1,2,3Sen Hu,4Yang Li,1Lixin Gu,1Xu Tang,1Ting Zhang,5Akira Yamaguchi,1,2,3 Yangting Lin,1Hitesh Changela
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.07.021]
1Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
2Innovation Academy for Earth Science, Chinese Academy of Sciences, Beijing 100029, China
3College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
4Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
5National Institute of Polar Research, Tokyo 190-8518, Japan
We report occurrences of coesite in a martian meteorite, expending previously-reported silica polymorphs such as stishovite (El Goresy et al., 2000), seifertite (Goresy et al., 2008; Sharp et al., 1999), and post-stishovite (El Goresy et al., 2000). The coesite was found in the shock-induced melt regions of NWA 8657, usually coexisting with deformed quartz and silica glass. Three morphological types of coesite have been identified: (I) in a silica-maskelynite assemblage, (II) needle grains, and (III) granular grains embedded in maskelynite. Transmission Electron Microscopy (TEM) shows that all types of coesite appear distributed in silica glass and/or nano-phase maskelynite. The stishovite-like morphology of Type II coesite and the presence of deformed quartz suggest coesite to have inverted from stishovite during decompression. The impact-induced peak pressures and temperatures are estimated at ∼ 18-30 GPa and ∼ 2000 ℃ respectively, based on static high pressure experiments (Langenhorst and Deutsch, 2012; Zhang et al., 1996). The polymorphs aggregates of silica in NWA 8657 indicate that the shock-induced melts in this meteorite cooled slower than those in other stishovite-bearing martian meteorites, but fast enough to preserve coesite.