Sample records for geobarometry
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Elastic geobarometry and the role of brittle failure on pressure release
Mazzucchelli, Mattia Luca; Angel, Ross John; Rustioni, Greta; Milani, Sula; Nimis, Paolo; Chiara Domeneghetti, Maria; Marone, Federica; Harris, Jeff W.; Nestola, Fabrizio; Alvaro, Matteo
2016-04-01
Mineral inclusions trapped in their hosts can provide fundamental information about geological processes. Recent developments in elastic geobarometry, for example, allow the retrieval of encapsulation pressures for host-inclusion pairs. In principle this method can be applied to any mineral-mineral pair so long as both the residual pressure on an inclusion (Pinc), and the equations of state for both host and inclusion are either known or determined (Angel et al., 2015). However, Angel et al. (2014) outlined some boundary conditions, one of which was that deformation in the host-inclusion pair has to be purely elastic. Thus this caveat would exclude from analysis all the inclusions that are surrounded by cracks, indicative of brittle deformation, which may result in partial or complete release of the Pinc. If however the effects of cracks surrounding trapped mineral inclusions could be quantitatively modelled, then the applicability of "elastic" geobarometry might be extended to a much larger number of inclusion-host pairs. We report the results of a pilot experiment in which the stress states (i.e. the residual pressure) have been determined for 10 olivine inclusions still entrapped in 5 diamonds. Inclusion pressures were determined from the unit-cell volumes of the olivines measured in-situ in the diamonds by X-ray diffraction. The olivine equations of state were determined from the olivine compositions by in-situ X-ray structure refinement. Values of Pinc range from 0.19 to 0.53 GPa. In order to quantify the degree of brittle failure surrounding the inclusions, the same set of samples were also investigated by synchrotron X-ray micro-tomography (SRXTM at TOMCAT, Swiss LightSource). Preliminary results showed that at the spatial resolution of our experiments (pixel size of 0.34μm), 90% of the inclusions trapped in our set of diamonds were surrounded by cracks. The volume of the cracks has been determined from 3D reconstruction with an accuracy of about 4%. Our
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A 3D Magnetotelluric Perspective on the Galway Granite, Western Ireland
Farrell, Thomas; Muller, Mark; Vozar, Jan; Feely, Martin; Hogg, Colin
2017-04-01
Magnetotelluric (MT) and audi-magnetotelluric (AMT) data were acquired at 75 locations across the exposed calc-alkaline Caledonian Galway granite batholith and surrounding country rocks into which the granite intruded. The Galway granite is located in western Ireland on the north shore of Galway bay, and has an ESE-WNW long axis. The granite is cut by trans-batholith faults, the Shannawona Fault Zone (SFZ) in the western part of the batholith, which has a NE-SW trend, and the Bearna Fault Zone (BFZ) in the eastern sector that has a NW-SE trend. Geobarometry data indicate that the central granite block between these fault zones has been uplifted, with the interpretation being that the granite in this central block is thinned. To the west of the SFZ, much of the Galway granite is below sea level, with the majority of the southern granite contact also beneath the sea in Galway bay. To the east of the batholith, the Carboniferous successions, consisting of mainly limestone with shale, overlie the basement rocks. The country rock to the north includes the metagabbro-gneiss suite, which itself intruded the deformed Dalradian successions that were deposited on the Laurentian margin of the Iapetus Ocean. The deformation of the Dalradian rocks, the intrusion of the metagabbro-gneiss suite and the intrusion of the Galway granite were major events in the protracted closure of the Iapetus Ocean. It is clear from geological mapping, from geobarometry and from the present submergence by the sea of a large part of the Galway granite, that inversion of MT data in this structurally complex geology is likely to require a 3D approach. We present a summary of 3D inversion of the Galway MT and AMT data. The study shows that the structure of the Galway granite is quite different from the pre-existing perspective. The central block, thought by its uplifting to be thinned, is shown to be the thickest part of the batholith. A geological model of granite intrusion is offered to explain this
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Sato, Y.; Ozawa, K.
2017-12-01
Mantle xenoliths are fragments of mantle materials entrapped in alkali basalts or kimberlites and transported to the surface (Nixon, 1987). They provide information on rheological, thermal, chemical, petrological structures of the upper mantle (e.g. Green et al., 2010; McKenzie and Bickle, 1988; O'Reilly and Griffin, 1996). They potentially represent materials from a boundary zone of lithosphere and asthenosphere (LABZ), where the heat transportation mechanism changes from convection to conduction (Sleep, 2005, 2006). However, difficulties in geobarometry for spinel peridotite (e.g. O'Reilly et al., 1997) have hampered our understanding of shallow LABZ. Ichinomegata located in the back-arc side of NE Japan is a latest Pleistocene andesitic-dacitic volcano yielding spinel peridotite xenoliths (Katsui et al., 1979). Through our works (Sato and Ozawa, 2016, 2017a, 2017b), we have overcome difficulties in geobarometry of spinel peridotites and gained accurate thermal structure (0.74-1.60 GPa, 832-1084 °C) from eight of the nine examined xenoliths. The rheological and chemical features suggest drastic changes: undeformed (granular), depleted, subsolidus mantle representing lithospheric mantle (ca. 28-35 km) and deformed (porphyroclastic), fertile, hydrous supersolidus mantle representing rheological LABZ (ca. 35-54 km). We investigate depth dependent variation of crystallographic preferred orientation (CPO) of constituent minerals of the xenoliths by electron back-scattered diffraction analysis (using JSM-7000F with a CCD detector and the CHANNEL5 software at the University of Tokyo). A shallower (ca. 32 km) sample with tabulargranular texture and coarse olivine size (0.92 mm) has A-type olivine CPO with [100] maximum as reported by Satsukawa and Michibayashi (2014) (hereafter SM14), whereas a deep (ca. 51 km) sample with porphyroclastic texture and finer olivine size (0.46 mm) has CPO with weaker fabric intensity characterized by a [100] girdle similar to AG-type and
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Nimis, Paolo
The crystal structures of 212 experimentally synthesized, igneous clinopyroxenes were modeled from electronprobe chemical data. The coexisting melts span a wide range of petrologically relevant, dry and hydrous compositions, characterized by variable enrichment in silica and alkalis. Experimental conditions pertain to Earth's crust and uppermost mantle (P=0-24kbar garnet absent) and a variety of fO2 values (from CCO-buffered to air-buffered) and mineral assemblages (Cpx+/-Opx+/-Pig+/-Ol+/-Plag+/-Spl +/-Mt+/-Amp+/-Ilm). Unit-cell volume (Vcell) versus M1-polyhedron volume (VM1) relations were investigated over a range of pressures and temperatures using data derived from structure modeling and corrected for thermal expansivity and compressibility. The relationships between pressure and clinopyroxene structural parameters were found to be dependent on the nature of the coexisting melt. To reduce compositional effects, only clinopyroxenes belonging to mildly alkaline (MA) and tholeiitic (TH) series were considered. Pressure was modeled as a linear function of Vcell, VM1, and Mg/(Mg+Fe2+)Cpx ratio. A calibration based on the whole data set (MA+TH) reproduced the experimental pressures within 1.4kbar at the 1-σ level. The maximum residuals were 3.5kbar and 3.9kbar for MA- and TH-clinopyroxenes, respectively. Better statistics were obtained by considering MA- and TH-clinopyroxenes separately. A calibration based on the 69 MA-clinopyroxenes reproduced the experimental pressures within 1.1kbar (1σ) and with a maximum residual of 2.7kbar. A calibration based on the 143 TH-clinopyroxenes reproduced the experimental pressures within 1.0kbar (1σ) and with a maximum residual of 3.4kbar. When these geobarometers are applied to natural samples for which P is unknown, the correction for compressibility is necessarily made through a trial-and-error procedure. This expedient propagates an additional error that increases the above uncertainties and residuals by a factor of about 2. Applications to natural, igneous rocks for which the pressures of crystallization could be constrained based on experimental, petrological or geological evidence yielded pressure estimates that reproduced the expected values to within ca. 2kbar. Compared to the MA-formulation, the TH-formulation appears to be less robust to variations in magma composition. When applied to high-pressure (>10kbar) clinopyroxenes synthesized from very low Na (Na2Opressure of crystallization from a clinopyroxene chemical analysis.
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Turova, Mariia; Plechov, Pavel; Scherbakov, Vasily; Larin, Nikolay
2017-04-01
The Lunar Crater volcanic field is located in a tension zone Basin and Range Province (USA). This tension is connected with dives oceanic plate under the continental plate [1]. Lunar Crater consists of flows basalt, basanite, trachybasalt has a different age [2]. In this work we investigate the youngest rock - basanite. The basanite is highly crystalline consisting of about megacrysts (3-10 cm) 30-60 wt% phenocrysts ( 800-1500 µm) and microphenocrysts (100-800 µm) and 40-60% microlites (Mathematical, Physical and Engineering Sciences. - 1981. - T. 300. - №. 1454. - C. 407-434. 2. Wood, X., and Keinle, Y., 1990, Volcanoes of North America: Cambridge,United Kingdom, Cambridge University Press, 354 p. 3. Nimis P. Clinopyroxene geobarometry of magmatic rocks. Part 2. Structural geobarometers for basic to acid, tholeiitic and mildly alkaline magmatic systems //Contributions to Mineralogy and Petrology. - 1999. - T. 135. - №. 1. - C. 62-74. 4. Ballhaus C., Berry R. F., Green D. H. High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobarometer: implications for the oxidation state of the upper mantle //Contributions to Mineralogy and Petrology. - 1991. - T. 107. - №. 1. - C. 27-40.
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Energy Technology Data Exchange (ETDEWEB)
Beyer, Christopher; Rosenthal, Anja; Myhill, Robert; Crichton, Wilson A.; Yu, Tony; Wang, Yanbin; Frost, Daniel J.
2018-02-01
We have performed an experimental cross calibration of a suite of mineral equilibria within mantle rock bulk compositions that are commonly used in geobarometry to determine the equilibration depths of upper mantle assemblages. Multiple barometers were compared simultaneously in experimental runs, where the pressure was determined using in-situ measurements of the unit cell volumes of MgO, NaCl, Re and h-BN between 3.6 and 10.4 GPa, and 1250 and 1500 °C. The experiments were performed in a large volume press (LVPs) in combination with synchrotron X-ray diffraction. Noble metal capsules drilled with multiple sample chambers were loaded with a range of bulk compositions representative of peridotite, eclogite and pyroxenite lithologies. By this approach, we simultaneously calibrated the geobarometers applicable to different mantle lithologies under identical and well determined pressure and temperature conditions. We identified discrepancies between the calculated and experimental pressures for which we propose simple linear or constant correction factors to some of the previously published barometric equations. As a result, we establish internally-consistent cross-calibrations for a number of garnet-orthopyroxene, garnet-clinopyroxene, Ca-Tschermaks-in-clinopyroxene and majorite geobarometers.
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McCarter, Renee L.; Fodor, R.V.; Trusdell, Frank A.
2006-01-01
Explosive eruptions at Mauna Loa summit ejected coarse-grained blocks (free of lava coatings) from Moku'aweoweo caldera. Most are gabbronorites and gabbros that have 0–26 vol.% olivine and 1–29 vol.% oikocrystic orthopyroxene. Some blocks are ferrogabbros and diorites with micrographic matrices, and diorite veins (≤2 cm) cross-cut some gabbronorites and gabbros. One block is an open-textured dunite.The MgO of the gabbronorites and gabbros ranges ∼ 7–21 wt.%. Those with MgO >10 wt.% have some incompatible-element abundances (Zr, Y, REE; positive Eu anomalies) lower than those in Mauna Loa lavas of comparable MgO; gabbros (MgO <10 wt.%) generally overlap lava compositions. Olivines range Fo83–58, clinopyroxenes have Mg#s ∼83–62, and orthopyroxene Mg#s are 84–63 — all evolved beyond the mineral-Mg#s of Mauna Loa lavas. Plagioclase is An75–50. Ferrogabbro and diorite blocks have ∼ 3–5 wt.% MgO (TiO2 3.2–5.4%; K2O 0.8–1.3%; La 16–27 ppm), and a diorite vein is the most evolved (SiO2 59%, K2O 1.5%, La 38 ppm). They have clinopyroxene Mg#s 67–46, and plagioclase An57–40. The open-textured dunite has olivine ∼ Fo83.5. Seven isotope ratios are 87Sr/86Sr 0.70394–0.70374 and 143Nd/144Nd 0.51293–0.51286, and identify the suite as belonging to the Mauna Loa system.Gabbronorites and gabbros originated in solidification zones of Moku'aweoweo lava lakes where they acquired orthocumulate textures and incompatible-element depletions. These features suggest deeper and slower cooling lakes than the lava lake paradigm, Kilauea Iki, which is basalt and picrite. Clinopyroxene geobarometry suggests crystallization at <1 kbar P. Highly evolved mineral Mg#s, <75, are largely explained by cumulus phases exposed to evolving intercumulus liquids causing compositional ‘shifts.’ Ferrogabbro and diorite represent segregation veins from differentiated intercumulus liquids filter pressed into rigid zones of cooling lakes. Clinopyroxene
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Mantle compositions below petit-spot volcanoes of the NW Pacific Plate
Hirano, N.
2017-12-01
Monogenetic petit-spot volcanoes of a few kilometers in diameter and geothermal gradient than the conventional GDH1 model; Machida et al., 2015; Yamamoto et al., 2014). The fact that the majority of the petit-spot lava samples do not contain olivine phenocrysts and have differentiated compositions (45-52 wt% SiO2, Mg# values of 50-65) indicates that these magmas have undergone differentiation in a magma chamber. However, geobarometry indicates that the deepest-sourced associated peridotitic xenoliths were derived from a depth of 42 km (Yamamoto et al., 2014). This indicates that melt fractionation must have occurred at depths greater than the middle lithosphere, a situation where the depth of fractionation could correlate with the rotation of the σ3 stress axis from the extensionally lower to the compressional upper part of the lithosphere. This rotation is the result of concave flexure prior to the outer rise of the subduction zone (Valentine & Hirano, 2010). Pilet et al. (2016) and Yamamoto et al. (2009) reported that these xenoliths were derived from a metasomatized region of the mantle, with this region metasomatized by prior melts of petit-spot magmas in the province. The strategic analysis of xenocrystic olivines from several petit-spot volcanoes also indicates that more depleted compositions are located in areas more proximal to the trench. This indicates that the lithospheric mantle in this region must have been significantly metasomatized prior to the onset of trench subduction.
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Cognate xenoliths in Mt. Etna lavas: witnesses of the high-velocity body beneath the volcano
Corsaro, Rosa Anna; Rotolo, Silvio Giuseppe; Cocina, Ornella; Tumbarello, Gianvito
2014-01-01
Various xenoliths have been found in lavas of the 1763 ("La Montagnola"), 2001, and 2002-03 eruptions at Mt. Etna whose petrographic evidence and mineral chemistry exclude a mantle origin and clearly point to a cognate nature. Consequently, cognate xenoliths might represent a proxy to infer the nature of the high-velocity body (HVB) imaged beneath the volcano by seismic tomography. Petrography allows us to group the cognate xenoliths as follows: i) gabbros with amphibole and amphibole-bearing mela-gabbros, ii) olivine-bearing leuco-gabbros, iii) leuco-gabbros with amphibole, and iv) Plg-rich leuco gabbros. Geobarometry estimates the crystallization pressure of the cognate xenoliths between 1.9 and 4.1 kbar. The bulk density of the cognate xenoliths varies from 2.6 to 3.0 g/cm3. P wave velocities (V P ), calculated in relation to xenolith density, range from 4.9 to 6.1 km/s. The integration of mineralogical, compositional, geobarometric data, and density-dependent V P with recent literature data on 3D V P seismic tomography enabled us to formulate the first hypothesis about the nature of the HVB which, in the depth range of 3-13 km b.s.l., is likely made of intrusive gabbroic rocks. These are believed to have formed at the "solidification front", a marginal zone that encompasses a deep region (>5 km b.s.l.) of Mt. Etna's plumbing system, within which magma crystallization takes place. The intrusive rocks were afterwards fragmented and transported as cognate xenoliths by the volatile-rich and fast-ascending magmas of the 1763 "La Montagnola", 2001 and 2002-03 eruptions.
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Depth of Formation of Ferropericlase Included in Super-Deep Diamonds
Anzolini, C.; Nestola, F.; Gianese, A.; Nimis, P.; Harris, J. W.
2017-12-01
Super-deep diamonds are believed to have formed at depths of at least 300 km depth (Harte, 2010). A common mineral inclusion in these diamonds is ferropericlase, (Mg,Fe)O (see Kaminsky, 2012 and references therein). Ferropericlase (fPer) is the second most abundant mineral in the lower mantle, comprising approximately 16-20 wt% (660 to 2900 km depth), and inclusions of fPer in diamond are often considered to indicate a lower-mantle origin (Harte et al., 1999). Samples from São Luiz/Juina, Brazil, are noteworthy for containing nanometer-sized magnesioferrite (Harte et al., 1999; Wirth et al., 2014; Kaminsky et al., 2015; Palot et al., 2016). Based upon a phase diagram valid for 1 atm, such exsolutions would place the origin of this assemblage in the uppermost part of the lower mantle. However, a newly reported phase diagram for magnesioferrite demonstrates that the latter is not stable at such pressures and, thus, it cannot exsolve directly from fPer at lower-mantle conditions (Uenver-Thiele et al., 2017). Here we report the investigation of two fPer inclusions, extracted from a single São Luiz diamond, by single-crystal X-ray diffraction and field emission scanning electron microscopy. Both techniques showed micrometer-sized exsolutions of magnesioferrite within the two fPers. We also completed elastic geobarometry (see Angel et al., 2015), which determined an estimate for the depth of entrapment of the two ferropericlase - diamond pairs. In the temperature range between 1273 and 1773 K, pressures varied between 9.88 and 12.34 GPa (325-410 km depth) for one inclusion and between 10.69 and 13.16 GPa (350-440 km depth) for the other one. These results strengthen the hypothesis that solitary fPer inclusions might not be reliable markers for a lower-mantle provenance. This work was supported by Fondazione CaRiPaRo and ERC-2012-StG 307322 to FN. Angel, R.J., et al. (2015) Russ Geol Geophys, 56, 211-220; Harte, B. (2010) Mineral Mag, 74, 189-215; Harte, B., et al
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International Nuclear Information System (INIS)
Allibone, A.H.; Tulloch, A.J.
2008-01-01
The character, timing, and significance of deformation within the Median Batholith has been debated since at least 1967, with allochthonous and autochthonous models proposed to account for internal variations in the character of the batholith. Stewart Island provides excellent exposures of intrabatholithic structures, allowing many aspects of the deformation history within the batholith to be analysed, far removed from the effects of later deformation related to the current plate boundary. Median Batholith rocks in northern and central Stewart Island are deformed by three major structures: the Freshwater Fault System, Escarpment Fault, and Gutter Shear Zone. Lineation orientations, Al in hornblende geobarometry, and Ar-Ar thermochronology indicate up to c. 7 km of NNE-directed uplift of the hanging wall of the Escarpment Fault between c. 110 and 105 Ma. Unlike the Escarpment Fault, a wide range of mineral elongation lineation orientations, including many oblique to the strike and dip of related foliations, characterise both the Gutter Shear Zone and Freshwater Fault System. Lineation and limited sense of shear data indicate dextral-reverse movement on both structures during development of their dominant ductile fabrics. Crosscutting and intrusive relationships indicate movement on the Freshwater Fault System after c. 130 Ma and on the Gutter Shear Zone between 120 and 112 Ma. The amount of movement on the Freshwater Fault System and Gutter Shear Zone remains largely unconstrained. However, the 342 ± 24 Ma age of a granite clast in a Paterson Group lithic tuff horizon at Abrahams Bay overlaps that of Carboniferous plutons in the block immediately south of the Freshwater Fault System, implying that the Paterson Group is little displaced from the basement rocks through which it was erupted. The three structures mapped on Stewart Island form part of a narrow transpressional mobile belt active within the Jurassic-Cretaceous arc on the outboard margin of the Western
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Directory of Open Access Journals (Sweden)
Houshang Pourkaseb
2017-07-01
used as an indicator for characterizing the conditions involved during the evaluation of magma crystallization i.e., pressure, temperature, liquid water content and oxygen fugacity. Most recent studies on the porphyry copper intrusions in the Urumieh- Dokhtar magmatic arc by (Zarasvandi et al., 2015a, indicate that all of the mineralized porphyry systems (Dalli porphyry is included consistently show high levels of La/Sm and Sm/Yb, with concave upward patterns in the rare earth elements’ spider diagrams. Importantly, such features indicate high crustal assimilation in a relatively thickened crust and provide insight into the contribution of hornblende during the development of mineralized porphyry systems in the Urumieh- Dokhtar belt. The results of this study indicate that amphiboles of Dalli intrusions belong to the calsic group and range in composition from magnesio- hornblende, to edenite, magnesiohastingsite, and tschermakite. (Ridolfi et al., 2010, indicating that the alumina content of amphibole could be used for geobarometry. The calculations of geobarometry for quartz diorite intrusions of Dalli indicate that they formed in the pressure range of 136 to 287 (MPa. Also, calculation of magmatic water content using amphibole geochemistry indicates that the water content of quartz diorite intrusions in the Dalli were between 4.6- 5.7 (wt. %. The results of plagioclase chemistry indicate that there is a little zoning in this mineral. Also, the plagioclase composition varies from Or0.01 to Ab 0.48, An 0.50, Or 0.018, Ab 0.62 and An 0.35. They mostly have Andesine and Labradorite compositions. Discussion Amphibole minerals of the Dalli intrusions are calcic type and exhibit geochemical signatures of subduction zones. Also, characterizing the source of ore-hosting intrusions with amphibole chemistry indicate that parental magma of Dalli intrusion were generated from mixing of mantle melts with crustal materials. It seems that in an ongoing process of closure of Neo
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Pleuger, J.; Podladchikov, Y.
2012-04-01
The Adula Nappe in the eastern Central Alps is one of the four units in the Alps from which ultrahigh-pressure rocks have been reported. Several very different models for its tectonic history have been published but none of these models is fully satisfactory. In the models of Schmid et al. (1996) and Engi et al. (2001), the main mechanism of exhumation is assumed to be extrusion. The extrusion models require top-to-the-hinterland, i.e. top-to-the-south faulting in the hanging wall of the exhuming nappe for which there is no evidence. Froitzheim et al. (2003) proposed a scenario with two different subduction zones, an internal one in which the South Penninic and Briançonnais domains were subducted, and an external one in which the North Penninc domain and the European margin, including the Adula nappe, were subducted. In this model, the exhumation of the Adula nappe results from the subduction of the overlying sub-Briançonnais and sub-South-Penninic mantle in the internal subduction zone. The Adula nappe would then have been exhumed from below into a top-to-the-north shear zone also affecting the overriding Briançonnais units. The main shortcoming of this model is that otherwise there is little evidence for two Alpine subduction zones. All the models cited above are based on the conversion of peak pressures obtained from geobarometry to depth by assuming lithostatic pressures. This results in a much greater burial depth of the Adula Nappe with respect to the surrounding units which poses major problems when trying to reconcile maximum burial depths of the Penninic nappes with their structural record. We performed a new restoration of the NFP20-East cross section (Schmid et al. 1996) without applying a lithostatic pressure-to-depth conversion but a purely geometrical restoration of deformation events in the Penninic nappe stack. The major constraints on these reconstructions are given by strain estimates for the major deformation phases in the units overlying the
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Thermobarometry for spinel lherzolite xenoliths in alkali basalts
Ozawa, Kazuhito; Youbi, Nasrrddine; Boumehdi, Moulay Ahmed; Nagahara, Hiroko
2016-04-01
geothermobarometry based on reactions with large and distinct volume changes, is necessary. Specification of mineral domains and their components representing the thermal state of the mantle just before xenolith extraction is one of the major tasks for the establishment of reliable geothermobarometry for spinel lherzolite xenoliths. Systematic variations of such mineralogical information among xenoliths transported by a single volcanic eruption guarantees proper estimation of a mantle geotherm. For the development of such geobarometry, it is important to choose appropriate xenolith locality, where previous studies provide enough information and where many xenolith samples are available for extending a range of derivation depth. Spinel lherzolite xenoliths in alkali basalts from Bou Ibalhatene maars in the Middle Atlas in Morocco are suitable study target. Geochemical, geochronological, petrological, and rheological aspects of the spinel lherzolite xenoliths have been studied (Raffone et al. 2009; El Messbahi et al., 2015; Witting et al., 2010; El Azzouzi et al., 2010), which show that they represent fragments of the lithospheric mantle formed and modified since 1.7Ga before their extraction from Miocene to recent. We have pinpointed portions of minerals in the xenolith samples and their components representing condition just before their entrapment in magmas, on which appropriate geothermobarometers are applied and detected ~0.5GPa pressure difference (1.5-2.0GPa) for ~100°C variation in temperatures (950-1050°C).
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Gerya, Taras
2014-05-01
On the one hand, the principle of lithostatic pressure is habitually used in metamorphic geology to calculate paleo-depths of metamorphism from mineralogical pressure estimates given by geobarometry. On the other hand, it is obvious that this lithostatic (hydrostatic) pressure principle should only be valid for an ideal case of negligible deviatoric stresses during the long-term development of the entire tectono-metamorphic system - the situation, which newer comes to existence in natural lithospheric processes. The question is therefore not "Do non-lithostatic pressure variations exist?" but " What is the magnitude of long-term non-lithostatic pressure variations in various lithospheric processes, which can be recorded by mineral equilibria of respective metamorphic rocks?". The later question is, in particular, relevant for various types of high-pressure (HP) and ultrahigh-pressure (UHP) rocks, which are often produced in convergent plate boundary settings (e.g., Hacker and Gerya, 2013). This question, can, in particular, be answered with the use of thermo-mechanical models of subduction/collision processes employing realistic P-T-stress-dependent visco-elasto-brittle/plastic rheology of rocks. These models suggest that magnitudes of pressure deviations from lithostatic values can range >50% underpressure to >100% overpressure, mainly in the regions of bending of rheologically strong mantle lithosphere (Burg and Gerya, 2005; Li et al., 2010). In particular, strong undepresures along normal faults forming within outer rise regions of subducting plates can be responsible for downward water suction and deep hydration of oceanic slabs (Faccenda et al., 2009). Weaker HP and UHP rocks of subduction/collision channels are typically subjected to lesser non-lithostatic pressure variations with characteristic magnitudes ranging within 10-20% from the lithostatic values (Burg and Gerya, 2005; Li et al., 2010). The strength of subducted crustal rocks and the degree of
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Directory of Open Access Journals (Sweden)
Mohammad Sayari
2016-07-01
authors would like to thank the University of Isfahan, and Dr. Seyed Mohsen Tabatabaei Manesh for his help for doing microprobe analyses. References Dilek, Y., Imamverdiyev, N. and Altunkaynak, S., 2010. Geochemistry and tectonics of Cenozoic volcanism in the Lesser Caucasus (Azerbaijan and the peri-Arabian region: collision-induced mantle dynamics and its magmatic fingerprint. International Geology Review, 52(4-6: 536-578. Nimis, P. and Ulmer, P., 1998. Clinopyroxene geobarometry of magmatic rocks. 1. An expanded structural geobarometer for anhydrous and hydrous, basic and ultrabasic systems. Contributions to Mineralogy and Petrology, 133(1-2: 122-135. Putirka, K., 2008. Thermometers and Barometers for Volcanic Systems. Reviews in Mineralogy and Geochemistry, 69(1: 61-120. Putirka, K., Johnson, M., Kinzler, R. and Walker, D., 1996. Thermobarometry of mafic igneous rocks based on clinopyroxene-liquid equilibria, 0-30 kbar. Contributions to Mineralogy and Petrology, 123(1: 92-108. Sayari, M., 2012. APG: An efficient software program for Amp-Pl thermobarometry based on graphical method. Journal of Sciences, Islamic Republic of Iran, 22(4: 345-349. Sayari, M., 2015. Petrogenesis and evolution of Oligocene-Pliocene volcanism in the central part of Urumieh-Dokhtar Magmatic Arc (NE of Isfahan. Ph.D Thesis, University of Isfahan, Isfahan, Iran, 195 pp (in Persian with English abstract. Sayari, M. and Sharifi, M., 2014. SCG: A computer application for single clinopyroxene geothermobarometry. Italian Journal of Geosciences 133(2: 315-322.
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Directory of Open Access Journals (Sweden)
Fatemeh Sarjoughian
2017-02-01
al., 1997. Actinolitic probably crystallized as a subsolidus phase. Pyroxene in the dikes is clinopyroxene with augite- diopside composition (Morimoto, 1988. Discussion The total Al content of hornblende is a sensitive linear function of crystallization pressure and temperature (Schmidt 1992; Holland and Blundy, 1994. However, the computed pressure may reflect the level at which the hornblende crystallizes rather than the pressure at which the granite consolidates. Therefore, Al content in hornblende geobarometer is only applicable in the presence of quartz and plagioclase; alkali feldspars, biotite, hornblende, clearly limit compositional influences (Ague, 1997. Oxygen fugacity has a marked effect on the mineral system, so only hornblendes with Fe/(Fe+Mg < 0.65, Si ≤7.5 and Ca ≥1.6 were used for geobarometry and are not applicable to subsolidus actinolite (Stein and Dietl, 2001. The average formation pressure in the intrusive rocks is evaluated to be 1.54 kbar by Schmidt (Schmidt, 1992 and Anderson and Smith (Anderson and Smith, 1995 equations, which is consistent with a depth of 5.9 Km, whereas the average pressure of amphibole crystallization in the dioritic dikes is calculated to be about 2.96 Kbar by the Ridolfi equation (Ridolfi et al., 2010, indicating 11.4 Km depth. The estimated pressure for clinopyroxene crystallization in the dikes is calculated to be about 4–8 kbar by the Soesoo (Soesoo, 1997 and Putirka (Putirka, 2008 equations which is reflecting the initial crystallization pressure of pyroxene from magma which corresponds to depths of about 15-30 km. The average formation temperature of the intrusive rocks and amphiboles in dioritic dikes is estimated to be 700 and 940 °C respectively, by the Holland and Blundy (Holland and Blundy, 1994, Vyhnal et al. (Vyhnal et al., 1991, and Ridolfi et al. (Ridolfi et al., 2010 equations. The highest temperatures from pyroxene thermometry in the dikes is about 1150 – 1250 °C by Soesoo (Soesoo, 1997 and