IRRADIATION RECORDS IN REGOLITH MATERIALS. I: ISOTOPIC COMPOSITIONS OF SOLAR-WIND NEON AND ARGON IN SINGLE LUNAR MINERAL GRAINS - CONSEQUENCES OF ASSUMING HYDRODYNAMIC LOSS DRIVEN BY GIANT IMPACT

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 Zurich 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 Zurich. 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 Zurich 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 Zurich and attributed by these workers to a deeply-sited component implanted by solar energetic particles.