COOLING RATES IN THE SHOCK VEINS OF CHONDRITES - CONSTRAINTS ON THE (MG, FE)(2)SIO4 POLYMORPH TRANSFORMATIONS

The occurrence of gamma-phase, a high-pressure polymorph of olivine (a lpha-phase), in the shock veins of Sixiangkou chondrite was due to a g reater cooling rate (>10 000 degrees C.s(-1)) in the veins. Because ga mma-phase partially reverted to beta-phase and no back-transformation from beta-phase to alpha-phase took place, the shock veins of Peace Ri ver chondrite with a cooling rate of 1 000-2 000 degrees C.s(-1) conta in a great amount of beta-phase. In the shock veins of Mbale chondrite with a cooling rate of <500 degrees C.s(-1), the majority of gamma-ph ase reverted to alpha-phase. The heat dissipation in shock veins took place after a stage of shock compression of chondrite parent body, and the parent body was broken into fragmental pieces. Cooling rate in th e shock veins constrained the hack-transformations of (Mg, Fe)(2)SiO4 high-pressure polymorphs.