HELIUM IN LUNAR SAMPLES ANALYZED BY HIGH-RESOLUTION STEPWISE ETCHING: IMPLICATIONS FOR THE TEMPORAL CONSTANCY OF SOLAR WIND ISOTOPIC COMPOSITION

摘要

The ~3He/~4He ratio in the present and past solar wind (SW) is of interest because it provides an estimate for the protosolar deuterium abundance. SW He and Ne composition has been analyzed in ilmenites of lunar soils (71501, 12001, 74241) and regolith breccias (79135, 79035, 14301) by closed system stepwise etching. The samples have SW antiquities ranging from ～0.1 to ～3.7 Gyr. The experiments have a very high depth resolution; thus, possible artifacts that may compromise the precise determination of the SW composition can be controlled. The nominal average (~3He/~4He)_(SW) ratios correlate with the SW antiquity, suggesting at first glance a (~3He/~4He)_(SW) increase of 6% per Gyr. In addition, also the (~(20)Ne/~(22)Ne)_(SW) ratio seems to have increased with time by about 2% per Gyr. However, there are reasons indicating that the apparent temporal increase of (~3He/~4He)_(SW) is an artifact. First, there is no straightforward explanation for a concurrent temporal change of (~(20)Ne/~(22)Ne)sw to the extent observed. Second, the average nominal (~3He/~4He)_(SW) and (~(20)Ne/~(22)Ne)_(SW) ratios correlate with each other, and this correlation parallels the one displayed by the single etch steps, indicating a progressive admixture of the deeper sited, isotopically heavier solar energetic particles with ongoing etching. We suggest that the nominal temporal variation of (~3He/~4He)_(SW) and (~(20)Ne/~(22)Ne)_(SW) is the result of secondary processes that cause erosion of the outermost layers of the regolith grains and thus a partial removal of the very surface-sited S W. Correcting this partial grain surface loss, assuming a constant SW Ne isotopic composition throughout the Sun's lifetime, leads to constant (~3He/~4He)_(SW) at (4.47 +- 0.13) x 10~(-4) within the last ～4 Gyr. The results suggest that the present-day (~3He/~4He)_(SW) can directly be used to deduce the protosolar (D+~3He)/~4He composition and that a possible mixing between radiative interior and convective zone must be restricted to a thin boundary layer.