Silica minerals in cumulate eucrites: Insights into their thermal histories

1Haruka ONO,2,3Atsushi TAKENOUCHI,4Takashi MIKOUCHI,3Akira YAMAGUCHI
Meteoritics & Planetary Science (in Press) Link to Article []
1Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
2Department of Basic Science, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan
3National Institute of Polar Research (NIPR), 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan
4The University Museum, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Published by arrangement with John Wiley & Sons

Some eucrites contain up to 10 vol% silica minerals; however, silica minerals havenot been studied in detail so far. We performed a mineralogical study of silica minerals inthree cumulate eucrites (Moore County, Moama, and Yamato [Y] 980433). Monoclinictridymite was common in all three samples. Moama contained orthorhombic tridymite aslamellae within monoclinic tridymite grains. Y 980433 included quartz around an impactmelt vein. The presence of orthorhombic tridymite in Moama indicates that Moama cooledmore rapidly than the other two samples at low temperatures (<400°C). This result isdifferent from the slower cooling rates of Moama (≳0.0004°Cyr1) than that of MooreCounty (>0.3°Cyr1, after the shock event) at high temperatures (>500°C) estimated fromcompositional profiles of pyroxene exsolution lamellae. The difference of the cooling ratesmay reflect their geological settings. Y 980433 cooled slowly at low temperature, as didMoore County. Quartz in Y 980433 could be a local product transformed from monoclinictridymite by a shock event. We suggest that silica minerals in meteorites record thermalhistories at low temperatures and shock events.


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