[P-CG22] Shock responses of planetary materials elucidated from meteorites and dynamic compression experiments

[PCG22-04] Linking occurrence and texture of dense silicate minerals in shocked meteorites with laser-shock experimental results of Mg₂SiO₄ analyzed by XFEL probe

*奥地 拓生¹、Narangoo Purevjav¹、尾崎 典雅²、松岡 健之²、瀬戸 雄介³、丹下 慶範⁴、薮内 俊毅⁵、富岡 尚敬⁶、関根 利守⁷、兒玉 了祐² (1.岡山大学惑星物質研究所、2.大阪大学工学研究科、3.神戸大学理学研究科、4.高輝度光科学研究センター、5.理化学研究所放射光科学研究センター、6.海洋研究開発機構高知コア研究所、7.Center for High Pressure Science and Technology Advanced Research)

キーワード：高密度鉱物、レーザー衝撃圧縮実験、衝撃変成、Mg2SiO4

Primitive meteorites often show features of shock metamorphism, including occurrences of dense polymorphs of major mineral phases. By previous transmission electron microscopy studies it was demonstrated that olivine (alpha-Mg₂SiO₄) often had transformed into its denser polymorphs such as ringwoodite (gamma-Mg₂SiO₄), and as a recently-discovered new structure (epsilon-Mg₂SiO₄: Tomioka and Okuchi, Sci. Rep. 2017). The impact events that produced these polymorphs played essential roles in the history of early solar system evolution. Here we tried to reproduce such a dynamic transformation process starting from forsterite olivine single crystals by using very strong laser beam and x-ray free electron laser (XFEL) beam in combination at SACLA, SPring-8, Japan. A transformation process has been successfully observed with sub-nanosecond resolution by x-ray diffraction using femtosecond XFEL pulses. We discovered an ultrafast transformation process completing only within few nanoseconds, which was not expected to occur during shock metamorphism of meteorites. It can be a shear-induced fast lattice slipping proceeding without any atomic diffusion process. We conclude that formation processes and environments of some of dense Mg₂SiO₄ polymorphs in heavily shocked meteorites are needed to be reconsidered to involve this fast process.