Day: November 3, 2020

Vanadium micro-XANES determination of oxygen fugacity in olivine-hosted glass inclusion and groundmass glasses of martian primitive shergottite Yamato 980459

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1,2Ryoichi Nakada,2Tomohiro Usui,3Masashi Ushioda,4Yoshio Takahashi
American Mineralogist 105, 1695–1703 Link to Article [http://www.minsocam.org/msa/ammin/toc/2020/Abstracts/AM105P1695.pdf]
1Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Monobe 200, Nankoku, Kochi 783-8502, Japan
2Earth-Life Science Institute, Tokyo Institute of Technology, Meguro, Tokyo 152-8550, Japan
3Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan
4Department of Earth and Planetary Science, The University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
Copyright: The Mineralogical Society of America

The redox condition of magma determines the stability and composition of crystallizing and volatile phases in martian meteorites, reflecting the evolution of the martian interior. In the current study, direct analyses on the oxidation states of V, Cr, and Fe were performed based on the X-ray absorption near- edge structure (XANES) measurements equipped with a micro-sized X-ray beam. We first applied the micro-XANES (μ-XANES) technique to the olivine-hosted glass inclusion and groundmass glass of martian meteorite Yamato 980459 (Y98), which is interpreted as representing a primary melt composi- tion. Mass-balance calculations and XANES spectra comparisons indicated that, while chromite and pyroxene affected Cr and Fe K-edge XANES spectra, the contribution of these minerals was minimal for V. The pre-edge peak intensity of V K-edge XANES enabled the estimation of the oxygen fugac- ity for inclusion and groundmass glasses. The calculated oxygen fugacity (fO2) of the glass inclusions was near the Iron-Wüstite (IW) buffer (IW-0.07 ± 0.32) for the glass inclusion, whereas it was 0.9 log units more oxidized (IW+0.93 ± 0.56) for the groundmass glasses. This result suggests that the redox condition of the parent magma of Y98 evolved during magma ascent and emplacement. Since Y98 is interpreted to have evolved in a closed system, our finding suggests that fractional crystallization and/or ascent of magma potentially induces the fO2 increase. This study shows that the μ-XANES technique enables us to determine the fO2 by only measuring a single phase of glassy compounds, and thus, it is useful to discuss the redox condition of volcanic rocks even if they do not crystallize out several equilibrium phases of minerals.

Evidence for dielectric breakdown weathering on the Moon

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1,2A.P.Jordan
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114199]
1Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH, USA
2Solar System Exploration Research Virtual Institute, NASA Ames Research Center, Moffett Field, CA, USA
Copyright Elsevier

Soil on the Moon is darkened by space weathering, a process generally assumed to be dominated by the solar wind and/or micrometeoroid impacts. Recent work, however, predicts that another process darkens the soil: large solar energetic particle events may cause dielectric breakdown (or “sparking”), melting, and vaporizing soil at a rate comparable to that of micrometeoroids. Unlike the solar wind and/or micrometeoroids, a combination of dielectric breakdown and micrometeoroid weathering can explain how the reflectance of the lunar maria varies with latitude at 750 and 1064 nm, and this combination provides a reasonable mechanism to explain how magnetic anomalies form prominent swirls in the maria. Consequently, space weathering in the lunar maria seems to be dominated by micrometeoroid impacts and dielectric breakdown.