Cooling rates in the shock veins of chondrites: Constraints on the (Mg, Fe)₂SiO₄ polymorph transformations

The occurrence of γ-phase, a high-pressure polymorph of olivine (α-phase), in the shock veins of Sixiangkou chondrite was due to a greater cooling rate ( > 10 000°C ·^-1) in the veins. Because γ-phase partially reverted to β-phase and no back-transformation from β-phase to α-phase took place, the shock veins of Peace River chondrite with a cooling rate of 1 000 - 2 000°C · s^-1 contain a great amount of β-phase. In the shock veins of Mbale chondrite with a cooling rate of <500°C · s^-1, the majority of γ-phase reverted to α-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 the shock veins constrained the back-transformations of (Mg, Fe)₂SiO₄ high-pressure polymorphs.