Irradiation records in regolith materials. I: isotopic compositions of solar-wind neon and argon in single lunar mineral grains |
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| Institution: | 1. Kamioka Observatory, Institute for Cosmic Ray Research, the University of Tokyo, Higashi-Mozumi, Kamioka, Hida, Gifu, 506–1205, Japan;2. Center for Underground Physics, Institute for Basic Science, 70 Yuseong-daero 1689-gil, Yuseong-gu, Daejeon, 305–811, South Korea;3. Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Aichi 464–8601, Japan;4. Kavli Institute for the Physics and Mathematics of the Universe (WPI), the University of Tokyo, Kashiwa, Chiba, 277–8582, Japan;5. Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi, 464–8602, Japan;6. Department of Physics, Kobe University, Kobe, Hyogo 657–8501, Japan;7. Korea Research Institute of Standards and Science, Daejeon 305–340, South Korea;8. Department of Physics, Miyagi University of Education, Sendai, Miyagi 980-0845, Japan;9. Department of Physics, Tokai University, Hiratsuka, Kanagawa 259–1292, Japan;10. Department of Physics, Tokushima University, 2-1 Minami Josanjimacho Tokushima city, Tokushima, 770–8506, Japan;11. Department of Physics, Faculty of Engineering, Yokohama National University, Yokohama, Kanagawa 240–8501, Japan;1. Department of Human Nutrition, Foods, and Exercise, Virginia Tech, Blacksburg, VA;2. Department of Medicine/Division of Endocrinology, University of Virginia, Charlottesville, VA;3. Metabolic Phenotyping Core, Virginia Tech, Blacksburg, VA |
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| Abstract: | We have applied a stepwise pyrolytic extraction technique to eleven individual lunar regolith grains to investigate the compositions of light noble gases embedded in grain surfaces by solar wind irradiation, with emphasis on the rather poorly known isotopic composition of solar-wind argon. Results are intriguing: average 20Ne/22Ne ratios observed in early pyrolytic releases from ilmenite grains separated from lunar soils 71501, 79035 and 10084 agree very well with both direct measures of the solar wind neon composition in the Apollo foils and with values obtained in first releases from acid-etched ilmenites by the Zürich laboratory, whereas these same pyrolytic and acid-etch fractions carry argon isotopic signatures that significantly disagree—average 36Ar/38Ar ratios near 5.8 for thermal extraction compared to 5.4–5.5 for chemical etching at Zürich. Consideration of the isotopic and elemental data from these grains in the context of first-order diffusive modeling calculations points to gas release at low temperatures, without significant isotopic or elemental fractionation, from isolated grain-surface reservoirs of solar wind composition. The physical nature of these reservoirs is presently unknown. In this interpretation the preferred solar wind 20Ne/22Ne and 21Ne/22Ne ratios deduced from this study are respectively 13.81 ± 0.08 and 0.0333 ± 0.0003, both within error of the Zürich acid-etch values, and 36Ar/38Ar = 5.77 ± 0.08. It may be possible to reconcile the discrepancy between the acid-etch and pyrolytic estimates for the solar wind 36Ar/38Ar ratio in the context of arguments originally advanced by Benkert et al. (1993) to account for their He and Ne isotopic compositions. At the other, high-temperature end of the release profile from one of these grains there are clear isotopic indications of the presence of a Ne constituent with 20Ne/22Ne close to the 11.2 ratio found at Zürich and attributed by these workers to a deeply-sited component implanted by solar energetic particles. |
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