Formation of high-pressure minerals in shocked L6 chondrite Yamato 791384: constraints on shock conditions and parent body size

We have studied the high-pressure phases observed in Yamato 791384 L6 chondrite. Host meteorite mainly consists of olivine, pyroxenes and plagioclase glass. Mineral fragments observed in the veins and the vein margin region of the host meteorite were partially or totally transformed into high-pressure phases; ringwoodite, majorite, akimotoite, NaAlSi3O8 hollandite and jadeite. Matrix of the shock vein contains majorite-pyrope solid solution. The spatial distribution indicates that high-pressure phases are present in the shock veins and host rocks adjacent to the shock veins. Investigation of the high-pressure phases revealed that, in Y791384, fragments and adjacent matrix were subjected to pressures around of 18-23 GPa and the vein experienced temperatures around 2300 degreesC during the shock event. Ringwoodite lamellae were observed in the host olivine adjacent to the vein. Kinetic investigation for ringwoodite lamellar growth in olivine indicates that the shocked meteorite experienced the pressure for at least 4 s. Such long duration of the pressure pulse indicates that the size of the parent body was greater than 10 km. (C) 2004 Elsevier B.V. All rights reserved.