Primordial neon, helium, and hydrogen in oceanic basalts

Abstract

A primordial neon component in neon from Kilauea Volcano and deep-sea tholeiite glass has been identified by the presence of excess 20Ne; relative to atmospheric neon the 20Ne enrichments are 5.4% in Kilauea neon and about 2.5% in the basalts. The 20Ne anomalies are associated with high 3He/ 4He ratios; the ratio in Kilauea helium is 15 times the atmospheric ratio, while mid-ocean ridge basalts from the Atlantic, Pacific, and Red Sea have uniform ratios about 10 times atmospheric. Mantle neon and helium are quite different in isotopic composition from crustal gases, which are highly enriched in radiogenic 21Ne and 4He. The 21Ne/ 4He ratios in crustal gases are consistent with calculated values based on G. Wetherill's 18O (α,n) reaction; the lack of 20Ne enrichment in these gases shows that the mantle 20Ne anomalies are not radiogenic. 21Ne enrichments in Kilauea neon and “high- 3He” Pacific tholeiites are much less than in crustal neon, about 2 ± 2% vs. present atmospheric neon, as expected from the much lower 4He/Ne ratios. Neon concentrations in two Atlantic tholeiites were found to be only 1–2% of the values obtained by Dymond and Hogan; helium concentrations are slightly greater and our He/Ne ratios are greater by a factor of 150. The large Ne excess relative to solar wind and meteoritic gases is thus not confirmed. Pacific and Atlantic basalts appear to be quite different in He/Ne ratios however, and He and Ne may be inversely correlated. He concentration variations due to diffusive loss can be distinguished from variations due to two-phase partitioning or mantle heterogeneity by the effects on 3He/ 4He ratios. The He isotopic and concentration measurements on “low- 3He” basalts are consistent with diffusive loss and dilution of the 3/4 ratio by in-situ radiogenic 4He, and may provide a method for dating basalt glasses. Deuterium/hydrogen ratios in Atlantic and Pacific tholeiite glasses are 77% lower than the ratio in seawater. The inverse correlation between deuterium and water content observed by Friedman in erupting Kilauea basalts is consistent with a Rayleigh separation process in which magmatic water is separated from an initial melt with the same D/H ratio as observed in deep-sea tholeiites. The consistency of the D/H ratios in tholeiites containing primordial He and Ne components indicates that these ratios are probably characteristic of primordial or juvenile hydrogen in the mantle.