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A shock-produced (Mg, Fe)SiO3 glass in the Suizhou meteoriteChen, Ming; Xie, Xiande; El Goresy, Ahmed (The Meteoritical Society, 2004-01-01)Ovoid grains consisting of glass of stoichiometric (Mg, Fe)SiO3 composition that is intimately associated with majorite were identified in the shock veins of the Suizhou meteorite. The glass is surrounded by a thick rim of polycrystalline majorite and is identical in composition to the parental low-Ca pyroxene and majorite. These ovoid grains are surrounded by a fine-grained matrix composed of majorite-pyrope garnet, ringwoodite, magnesiowstite, metal, and troilite. This study strongly suggests that some precursor pyroxene grains inside the shock veins were transformed to perovskite within the pyroxene due to a relatively low temperature, while at the rim region pyroxene grains transformed to majorite due to a higher temperature. After pressure release, perovskite vitrified at post-shock temperature. The existence of vitrified perovskite indicates that the peak pressure in the shock veins exceeds 23 GPa. The post-shock temperature in the meteorite could have been above 477 degrees C. This study indicates that the occurrence of high-pressure minerals in the shock veins could not be used as a ubiquitous criterion for evaluating the shock stage of meteorites.
Fracture-related intracrystalline transformation of olivine to ringwoodite in the shocked Sixiangkou meteoriteChen, Ming; Li, Hui; El Goresy, Ahmed; Liu, Jing; Xie, Xiande (The Meteoritical Society, 2006-01-01)Magnesium-iron olivine in the Sixiangkou L6 chondrite contains abundant fractures induced by plastic deformation during shock metamorphism. This study reports the discovery of lamellar ringwoodite that incoherently nucleated and grew along planar and irregular fractures in olivine. Magnesium-iron interdiffusion took place between olivine matrix and crystallizing ringwoodite at high pressures and high temperatures, which resulted in higher FeO content in ringwoodite lamellae than in olivine. This suggests that a quasi-hydrostatic high pressure lasting for several minutes should have been produced in the shock veins of the meteorite. The intracrystalline transformation of olivine to ringwoodite also has implications for phase transitions in subducting lithospheric slabs because planar and irregular fractures are commonly produced in olivine that suffered plastic deformation.