Helium and xenon in deep-sea basalts as a measure of magmatic differentiation

Abstract

There has been much recent interest in the rare gas abundances of terrestrial materials, due mainly to the discovery of ‘excess’ radiogenic Ar and He trapped in deep-sea glasses^1–4. The abundances of these gases indicated that such materials might effectively trap the internal Earth's atmosphere, and evidence of trapped primordial gases has by now been found not only in deep-sea glasses^5–10, but also in xenolithic inclusions^11–15, deep ocean waters¹⁶, CO₂ well gas^17–19, volcanic gas²⁰ and diamonds²¹. The rare gas elemental abundances measured in these samples may not quantitatively reflect the deep-Earth pattern. Several elemental fractionation processes have been suggested, ranging from selective enrichment of the lighter gases in an upward-rising magma^22,23 to posteruptive diffusion loss which would selectively deplete the lighter gases²⁴. Each of these processes will affect concentrations of the helium and xenon isotopes in a characteristic manner, thus leaving in the glasses an isotopic fingerprint of what has occurred. New data are reported here which indicate extremely small concentrations of fissiogenic xenon in deep-sea basalts containing large concentrations of trapped excess radiogenic helium and argon. The resulting high ⁴He_r/¹³⁶Xe_f ratios are diagnostic of frac-tionation during the magmatic processes leading to eruption rather than of posteruptive loss of gas (which would show the opposite effect). The results indicate that studies of volatiles in general and of the rare gases in particular in such mantle-derived samples, even in those with no subsequent alteration or contamination, may be profoundly influenced by mass-dependent pre-eruptive effects. The consequences of this fractionation for the deep-Earth K/U ratio are discussed.