Raman Spectroscopy of High-Pressure Phases in Shocked L6 Chondrite NWA 5011

摘要

The paper presents the results of studies of thick shock melt veins in the L6 chondrite NWA 5011. The veins contain a wide variety of high-pressure phases that correspond to contrast values of pressure-temperature parameters on equilibrium phase diagrams. Olivine was transformed to ringwoodite and wadsleyite, orthopyroxene to majorite, akimotoite, and bridgmanite glass, maskelynite is converted to jadeite (+SiO2) and lingunite, apatite to tuite, and chromite to the phase with the calcium ferrite (mCF-FeCr2O4) structure. The peak PT shock parameters for NWA 5011 seem to be the highest among the ones for other shocked chondrites according to the wide occurrence of lingunite and bridgmanite glass and are considerably higher than 25 GPa and 2500 K. Akimotoite crystals in a quenched matrix of the shock melt veins were found for the first time. Probably, they initially crystallized as bridgmanite, since akimotoite is not a liquidus phase in the related systems. Plagioclase-chromite aggregates have been established, which characterize the late stages of the shock process and are formed during successive crystallization from isolated pockets of the impact melt.