Back-transformation processes in high-pressure minerals: implications for planetary collisions and diamond transportation from the deep Earth

We conducted back-transformation experiments in ringwoodite, bridgmanite, and lingunite at 0.47-8.1 GPa and 310-920 degrees C by in situ X-ray observation method. Ringwoodite back-transformed to olivine by grain-boundary nucleation and growth mechanism. The site saturation occurred at the early stage under the conditions far from the equilibrium boundary, and we observed the growth-controlled back-transformation kinetics in ringwoodite. The growth kinetics determined in the present study is largely different from that in the previous study (Reynard et al. in Am Min 81:585-594, 1996), which may be due to the effects of water. Bridgmanite did not directly back-transform to the stable phase orthoenstatite at similar to 1-4 GPa, but first becomes amorphous with increasing temperatures. We observed kinetics of the orthoenstatite crystallization from amorphous bridgmanite that was controlled by both nucleation and growth processes. The temperature range in the amorphous state became narrow with increasing pressures, and the direct back-transformation to high-P clinoenstatite without amorphization eventually occurred at 8 GPa. Amorphization was also observed in lingunite when increasing temperature at similar to 1.5 GPa; however, the plagioclase crystallization proceeded before the complete amorphization. The back-transformation in ringwoodite variedly occurs in shocked meteorites depending on the degree of the post-shock annealing, which can be reasonably interpreted based on the growth kinetics. On the other hand, the presence of hydrous ringwoodite in diamond inclusions cannot be explained without the help of residual stress. The present study also indicates that complete amorphization or the back-transformation to enstatite is unavoidable in bridgmanite during the post-shock annealing. This is inconsistent with the presence of crystalline bridgmanite in shocked meteorites, still requiring further investigations of kinetic behaviors in shorter timescales.