Shock Features in L6 Chondrite NWA 091: Search for Evidence of Very High Shock Pressure in Chondrites

Many L chondrites are thought to be highly shocked by the giant impact event on L chondrite parent body at 500Ma. Such a hypervelocity impact on the L chondrite parent body would generate very high pressures and abundant melting. However, the high-pressure phases in shock veins of L chondrites constrain the shock pressure up to a maximum of ~26GPa (Hu et al., 2012). NWA 091 is a L6 shock-melt breccia from the 500Ma event that is highly blackened (Welrich et al., 2012). It was originally classified as shock stage S4, based on the presence of plagioclase rather than maskelynite. We used Raman spectroscopy, field-emission SEM and analytical TEM to investigate the mineralogy and microstructure in NWA 091 to better estimate its shock pressure. Olivine and pyroxene in NWA 091 are laced with metal and sulfide veins and droplets, suggesting these fragments are highly shocked. The silicate melt veins and pockets include very fine-grained crystals with compositions of olivine, pyroxene and plagioclase. No high-pressure phases have been found in the veins. Plagioclase in this sample is poikilitic with inclusions of metal, sulfide and chromite. We will characterize the microstructure of plagioclase to determine if it is primary or has transformed from a high pressure phases after pressure release. Based on the microstructures observed so far, we interpret NWA091 as a very highly shocked (S6-7) chondrite that was annealed at high temperature after decompression. Thus the high-pressure polymorphs of rock-forming minerals are not preserved. Such post-shock annealing can explain why mineralogical evidence for pressure above 26GPa is lacking in L chondrites.