title: Coesite and stishovite in a shocked lunar meteorite, Asuka-881757, and impact events in lunar surface.
abstract: Microcrystals of coesite and stishovite were discovered as inclusions in amorphous silica grains in shocked melt pockets of a lunar meteorite Asuka-881757 by micro-Raman spectrometry, scanning electron microscopy, electron back-scatter diffraction, and transmission electron microscopy. These high-pressure polymorphs of SiO(2) in amorphous silica indicate that the meteorite experienced an equilibrium shock-pressure of at least 8-30 GPa. Secondary quartz grains are also observed in separate amorphous silica grains in the meteorite. The estimated age reported by the (39)Ar/(40)Ar chronology indicates that the source basalt of this meteorite was impacted at 3,800 Ma ago, time of lunar cataclysm; i.e., the heavy bombardment in the lunar surface. Observation of coesite and stishovite formed in the lunar breccias suggests that high-pressure impact metamorphism and formation of high-pressure minerals are common phenomena in brecciated lunar surface altered by the heavy meteoritic bombardment.
authors.affiliation: Department of Earth Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan. ohtani@m.tohoku.ac.jp
authors.forename: E
authors.initials: E
authors.lastname: Ohtani
authors.forename: S
authors.initials: S
authors.lastname: Ozawa
authors.forename: M
authors.initials: M
authors.lastname: Miyahara
authors.forename: Y
authors.initials: Y
authors.lastname: Ito
authors.forename: T
authors.initials: T
authors.lastname: Mikouchi
authors.forename: M
authors.initials: M
authors.lastname: Kimura
authors.forename: T
authors.initials: T
authors.lastname: Arai
authors.forename: K
authors.initials: K
authors.lastname: Sato
authors.forename: K
authors.initials: K
authors.lastname: Hiraga
mesh_terms: D019110:Meteoroids; D008855:Microscopy, Electron, Scanning / Q000379:methods; D046529:Microscopy, Electron, Transmission / Q000379:methods; D008903:Minerals / Q000032:analysis; D016081:Moon; D011312:Pressure; D011791:Quartz; D012822:Silicon Dioxide; D013059:Spectrum Analysis, Raman / Q000379:methods*; D013499:Surface Properties; D013997:Time Factors
chemical_list: D008903:Minerals; D011791:Quartz; D012822:Silicon Dioxide