[P-PS07] Planetary Sciences

[PPS07-P07] Total assessment of metamorphism in laser-shocked SiO₂ based on the multiple analysis

*Tadashi Kondo¹, Masakazu Ohno¹, Tatsuhiro Sakaiya¹, Akira Miyake², Masayuki Nishi¹, Keisuke Shigemori³ (1.Department of Earth and Space Science, Graduate School of Science, Osaka University, 2.Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, 3.Institute of Laser Engineering, Osaka University)

Keywords:shock metamorphism, x-ray diffraction, SiO2

Impact events in the space are most important as one of the elementary processes of planet formation and evolution, and various shock metamorphism have been found in meteorites that have experienced impact events. Since the impact velocity and parent body size of meteorites and planetesimals are estimated from these structures, it is very interesting to experimentally recover the shock-metamorphosed minerals to investigate their detailed formation. Our group has succeeded in almost complete recovery of a SiO₂ sample which have laser-shocked to over 100 GPa [1]. In this study, we observed the recovered samples using several techniques such as optical microscopy, micro-focused X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) to elucidate the metamorphic mechanism, and comprehensively evaluate the impact metamorphism. The results of microstructural observations by optical microscopy and SEM showed that the recovered samples could be divided into basically four metamorphic regions, Regions I-IV, in order from the impact point. The phase transformation of SiO₂ to high-pressure /high-temperature phases are examined by micro-focused X-ray diffraction, and it was found that in the region I, stishovite, a high-pressure phase, was locally distributed in crystalline or amorphized quartz. The lattice volume of quartz and the line width of diffraction peaks were also examined as a function of the distance from the impact point, and it was found that the lattice volume increased in regions I~II and the full width at half maximum of diffraction peaks increased in regions I~III. The results of SEM/TEM observations also show a quite distinguishable texture in each region. Based on these detailed observations in multiple analysis, we will present a classification of impact metamorphism and a discussion of its metamorphic mechanism.