Effect of grain size on amorphization mechanism and kinetics of bridgmanite in shocked meteorites

Abstract Bridgmanite formation and amorphization in shocked meteorites constrains the pressure and temperature conditions during planetary impact process. However, the effect of grain size of bridgmanite on its amorphization kinetics is still unclear. In this study, the amorphization mechanism and kinetics of fine-grained polycrystalline bridgmanite were studied at high temperatures up to 1080 K. High-temperature time-resolved synchrotron X-ray diffraction measurements showed that significant volume expansion due to temperature-induced amorphization caused static stress, which then hindered amorphization progress. Further, the temperature required for the amorphization of fine-grained bridgmanite (~1 mm) was found to be approximately 100 K higher than that required for the amorphization of coarse-grained samples (> 10 mm). We also noted that amorphization preferentially commenced at the twin and subgrain boundaries of bridgmanite grains, resulting in lower amorphization temperatures for the coarse-grained samples. The limited number of such specific locations in fine-grained natural bridgmanite suggested that grain boundary amorphization may be the dominant mechanism for bridgmanite amorphization in shocked meteorites. This unique amorphization kinetics would support the preservation of bridgmanite during the post-shock annealing in the shocked meteorite. Although bridgmanite amorphization starts easily at temperature above ~420 K, small amount of bridgmanite grains can survive at temperatures above 800 K by the effect of the amorphization-induced stress.