^1,2,3Katherine H. Joy, ⁴Channon Visscher, ⁵Michael E. Zolensky, ⁶Takashi Mikouchi, ⁷Kenji Hagiya, ⁸Kazumasa Ohsumi, ^1,2David A. Kring
¹Center for Lunar Science and Exploration, The Lunar and Planetary Institute—USRA, Houston, Texas, USA
² NASA Solar System Exploration Research Virtual Institute
³School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK
⁴ Dordt College, Iowa, USA
⁵ARES, NASA Johnson Space Center, Houston, Texas, USA
⁶Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo, Japan
⁷Graduate School of Science, University of Hyogo, Ako-gun, Hyogo, Japan
⁸Japan Synchrotron Radiation Research Institute (JASRI), Sayo-gun, Hyogo, Japan

Lunar regolith breccias are temporal archives of magmatic and impact bombardment processes on the Moon. Apollo 16 sample 60016 is an “ancient” feldspathic regolith breccia that was converted from a soil to a rock at ~3.8 Ga. The breccia contains a small (70 × 50 μm) rock fragment composed dominantly of an Fe-oxide phase with disseminated domains of troilite. Fragments of plagioclase (An95-97), pyroxene (En74-75, Fs21-22,Wo3-4), and olivine (Fo66-67) are distributed in and adjacent to the Fe-oxide. The silicate minerals have lunar compositions that are similar to anorthosites. Mineral chemistry, synchrotron X-ray absorption near edge spectroscopy (XANES) and X-ray diffraction (XRD) studies demonstrate that the oxide phase is magnetite with an estimated Fe3+/ΣFe ratio of ~0.45. The presence of magnetite in 60016 indicates that oxygen fugacity during formation was equilibrated at, or above, the Fe-magnetite or wüstite–magnetite oxygen buffer. This discovery provides direct evidence for oxidized conditions on the Moon. Thermodynamic modeling shows that magnetite could have been formed from oxidization-driven mineral replacement of Fe-metal or desulphurisation from Fe-sulfides (troilite) at low temperatures (<570 °C) in equilibrium with H₂O steam/liquid or CO₂ gas. Oxidizing conditions may have arisen from vapor transport during degassing of a magmatic source region, or from a hybrid endogenic–exogenic process when gases were released during an impacting asteroid or comet Impact.

Reference
Joy KH, Visscher C, Zolensky ME, Mikouchi T, Hagiya K, Ohsumi K, Kring DA (2015) Identification of magnetite in lunar regolith breccia 60016: Evidence for oxidized conditions at the lunar surface. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12462]

Published by arrangement with John Wiley&Sons