Non-Equilibrium Degassing and a Primordial Source for Helium in Ocean-Island Volcanism

Abstract

Radioactive decay of uranium and thorium produces He-4, whereas He-3 in the Earth's mantle is not produced by radioactive decay and was only incorporated during accretion-that is, it is primordial(1). He-3/He-4 ratios in many ocean-island basalts (OIBs) that erupt at hotspot volcanoes, such as Hawaii and Iceland, can be up to sixfold higher than in mid-ocean ridge basalts (MORBs). This is inferred to be the result of outgassing by melt production at mid-ocean ridges in conjunction with radiogenic ingrowth of He-4, which has led to a volatile-depleted upper mantle (MORB source) with low He-3 concentrations and low He-3/He-4 ratios(2-6). Consequently, high He-3/He-4 ratios in OIBs are conventionally viewed as evidence for an undegassed, primitive mantle source, which is sampled by hot, buoyantly upwelling deep-mantle plumes(3,6,7). However, this conventional model provides no viable explanation of why helium concentrations and elemental ratios of He/Ne and He/Ar in OIBs are an order of magnitude lower than in MORBs. This has been described as the 'helium concentration paradox'(8) and has contributed to a long-standing controversy about the structure and dynamics of the Earth's mantle. Here we show that the helium concentration paradox, as well as the full range of noble-gas concentrations observed in MORB and OIB glasses, can self-consistently be explained by disequilibrium open-system degassing of the erupting magma. We show that a higher CO2 content in OIBs than in MORBs leads to more extensive degassing of helium in OIB magmas and that noble gases in OIB lavas can be derived from a largely undegassed primitive mantle source.