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 an d atmosphere-derived neon components, we calculate the amounts of solar-der ived 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 b asaltic glasses from mid-oceanic ridges (MORBs) vary by almost 2 orders of magnitude and define a linear correlation with a slope of unity which passe s 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 ele mental 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 abunda nce and the degree of helium enrichment. Specifically, the data show positi ve 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 h elium abundance and elevated He/Ne and He/Ar ratios in MORE glasses are inc onsistent with elemental fractionation associated with any form of solubili ty controlled gas loss process, as these would lead to negative correlation s. Additionally there is relatively little fractionation of neon from argon . This observation excludes any type of simple mass dependent process. Rath er, it requires some form of "threshold" fractionation process which effect s only helium. Although the origin of the correlation between helium abunda nce and helium fractionation is unclear, we speculate that the systematic h elium enrichment observed in MORE glasses may reflect preferentially concen trating helium liberated from the crystallising oceanic crustal section int o the relatively small volume of residual magma that is erupted to form gla sses. (C) 1999 Elsevier Science Ltd.