High-pressure polymorphs in Yamato-790729 L6 chondrite and their significance for collisional conditions

1Yukako Kato,1,2Toshimori Sekine,1,3Masahiko Kayama,1,4Masaaki Miyahara,5,6Akira Yamaguchi
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12957]
1Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Japan
2Center for High Pressure Science and Technology Advanced Research, Shanghai, China
3Creative Interdisciplinary Research Division, Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, Japan
4Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai, Japan
5National Institute of Polar Research, Tokyo, Japan
6Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo, Japan
Published by arrangement with John Wiley & Sons

Shock pressure recorded in Yamato (Y)-790729, classified as L6 type ordinary chondrite, was evaluated based on high-pressure polymorph assemblages and cathodoluminescence (CL) spectra of maskelynite. The host-rock of Y-790729 consists mainly of olivine, low-Ca pyroxene, plagioclase, metallic Fe-Ni, and iron-sulfide with minor amounts of phosphate and chromite. A shock-melt vein was observed in the hostrock. Ringwoodite, majorite, akimotoite, lingunite, tuite, and xieite occurred in and around the shock-melt vein. The shock pressure in the shock-melt vein is about 14–23 GPa based on the phase equilibrium diagrams of high-pressure polymorphs. Some plagioclase portions in the host-rock occurred as maskelynite. Sixteen different CL spectra of maskelynite portions were deconvolved using three assigned emission components (centered at 2.95, 3.26, and 3.88 eV). The intensity of emission component at 2.95 eV was selected as a calibrated barometer to estimate shock pressure, and the results indicate pressures of about 11–19 GPa. The difference in pressure between the shock-melt vein and host-rock might suggest heterogeneous shock conditions. Assuming an average shock pressure of 18 GPa, the impact velocity of the parent-body of Y-790729 is calculated to be ~1.90 km s−1. The parent-body would be at least ~10 km in size based on the incoherent formation mechanism of ringwoodite in Y-790729.

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