Implanted solar helium, neon, and argon in individual lunar ilmenite grains: Surface effects and a temporal variation in the solar wind composition

Abstract

Solar implanted helium, neon, and argon were extracted from individual ilmenite grains from lunar soil 71501 and soil breccia 79035 using laser vaporization and analyzed by static mass spectrometry. Clear differences were observed for these two grain populations, one from a contemporary soil and the other from an ancient soil exposed on the lunar surface approximately 1 Ga ago. The different trends between the 20Ne 22Ne ratios and the He Ne ratios do not simply reflect differences either in regolith gardening or in diffusive losses but rather suggest a greater relative helium abundance in the ancient solar wind by a factor of about 1.8. The majority of the grains are enriched in solar energetic particle (SEP) Ne relative to solar wind (SW) Ne in a manner that increases with surface exposure. The progressive enrichment in retained SEP Ne relative to SW Ne is explained by a combination of diffusion and nonfractionating losses of the less deeply implanted SW component. The neon isotopic differences observed in various analyses of ilmenite separates from these two soils and previously attributed to a secular variation of either the SW SEP flux ratio or the SW Ne isotopic composition may alternatively be a natural consequence of greater SW losses which accompany an enhanced helium flux in the ancient solar wind.