Bridgmanite Freezing in Shocked Meteorites Due To Amorphization‐Induced Stress

Bridgmanite, the most abundant mineral in the Earth's lower mantle, can be found in meteorites that experienced instantaneous high shock pressure during parent body impact. However, the presence of bridgmanite in meteorites is unusual because bridgmanite grains should be amorphized under residual post‐shock temperatures at ambient pressure. Here, we report the results of time‐resolved synchrotron X‐ray diffraction measurements at high temperatures to analyze the amorphization mechanisms and kinetics of bridgmanite. The thermal expansion coefficient of bridgmanite before the amorphization is 2.1 × 10−5 K−1. At higher temperatures, our results show that the significant volume expansion due to the amorphization induces static stress that can reach up to ∼0.5 GPa, which prevents the progress of the amorphization. This time‐insensitive amorphization kinetics may have enabled the preservation of bridgmanite in the shocked meteorite that fell on Earth. Also, the reaction progress estimated based on the amorphous fraction provides the residual post‐shock temperature. Plain Language Summary Bridgmanite is the most abundant mineral in the Earth's lower mantle. Natural bridgmanite in the shocked meteorite is used to constrain the impact histories of parent bodies since this mineral can form at very high pressure. However, the presence of bridgmanite in meteorites is mysterious because it should be easily amorphized at atmospheric pressure after the shock events. This study reports the amorphization mechanisms and kinetics of bridgmanite based on time‐resolved synchrotron X‐ray diffraction techniques. We found that the significant volume expansion due to the amorphization induces static stress, which hinders the amorphization from progressing. The preservation of bridgmanite in meteorites can be explained by this unique amorphization kinetics. Key Points Amorphization kinetics of bridgmanite was studied via time‐resolved synchrotron X‐ray diffraction measurements Significant volume expansion due to the bridgmanite amorphization induces static stress, which prevents the progress of the amorphization Preservation of bridgmanite in the meteorites can be explained by the unique amorphization kinetics