Helium solubility in olivine and implications for high He/He in ocean island basalts

High He/He ratios found in ocean island basalts are the main evidence for the existence of an undegassed mantle reservoir. However, models of helium isotope evolution depend critically on the chemical behaviour of helium during mantle melting. It is generally assumed that helium is strongly enriched in mantle melts relative to uranium and thorium, yet estimates of helium partitioning in mantle minerals have produced conflicting results. Here we present experimental measurements of helium solubility in olivine at atmospheric pressure. Natural and synthetic olivines were equilibrated with a 50% helium atmosphere and analysed by crushing in vacuo followed by melting, and yield a minimum olivine–melt partition coefficient of 0.0025 6 0.0005 (s.d.) and a maximum of 0.0060 6 0.0007 (s.d.). The results indicate that helium might be more compatible than uranium and thorium during mantle melting and that high He/He ratios can be preserved in depleted residues of melting. A depleted source for high He/He ocean island basalts would resolve the apparent discrepancy in the relative helium concentrations of ocean island and mid-ocean-ridge basalts. Although there is a general consensus that Earth started with an initial He/He ratio of ,120R a (R a 1⁄4 (He/He)sample/ (He/He)atmosphere), its subsequent evolution is not well constrained. In the ‘standard’ model, He is assumed to be more incompatible than U þ Th (the parent isotopes of He) during melting. If so, any melting event will leave the mantle residue enriched in U þ Th relative to He, and it will evolve radiogenic He isotope ratios (low He/He; Fig. 1a). In this model, the unmelted, undegassed mantle has the highest He/He ratios and highest He concentrations of any mantle reservoir and is the likely source of high He/He in ocean island basalt (OIB). In an alternative model, He is assumed to be more compatible than U þ Th during melting, in which case melting decreases the parent/daughter ratio (U þ Th)/He of the mantle. Therefore the He/He ratio of a depleted residue will decrease more slowly with time than an undegassed reservoir (Fig. 1b). In this case, the highest He/He mantle reservoirs are the depleted residues of melting, but unlike in the standard model, these high He/He sources will have low He concentrations. The older the depletion event and the greater the degree of melting, the larger the difference will be between a depleted and undepleted source. Most OIBs have higher He/He ratios and lower gas contents than mid-ocean-ridge basalt (MORB). This has been termed the ‘helium paradox’ and is more consistent with the alternative model than the standard model. However, concentrations of noble gases in lavas are dominated by shallow degassing processes, and the relative gas concentrations in OIBs and MORBs may not reflect the gas concentrations in their sources. Studies of concentration ratios (for example He/Ne) indicate that simple degassing cannot explain LETTERS