THE MAJORITE-PYROPE PLUS MAGNESIOWUSTITE ASSEMBLAGE - CONSTRAINTS ON THE HISTORY OF SHOCK VEINS IN CHONDRITES

Shock veins in the Sixiangkou (L6) chondrite contain two high-pressure assemblages: (i) majorite-pyrope solid solution plus magnesiowustite that crystallized at high pressures and temperatures from a shock-indu ced silicate melt of bulk Sixiangkou composition and (ii) ringwoodite plus low-calcium majorite that were produced by solid-state transforma tion of olivine and low-calcium pyroxene. The morphology and chemistry of the majorite-pyrope garnet and the size of the magnesiowustite cry stals indicate a longer duration at high pressure and temperature than predicted by impact scenarios. This pressure-temperature regime is co nstrained by the olivine-ringwoodite and orthopyroxene-majorite phase transformations, fusion of the meteorite constituents, and crystalliza tion of majorite-pyrope solid solution plus magnesiowustite from that melt under high pressure.