The Majorite-Pyrope + Magnesiowüstite Assemblage: Constraints on the History of Shock Veins in Chondrites

Abstract

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