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Shoji Arai - One of the best experts on this subject based on the ideXlab platform.
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silica and lree enriched spinel peridotite xenoliths from the quaternary intraplate alkali basalt jeju island south korea old subarc fragments
Lithos, 2014Co-Authors: Yonghoon Woo, Kyounghee Yang, Youngwoo Kil, Sunghyo Yun, Shoji AraiAbstract:Abstract Spinel harzburgite to lherzolite xenoliths are entrapped in Quaternary intraplate alkali basalts on Jeju Island, South Korea. These xenoliths are unusual in containing late-stage secondary Orthopyroxene, free of deformation and exsolution that is replacing olivine as the main pervasive metasomatic mineral. These xenoliths are characterized by high Mg# in olivine, Orthopyroxene, and clinopyroxene (89–93) and variable Cr# of spinel (9–53), representing residues left after variable degrees of melt extraction (~ 25%). In contrast to their depleted major-element compositions, clinopyroxenes in the xenoliths are enriched in most incompatible trace elements. Clinopyroxenes display enrichment in light rare earth elements (LREE) or spoon-shaped REE with a general enrichment in La over Ce, and depletion in high field strength elements (HFSE; e.g., Nb-Ta, Zr-Hf, Ti). Orthopyroxenes (either primary or secondary) are characterized by low TiO2, high Al2O3, and moderate CaO contents, and resemble those of sub-continental arc peridotites from the eastern Pacific. The geochemical evidence, in addition to the formation of secondary Orthopyroxene, indicates that Jeju peridotite xenoliths have been subjected to different degrees of metasomatism by subduction-related silica- and LREE-enriched fluids (or melts). However, chemical equilibrium is evident between the primary and secondary Orthopyroxene, implying that the duration of post-metasomatic high temperatures enabled complete resetting/reequilibration of the mineral compositions. The metasomatic enrichment pre-dates the host Jeju Quaternary magmatism, and a genetic relationship with the host magmas is considered unlikely. We therefore propose that the Jeju peridotite xenoliths went through a two-stage evolution, with their composition primarily controlled by early fractional melt extraction, which was subsequently modified by residual slab-derived fluids (or melts). Following enrichment in the peridotite protolith in the mantle wedge, the upper mantle beneath proto-Jeju Island was transformed from a subarc environment to an intraplate environment. The Jeju peridotites, representing old subarc fragments, were subsequently transported to the surface, incorporated into ascending Quaternary intraplate alkali basalt. The result of this study implies that long term material transfer in the transformation of geotectonic setting from a subarc to intraplate may have played a significant role in the evolution of the subcontinental lithospheric mantle, resulting in the enriched mantle domains, such as EMI or EMII.
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Hydrous peridotites with Ti-rich chromian spinel as a low-temperature forearc mantle facies: evidence from the Happo-O’ne metaperidotites (Japan)
Contributions to Mineralogy and Petrology, 2009Co-Authors: Mohamed Zaki Khedr, Shoji AraiAbstract:The Happo-O’ne peridotite complex is situated in the northeastern part of the Hida Marginal Tectonic Zone, central Japan, characterized by the high- P / T Renge metamorphism, and is considered as a serpentinite mélange of Paleozoic age. Peridotitic rocks, being massive or foliated, have been subjected to hydration and metamorphism. Their protoliths are mostly lherzolites to harzburgites with subordinate dunites. We found a characteristic mineral assemblage, olivine + Orthopyroxene + tremolite + chlorite + chromian spinel, being stable at low- T , from 650 to 750°C, and high- P , from 16 to 20 kbar, tremolite–chlorite peridotites of the tremolite zone. Olivines are Fo_88–Fo_91, and Orthopyroxenes (Mg# = 0.91) show low and homogenous distributions of Al_2O_3 (up to 0.25 wt%), Cr_2O_3 (up to 0.25 wt%), CaO (up to 0.36 wt%) and TiO_2 (up to 0.06 wt%) due to the low equilibration temperature. Chromian spinels, which are euhedral and enclosed mainly in the Orthopyroxenes, have high TiO_2, 3.1 wt% (up to 5.7 wt%) on average, and high Cr# [=Cr/(Cr + Al) atomic ratio], 0.95 on average but low Fe^3+ [=Fe^3+/(Cr + Al + Fe^3+) atomic ratio,
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A new type of orthopyroxenite xenolith from Takashima, Southwest Japan: silica enrichment of the mantle by evolved alkali basalt
Contributions to Mineralogy and Petrology, 2006Co-Authors: Shoji Arai, Yohei Shimizu, Tomoaki Morishita, Yoshito IshidaAbstract:We found fine-grained Fe-rich orthopyrox- ene-rich xenoliths (mainly orthopyroxenite) containing partially digested dunite fragments of Group I from Takashima, Southwest Japan. Orthopyroxenite vein- lets, some of which contain plagioclase at the center, also replace olivine in dunite and wehrlite xenoliths of Group I. This shows high reactivity with respect to olivine of the melt involved in orthopyroxenite for- mation, indicating its high SiO2 activity. The secondary Orthopyroxene of this type is characterized by low Mg# (= Mg/(Mg + total Fe) atomic ratio) (down to 0.73) and high Al2O3 contents (5-6 wt%). It is differ- ent in chemistry from other secondary Orthopyroxenes found in peridotite xenoliths derived from the mantle wedge. Clinopyroxenes in the Fe-rich orthopyroxenite show a convex-upward REE pattern with a crest around Sm. This pattern is strikingly similar to that of clinopyroxenes of Group II pyroxenite xenoliths and of phenocrystal and xenocrystal clinopyroxenes, indicat- ing involvement of similar alkali basaltic melts. The Fe- rich orthopyroxenite xenoliths from Takashima formed by reaction between evolved alkali basalt melt and mantle olivine; alkali basalt initially slightly undersat- urated in silica might have evolved to silica-oversatu- rated compositions by fractional crystallization at high- pressure conditions. The Fe-rich orthopyroxenites oc- cur as dikes within the uppermost mantle composed of dunite and wehrlite overlying pockets of Group II pyroxenites. The Orthopyroxene-rich pyroxenites of this type are possibly common in the uppermost mantle beneath continental rift zones where alkali basalt magmas have been prevalent.
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Reaction of Orthopyroxene in peridotite xenoliths with alkali basalt melt and its implication for genesis of Alpine-type chromitite
American Mineralogist, 1995Co-Authors: Shoji Arai, Natsue AbeAbstract:The Kawashimo alkali basalt of the southwest Japan arc has peridotite xenoliths with a wide lithological range, from lherzolite {Fo of olivine, 89; Cr' [=Cr/(Cr + AI) atomic ratio] of spinel O.IO}to harzburgite (Fo of olivine, 91; Cr' of spinel, 0.54). Reaction zones between Orthopyroxene and alkali-basalt melt are low-pressure analogues of mantle-melt interaction products and consist of two subzones: a fme-grained inner subzone (adjacent to Orthopyroxene) and a relatively coarse-grained outer subzone. In both xenolith types the reaction products are olivine + diopsidic clinopyroxene :t spinel :t glass, but the spinel concentrations are remarkably different around lherzolite from those around harzburgite. Cr-bearing spinel is concentrated only in the outer subzone on harzburgite Orthopyroxene (Cr' > 0.14); the inner subzone on harzburgite Orthopyroxene and both the inner and outer subzones on lherzolite Orthopyroxene (Cr' = 0.05) are almost free of spinel. The remarkable enrichment of spinel in the outer subzone on the harzburgite Orthopyroxene suggests a mechanism of spinel concentration, Le., the origin of podiform chromitite is related to interaction between Cr-rich Orthopyroxene and basaltic melt. This observation for the Kawashimo xenoliths is concordant with the near absence ofpodiform chromitite in Iherzolitic mantle. Chromian spinel could be concentrated if a relatively silica-rich secondary melt, produced by interaction between pyroxene-undersaturated magma and harzburgite Orthopyroxene, is mixed with a primitive magma in the upper mantle.
Jibamitra Ganguly - One of the best experts on this subject based on the ideXlab platform.
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Compositional properties of coexisting Orthopyroxene and spinel in some Antarctic diogenites: Implications for thermal history
Meteoritics & Planetary Science, 2001Co-Authors: H‐p. Hermann, Jibamitra GangulyAbstract:— We analyzed the compositional profiles of coexisting Orthopyroxenes and spinels in six diogenite samples from the Antarctic meteorite collection and used the data to constrain their thermal histories. The closure temperatures of Fe2+-Mg exchange between spinel and Orthopyroxene in these samples vary between ∼630 and 830 °C. However, those in other diogenite samples, for which the compositional data are available in the literature, extend up to ∼1125 °C. This wide range of closure temperatures suggests repeated excavation of the diogenites from their original sites over a long time interval during cooling. The Orthopyroxene grains were found to be homogeneous in composition while two of the relatively large spinel grains in the samples Elephant Moraine (EET) 87530 and Thiel Mountains (TIL) 82410 showed compositional zoning near the rim. Modeling of the spinel zoning in TIL 82410 suggests that it developed during cooling under a regolith or ejecta blanket, possibly at a depth of ∼80–120 m, and that the spinel composition was homogeneous at ∼900 °C. A nonlinear cooling model in which the cooling rate is given by ηT(K)2, with η = 5.8 times 10−3 K−1Ma−1, leads to simulated retrograde zoning profile in spinel which match the observed profile in TIL 82410 very well.
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Cation ordering in Orthopyroxenes from two stony-iron meteorites: implications for cooling rates and metal-silicate mixing
Geochimica et Cosmochimica Acta, 2000Co-Authors: Jibamitra Ganguly, M. StimpflAbstract:Abstract We have determined the cooling rates of Orthopyroxene crystals from two group IVA stony iron meteorites—Steinbach (ST) and Sao Joao Nepomuceno (SJN)—on the basis of their Fe–Mg ordering states. The rate constant was calibrated as a function of temperature by controlled cooling experiments using Orthopyroxene crystals separated from ST. These data were used along with earlier calibrations of the equilibrium intracrystalline fractionation of Fe and Mg as a function of temperature for crystals separated from both meteorites to calculate their cooling rates. The site occupancies of the Orthopyroxene crystals were determined by single-crystal X-ray diffraction subject to the bulk compositional constraints. The closure temperatures ( T c ) of cation ordering for the untreated crystals from SJN are ∼400°C, whereas those from ST vary between ∼430 and 470°C. Reconciliation of the metallographic and Orthopyroxene cooling rate data, within the framework of the metal–silicate mixing model of Haack et al. (1995) , suggests that these two stony irons had cooled at a similar rate of ∼400°C/Ma through the closure temperatures for cation ordering in the Orthopyroxenes. This was followed by slow cooling for ST at ∼50°C/Ma at T (Rasmussen et al., 1995) of distinctly different cooling rates for the high and low Ni IVA irons and stony irons. The cation ordering and metallographic cooling rate data are also amenable to an alternative interpretation, which requires two different parent bodies for the two stony irons, and mixing of the metal and silicate components of ST after the metals had cooled below the closure temperature of Fe–Ni interdiffusion. However, the available textural data for ST seems to argue against such metal-silicate mixing model.
Patrick K. Schelling - One of the best experts on this subject based on the ideXlab platform.
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Atomic‐scale simulation of space weathering in olivine and Orthopyroxene
Journal of Geophysical Research: Planets, 2015Co-Authors: Abrar H. Quadery, Shaun Pacheco, Natalie Rizzacasa, Joshua Nichols, Cameron Glasscock, Patrick K. SchellingAbstract:Classical molecular dynamics was used to study the annealing of anion and cation Frenkel defects in olivine and Orthopyroxene minerals. While it was found that for both minerals, reorganization of the Si–O bonds, often accompanied by large Si displacements, occurs to maintain the fourfold coordination of the SiO4 tetrahedra, important differences are observed in their annealing behavior. Specifically, cation defects are substantially more mobile in olivine than in Orthopyroxene leading to rapid annihilation of cation Frenkel defects and formation of extended defects in olivine. By contrast, the diffusion rate of anion defects in Orthopyroxene is much higher than that in olivine and also exhibits large anisotropy. Consequently, it was found that diffusion in Orthopyroxene occurs without significant annihilation of anion Frenkel defects or trapping of anion interstitials or vacancies into clusters. The results obtained here are discussed in the context of space weathering of olivine and Orthopyroxene. Specifically, two important observations are made which may explain previous experimental results. First, ion irradiation experiments that show reduced tolerance for radiation damage in Orthopyroxene may be explained by the rapid, one-dimensional anion mobility which prevents healing of the lattice. Second, laser heating experiments which show that Orthopyroxene has enhanced tolerance to reduction and the evolution of nanophase Fe inclusions could be due to the observed rapid anion diffusion in Orthopyroxene, which might allow the bulk to act as a reservoir for the surface.
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atomic scale simulation of space weathering in olivine and Orthopyroxene
Journal of Geophysical Research, 2015Co-Authors: Abrar H. Quadery, Shaun Pacheco, Natalie Rizzacasa, Joshua Nichols, Cameron Glasscock, Patrick K. SchellingAbstract:Classical molecular dynamics was used to study the annealing of anion and cation Frenkel defects in olivine and Orthopyroxene minerals. While it was found that for both minerals, reorganization of the Si–O bonds, often accompanied by large Si displacements, occurs to maintain the fourfold coordination of the SiO4 tetrahedra, important differences are observed in their annealing behavior. Specifically, cation defects are substantially more mobile in olivine than in Orthopyroxene leading to rapid annihilation of cation Frenkel defects and formation of extended defects in olivine. By contrast, the diffusion rate of anion defects in Orthopyroxene is much higher than that in olivine and also exhibits large anisotropy. Consequently, it was found that diffusion in Orthopyroxene occurs without significant annihilation of anion Frenkel defects or trapping of anion interstitials or vacancies into clusters. The results obtained here are discussed in the context of space weathering of olivine and Orthopyroxene. Specifically, two important observations are made which may explain previous experimental results. First, ion irradiation experiments that show reduced tolerance for radiation damage in Orthopyroxene may be explained by the rapid, one-dimensional anion mobility which prevents healing of the lattice. Second, laser heating experiments which show that Orthopyroxene has enhanced tolerance to reduction and the evolution of nanophase Fe inclusions could be due to the observed rapid anion diffusion in Orthopyroxene, which might allow the bulk to act as a reservoir for the surface.
Jay D. Bass - One of the best experts on this subject based on the ideXlab platform.
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High-temperature elastic softening of Orthopyroxene and seismic properties of the lithospheric upper mantle
Geophysical Journal International, 2010Co-Authors: Bruno Reynard, Jay D. Bass, Jon BrenizerAbstract:Mineralogical models suggest that low velocity zones of the upper mantle may be accounted for by elasticity of dry rocks along geotherms away from the mid-oceanic ridges (MOR) while, closer to MOR, anelasticity plays a significant role in reducing shear wave velocities V(S) and generating attenuation. We investigate the potential influence of elastic softening, precursor of high-temperature phase transition in Orthopyroxenes, on the seismic properties of the upper mantle. In situ Brillouin and Raman spectroscopy were used to evidence pre-transitional behaviour at high temperature in natural San Carlos Orthopyroxene. Pre-transitional behaviour induces a large softening of the acoustic and low frequency modes, resulting in a large anharmonic decrease of sound velocities, similar to that observed in the MgSiO(3) end-member orthoenstatite. The high-temperature high-pressure phase diagram of enstatite is revised to account for the new phase transition, and a simple model is developed to evaluate elastic softening effects on the upper-mantle seismic properties, whose results depend much on the as yet poorly constrained pressure and compositional dependence of the phase transition boundary. Within the tested range of parameters, OPx softening is likely to affect the seismic properties of mantle rocks at depths shallower than 80 km in hot regions. Thus elastic softening of pyroxene is unlikely to affect the LVZ or continental lithospheric mantle unless the transition temperature is drastically reduced by incorporation of aluminium in Orthopyroxene. It will contribute to a decrease of V(S) near MOR, hot spots and evolved continental rifts, where it can explain part of the non-linear high-temperature decrease of V(S) in the lithospheric oceanic mantle. It will affect the magnitude of anelastic effects and the nature of the likely mechanisms of attenuation required to match seismological observations.
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Single-crystal elastic properties of Ca0.07Mg1.93Si2O6 Orthopyroxene
American Mineralogist, 2007Co-Authors: Jean-philippe Perrillat, Fabrizio Nestola, Stanislav V. Sinogeikin, Jay D. BassAbstract:The single-crystal elastic properties of Ca 0.07 Mg 1.93 Si 2 O 6 Orthopyroxene (space group Pbca ) have been investigated by Brillouin spectroscopy at ambient conditions. The aggregate bulk and shear moduli, K 0 ,S = 102.5 GPa (1.5) and μ = 74.2 GPa (1.1), respectively, are ~5% and ~3% lower than commonly accepted values for MgSiO 3 end-member ( K 0 ,S = 107.6, μ = 76.8 GPa). These results indicate that the incorporation of small amount of Ca in the orthoenstatite structure does not greatly affect its elastic properties. As a consequence, the increase in bulk modulus reported in natural Orthopyroxenes relative to the Mg-end-member is not related to the substitution of Ca in the M2 octahedral sites, but more probably to the substitution of Al in tetrahedral sites.
George R. Rossman - One of the best experts on this subject based on the ideXlab platform.
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Water in boninite glass and coexisting Orthopyroxene: concentration and partitioning
Contributions to Mineralogy and Petrology, 1995Co-Authors: Patrick F. Dobson, Henrik Skogby, George R. RossmanAbstract:Spectroscopic measurements of water in glass inclusions in pyroxene from boninite samples from the Bonin Islands conclusively document the high (2.8–3.2 wt%) primary water contents of boninite magmas. Associated quenched glass from pillow lava rims have slightly lower (2.2–2.4 wt%) water contents, suggesting that minor amounts of degassing occurred between the time of melt entrapment in the Orthopyroxenes and subsequent eruption on the sea floor. Some zonation of molecular water contents in pillow rim glasses was observed. OH contents of the host Orthopyroxene phenocrysts were also measured, allowing for the calculation of partition coefficients for water between boninite melt and Orthopyroxene. These values (0.003–0.004) for water partitioning between Orthopyroxene and mafic melts may help constrain petrogenetic models of mantle-derived magmas.
