Shock veins and brown olivine in Martian meteorites: Implications for their shock pressure–temperature histories

1,2Atsushi Takenouchi,1,3Takashi Mikouchi,4,5Akira Yamaguchi
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13120]
1Department of Earth and Planetary Science, Graduate School of Arts and Sciences, The University of Tokyo, Bunkyo‐ku, Tokyo, Japan
2Department of Basic Sciences, Graduate School of Science, The University of Tokyo, Meguro‐ku, Tokyo, Japan
3The University Museum, The University of Tokyo, Bunkyo‐ku, Tokyo, Japan
4National Institute of Polar Research, Tachikawa‐shi, Tokyo, Japan
5Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies)Tachikawa‐shi, Tokyo, Japan
Published by arrangement with John Wiley & Sons

Shergottite Martian meteorites are known to contain brown‐colored olivine (brown olivine), which is considered to form during a shock event on Mars. In order to constrain the formation conditions of brown olivine, four shergottites with brown olivine and four shergottites without brown olivine are analyzed in this study. Based on our observations, brown olivine is often accompanied by thin (<10 μm) melt veins indicating local temperature increase (1750–1870 K). Even in shergottites without brown olivine, olivine around shock melt veins/pockets is partly darkened and shows similar features to those of brown olivine. These observations support that brown olivine is formed under conditions similar to those around shock melt veins/pockets. Components of shock melt veins (vesicles, quench crystals, etc.) and the absence of high‐pressure phases in shergottites with brown olivine indicate that they have high postshock temperature (>1200–1170 K). Such high postshock temperature may indicate that shergottites with brown olivine experienced high pressure (around 55 GPa), while shergottites without brown olivine experienced lower shock pressure (>20–35 GPa). Therefore, brown olivine may be a good indicator for strong shock events (peak shock pressure: ~55 GPa; postshock temperature: >1200–1170 K) and such shock events could be induced by small but rapid projectiles onto Mars.

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