1. Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, UK
2. School of GeoSciences, University of Edinburgh, Edinburgh EH9 3JW, UK

Correspondence to: Finlay M. Stuart¹ Correspondence and requests for materials should be addressed to F.M.S. (Email: f.stuart@suerc.gla.ac.uk).

Abstract

The high ³He/⁴He ratio of volcanic rocks thought to be derived from mantle plumes is taken as evidence for the existence of a mantle reservoir that has remained largely undegassed since the Earth's accretion^{1, 2, 3}. The helium isotope composition of this reservoir places constraints on the origin of volatiles within the Earth and on the evolution and structure of the Earth's mantle. Here we show that olivine phenocrysts in picritic basalts presumably derived from the proto-Iceland plume at Baffin Island, Canada, have the highest magmatic ³He/⁴He ratios yet recorded. A strong correlation between ³He/⁴He and ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd and trace element ratios demonstrate that the ³He-rich end-member is present in basalts that are derived from large-volume melts of depleted upper-mantle rocks. This reservoir is consistent with the recharging of depleted upper-mantle rocks by small volumes of primordial volatile-rich lower-mantle material at a thermal boundary layer between convectively isolated reservoirs. The highest ³He/⁴He basalts from Hawaii and Iceland plot on the observed mixing trend. This indicates that a ³He-recharged depleted mantle (HRDM) reservoir may be the principal source of high ³He/⁴He in mantle plumes, and may explain why the helium concentration of the 'plume' component in ocean island basalts is lower than that predicted for a two-layer, steady-state model of mantle structure.