Association of silica phases as geothermobarometer for eucrites: Implication for two-stage thermal metamorphism in the eucritic crust

1,2Haruka Ono,3Atsushi Takenouchi,1,4Takashi Mikouchi,3,5Akira Yamaguchi,6,7Masahiro Yasutake,8Akira Miyake,8,9,10Akira Tsuchiyama
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
2Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1, Tsudanuma, Narashino, Chiba, 275-0016 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
5Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo, 190-8518 Japan
6Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo, 679-5198 Japan
7Research Organization of Science and Technology, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga, 525-8577 Japan
8Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwake, Sakyo-ku, Kyoto, 606-8502 Japan
9CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (CAS), Guangzhou, 510640 People’s Republic of China
10CAS Center for Excellence in Deep Earth Science, Guangzhou, 510640 People’s Republic of China
Published by arrangement with John Wiley & Sons

Silica mineral is present in different stable polymorphs depending on the temperature and pressure conditions of crystallization. We suggest using silica mineral phases to constrain the thermal history of eucrites. We focused on silica minerals in basaltic clasts of nine non-cumulate eucrites to compare with previously studied cumulate eucrites. Our observations indicate an apparent relationship between thermal metamorphic degrees and silica phase texture in basaltic clasts of non-cumulate eucrites. To reveal complex transformation relations between silica polymorphs in eucrites, we performed cooling experiments (cooling rate: 1 and 0.1 °C h−1) and heating experiments (heating 500 °C for 168 h and 800 °C for 96 h) using eucrites. The cooling experiments show that cristobalite is an initial silica phase crystallized from eucritic magma and transforms to quartz at the cooling rate between 0.1 and 1 °C h−1. Based on the cooling experiments and observations of eucrites, we suggest that a combination of silica minerals varies depending mainly on cooling rates. According to the heating experiments, monoclinic tridymite hardly transforms to other phases at low temperature by short reheating events such as brecciation. Monoclinic tridymite can partially transform to quartz with a “hackle” fracture. We conclude that a reheating event partially transformed monoclinic tridymite to quartz to form aggregates of monoclinic tridymite and quartz with the hackle fracture in eucrites. We suggested that some basaltic clasts in non-cumulate eucrites experienced two-stage thermal metamorphism in the eucritic crust. The first metamorphic event has resulted from burial under lava produced by successive eruptions. Igneous intrusions into the preformed crust may have caused the second metamorphic event. The intrusions heated the deep eucritic crust and induced the transformation from monoclinic tridymite to quartz.


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