FT-IR/PAS Studies of Lunar Regolith Samples

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 9 50 lm) rock fragment composed dominantly of an Fe-oxide phase with disseminated domains of troilite. Fragments of plagioclase (An 95-97 ), pyroxene (En 74-75 , Fs 21-22 ,Wo 3-4 ), and olivine (Fo 66-67 ) 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 Fe 3+ /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 w€ ustite-magnetite oxygen buffer. This discovery provides direct evidence for oxidized conditions on the Moon. Thermodynamic modeling shows that magnetite could have been formed from oxidizationdriven mineral replacement of Fe-metal or desulphurisation from Fe-sulfides (troilite) at low temperatures (<570°C) in equilibrium with H 2 O steam/liquid or CO 2 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. BACKGROUND

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The oxidation state of nanophase Fe particles in lunar soil: Implications for space weathering

Thompson

Zega

Becerra

et al. 2016

Meteorit & Planetary Scien

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We report measurements of the oxidation state of Fe nanoparticles within lunar soils that experienced varied degrees of space weathering. We measured >100 particles from immature, submature, and mature lunar samples using electron energy‐loss spectroscopy (EELS) coupled to an aberration‐corrected transmission electron microscope. The EELS measurements show that the nanoparticles are composed of a mixture of Fe0, Fe2+, and Fe3+ oxidation states, and exhibit a trend of increasing oxidation state with higher maturity. We hypothesize that the oxidation is driven by the diffusion of O atoms to the surface of the Fe nanoparticles from the oxygen‐rich matrix that surrounds them. The oxidation state of Fe in the nanoparticles has an effect on modeled reflectance properties of lunar soil. These results are relevant to remote sensing data for the Moon and to the remote determination of relative soil maturities for various regions of the lunar surface.

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FT-IR/PAS Studies of Lunar Regolith Samples

Cited by 3 publications

References 5 publications

Thermally induced phase transition of troilite during micro-raman spectroscopy analysis

Thermally induced phase transition of troilite during micro-raman spectroscopy analysis

Identification of magnetite in lunar regolith breccia 60016: Evidence for oxidized conditions at the lunar surface

The oxidation state of nanophase Fe particles in lunar soil: Implications for space weathering

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