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

Abstract

Based on the proposition that the Earth contains solar-like, nucleogenic and atmosphere-derived neon components, we calculate the amounts of solar-derived22Ne(s) and nucleogenic21Ne(*) present in a sample. The amounts of22Ne(s) and21Ne(*) can then be compared with the amounts of primordial3He and radiogenic4He. The3He/22Ne(s) and4He/21Ne(*) 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 primordial3He/22Ne ratio in the Earth (= 7.7) and the radiogenic4He to nucleogenic21Ne production ratio (= 2.2 X 107). This indicates that there has been significant recent elemental fractionation which has enriched MORB glasses in both primordial3He and radiogenic4He with respect to mantle neon. A similar enrichment of helium in MORB glasses is observed relative to mantle-derived40Ar(*). 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 [3He] vs.3He/22Ne(s), [4He] vs.4He/21Ne(*), and [4He] vs.4He/40Ar(*). These positive correlations between helium abundance and elevated He/Ne and He/Ar ratios in MORB 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 MORB 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. Show more