Identification of Magnetite in Lunar Regolith Breccia 60016: evidence for oxidised conditions at the lunar surface

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Abstract

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+/Fe 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 wustite-magnetite oxygen buffer. This discovery provides direct evidence for oxidised conditions on the Moon. Thermodynamic modelling shows that magnetite could have been formed from oxidisation-driven mineral replacement of Fe-metal or desulphurisation from Fe-sulphides (troilite) at low temperatures (<570 °C) in equilibrium with H2O steam/liquid or CO2 gas. Oxidising conditions may have arisen from vapour 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.

Keyword(s)

lunar-regolith, regolith-breccia, magnetite, lunar-volatiles, lunar-oxidation

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