Apollo 15 regolith breccia provides first natural evidence for olivine incongruent melting

1,2Niccolò Satta,3Masaaki Miyahara,4Shin Ozawa,2Hauke Marquardt,5Masahiko Nishijima,6Tomoko Arai,4Eiji Ohtani
American Mineralogist 107, 1661-1667 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2022/Abstracts/AM107P1661.pdf]

1Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth 95440, Germany
2Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, U.K.
3Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
4Department of Earth Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
5Laboratory for Protein Crystallography and Laboratory for Electron Structural Biology Institute for Protein Research, Osaka University, Suita-shi Osaka 565-0871, Japan
6Planetary Exploration Research Center, Chiba Institute of Technology, Chiba 275-0016, Japan
Copyright: The Mineralogical Society of America

The Apollo 15 mission returned various samples of regolith breccias, typical lunar rocks lithified
by impact events on the Moon’s surface. Here we report our observations on shock features recorded
in a section of the Apollo Sample 15299. We observe the presence of ferropericlase crystals confined
in a shock-melt pocket and conclude that their formation is related to a shock-induced incongruent
melting of olivine. While predicted by experiments, this phenomenon has never been observed in a
natural sample. The incongruent melting of olivine provides an important signature of melting under
high-pressure conditions and allows for estimating the pressure-temperature (P-T) experienced by the
studied sample during the impact event. We infer that the fracture porosity that likely characterized the
studied sample prior to the shock event critically affected the P-T path during the shock compression
and allowed the studied sample to be subjected to elevated temperature during relatively low shock
pressures.

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