レーザー衝撃圧縮によるMg₂SiO₄の構造相転移その場計測

抄録

Primitive materials in the Solar System, such as meteorites and return samples from near-Earth asteroids, often preserve unequivocal records of their evolution histories in the microtextural and crystallographic properties of their constituent minerals. In strongly shocked meteorites, the formation of dense polymorphs provides clear evidence for impacts on their parent bodies. Here, we experimentally evaluate these processes as functions of shock pressure and its duration. We successfully observed the structure transformation process from olivine［α-（Mg, Fe）₂SiO₄］to its high-pressure polymorph ringwoodite［γ-（Mg, Fe）₂SiO₄］, which is the most common high-pressure polymorph in meteorites, during shock compression. In these experiments, a high-power optical laser pulse was focused onto a single crystal of α-Mg₂SiO₄ to induce strong shock compression, and the transformation process was time-resolved using ultrafast diffractometry using SACLA X-ray free electron laser pulse with femtosecond-order duration. We found that a lattice-shear mechanism was activated in the α-（Mg, Fe）₂SiO₄ structure within several nanoseconds. This timescale corresponds to shock compression associated with impact events of sub-kilometer-sized bodies. Since shorter-duration compression events occur more frequently in asteroid impacts, such an ultrafast lattice-shear mechanism is likely to be widely recorded. Furthermore, the lattice-shear mechanism predominantly operates during shock releases. Therefore, the peak shock pressures of meteorites previously deduced from the existence of shear-produced ringwoodite may have been significantly underestimated.