High-pressure minerals in shocked L6-chondrites: Constraints on impact conditions

We have studied the high-pressure phases observed in Yamato 791384 and ALH78003 L6-chondrites. Host meteorite consists mainly of olivine, pyroxenes, and plagioclase glass. Mineral fragments observed in the veins and the vein margin region of these meteorites were partially or totally transformed into high-pressure phases wadsleyite, ringwoodite, majorite, akimotoite, NaAlSi ₃O₈ hollandite and jadeite. Whereas matrix of the shock vein contains majorite-pyrope solid solution in both meteorites. 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 18-23 GPa and the vein experienced temperatures around 2,000-2,300°C during the shock event. ALH78003 experienced the shock pressure of about 15-18 GPa at 2,000°C. Ringwoodite lamellae were observed in the host olivine adjacent to the vein in Y791384. Kinetic investigation for ringwoodite lamellar growth in olivine indicates that the meteorite experienced an impact with a pressure around 20 GPa for more than 4 s of the pressure pulse indicating a large impactor with the size greater than 10 km. ALH78003 contains wadsleyite-ringwoodite aggregates in the shock veins. The ringwoodite grains have wadsleyite rim enriched in Mg₂SiO₄ component. The compositional profiles of wadsleyite rim and ringwoodite core of the fragments in the shock veins in ALH78003 cannot be explained by a simple Mg-Fe inter-diffusion process.