Systematic elemental fractionation of mantle-derived helium, neon, and argon in mid-oceanic ridge glasses

Based on the proposition that the Earth contains solar-like, nucleogenic and atmosphere-derived neon components, we calculate the amounts of solar-derived Ne-22(s) and nucleogenic Ne-21* present in a sample. The amounts of Ne-22(s) and Ne-21* can then be compared with the amounts of primordial He-3 and radiogenic He-4. The He-3/Ne-22(s) and He-4/Ne-21* ratios observed in basaltic glasses from mid-oceanic ridges (MORBs) vary by almost 2 orders of magnitude and define a linear correlation with a slope of unity which passes through the point defined by the mean primordial He-3/Ne-22 ratio in the Earth (= 7.7) and the radiogenic He-4 to nucleogenic Ne-21 production ratio (= 2.2 x 10(7)). This indicates that there has been significant recent elemental fractionation which has enriched MORE glasses in both primordial He-3 and radiogenic He-4 with respect to mantle neon. A similar enrichment of helium in MORE glasses is observed relative to mantle-derived Ar-40*. Importantly, there is a positive correlation between absolute helium abundance and the degree of helium enrichment. Specifically, the data show positive linear correlations in plots of [He-3] vs. He-3/Ne-22(s), [He-4] vs. He-4/Ne-21*, and [He-4] vs. He-4/Ar-40*. These positive correlations between helium abundance and elevated He/Ne and He/Ar ratios in MORE glasses are inconsistent with elemental fractionation associated with any form of solubility controlled gas loss process, as these would lead to negative correlations. Additionally there is relatively little fractionation of neon from argon. This observation excludes any type of simple mass dependent process. Rather, it requires some form of "threshold" fractionation process which effects only helium. Although the origin of the correlation between helium abundance and helium fractionation is unclear, we speculate that the systematic helium enrichment observed in MORE glasses may reflect preferentially concentrating helium liberated from the crystallising oceanic crustal section into the relatively small volume of residual magma that is erupted to form glasses. (C) 1999 Elsevier Science Ltd.