1. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Building 54, Cambridge, Massachusetts 02139, USA
2. Woods Hole Oceanographic Institute, Clark Building, Woods Hole, Massachusetts 02543, USA
3. †Present address: Department of Earth Sciences, University of Durham, Science Labs, Durham DH1 3LE, UK

Correspondence to: Stephen W. Parman^1,3 Correspondence and requests for materials should be addressed to S.W.P. (Email: stephen.parman@durham.ac.uk).

Abstract

High ³He/⁴He ratios found in ocean island basalts are the main evidence for the existence of an undegassed mantle reservoir^{1, 2, 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, 5, 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 ³He/⁴He ratios can be preserved in depleted residues of melting. A depleted source for high ³He/⁴He ocean island basalts would resolve the apparent discrepancy⁷ in the relative helium concentrations of ocean island and mid-ocean-ridge basalts.

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