T conditions in multianvil experiments (Dobrzhinetskaya et al., 2000) - findings that are strengthened by the experiments of Tinker & Lesher (2001). Using Focused Ion Beam we prepared TEM foils of the later experiments and confirm that the concentration of TiO2 in Ol increases with pressure under nearly isothermal conditions. Two reactions are proposed to account for these observations. If Ti enters the octahedral site of olivine, either an oxide phase must be formed or a phase richer in SiO2 than olivine must be consumed, leading to reaction 1: 6MgSiO3 + 2FeTiO3 = 2Ti△SiO4 + 3Mg2SiO4 + Fe2SiO4 [1

  11. Modern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability (United States)

    Miller, Hannah M.; Matter, Jürg M.; Kelemen, Peter; Ellison, Eric T.; Conrad, Mark E.; Fierer, Noah; Ruchala, Tyler; Tominaga, Masako; Templeton, Alexis S.


    The Samail ophiolite in Oman is undergoing modern hydration and carbonation of peridotite and may host a deep subsurface biosphere. Previous investigations of hyperalkaline fluids in Oman have focused on fluids released at surface seeps, which quickly lose their reducing character and precipitate carbonates upon contact with the O2/CO2-rich atmosphere. In this work, geochemical analysis of rocks and fluids from the subsurface provides new insights into the operative reactions in serpentinizing aquifers. Serpentinite rock and hyperalkaline fluids (pH > 10), which exhibit millimolar concentrations of Ca2+, H2 and CH4, as well as variable sulfate and nitrate, were accessed from wells situated in mantle peridotite near Ibra and studied to investigate their aqueous geochemistry, gas concentrations, isotopic signatures, mineralogy, Fe speciation and microbial community composition. The bulk mineralogy of drill cuttings is dominated by olivine, pyroxene, brucite, serpentine and magnetite. At depth, Fe-bearing brucite is commonly intermixed with serpentine, whereas near the surface, olivine and brucite are lost and increased magnetite and serpentine is detected. Micro-Raman spectroscopy reveals at least two distinct generations of serpentine present in drill cuttings recovered from several depths from two wells. Fe K-edge X-ray absorption near-edge spectroscopy (XANES) analysis of the lizardite shows a strong tetrahedral Fe coordination, suggesting a mixture of both Fe(II) and Fe(III) in the serpentine. Magnetite veins are also closely associated with this second generation serpentine, and 2-10 μm magnetite grains overprint all minerals in the drill cuttings. Thus we propose that the dissolved H2 that accumulates in the subsurface hyperalkaline fluids was evolved through low temperature oxidation and hydration of relict olivine, as well as destabilization of pre-existing brucite present in the partially serpentinized dunites and harzburgites. In particular, we hypothesize

  12. Bases of the Mantle-Carbonatite Conception of Diamond Genesis (United States)

    Litvin, Yuriy; Spivak, Anna; Kuzyura, Anastasia


    In the mantle-carbonatite conception of diamond genesis, the results of physic-chemical experiments are coordinated with the data of analytic mineralogy of primary inclusions in natural diamonds. Generalization of the solutions of principal genetic problems constitutes the bases of the conception. The solutions are following: (1) it is grounded that diamond-parental melts of the upper mantle have peridotite/eclogite - carbonatite - carbon compositions, of the transition zone - (wadsleite↔ringwoodite) - majorite - stishovite - carbonatite - carbon compositions, and of the lower mantle - periclase/wustite - bridgmanite - Ca-perovskite -stishovite - carbonatite - carbon compositions; (2) a construction of generalized diagrams for the diamond-parental media, which reveal changeable compositions of the growth melts of diamonds and associated phases, their genetic relations to the mantle substance, and classification connections of the primary inclusions in natural diamonds; (3) experimental equilibrium phase diagrams of syngenesis of diamonds and primary inclusions, which characterize the nucleation and growth conditions of diamonds and a capture of paragenetic and xenogenetic minerals by the growing diamonds; (4) a determination of the phase diagrams of diamonds and inclusions syngenesis under the regime of fractional crystallization, which discover the regularities of ultrabasic-basic evolution and paragenesis transitions in the diamond-forming systems of the upper and lower mantle. The evidence of the physic-chemically united mode of diamond genesis at the mantle depths with different mineralogy is obtained. References. Litvin Yu.A. (2007). High-pressure mineralogy of diamond genesis. In: Advances in High-Pressure Mineralogy (edited by Eiji Ohtani), Geological Society of America Special paper 421, 83-103. Litvin Yu.A. (2012). Experimental study of physic-chemical conditions of natural diamond formation on an example of the eclogite

  13. The Elephants' Graveyard: Constraints from Mantle Plumes on the Fate of Subducted Slabs and Implications for the Style of Mantle Convection (United States)

    Lassiter, J. C.


    The style of mantle convection (e.g., layered- vs. whole-mantle convection) is one of the most hotly contested questions in the Geological Sciences. Geochemical arguments for and against mantle layering have largely focused on mass-balance evidence for the existence of "hidden" geochemical reservoirs. However, the size and location of such reservoirs are largely unconstrained, and most geochemical arguments for mantle layering are consistent with a depleted mantle comprising most of the mantle mass and a comparatively small volume of enriched, hidden material either within D" or within seismically anomalous "piles" beneath southern Africa and the South Pacific. The mass flux associated with subduction of oceanic lithosphere is large and plate subduction is an efficient driver of convective mixing in the mantle. Therefore, the depth to which oceanic lithosphere descends into the mantle is effectively the depth of the upper mantle in any layered mantle model. Numerous geochemical studies provide convincing evidence that many mantle plumes contain material which at one point resided close to the Earth's surface (e.g., recycled oceanic crust ± sediments, possibly subduction-modified mantle wedge material). Fluid dynamic models further reveal that only the central cores of mantle plumes are involved in melt generation. The presence of recycled material in the sources of many ocean island basalts therefore cannot be explained by entrainment of this material during plume ascent, but requires that recycled material resides within or immediately above the thermo-chemical boundary layer(s) that generates mantle plumes. More recent Os- isotope studies of mantle xenoliths from OIB settings reveal the presence not only of recycled crust in mantle plumes, but also ancient melt-depleted harzburgite interpreted to represent ancient recycled oceanic lithosphere [1]. Thus, there is increasing evidence that subducted slabs accumulate in the boundary layer(s) that provide the source

  14. Flow in the shallow mantle in the westernmost Mediterranean: insights from xenoliths in Plio-Pleistocene alkali basalts from the eastern Betic Cordillera (SE Spain) (United States)

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


    Peridotite mantle xenoliths in Plio-Pleistocene alkali basalts of the eastern Betic Cordillera (Cartagena area, Murcia, SE Spain) provide a snapshot of the structure and composition of the lithospheric mantle at the northern limb of the Alpine Betic-Rif arched belt in the westernmost Mediterranean. The xenoliths are spinel and plagioclase lherzolite with minor harzburgite and wehrlite, displaying porphyroclastic to equigranular textures. Regardless of composition and texture, the Crystal Preferred Orientation (CPO) of olivine shows an axial-[100] pattern characterized by a strong alignment of [100]-axes near or parallel to the peridotite lineation and a girdle distribution of [010]-axes with a maximum normal to the peridotite foliation. This CPO pattern is consistent with ductile deformation accommodated by dislocation creep with dominant activation of the high temperature {0kl}[100] olivine slip system, indicative of deformation by simple shear or combinations of simple shear and pure shear with a transtensional component. Calculated seismic properties are characterized by fast propagation of P-waves and polarization of fast S-waves parallel to olivine [100]-axis, indicating the flow direction. SKS and Pn anisotropy in the eastern Betics can be explained by a lithospheric mantle peridotite with similar fabric to the one displayed by the studied mantle xenoliths. Considering the limited thickness of the mantle lithosphere in the Betics (40-80 km), the measured azimuths and delays of SKS waves in the eastern Betics are consistent with a steeply dipping mantle foliation and a subhorizontal lineation with ENE strike. This geometry of the lithospheric fabrics implies active or frozen mantle flow with a dominantly strike-slip component subparallel to the paleo-Iberian margin. Synkinematic overprinting of mineral assemblages from the garnet-spinel to the plagioclase facies demonstrates 36-40 km uplift continuously accommodated by ductile shear thinning of the

  15. Sharpness of upper-mantle discontinuities determined from high-frequency reflections (United States)

    Benz, H.M.; Vidale, J.E.


    AN understanding of the nature of seismic discontinuities in the Earth's upper mantle is important for understanding mantle processes: in particular, the amplitude and sharpness of these discontinuities are critical for assessing models of upper-mantle phase changes and chemical layering. So far, seismic studies aimed at determining the thickness and lateral variability of upper-mantle discontinuities have yielded equivocal results, particularly for the discontinuity at 410km depth1,2. Here we present short-period (0.8-2.0 s) recordings of upper-mantle precursors to the seismic phase P???P??? (PKPPKP) from two South American earthquakes recorded by the ???700-station short-period array in California. Our results show that the 410- and 660-km discontinuities beneath the Indian Ocean are locally simple and sharp, corresponding to transi-tion zones of 4 km or less. These observations pose problems for mineral physics models3-5, which predict a transitional thickness greater than 6 km for the peridotite to ??-spinel phase transition. In contrast to the results of long-period studies6,7, we observe no short-period arrivals from near 520 km depth. ?? 1993 Nature Publishing Group.

  16. Extreme incompatibility of helium during mantle melting: Evidence from undegassed mid-ocean ridge basalts (United States)

    Graham, David W.; Michael, Peter J.; Shea, Thomas


    We report total helium concentrations (vesicles + glass) for a suite of thirteen ultradepleted mid-ocean ridge basalts (UD-MORBs) that were previously studied for volatile contents (CO2, H2O) plus major and trace elements. The selected basalts are undersaturated in CO2 + H2O at their depths of eruption and represent rare cases of undegassed MORBs. Sample localities from the Atlantic (2), Indian (1) and Pacific (7) Oceans collectively show excellent linear correlations (r2 = 0.75- 0.92) between the concentrations of helium and the highly incompatible elements C, K, Rb, Ba, Nb, Th and U. Three basalts from Gakkel Ridge in the Arctic were also studied but show anomalous behavior marked by excess lithophile trace element abundances. In the Atlantic-Pacific-Indian suite, incompatible element concentrations vary by factors of 3-4.3, while helium concentration varies by a factor of 13. The strong correlations between the concentrations of helium and incompatible elements are explained by helium behavior as the most incompatible element during mantle melting. Partial melting of an ultradepleted mantle source, formed as a residue of earlier melt extraction, accounts for the observed concentrations. The earlier melting event involved removal of a small degree melt (∼1%) at low but non-zero porosity (0.01-0.5%), leading to a small amount of melt retention that strongly leveraged the incompatible element budget of the ultradepleted mantle source. Equilibrium melting models that produce the range of trace element and helium concentrations from this source require a bulk solid/melt distribution coefficient for helium that is lower than that for other incompatible elements by about a factor of ten. Alternatively, the bulk solid/melt distribution coefficient for helium could be similar to or even larger than that for other incompatible elements, but the much larger diffusivity of helium in peridotite leads to its more effective incompatibility and efficient extraction from a

  17. Oceanic crust recycling and the formation of lower mantle heterogeneity (United States)

    van Keken, Peter E.; Ritsema, Jeroen; Haugland, Sam; Goes, Saskia; Kaneshima, Satoshi


    The Earth's lower mantle is heterogeneous at multiple scales as demonstrated for example by the degree-2 distribution of LLSVPs seen in global tomography and widespread distribution of small scale heterogeneity as seen in seismic scattering. The origin of this heterogeneity is generally attributed to leftovers from Earth's formation, the recycling of oceanic crust, or a combination thereof. Here we will explore the consequences of long-term oceanic crust extraction and recycling by plate tectonics. We use geodynamical models of mantle convection that simulate plates in an energetically consistent manner. The recycling of oceanic crust over the age of the Earth produces persistent lower mantle heterogeneity while the upper mantle tends to be significantly more homogeneous. We quantitatively compare the predicted heterogeneity to that of the present day Earth by tomographic filtering of the geodynamical models and comparison with S40RTS. We also predict the scattering characteristics from S-P conversions and compare these to global scattering observations. The geophysical comparison shows that lower mantle heterogeneity is likely dominated by long-term oceanic crust recycling. The models also demonstrate reasonable agreement with the geochemically observed spread between HIMU-EM1-DMM in ocean island basalts as well as the long-term gradual depletion of the upper mantle as observed in Lu-Hf systematics.

  18. The Kea- and Loa- trends and magma genesis in the Hawaiian mantle plume (United States)

    Ren, Z.; Ingle, S.; Takahashi, E.; Hirano, N.; Hirata, T.; Tatsumi, Y.


    the shield volcanoes likely reflect different mixing proportions of subducted recycled gabbroic oceanic crust and peridotite from the lower mantle. The dominant component sampled at a given shield volcano is likely controlled by the thermal structure. As the volcano grows, it migrates away from the hot plume axis with plate motion. In the plume core, higher temperatures are able to generate melts with Kea-like composition from the more refractory component (i.e. peridotite from lower mantle) during the early shield stages. In contrast, lavas from some of the late-stages of the Hawaiian shields are Loa-like with isotopically enriched characteristics, implying that the proportion of the subducted oceanic crust (eclogite) component contributing to the melt may be higher relative to the peridotitic matrix. This is because during the generation of the later-stage lavas, their mantle source is located significantly away from the mantle plume axis where the temperature is lower than in the center of plume, and therefore the lower melting point component (eclogite) is preferentially sampled by the melt.

  19. Volatile-rich Melts in the Earth's Upper Mantle (AGU Kuno Medal) (United States)

    Dasgupta, Rajdeep


    The onset of silicate magma generation in the Earth's upper mantle influences the thermal evolution of the planet, fluxes of key volatiles to the exosphere, and geochemical and geophysical properties of the mantle. Although carbonatitic fluid with variable water content could be stable ≤250 km beneath mid oceanic ridges [1-3], owing to the small fraction (oxygen fugacity of the mantle in the garnet peridotite field [2, 3], we suggest that on a global scale, carbonated silicate melt generation at ~250-180 km deep redox solidus, with destabilization of metal and majorite in the upwelling mantle, explains oceanic low-velocity zone and electrical conductivity structure of the mantle. In locally oxidized domains (i.e., higher than average Fe3+/Fetotal), deeper carbonated silicate melt may contribute to the X-discontinuity. Furthermore, the new experimental results along with the electrical conductivity of molten carbonated peridotite [8] and that of the oceanic upper mantle [6] suggest that if CO2-rich melt is the only possible agent to explain the high electrical conductivity of the asthenospheric mantle then the mantle at depth is CO2-rich but H2O-poor; higher H2O content in the mantle enhances melting, lowers the CO2 content and likely the conductivity of such melts. Finally, carbonated silicate melts restrict the stability of carbonatite in the Earth's deep oceanic upper mantle and the inventory of carbon, water, and other highly incompatible elements at ridges becomes controlled by flux of the former [7]. Although the stability of carbonatitic melt may be eliminated beneath oceanic ridges at all depths, beneath continents stability of carbonatitic melt is expected. Archean cratonic mantle (geotherms corresponding to surface heat flux of 40-50 mW m-2) crosses the carbonated peridotite solidus, at a depth of ~100-220 km [9]; thus considering the oxygen fugacity profile for cratons [3], carbonatitic melt is expected to be stable at 100-180 km depths, at a narrow

  20. The North American upper mantle: density, composition, and evolution (United States)

    Mooney, Walter D.; Kaban, Mikhail K.


    . The thermally corrected mantle density map reveals density anomalies that are chiefly due to compositional variations. These compositional density anomalies cause gravitational anomalies that reach ~250 mGal. A pronounced negative anomaly (−50 to −200 mGal) is found over the Canadian shield, which is consistent with chemical depletion and a corresponding low density of the lithospheric mantle, also referred to as the mantle tectosphere. The strongest positive anomaly is coincident with the Gulf of Mexico and indicates a positive density anomaly in the upper mantle, possibly an eclogite layer that has caused subsidence in the Gulf. Two linear positive anomalies are also seen south of 40°N: one with a NE-SW trend in the eastern United States, roughly coincident with the Grenville-Appalachians, and a second with a NW-SE trend beneath the states of Texas, New Mexico, and Colorado. These anomalies are interpreted as being due to (1) the presence of remnants of an oceanic slab in the upper mantle beneath the Grenville-Appalachian suture and (2) mantle thickening caused by a period of shallow, flat subduction during the Laramie orogeny, respectively. Based on these geophysical results, the evolution of the NA upper mantle is depicted in a series of maps and cartoons that display the primary processes that have formed and modified the NA crust and lithospheric upper mantle.

  1. Petrology of metabasic and peridotitic rocks of the Songshugou ophiolite, Qinling orogen, China (United States)

    Belic, Maximilian; Hauzenberger, Christoph; Dong, Yunpeng


    The Proterozoic Songshugou ophiolite outcrops as a rootless nappe which was emplaced into the southern margin of the Qinling Group. It consists mainly of amphibolite facies metamafic and -ultramafic rocks. Trace element geochemistry and isotope composition show that the mafic rocks are mainly E-MORB and T-MORB metabasalts (Dong et al., 2008b). Within the ophiolite sequence, ultramafic rocks consist mainly of peridotites and serpentinites. Particularly, extremely fresh dunites and harzburgites, are found which do not display a conspicuous metamorphic overprint. The low CaO (non-fertile mantle rocks. Chromite is found as disseminated phase but can sometimes form massive chromite bands. The platinumgroup mineral Laurite (RuS2) could be identified as inclusion in chromites. Usually part of Ru is substituted by Os and Ir. The metamafic rocks consist of garnet, amphibole, symplectitic pyroxenes, ilmenite, apatite, ±zoisite, ±sphene and show a strong metamorphic overprint. Garnet contains numerous inclusions in the core but are nearly inclusion free at the rim. The cores have sometimes snowball textures indicating initially syndeformative growth. Pure albite and prehnite were found in the central parts of the garnets. In the outer portions, pargasitic amphibole, rutile and rarely glaukophane were found. The symplectitic pyroxenes are of diopsidic composition which enclose prehnite and not albite, as common in retrograde eclogitic rocks. Different stages of garnet breakdown to plagioclase and amphibole, from thin plagioclase rims surrounding the garnets to plagioclase rich pseudomorphs, can be observed in different samples. Based on the glaukophane inclusions and symplectitic pyroxenes a high pressure metamorphic event can be concluded. The garnet breakdown to plagioclase and the symplectites clearly indicate a rapid exhumation phase. The age of the metamorphic event is unclear but probably related to the closure of the Shangdan ocean during the early Paleozoic. The

  2. 西南天山东德沟橄榄岩的岩相学、矿物学演化特征及其地质意义%Petrography, mineralogy and the evolution of peridotites from the Dongdegou, southwestern Tianshan and its geological significance

    Institute of Scientific and Technical Information of China (English)

    孔凡梅; 李旭平; 吴苏; 李守军; 徐衍明


    Peridotites of the Dongdegou are located at the northern of the orogenic belt between the Tarim plate and the Yili-central Tianshan plate, southwestern Tianshan, China, and are generally metasomatized by hydrous melt or fluid. The main minerals in the rocks are olivine, spinel, orthopyroxene, amphibole, phlogopite and diopside. Studies suggested that compositions of olivine and orthopyroxene are similar to those from mantle wedge peridotite, and the primary chromian spinel carries characteristics of mantle wedge above subduction zone peridotites, whiles secondary green spinels form later in granulite/amphibolite facies. Comprehensive analyses of petrography, mineralogy and mineral assemblage provide insight into a very complicated evolutional history of the peridotites from the Dongdegou. In generally, after the formation of peridotites, they experienced early mantle metasomatism, retrogressive metamorphism of granulite/amphibolite facies and finally followed by an episode of serpentinization. The peridotites from the Dongdegou were considered formed at mantle environment at temperature of 915 ~ 961 ℃ , and the mantle metasomatism may be under PT conditions of 770 ~900℃ , 10 ~ 18kbar. The features of the peridotite, such as lower TiO2 in both whole rock composition and in spinel minerals, and mineral assemblages, imply that the rocks has been cooled effectively by fluid released from the subducted slab, and are most likely to be the fragments of peridotite from SSZ volcanic arc environment.%中国西南天山东德橄榄岩出露于天山伊犁-中天山板块和塔里木板块碰撞造山带北缘,普遍经历了复杂多期的含水流体(熔体)交代作用.主要矿物为橄榄石、斜方辉石、尖晶石、角闪石、金云母和单斜辉石,研究发现,橄榄石和斜方辉石与地幔楔橄榄岩中橄榄石和斜方辉石成份相近,原生铬尖晶石具有SSZ环境橄榄岩的特征,次生绿色尖晶石形成于麻粒岩相-角闪相

  3. Evidence for recycled Archaean oceanic mantle lithosphere in the Azores plume. (United States)

    Schaefer, Bruce F; Turner, Simon; Parkinson, Ian; Rogers, Nick; Hawkesworth, Chris


    The compositional differences between mid-ocean-ridge and ocean-island basalts place important constraints on the form of mantle convection. Also, it is thought that the scale and nature of heterogeneities within plumes and the degree to which heterogeneous material endures within the mantle might be reflected in spatial variations of basalt composition observed at the Earth's surface. Here we report osmium isotope data on lavas from a transect across the Azores archipelago which vary in a symmetrical pattern across what is thought to be a mantle plume. Many of the lavas from the centre of the plume have lower 187Os/188Os ratios than most ocean-island basalts and some extend to subchondritic 187Os/188Os ratios-lower than any yet reported from ocean-island basalts. These low ratios require derivation from a depleted, harzburgitic mantle, consistent with the low-iron signature of the Azores plume. Rhenium-depletion model ages extend to 2.5 Gyr, and we infer that the osmium isotope signature is unlikely to be derived from Iberian subcontinental lithospheric mantle. Instead, we interpret the osmium isotope signature as having a deep origin and infer that it may be recycled, Archaean oceanic mantle lithosphere that has delaminated from its overlying oceanic crust. If correct, our data provide evidence for deep mantle subduction and storage of oceanic mantle lithosphere during the Archaean era.

  4. Crystal chemistry of amphiboles: implications for oxygen fugacity and water activity in lithospheric mantle beneath Victoria Land, Antarctica (United States)

    Bonadiman, C.; Nazzareni, S.; Coltorti, M.; Comodi, P.; Giuli, G.; Faccini, B.


    Amphibole is the hydrous metasomatic phase in spinel-bearing mantle xenoliths from Baker Rocks, Northern Victoria Land, Antarctica. It occurs in veins or in disseminated form in spinel lherzolites. Both types derive from reaction between metasomatic melts and the pristine paragenesis of the continental lithospheric mantle beneath Northern Victoria Land. To determine the effective role of water circulation during the metasomatic process and amphibole formation, six amphibole samples were fully characterized. Accurate determination of the site population and the state of dehydrogenation in each of these amphiboles was carried out using single-crystal X-ray diffraction, electron microprobe and secondary ion mass spectroscopy on the same single crystal. The Fe3+/ΣFe ratio was determined by X-ray absorption near edge spectroscopy on amphibole powder. The degree of dehydrogenation determined by SIMS is 0.870-0.994 O3(O2-) a.p.f.u., primary and ascribed to the Ti-oxy component of the amphibole, as indicated by atom site populations; post-crystallization H loss is negligible. Estimates of aH2O (0.014-0.054) were determined from the dehydration equilibrium among end-member components assuming that amphiboles are in equilibrium with the anhydrous peridotitic phases. A difference up to 58 % in determination of aH2O can be introduced if the chemical formula of the amphiboles is calculated based on 23 O a.p.f.u. without knowing the effective amount of dehydrogenation. The oxygen fugacity of the Baker Rocks amphibole-bearing mantle xenoliths calculated based upon the dissociation constant of water (by oxy-amphibole equilibrium) is between -2.52 and -1.32 log units below the fayalite-magnetite-quartz (FMQ) buffer. These results are systematically lower and in a narrow range of values relative to those obtained from anhydrous olivine-orthopyroxene-spinel equilibria ( fO2 between -1.98 and -0.30 log units). A comparative evaluation of the two methods suggests that when amphibole

  5. Experimental study of diamond resorption during mantle metasomatism (United States)

    Fedorchuk, Yana; Schmidt, Max W.; Liebske, Christian


    and CO2 diamond crystals develop ditrigonal outline of {111} faces, striation or hillocks along the edges, and shallow negatively oriented trigonal etch pits with flat or pointed bottom. Presence of SiO2 in the fluid resulted in multi-corner morphology, layering and / or more intensive etching of {111} faces. Dissolution in carbonate melt in the absence of water produced deep stepped-wall hexagonal and trigonal etch pits with negative orientation. The three resorption styles show strong resemblance to the secondary morphology of natural diamonds. This suggests that CHO fluid, aqueous silicate melt, and carbonatatitic melt are metasomatic agents reacting with natural diamonds in subcratonic mantle. The comparison of the experimentally-induced resorption styles to those on natural diamonds show that mantle metasomatism induced by CHO fluid develop morphologies similar to those developed in kimberlite magma. This indicates that significant proportion of kimberlite-hosted diamonds may show resorption features of mantle origin. Aqueous silicate melt induces step-faced multi-corner resorption morphology, similar to natural diamonds with nitrogen aggregation corresponding to eclogitic diamonds. Dissolution in carbonatitic melt results in complex morphologies with deep hexagonal pits similar to natural diamonds with nitrogen data corresponding to peridotitic diamonds.

  6. The Oxidation State of Terrestrial Basalts and its Link with the Mantle (United States)

    Mallmann, G.; O'Neill, H. S.; Berry, A. J.; Norman, M. D.; Eggins, S. M.; Kamenetsky, V.; Turner, S.; Smith, I. E.; Ballhaus, C.


    The prevailing paradigm is that the Earth's mantle is both laterally and vertically heterogeneous in regards to its oxidation state. This view has been motivated by the observation that, on average, primitive island arc basalts (IAB) preserve Fe3+/Fe2+ higher than ocean island (OIB) and, particularly, mid-ocean ridge basalts (MORB), and reinforced by the higher oxygen fugacities (fO2) determined in lithospheric (mantle wedge) arc peridotites. fO2 measurements in peridotites equilibrated over a range of pressures have also led to the notion that the mantle becomes more reduced with depth. V and Sc behave very similarly during partial melting of the mantle, but while V is redox-sensitive Sc is not. Their ratio in basalts has therefore a memory of the redox conditions during partial melting. Within the many assumptions involved in forward trace-element modeling, the bulk-rock V/Sc of MORBs, OIBs and IABs indicate that the partial melting events responsible for their genesis occurred at a relatively narrow range of fO2s between QFM and QFM-1. V olivine-liquid partition coefficients are also sensitive to oxidation state (normalization to Sc is useful to minimize the effect of variables other than fO2), and the values determined between olivine phenocrysts (Fo76-90) and quenched basaltic melts suggest that, at the time of olivine crystallization, terrestrial basalts have already oxidized about 1 log fO2 unit (IABs even more so, approximately 2 log fO2 units). The results reveal no statistically significant distinction between the oxidation states of MORBs and OIBs. This has been confirmed by Fe3+/Fe2+ determined by XANES.

  7. Crust-Mantle Interaction in Dabie Orogenic Belt, Central China: Geochemical Evidence from Late Cretaceous Basalts

    Institute of Scientific and Technical Information of China (English)

    匡少平; 张本仁


    It has been suggested that eclogites in the Dabie orogenic belt are exhumation prod-ucts, which had subducted into the deep-seated mantle and undergone ultra-high pressure meta-morphism during the Triassic. But no direct evidence supports this process except the calculatedp-T conditions from mineral thermobarometers. The Late Cretaceous basalts studied in the pres-ent paper, however, have provided some geochemical evidence for crust-mantle interaction inthe area. These basalts are distributed in Mesozoic faulted basins in central and southern Dabieorogenic belt. Since little obvious contamination from continental crust and differentiation-crys-tallization were observed, it is suggested, based on a study of trace elements, that the basaltsare alkaline and resultant from batch partial melting of the regional mantle rocks, and share thesame or similar geochemical features with respect to their magma source. In the spider diagramnormalized by the primitive mantle, trace element geochemistry data show that their mantlesources are enriched in certain elements concentrated in the continental crust, such as Pb, K,Rb and Ba, and slightly depleted in some HFSE such as Hf, P and Nb. Pb-Sr-Nd isotopic com-positions further suggest the mantle is the mixture of depleted mantle (DM) and enriched one( EMI + EMII). This interaction can.explain the trace element characteristics of basaltic mag-mas, i.e. , the enrichment of Pb and the depletion of Hr, P and Nb in basalts can be interpre-ted by the blending of the eclogites in DOB (enriched in Pb and depleted in Hf, P and Nd)with the East China depleted mantle (As compared to the primitive mantle, it is neither en-riched in Pb nor depleted in Hf, P and Nb). It is also indicated that the eclogites in the Dabieorogenic belt were surely derived from the exhumation materials, which had delaminated into thedeep-seated mantle. Moreover, the process subsequently resulted in compositional variation ofthe mantle (especially in trace elements

  8. Newania carbonatites, Western India: example of mantle derived magnesium carbonatites (United States)

    Doroshkevich, Anna Gennad'evna; Ripp, German; Viladkar, Shrinivas


    The key mineralogical features of the Newania carbonatites, that illustrate their derivation from primary mantle melts (Gruau et al. Terra Nova, Abstract Suppl 1:336, 1995; Viladkar Petrology 6(3):272-283, 1998; Basu and Murty Abstracts of Goldschmidt Conference A40, 2006), are the presence of magnesite, graphite and Cr-rich magnetite. Magnesite is an early crystallizing phase. Cr-rich magnetite and graphite coexist with carbonatite minerals and precipitated from carbonate magma. Graphite, as well as gaseous CO2 and carbonate minerals such as dolomite and magnesite, can be stable in peridotite mantle. Coexistence of these minerals is controlled by fO 2 and PT-conditions. Mineral geothermometers for the Newania carbonatite give temperatures from 463 to 950°C. The parental source for Newania carbonatites was characterized by a relatively high log ( fHF/ fH2O) level which increased during the crystallization history of Newania. The estimated oxygen fugacity (for ilmenite-magnetite pairs) varies from -1.5 to +3.5 (log-bar unit deviation from FMQ buffer), which is supported by the presence of Fe-columbite, and the composition of phlogopite, amphibole and pyroxene that have an elevated concentration of Fe3+. However, the oxygen fugacity range represented by co-existing early-crystallized graphite and magnesite is below that of the FMQ buffer and lies on the CCO buffer.

  9. Intrinsic Depletion or Not

    DEFF Research Database (Denmark)

    Klösgen, Beate; Bruun, Sara; Hansen, Søren;

      The presence of a depletion layer of water along extended hydrophobic interfaces, and a possibly related formation of nanobubbles, is an ongoing discussion. The phenomenon was initially reported when we, years ago, chose thick films (~300-400Å) of polystyrene as cushions between a crystalline...... giving rise to depletion layers, and the mechanisms and border conditions that control their presence and extension require still clarification. Recently, careful systematic reflectivity experiments were re-done on the same system. No depletion layers were found, and it was conjectured that the whole...

  10. The Deep Mantle Volatile Cycle Revealed in Superdeep Diamonds and their Mineral Inclusions (United States)

    Walter, Michael; Thomson, Andrew; Frost, Jennifer; Bulanova, Galina; Smith, Chris; Kohn, Simon; Burnham, Antony


    Diamonds crystallize in the mantle primarily as a consequence of fluid or melt metasomatism. In doing so they sample the fluid-melt-solid equilibria directly by incorporation of carbon and its isotopic flavours, and by entrapping other phases as they grow. Superdeep diamonds from the transition zone and lower mantle provide evidence for crystallization from melts derived from subducted materials [1, 2]. The presence of deeply subducted volatile components such as carbon and water are important because they lower the solidus of subducted materials. The source of carbon may ultimately be via deposition of biogenic or abiogenic carbon in subducted crust, and water may become available via dehydration of high-pressure hydrous phases in the slab (e.g. superhydrous B, Phase D) [3]. Foundering of slabs around 700 km due to density inversion and thermalization with surrounding mantle leads to the generation of low-degree, volatile-charged melts. Melts from subducted oceanic crust may be carbonated, and diamond crystallization occurs as a consequence of 'redox freezing' when the oxidized slab melts react with reducing mantle rocks [4]. Reaction of slab melts with mantle peridotite may precipitate phases such as Ca-perovskite, Mg-perovskite, majorite and ferropericlase. Here we will survey evidence from the chemistry of superdeep mineral inclusions for a record of this deep mantle reactive transport process, and speculate on the role of deep mantle volatiles. 1. Bulanova, G.P., et al., Contributions to Mineralogy and Petrology, 2010. 160: p. 489-510. 2. Walter, M.J., et al., Nature, 2008. 454: p. 622-U30. 3. Harte, B., Mineralogical Magazine, 2010. 74: p. 189-215. 4. Rohrbach, A. and M.W. Schmidt, Nature, 2011. 472: p. 209-212.

  11. Addressing Ozone Layer Depletion (United States)

    Access information on EPA's efforts to address ozone layer depletion through regulations, collaborations with stakeholders, international treaties, partnerships with the private sector, and enforcement actions under Title VI of the Clean Air Act.

  12. Tracking the evolution of mantle sources with incompatible element ratios in stagnant-lid and plate-tectonic planets (United States)

    Condie, Kent C.; Shearer, Charles K.


    The distribution of high field strength incompatible element ratios Zr/Nb, Nb/Th, Th/Yb and Nb/Yb in terrestrial oceanic basalts prior to 2.7 Ga suggests the absence or near-absence of an enriched mantle reservoir. Instead, most oceanic basalts reflect a variably depleted mantle source similar in composition to primitive mantle. In contrast, basalts from hydrated mantle sources (like those associated with subduction) exist from 4 Ga onwards. The gradual appearance of enriched mantle between 2 and 3 Ga may reflect the onset and propagation of plate tectonics around the globe. Prior to 3 Ga, Earth may have been in a stagnant-lid regime with most basaltic magmas coming from a rather uniform, variably depleted mantle source or from a non-subduction hydrated mantle source. It was not until the extraction of continental crust and accompanying propagation of plate tectonics that ;modern type; enriched and depleted mantle reservoirs developed. Consistent with the absence of plate tectonics on the Moon is the near absence of basalts derived from depleted (DM) and enriched (EM) mantle reservoirs as defined by the four incompatible element ratios of this study. An exception are Apollo 17 basalts, which may come from a mixed source with a composition similar to primitive mantle as one end member and a high-Nb component as the other end member. With exception of Th, which requires selective enrichment in at least parts of the martian mantle, most martian meteorites can be derived from sources similar to terrestrial primitive mantle or by mixing of enriched and depleted mantle end members produced during magma ocean crystallization. Earth, Mars and the Moon exhibit three very different planetary evolution paths. The mantle source regions for Mars and the Moon are ancient and have HFS element signatures of magma ocean crystallization well-preserved, and differences in these signatures reflect magma ocean crystallization under two distinct pressure regimes. In contrast, plate

  13. Using nanogranitoids and phase equilibria modeling to unravel anatexis in the crustal footwall of the Ronda peridotites (Betic Cordillera, S Spain) (United States)

    Bartoli, Omar; Acosta-Vigil, Antonio; Tajčmanová, Lucie; Cesare, Bernardo; Bodnar, Robert J.


    Anatexis in the crustal footwall of Ronda peridotites (Betic Cordillera, S Spain) is apparently related to the hot emplacement of this mantle slab over metasedimentary rocks. In this study, we combine the analysis of melt inclusions (MI) and phase equilibria calculations on quartzo-feldspathic mylonites (former migmatites) occurring at the contact with the mantle rocks, in the region of Sierra Alpujata (Ojén unit). The goal is to better characterize anatexis in these rocks and to provide new constraints on the geodynamic evolution of the crustal footwall. Such data are important for understanding the mechanisms of crustal emplacement of the mantle slice. The quartzo-feldspathic mylonites are characterized by the mineral assemblage Qtz + Pl + Kfs + Sil + Grt + Ilm + Bt ± Ap ± Gr. Clusters of MI are observed both at the core and toward the rim of peritectic garnet. In each cluster, MI range from totally glassy to nanogranitoids, consisting of Qtz + Kfs + Bt + Ms + Pl aggregates. The trapped melt is leucogranitic and peraluminous with variable Na2O/K2O values and low H2O contents (≈ 2-4 wt%). Phase equilibria modeling in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2-TiO2-O2-C (MnNCaKFMASHTOC) system with graphite-saturated fluid constrains the P-T conditions of melting at ≈ 6 kbar, ≈ 820 °C. MI data support the fluid-absent character of melting. The investigated MI represent the primary anatectic melts produced during prograde anatexis of the host rocks via biotite dehydration melting. Field, compositional, and textural observations indicate that mylonitic migmatites represent strongly deformed former diatexites. The comparison between the new data and some recently published information on migmatites located further from the contact with the peridotites and toward the bottom of the crustal footwall, raises some important issues which question the previously proposed geodynamic models for this region. Among them, (i) the crustal footwall at Sierra Alpujata

  14. 186Os and 187Os enrichments and high-3He/4He sources in the Earth's mantle

    DEFF Research Database (Denmark)

    Brandon, A.D.; Graham, D.W.; Waight, Tod Earle


    . These Os isotope systematics are best explained by ancient recycled crust or melt enrichment in the mantle source region. If so, then the coupled enrichments displayed in 186Os/188Os and 187Os/188Os from lavas of other plume systems must result from an independent process, the most viable candidate...... picrites is best modeled as mixtures of 1 Ga or older ancient recycled crust mixed with primitive mantle or incompletely degassed depleted mantle isolated since 1-1.5 Ga, which preserves the high 3He/4He of the depleted mantle at the time. These mixtures create a hybrid source region that subsequently...... be interpreted as an increase in the proportion of ancient recycled crust in the upwelling plume over this time period. The positive correlation between 187Os/188Os and 3He/4He demonstrates that the Iceland lava He isotopic compositions do not result from simple melt depletion histories and consequent removal...

  15. Geochemical Constraints for Mechanisms of Planetary Differentiation and Volatile Depletion


    Dhaliwal, Jasmeet Kaur


    The evolution of the terrestrial planets involved a range of complex processes, including accretion, core formation, post-core formation accretion, mantle differentiation and volatile depletion. The earliest processes of accretion and core formation have largely been overprinted on Earth and Mars, but can be investigated using geochemical measurements of extraterrestrial materials. Highly siderophile elements (HSE; Os, Ir, Ru, Rh, Pt, Pd, Re, Au) preferentially partition into metal phases an...

  16. Petrological processes in mantle plume heads: Evidence from study of mantle xenoliths in the late Cenozoic alkali Fe-Ti basalts in Western Syria (United States)

    Sharkov, Evgenii


    It is consensus now that within-plate magmatism is considered with ascending of mantle plumes and adiabatic melting of their head. At the same time composition of the plumes' matter and conditions of its adiabatic melting are unclear yet. The major source of objective information about it can be mantle xenoliths in alkali basalts and basanites which represent fragments of material of the plume heads above magma-generation zone. They are not represent material in melting zone, however, carry important information about material of modern mantle plumes, its phase composition and components, involved in melting. Populations of mantle xenoliths in basalts are characterized by surprising sameness in the world and represented by two major types: (1) dominated rocks of ``green'' series, and (2) more rare rocks of ``black'' series, which formed veins in the ``green'' series matrix. It can evidence about common composition of plume material in global scale. In other words, the both series of xenoliths represent two types of material of thermochemical mantle plumes, ascended from core-mantle boundary (Maruyama, 1994; Dobretsov et al., 2001). The same types of xenoliths are found in basalts and basanites of Western Syria (Sharkov et al., 1996). Rocks of ``green'' series are represented by Sp peridotites with cataclastic and protogranular structures and vary in composition from dominated spinel lherzolites to spinel harzburgites and rare spinel pyroxenites (websterites). It is probably evidence about incomplete homogenizing of the plume head matter, where material, underwent by partial melting, adjoins with more fertile material. Such heterogeneity was survived due to quick cooling of upper rim of the plume head in contact with relatively cold lithosphere. Essential role among xenoliths of the ``black'' series play Al-Ti-augite and water-bearing phases like hornblende (kaersutute) and Ti-phlogopite. Rocks of this series are represented by wehrlite, clinopyroxenite, amphibole

  17. Storage and recycling of water in the Earth's mantle (United States)

    Bolfan-Casanova, N.


    Most natural samples originating from the mantle contain traces of water. It can be observed that water content varies laterally as a function of the geodynamic context, but also with depth in cratons. Basalts from mid-ocean ridges, which sample the convecting upper mantle, contain generally below 0.6 wt% H2O leading to 50-330 parts per million by weight in the source. Oceanic Islands Basalts are more hydrated with contents ranging from 0.6 to 1.1 wt%, leading to 350-1100 ppm wt H2O in the source. Arc basalts are even more hydrated with water contents ranging from 0.2 to 5-6 wt% H2O testifying of the recycling of water by subduction. Kimberlite magmas are also the proof that local saturation in volatiles is possible. Among xenoliths, the samples from cratons are very interesting because they may provide a depth profile of water. However, the variation of water content in olivine with depth differs from craton to craton, and is the result of a complex geological history. Also, olivine inclusions in diamond and olivine from peridotite xenoliths do not give the same message regarding to water activity. The water storage capacity of the mantle is defined as the maximum water or hydroxyl that can be incorporated in its constitutive minerals before a free fluid phase appears. It can be determined experimentally and confronted to geophysical observations, such as low seismic velocities, and electrical conductivity. In this talk we will review our current knowledge of water incorporation in NAMs as determined experimentally and compare it with available observations. New data concerning clinopyroxenes will be shown. The aim being to understand the deep water cycle.

  18. Experiments on melt-rock reaction in the shallow mantle wedge (United States)

    Mitchell, Alexandra L.; Grove, Timothy L.


    This experimental study simulates the interaction of hotter, deeper hydrous mantle melts with shallower, cooler depleted mantle, a process that is expected to occur in the upper part of the mantle wedge. Hydrous reaction experiments ( 6 wt% H2O in the melt) were conducted on three different ratios of a 1.6 GPa mantle melt and an overlying 1.2 GPa harzburgite from 1060 to 1260 °C. Reaction coefficients were calculated for each experiment to determine the effect of temperature and starting bulk composition on final melt compositions and crystallizing assemblages. The experiments used to construct the melt-wall rock model closely approached equilibrium and experienced phase equilibria, melt compositions, and reaction coefficients provide a framework for understanding how melt-wall rock reaction occurs in the natural system during melt ascent in the mantle wedge.

  19. U-Pb isotopic geochemistry of the post-collisional mafic-ultramafic rocks from the Dabie Mountains--Crust-mantle interaction and LOMU component

    Institute of Scientific and Technical Information of China (English)

    黄方; 李曙光; 周红英; 李惠民


    The U-Pb isotope geochemical study of the pyroxenite-gabbro intrusion in the Dabie Mountains shows that the post-collisional mafic-ultramafic rocks of the Dabie Mountains are characterized by relative high Pb contents, low U contents and low U/Pb ratios. These characters may be results of interaction between lithosphere or depleted asthenospheric mantle (DMM) and lower crust, but have nothing to do with mantle plume and subducted continental crust. It was first observed that some samples with lower 206Pb/204Pb and higher 207Pb/204Pb ratios show typical characters of the LOMU component. The Pb, Sr, and Nd isotopic tracing shows that three components are needed in the source of the Zhujiapu pyroxenite-gabbro intrusion. They could be old enriched sub-continental lithospheric mantle (LOMU component), lower crust and depleted asthenospheric mantle. The crust-mantle interaction process producing primitive magma of post-collisional mafic-ultramafic rocks in the Dabie Mountains could be described by a lithospheric delamination and magma underplating model. After continent-continent collision, delamination of the thickened lithosphere induced the upwelling of depleted asthenospheric mantle, which caused partial melting of asthenospheric mantle and residual sub-continental lithospheric mantle. The basaltic magma produced in this process underplated in the boundary between the crust and mantle and interacted with lower crust resulting in the geochemical characters of both enriched lithospheric mantle and lower crust.

  20. Geochemical, Petrographic and Magnetic Characteristics of Spinel Lherzolite Mantle Xenoliths from Jabal Remah Volcano, Jordan

    Directory of Open Access Journals (Sweden)

    Ahmad Al-Malabeh


    Full Text Available Peridotite ultramafic mantle xenoliths are occurring abundantly in the Harrat Al-Shaam basaltic province. Jabal Remah volcano is located in the Jordanian parts of the plateau and contains considerable amounts of mantle xenoliths within its pyroclastic successions. Mineralogical investigations show that the xenoliths are mostly of the spinel lherzolite type, which are characterized by a protogranular texture. Modally, the xenoliths are composed of olivine (55-65%, orthopyroxene (12-16%, clinopyroxene (10-15% and spinel (~ 5%. The xenoliths are classified as Type I and belong to the Cr-diopside group. They are characterized by a high content of MgO (42.1-43.2%, Cr (2465-2538 ppm and Ni (2196-2301 ppm. Three selected mantle-xenolith samples were analyzed for their magnetic characteristics. They were found to behave similarly in their ferrimagnetic phase, as indicated by their narrow hysteresis curves and because they never reach saturation even at high applied field. They exhibit ferrimagnetic hystertesis curves similar to world known mantle xenoliths, which is due to the presence of Fe in the form of oxides.

  1. Origin of alkali rocks in the light of data on the fluid mantle flow

    Energy Technology Data Exchange (ETDEWEB)

    Kushev, V.G.; Mironov, A.G.


    The hypotheses on the alkali-basic melts as formed through partial fusion of either the hydroxyl-containing alkali peridotite (or pyroxenite), or the phlogopite- or amphibole-enriched rocks in the mantle turn out most relevant as for the alkali rock peculiarities. Still, the hypotheses do not explain the appearance of alkalies and water involved in fusion. It is supposed that the alkalies-enriched rocks result from the dynamometamorphic segregation of the mantle fluid (CH/sub 4/, H/sub 2/, CO), which reacts with the mantle rocks (magnetite+olivine+pyroxene) with subsequent release of H/sub 2/O and liberation of K and Na from the gaseous phase and pyroxenes lattice. The start of fusion is induced by sharp decrease of pressure and heat incoming along the deep dislocation zones in the mantle. Once produced, the alkali melt evolved through differetiation or interaction with enclosing rocks. This method provides interpretation of most petrological and geochemical peculiarities of different series of alkali rocks. 65 references.

  2. Experimental Constraints on Mantle Heterogeneity and Mantle-Melt Equilibration Depths along the Volcanic Front of the Trans-Mexican Volcanic Belt (United States)

    Weaver, S.; Wallace, P. J.; Johnston, A.


    petrologic evidence to suggest that a large proportion of primitive magmas that initially form in the deep portion of the wedge subsequently equilibrate in the shallow mantle. Experimentally constrained equilibration pressures are consistent with the interpretation that primitive magmas tend to equilibrate in the upper portion of the mantle wedge at depths shallower than those of the predicted peak wedge temperature. Advective heating of the upper mantle by upwelling magmas is required by our results, and this process may cause melting of non-peridotite lithologies such as metasomatized, pyroxene-rich veins. It is apparent that the upper mantle between the so-called "hot nose" of the wedge and the overlying crust is a key component in understanding the geochemical complexities of subduction zone magmas.

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


    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 different among the different samples with one Libyan group (LB I) showing Tc in the range 490-640 °C and the other 680-950 °C (LB II). Cameroon and 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

  4. Slab-derived metasomatism in the Carpathian-Pannonian mantle revealed by investigations of mantle xenoliths from the Bakony-Balaton Highland Volcanic Field (United States)

    Créon, Laura; Delpech, Guillaume; Rouchon, Virgile; Guyot, François


    A suite of fifteen peridotite xenoliths from the Bakony-Balaton Highland Volcanic Field (BBHVF, Pannonian Basin, Central Europe) that show abundant petrographic evidence of fluid and melt percolation were studied in order to decipher the formation of their melt pockets and veins. The suite mainly consists of ;fertile; lherzolites (5.8-19.9 vol.% clinopyroxene) and a few harzburgites (1.9-5.4 vol.% clinopyroxene) from well-known localities (Szentbékkálla, Szigliget) and two previously unreported localities (Füzes-tó and Mindszentkálla). Major and trace element data indicate that most of the peridotites record variable degrees of partial melt extraction, up to > 15% for the harzburgites. Subsequently, the xenoliths experienced at least two stages of metasomatic modification. The first stage was associated with percolation of a volatile-bearing silicate melt and resulted in crystallization of amphibole, enrichment in the most incompatible trace elements (Ba, Th, U, Sr), and development of negative Nb-Ta anomalies in clinopyroxene. The second and last metasomatic event, widespread beneath the BBHVF, is associated with the formation of silicate melt pockets, physically connected to a network of melt veins, with large and abundant CO2 vesicles. The glass in these veins has sub-alkaline trachy-andesitic composition and displays an OIB-like trace element signature. Its composition attests to the migration through a supra-subduction zone mantle wedge of silicic melt highly enriched in volatiles (CO2, H2O, Cl, F), LILE, REE and HFSE and consistent with compositions of natural and experimental examples of slab melting-derived magma. In the present case, however, melt was likely derived from melting of oceanic crust and carbonated sediments under conditions where Nb-rich mineral phases were not stable in the residue. A likely scenario for the origin such melts involves melting after subduction ceased as the slab thermally equilibrated with the asthenosphere. Melt

  5. Coupled Petrological and Geodynamic Models of Mantle Flow in Subduction Zones; the Importance of Chlorite in the Emergence of a Low-Viscosity Channel (United States)

    Smith, P. M.; Baker, L. J.; Asimow, P. D.; Gurnis, M. C.


    Seismic velocity and attenuation studies have shown that 5-20 km thick low velocity layers exist above seismically fast slabs and are associated with broad zones of high attenuation in many subduction zones. These observations are generally interpreted as formation of hydrous phases by dehydration of the slab, although the impact of water in nominally anhydrous minerals (NAM) on seismic wave propagation is largely unknown. Recent petrological experiments on hydrous peridotite at subduction zone conditions suggest that chlorite will be stable adjacent to the subducting slab in sufficient quantities to be a significant water sink. We use a scheme that couples a petrological model (pHMELTS) with a 2-D thermal and variable viscosity flow model (ConMan) to model energy and mass transfer within a subduction zone. By varying input parameters including the convergence rate and slab dip we have developed models for cases in the Costa-Rica and Izu- Bonin-Marianas arc systems and are able to predict major and trace element compositions of primary melts, as well as geophysical observables, such as the topography and geoid. We find that the emergence of a slab- adjacent low-viscosity channel (LVC) is a natural consequence of the thermal and chemical controls on mantle dynamics and feedback between them. In our earlier models, as the LVC is dragged downwards by the subducting slab, hornblende breaks down at about 2.5 GPa and other hydrous phases such as serpentine are secondary in importance to the NAM water reservoir. The spatial limit of the LVC is the water-saturated solidus of the hydrated peridotite; the LVC thickens as the peridotite is progressively depleted by melting and the solidus migrates into the warmer wedge, despite water replenishment at depth. pHMELTS is a hybrid of the pMELTS model of Ghiorso and co-workers and includes amphiboles, serpentines and micas. Chlorite was lacking but we have recently rectified this omission. Following De Capitani and co- workers, we

  6. The depth of sub-lithospheric diamond formation and the redistribution of carbon in the deep mantle (United States)

    Beyer, Christopher; Frost, Daniel J.


    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

  7. Thermal and metasomatic rejuvenation and dunitization in lithospheric mantle beneath Central Europe - The Grodziec (SW Poland) case study (United States)

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


    The 32 Ma Grodziec nephelinite (Lower Silesia, SW Poland) contains xenolith of peridotite (mostly lherzolite) and clinopyroxenite/olivine clinopyroxenite composition. The forsterite content in olivine classifies these rocks into three groups: groups A and B consist of peridotites, while group C xenoliths are pyroxenitic cumulates. Group A xenoliths contain olivine Fo 87.90-91.8% and pyroxenes with high Mg# ( 0.91-0.92); clinopyroxene is strongly LREE-enriched (LaN/LuN = 2.19-17.74) and strongly impoverished in Zr, Hf and Ti relative to primitive mantle. The group B xenoliths (dunites and wehrlite) are orthopyroxene-free, olivine and clinopyroxene are less magnesian than those in the A group (Fo = 85.2-87.2%, Mg# = 0.86-0.88), clinopyroxene is less LREE-enriched (LaN/LuN = 4.07-4.15) and only slightly impoverished in Zr, Hf and Ti. Group C xenoliths contain olivine with forsterite content from 78.6 to 86.6% and clinopyroxene of Mg# from 0.84 to 0.85, with LREE/trace element characteristics similar to those of B group (LaN/LuN = 1.96-3.10). Group A xenoliths from Grodziec record migration of mixed carbonatite-alkaline silicate melts through the subcontinental lithospheric mantle beneath Lower Silesia, which preceded the migration of melts similar to the Grodziec nephelinite. The peridotitic protoliths were dunitized at the direct contacts with the migrating nephelinite melt and are now represented by group B. Group C pyroxenites originated in mantle conditions by crystal settling in places of transient nephelinite melt stagnation. The mantle section beneath Grodziec was reheated to ca 1000-1100 °C. The Grodziec scenario is similar to that of Księginki (northern extension of Eger Rift, SW Poland), which shares a similar age of xenolith entrainment. Both sites show that the processes of mantle metasomatism and thermal rejuvenation of subcontinental lithospheric mantle were more intense during the Lower Oligocene volcanic climax compared to those recorded in younger

  8. Low Ni olivine in silica-undersaturated ultrapotassic igneous rocks as evidence for carbonate metasomatism in the mantle (United States)

    Ammannati, Edoardo; Jacob, Dorrit E.; Avanzinelli, Riccardo; Foley, Stephen F.; Conticelli, Sandro


    Subduction drags a large amount of CO2 into the Earth's interior, which is partly returned to the atmosphere by arc volcanism. Processes involved in the recycling of subducted carbon within the upper mantle are mainly related to mineralogical transformation. Subducted CO2 may dramatically affect the equilibria among peridotitic minerals (olivine vs. pyroxenes) changing their stability fields and hence their modal abundances. This process is accompanied by a subduction-induced change in the budget of some incompatible trace and major elements (e.g., K, Ca, HFSE), whereas it has a minimal effect on the mass balance of compatible elements (e.g., Ni). We report trace elements in olivine in subduction-related mafic alkaline ultrapotassic rocks from Italy, which are used as a proxy to define mantle wedge mineralogy and metasomatic processes. Minor element concentrations, and in particular the high Li and low Ti of all the olivines, confirm a major role for recycled sediment in the generation of Italian ultrapotassic magmas. The distinct contents of Ni, Mn, and Ca in olivine reflect the bimodal character of silica-rich and silica-poor ultrapotassic Italian rocks and constrain two distinct mineralogical reactions between metasomatic agents and peridotite. Olivine chemistry from silica-saturated rocks reflects the reaction of silicate melts with the ambient mantle, with consequent consumption of olivine in favour of orthopyroxene. In contrast, the low-Ni, high-Mn/Fe of olivine crystallised from silica-undersaturated leucitites require a mantle source enriched in olivine (and clinopyroxene) compared to orthopyroxene, as a result of the interaction between the ambient peridotitic mantle and CaCO3-rich metasomatic agents. The change from silica-oversaturated lamproites to silica-undersaturated leucitites and thus the difference in the olivine composition is due to a change in composition of the subducting sediment from pelitic to carbonate-rich. The results of this study

  9. Intrinsic Depletion or Not

    DEFF Research Database (Denmark)

    Klösgen, Beate; Bruun, Sara; Hansen, Søren;

    with an AFM (2).    The intuitive explanation for the depletion based on "hydrophobic mismatch" between the obviously hydrophilic bulk phase of water next to the hydrophobic polymer. It would thus be an intrinsic property of all interfaces between non-matching materials. The detailed physical interaction path......  The presence of a depletion layer of water along extended hydrophobic interfaces, and a possibly related formation of nanobubbles, is an ongoing discussion. The phenomenon was initially reported when we, years ago, chose thick films (~300-400Å) of polystyrene as cushions between a crystalline...

  10. Shear-affected depletion interaction

    NARCIS (Netherlands)

    July, C.; Kleshchanok, D.; Lang, P.R.


    We investigate the influence of flow fields on the strength of the depletion interaction caused by disc-shaped depletants. At low mass concentration of discs, it is possible to continuously decrease the depth of the depletion potential by increasing the applied shear rate until the depletion force i

  11. Exsolutions of Diopside and Magnetite in Olivine from Mantle Dunite, Luobusa Ophiolite, Tibet, China

    Institute of Scientific and Technical Information of China (English)

    REN Yufeng; CHEN Fangyuan; YANG Jingsui; GAO Yuanhong


    The exsolutions of diopside and magnetite occur as intergrowth and orient within olivine from the mantle dunite, Luobusa ophiolite, Tibet. The dunite is very fresh with a mineral assemblage of olivine (>95%) + chromite (1%-4%) + diopside (<1%). Two types of olivine are found in thin sections: one (Fo = 94) is coarse-grained, elongated with development of kink bands, wavy extinction and irregular margins; and the other (Fo = 96) is fine-grained and poly-angled. Some of the olivine grains contain minor Ca, Cr and Ni. Besides the exsolutions in olivine, three micron-size inclusions are also discovered. Analyzed through energy dispersive system (EDS) with unitary analytical method, the average compositions of the inclusions are: Na20, 3.12%-3.84%; MgO, 19.51%-23.79%; Al2O3,9.33%-11.31%; SiO2, 44.89%-46.29%; CaO, 11.46%-12.90%; Cr2O3, 0.74%-2.29%; FeO, 4.26%-5.27%, which is quite similar to those of amphibole. Diopside is anhedral filling between olivines, or as micro-inclusions oriented in olivines. Chromite appears euhedral distributed between olivines,sometimes with apparent compositional zone. From core to rim of the chromite, Fe content increases and Cr decreases; and Al and Mg drop greatly on the rim. There is always incomplete magnetite zone around the chromite. Compared with the nodular chromite in the same section, the euhedral chromite has higher Fe3O4 and lower MgCr2O4 and MgAl2O4 end member contents, which means it formed under higher oxygen fugacity environment. With a geothermometer estimation, the equilibrium crystalline temperature is 820℃-960℃ for olivine and nodular chromite, 630℃-770℃ for olivine and euhedral chromite, and 350℃-550℃ for olivine and exsoluted magnetite, showing that the exsolutions occurred late at low temperature. Thus we propose that previously depleted mantle harzburgite reacted with the melt containing Na, Al and Ca, and produced an olivine solid solution added with Na+,Al3+, Ca2+, Fe3+, Cr3+. With temperature

  12. Depleted and metasomatized oceanic lithosphere beneath La Palma, Canary Islands (United States)

    Janisch, Astrid; Ntaflos, Theodoros


    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

  13. Partial melting and the efficiency of mantle outgasing in one-plate planets (United States)

    Plesa, Ana-Catalina; Breuer, Doris


    The generation of partial melting can have a major impact on the thermo-chemical evolution of a terrestrial body by the depletion of the mantle material in incompatible elements such as radioactive elements and volatiles, crust formation and volcanic outgassing. During some period in the thermal history of a terrestrial planet, the temperature in regions of the upper mantle, either below tectonic plates or a stagnant lid, rises above the solidus - the temperature at which the mineral with the lowest melting temperature among those that form the silicate mantle mixture starts to melt. The melt than rises toward the surface, forms the crust, and releases volatiles into the atmosphere. In case of one-plate (stagnant lid) planets the thickness of the present-day crust can 'tell' us already about the efficiency of mantle melting and mantle degassing - the thicker the crust the more mantle material experienced melting and thus the more efficient can be the outgassing. However, it has been shown with parameterized convection models [1] but also 2-3D convection models [2] that crustal delamination is a common process in one-plate planets. Crustal delamination allows that possibly much more crust is produced during the entire evolution (and thus more mantle material experienced differentiation) than what is observed today, implying also more efficient outgassing than expected. Crustal delamination is therefore a process that may help to generate a substantial planetary atmosphere. In the present work we investigate the influence of partial melt on mantle dynamics and the volcanic outgassing of one-plate planets using the mantle convection code GAIA [3] in a 2D cylindrical geometry. We consider the depletion of the mantle, redistribution of radioactive heat sources between mantle and crust, as well as mantle dehydration and volcanic outgassing [4]. When melt is extracted to form the crust, the mantle material left behind is more buoyant than its parent material and depleted

  14. Temporal distribution of mantle-derived potassic rocks and carbonatites linked to stabilization of mantle lithosphere and redox states during subduction (United States)

    Foley, S. F.


    Mantle-derived potassic igneous rocks and carbonatites first appear in the geological record in the late Archean, coinciding with major crust-forming events on most continents. The compositions of potassic rocks require sources including discrete ultramafic rocks with phlogopite and pyroxenes, whereas carbonatites and ultramafic lamprophyres (carbonate-rich potassic rocks) require oxidizing conditions in which carbonate is stable. The presence of these source rocks from this time is probably related to the stabilization of mantle lithosphere. If mantle lithosphere had not been stable for considerable periods of time, then melting would be restricted to peridotite, which is not a viable option for strongly potassic rocks. The phlogopite-rich source-rock assemblages that are necessary precursors for potassic melts could be introduced into the lithosphere by either subduction processes or by multiple stages of low-degree melting. Many modern examples involve subducted sedimentary material, which concentrates potassium by the stabilization of micas in subduction metamorphism. Subduction involves a great variety of redox states, but the bulk effect is the return of oxidized material from the surface into the mantle. However, we cannot apply uniformitarianism unthinkingly, because subduction processes at and before 2.7 Ga may have had different redox states. Before the Great Oxidation Event the distribution and abundances of geological formations such as banded iron formations, red beds, and uraninites indicate that geological reservoirs became gradually oxidized, preventing an earlier increase in atmospheric oxygen. This means that the function of the subduction process to oxidize the upper mantle by the return of oxidized rocks from the surface was much weaker in the early Earth. Early continental mantle lithosphere was, therefore, likely to accumulate carbon in reduced form, which would be more easily remobilized in melts through low-temperature redox melting much

  15. Preliminary Statistics of Temperatures and Pressures for Formation of Eclogites,Granulites and Peridotites in China

    Institute of Scientific and Technical Information of China (English)

    Hu Baoqun; Wang Fangzheng; Sun Zhanxue; Liu Chengdong; Bai Lihong


    The rock-forming temperatures and pressures represent the p-T points of the local regions in the lithosphere at a certain age, providing some important information on rock formation. Based on the preliminary statistics on the temperatures and pressures for the formation of eclogites, granulites and peridotites in China, the variant ranges are given, in this paper, of temperatures, pressures and linear geothermal gradients of eclogites, granulites and peridotites. In addition, since the eclogite is different from granulite and peridotite in the p-T diagram, these three rocks can be classified into two groups: the first group includes eclogites and the second group granulites and peridotites. Then, the p-T correlation functions of these two groups of rocks are provided. Finally, the two groups of rocks have different geothermal gradients at the same pressure gradient or have different pressure gradients at the same geothermal gradient. The temperatures and pressures for the formation of the rocks can be calculated from the mineral chemical compositions, but the depths (H) for the rock formation can be calculated only under the hypotheses of given p-H (or T-H) correlation functions. The explanations for the ultrahigh pressure metamorphism vary obviously with different hypotheses.

  16. Garnet peridotite xenoliths in a Montana, U.S.A., kimberlite (United States)

    Carter, Hearn B.; Boyd, F.R.


    Within a swarm of late middle Eocene subsilicic-alkalic diatremes, one diatreme 270 by 370 m and an associated dike contain common xenoliths of granulite and rare xenoliths of spinel peridotite and garnet peridotite. Six garnet lherzolite xenoliths have been found and these show a range of textures. Four are granular, and two are intensely sheared. Phlogopite is absent from the intensely sheared xenoliths and is thought to be primary in part in the granular xenoliths. Estimated temperatures and depths of equilibration of xenolith pyroxenes range from 920??C, 106 km (32 kbar) to 1315??C, 148 km (47 kbar). The xenoliths show increasing amounts of deformation with greater inferred depths of origin. The temperature-depth points suggest a segment of an Eocene geotherm for Montana which is similar in slope to the steep portion of the pyroxene-determined Lesotho geotherm (Boyd and Nixon, this volume) and is considerably steeper than typical calculated shield and continental geotherms at present. The steep trend could be a result of plate-tectonic shearing and magma ascension within an Eocene low-velocity zone. Preservation of intensely sheared textures requires rapid transport of material from about 150 km depth during active deformation of relatively dry rock. The occurrence of monticellite peridotite in this kimberlite diatreme suggests that magmas which crystallized to monticellite peridotite at relatively shallow depth could be one of the primitive types of kimberlite magma. ?? 1975.

  17. Mineralogy and geochemistry of the Neo-Tethyan Orhaneli ultramafic suite, NW Turkey: Evidence for the initiation and evolution of magmatic processes in a developing crust-mantle boundary (United States)

    Uysal, Ibrahim; Dokuz, Abdurrahman; Kapsiotis, Argyris; Kaliwoda, Melanie; Karsli, Orhan; Müller, Dirk; Aydin, Faruk


    and Al2O3 contents and high Mg#) identical of clinopyroxene in arc-derived peridotites. These magmatically formed dunites are rich in light rare earth elements (LREE) and commonly carry elevated Pt+Pd concentrations (up to 17.92 ppb), especially compared to replacive dunites that are almost deprived of Pt and Pd (up to 3.92 ppb). Further upward movement and differentiation of this melt caused the formation of clinopyroxenites, containing spinel similar in composition to that of cumulate dunites, with elevated Pt+Pd abundances (up to 499.75 ppb) and LREE-depleted multi-element patterns typical of crystallization from a melt with comparable composition between boninite and IAT. Overall data indicate that the studied ultramafic suite represents part of a sub-oceanic Moho transition zone, which preserves mixed mantle and cumulate characteristics.

  18. Evidence for an upper mantle low velocity zone beneath the southern Basin and Range-Colorado Plateau transition zone (United States)

    Benz, H.M.; McCarthy, J.


    A 370-km-long seismic refraction/wide-angle reflection profile recorded during the Pacific to Arizona Crustal Experiment (PACE) detected an upper mantle P-wave low-velocity zone (LVZ) in the depth range 40 to 55 km beneath the Basin and Range in southern Arizona. Interpretation of seismic data places constraints on the sub-crustal lithosphere of the southern Basin and Range Province, which is important in light of the active tectonics of the region and the unknown role of the sub-crustal lithosphere in the development of the western United States. Forward travel time and synthetic seismogram techniques are used to model this shallow upper mantle LVZ. Modeling results show that the LVZ is defined by a 5% velocity decrease relative to a Pn velocity of 7.95 km s−1, suggesting either a ∼3–5% mafic partial melt or high-temperature, sub-solidus peridotite.

  19. The survival of early Earth mantle reservoirs: Evidence from