IMPLANTED SOLAR HELIUM, NEON, AND ARGON IN INDIVIDUAL LUNAR ILMENITE GRAINS - SURFACE EFFECTS AND A TEMPORAL VARIATION IN THE SOLAR-WIND COMPOSITION

Solar implanted helium, neon, and argon were extracted from individual ilmenite grains from lunar soil 71501 and soil breccia 79035 using la ser vaporization and analyzed by static mass spectrometry. Clear diffe rences were observed for these two grain populations, one from a conte mporary soil and the other from an ancient soil exposed on the lunar s urface approximately 1 Ga ago. The different trends between the Ne-20/ Ne-22 ratios and the He/Ne ratios do not simply reflect differences ei ther in regolith gardening or in diffusive losses but rather suggest a greater relative helium abundance in the ancient solar wind by a fact or of about 1.8. The majority of the grains are enriched in solar ener getic particle (SEP) Ne relative to solar wind (SW) Ne in a manner tha t increases with surface exposure. The progressive enrichment in retai ned SEP Ne relative to SW Ne is explained by a combination of diffusio n and nonfractionating losses of the less deeply implanted SW componen t. The neon isotopic differences observed in various analyses of ilmen ite separates from these two soils and previously attributed to a secu lar variation of either the SW/SEP flux ratio or the SW Ne isotopic co mposition may alternatively be a natural consequence of greater SW los ses which accompany an enhanced helium flux in the ancient solar wind.