Transformation textures,mechanisms of formation of high‐pressure minerals in shock melt veins of L6 chondrites,and pressure‐temperature conditions of the shock events     

Shin Ozawa  Eiji Ohtani  Masaaki Miyahara  Akio Suzuki  Makoto Kimura  Yoshinori Ito 《Meteoritics & planetary science》2009,44(11):1771-1786

Abstract— The high‐pressure polymorphs of olivine, pyroxene, and plagioclase in or adjacent to shock melt veins (SMVs) in two L6 chondrites (Sahara 98222 and Yamato 74445) were investigated to clarify the related transformation mechanisms and to estimate the pressure‐temperature conditions of the shock events. Wadsleyite and jadeite were identified in Sahara 98222. Wadsleyite, ringwoodite, majorite, akimotoite, jadeite, and lingunite (NaAlSi₃O₈‐hollandite) were identified in Yamato 74445. Wadsleyite nucleated along the grain boundaries and fractures of original olivine. The nucleation and growth of ringwoodite occurred along the grain boundaries of original olivine, and as intracrystalline ringwoodite lamellae within original olivine. The nucleation and growth of majorite took place along the grain boundaries or fractures in original enstatite. Jadeite‐containing assemblages have complicated textures containing “particle‐like,” “stringer‐like,” and “polycrystalline‐like” phases. Coexistence of lingunite and jadeite‐containing assemblages shows a vein‐like texture. We discuss these transformation mechanisms based on our textural observations and chemical composition analyses. The shock pressure and temperature conditions in the SMVs of these meteorites were also estimated based on the mineral assemblages in the SMVs and in comparison with static high‐pressure experimental results as follows: 13–16 GPa, >1900 °C for Sahara 98222 and 17–24 GPa, >2100 °C for Yamato 74445.  相似文献   

Origin of a late Neoproterozoic (605 ± 13 Ma) intrusive carbonate–albitite complex in Southern Sinai, Egypt     

Mokhles Kamal Azer  Robert J. Stern  Jun-Ichi Kimura 《International Journal of Earth Sciences》2010,99(2):245-267

New geochemical, isotopic, and geochronological data and interpretations are presented for late Neoproterozoic intrusive carbonates and related rocks of southern Sinai, Egypt (northernmost Arabian–Nubian Shield). The Tarr carbonates are coarsely crystalline and related to explosive emplacement of hypabyssal and volcanic albitite at 605 ± 13 Ma. The carbonates associated with the albitites are divisible into two types: primary dolomitite and secondary breunneritite (Fe-rich magnesite). The dolomitite was clearly intrusive but differs from classic igneous carbonatites, containing much lower abundances of incompatible elements, such as REE, U, Th, Rb, Nb, Y, P, Sr, Zr, Ba, and total alkalies. The breunneritite is a secondary replacement of dolomitite, probably marking the roots of a vigorous hydrothermal system. Albitites show pristine abundances of major and trace elements and were not subjected to a major metamorphic overprint. They are relatively more fractionated, alkaline and related to within-plate A-type magmas, were emplaced in an extensional or non-compressive tectonic regime in the cupola of high-level A-type granite. Tarr albitites may represent residual magma remaining after near-total crystallization of an A-type granite pluton at depth, forcibly emplaced into the roof above the cooling pluton. The intrusive dolomitite exsolved from highly differentiated albitite melt, in the apical regions of a still-buried alkaline “A-type” granite pluton that was rich in CO₂; these volatiles migrated upwards and towards the cooler margins of the magma body. Late NNE-SSW extension allowed a shallow-level cupola to form, into which albitite melts and carbonate fluids migrated, culminating in explosive emplacement of albitite breccia and intrusive carbonate. Isotopic compositions of Tarr dolomitite and albitite indicate these are consanguineous and ultimately of mantle origin. Magmatic volatiles fenitized the wall rock, while submarine hydrothermal activity transformed some of the dolomitite into breunneritite. Recognition of Tarr-type should encourage similar hypabyssal complex intrusions to be sought for in association with A-type granitic plutons elsewhere.  相似文献   

Condensation and aggregation of solar corundum and corundum‐hibonite grains     

T. M. Nakamura  N. Sugiura  M. Kimura  A. Miyazaki  A. N. Krot 《Meteoritics & planetary science》2007,42(7-8):1249-1265

Abstract— Forty‐three corundum grains (1–11 μm in size) and 5 corundum‐hibonite grains with corundum overgrown by hibonite (4–7 μm in size), were found in the matrix of the mineralogically pristine, ungrouped carbonaceous chondrite Acfer 094 by using cathodoluminescence imaging. Some of the corundum and corundum‐hibonite grains occur as aggregates of 2 to 6 grains having similar sizes. The oxygen isotopic compositions of some of the corundum‐bearing grains suggest their solar nebula origin. ²⁶Al‐²⁶Mg systematics of one corundum grain showed the canonical initial ²⁶Al/²⁷Al ratio, also suggesting a solar nebula origin. Quantitative evaluation of condensation and accretion processes made based on the homogeneous nucleation of corundum, diffusion‐controlled hibonite formation, collisions of grains in the nebula, and critical velocity for sticking, indicates that, in contrast to the hibonite‐bearing aggregates of corundum grains, the hibonite‐free corundum aggregates could not have formed in the slowly cooling nebular region with solar composition. We suggest instead that such aggregates formed near the protosun, either in a region that stayed above the condensation temperature of hibonite for a long time or in a chemically fractionated, Ca‐depleted region, and were subsequently physically removed from this hot region, e.g., by disk wind.  相似文献   

Non‐mass‐dependent oxygen isotopic fractionation in smokes produced in an electrical discharge     

Yuki Kimura  Joseph A. Nuth  Subrata Chakraborty  Mark H. Thiemens 《Meteoritics & planetary science》2007,42(7-8):1429-1439

Abstract— We report the first production of non‐mass‐dependently fractionated silicate smokes from the gas phase at room temperature from a stream of silane and/or pentacarbonyl iron in a molecular hydrogen (or helium) flow mixed with molecular oxygen (or nitrous oxide). The smokes were formed at the Goddard Space Flight Center (GSFC) at total pressures of just under 100 Torr in an electrical discharge powered by a Tesla coil, were collected from the surfaces of the copper electrodes after each experiment and sent to the University of California at San Diego (UCSD) for oxygen isotopic analysis. Transmission electron microscopy studies of the smokes show that they grew in the gas phase rather than on the surfaces of the electrodes. We hypothesize at least two types of fractionation processes occurred during formation of the solids: a mass‐dependent process that made isotopically lighter oxides compared to our initial oxygen gas composition followed by a mass‐independent process that produced oxides enriched in ¹⁷O and ¹⁸O. The maximum Δ¹⁷O observed is + 4.7‰ for an iron oxide produced in flowing hydrogen, using O2 as the oxidant. More typical displacements are 1–2‰ above the equilibrium fractionation line. The chemical reaction mechanisms that yield these smokes are still under investigation.  相似文献   

Geology and geochemistry of lavas at Nekoma volcano: Implications for origin of Quaternary low-K andesite in the north-eastern Honshu arc, Japan   总被引：4，自引：0，他引：4  

Jun-Ichi Kimura  Tomoyuki Tanji  Takeyoshi Yoshida  Shigeru Iizumi 《Island Arc》2001,10(2):116-134

Abstract Nekoma volcano forms part of the arc axis volcanic array of the North-eastern Honshu arc, Japan, which is commonly characterized by medium-K lava suites. However, Nekoma is exceptional because many of its lavas are low-K. This anomaly has been a matter of debate. Nekoma was active from 1.1 to 0.35 Ma. The volcano consists of thick andesite flows and domes associated with block and ash flow deposits produced during lava dome formation. A horseshoe-shaped collapse caldera was formed at the summit and small lava domes extruded into the caldera. Stratigraphy, published K–Ar ages, and tephrochronology define three stages of volcanic activity, about 1.1 Ma (Stage 1), 0.8–0.6 Ma (Stage 2) and 0.45–0.35 Ma (Stage 3; post caldera stage). Low-K andesites occur in all stages. Extremely low-K andesite was also associated in Stage 2 and medium-K andesite was dominant in Stage 3. In general, lavas changed from low-K to medium-K after caldera formation. Geochemical study of the Nekoma lavas shows that both low-K and medium-K lavas are isotopically similar and were derived from a common source. Adatara and Azuma volcanoes, which lie close to Nekoma, also have both low-K and medium-K andesites. However, Sr isotope ratios or temporal-spatial variations in K-level lava classification vary between the three centers. Comparisons of K suites and Sr isotope ratios with frontal arc volcanoes in North-east–Honshu suggest source heterogeneity existed in both medium- and low-K suites. The K contents of lavas and their Sr isotopes are not simply related. This requires re-examination of models for chemical variation of andesites in arcs.  相似文献   

Crystallization of chondrules in ordinary chondrites     

Makoto Kimura  Kenzo Yagi 《Geochimica et cosmochimica acta》1980,44(4):589-602

The process of crystallization and the origin of chondrules are discussed, in terms of the phase relations of the minerals in chondrules in six ordinary chondrites of the Yamato-74 meteorites, especially the Yamato-74191 (L3).Chondrules are classified into six types. The bulk compositions of chondrules projected onto the MgO-FeO-SiO₂ system show that the compositions of chondrules vary widely. Investigations by means of the MgO-Al₂O₃-SiO₂ system indicate that porphyritic chondrules can be regarded as products of supercooling crystallization. The growth rates of crystals in porphyritic chondrules were fairly small. The difference between types of chondrules is interpreted in terms of the compositions of chondrules and the nucleation temperatures of the supercooled droplets.All these observations and estimations must be taken into account for discussing the origin of chondrules. The impact and dust fusion theories do not appear to be plausible. Molten droplets due to these mechanisms will be glassy spherules, or crystallize at equilibrium. Only a liquid condensation theory can well explain the characteristic features and the process of the crystallization of chondrules.  相似文献   

The Dependence of Constitutive Properties on Temperature and Effective Normal Stress in Seismogenic Environments     

Aitaro Kato  Shingo Yoshida  Mitiyasu Ohnaka  Hiromine Mochizuki 《Pure and Applied Geophysics》2004,161(9-10):1895-1913

— We have evaluated how the parameters prescribing the slip-dependent constitutive law are affected by temperature and effective normal stress, by conducting the triaxial fracture experiments on Tsukuba-granite samples in seismogenic environments, which correspond to a depth range to 15 km. The normalized critical slip displacement D _c almost remains constant below 300^oC (insensitive to both temperature and effective normal stress σ _n ^eff); D _c increases with increasing temperature above 300 °C, and the rate of D _c increase with temperature tends to be largest at higher σ _n ^eff. The breakdown stress drop Δτ _b for the granite at constant σ _n ^eff is roughly 80 MPa below 300 °C, and does not depend on σ _n ^eff. Above 300 °C, Δτ _b decreases gradually with increasing temperature, and the rate of Δτ _b reduction with temperature increases at higher σ _n ^eff. The peak shear strength τ _p increases nearly linearly with increasing σ _n ^eff below 300 °C. However, τ _p becomes lower above 300 °C, deviating from the linear relation extrapolated from below 300 °C. This is consistent with the onset of crystal plastic deformation mechanisms of Tsukuba granite.  相似文献   

Orogeny of the Japanese Islands     

Yukio Isozaki  Shigenori Maruyama  Gaku Kimura 《Island Arc》1997,6(1):1-1

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Cretaceous episodic growth of the Japanese Islands   总被引：1，自引：0，他引：1  

G. Kimura 《Island Arc》1997,6(1):52-68

Abstract The Japanese Islands formed rapidly in situ along the eastern Asian continental margin in the Cretaceous due to both tectonic and magmatic processes. In the Early Cretaceous, huge oceanic plateaus created by the mid-Panthalassa super plume accreted with the continental margin. This tectonic interaction of oceanic plateau with continental crust is one of the significant tectonic processes responsible for continental growth in subduction zones. In the Japanese Islands, Late Cretaceous-Early Paleogene continental growth is much more episodic and drastic. At this time the continental margin uplifted regionally, and intra-continent collision tectonics took place in the northern part of the Asian continent. The uplifting event appears to have been caused by the subduction of very young oceanic crust (i.e. the Izanagi-Kula Plate) along the continental margin. Magmatism was also very active, and melting of the young oceanic slab appears to have resulted in ubiquitous plutons in the continental margin. Regional uplift of the continental margin and intra-continent collision tectonics promoted erosion of the uplifted area, and a large amount of terrigenous sediment was abruptly supplied to the trench. As a result of the rapid supply of terrigenous detritus, the accretionary complexes (the Hidaka Belt in Hokkaido and the Shimanto Belt in Southwest Japan) grew rapidly in the subduction zone. The rapid growth of the accretionary complexes and the subduction of very young, buoyant oceanic crust caused the extrusion of a high-P/T metamorphic wedge from the deep levels of the subduction zone. Episodic growth of the Late Cretaceous Japanese Islands suggests that subduction of very young oceanic crust and/or ridge subduction are very significant for the formation of new continental crust in subduction zones.  相似文献   

Effect of the contraction of Zn-O bonds on X-ray emission spectra     

Kimura  Makoto  Ohashi  Haruo  Osawa  Toshikazu 《Physics and Chemistry of Minerals》1988,15(6):545-547

Physics and Chemistry of Minerals - The X-ray emission spectra were measured for tetrahedrally coordinated Zn ion in several synthetic compounds. Intensity ratios of L β and L α spectral...  相似文献