Helium solubility in olivine and implications for high 3He/4He in ocean island basalts

High (3)He/(4) He ratios found in ocean island basalts are the main evidence for the existence of an undegassed mantle reservoir(1-3). 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(4-6). 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 +/- 0.0005 (s.d.) and a maximum of 0.0060 +/- 0.0007 (s.d.). The results indicate that helium might be more compatible than uranium and thorium during mantle melting and that high (3)He/(4) He ratios can be preserved in depleted residues of melting. A depleted source for high (3)He/(4) He ocean island basalts would resolve the apparent discrepancy(7) in the relative helium concentrations of ocean island and mid-ocean-ridge basalts.