Formation and decomposition of vacancy-rich clinopyroxene in a shocked eucrite: New insights for multiple impact events

1,2Ai-Cheng Zhang,1Jie-Ya Li,1Jia-Ni Chen,3Yuan-Yun Wen,4Yan-Jun Guo,2,3Yang Li,5Naoya Sakamoto,5,6Hisayoshi Yurimoto
Geochimica et Cosmochimica Acta (in Press) Link to Article []
1State Key Laboratory for Mineral Deposits Research, School of Earth Science and Engineering, Nanjing University, Nanjing 210023, China
2CAS Center for Excellence in Comparative Planetary, Hefei 230026, China
3Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
4CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing, China
5Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 010-0021, Japan
6Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
Copyright Elsevier

Impact is a fundamental process shaping the formation and evolution of planets and asteroids. It is inevitable that some materials on the surface of planets and asteroids have been impacted for many times. However, unambiguous petrological records for multiple post-formation impact events are rarely described. Here, we report that the thin shock melt veins of the shocked eucrite Northwest Africa 8647 are dominated by a fine-grained intergranular or vermicular pigeonite and anorthite assemblage, rather than compact vacancy-rich clinopyroxene. Vacancy-rich clinopyroxene in the veins instead is ubiquitous as irregularly-shaped, relict grains surrounded by intergranular or vermicular pigeonite and anorthite assemblage. The silica fragments entrained in shock melt veins contain a coesite core and a quartz rim. The occurrences of vacancy-rich clinopyroxene and coesite can be best explained by two impact events. The first impact event produced the shock melt veins and lead to the formation of vacancy-rich clinopyroxene and coesite. The second impact event heated the fine-grained melt veins and lead to the widespread partial decomposition of vacancy-rich clinopyroxene and the partial back-transformation of coesite. This paper is the first report of the decomposition reaction of shock-induced vacancy-rich clinopyroxene in extraterrestrial materials. We propose that widespread decomposition and/or back-transformation of high-pressure minerals in shocked meteorites can be considered as important records of multiple impact events.


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