1. Lamont-Doherty Earth Observatory of Columbia University,
2. Department of Earth and Environmental Sciences, Columbia University, Palisades, New York 10964, USA

Correspondence to: Cornelia Class¹ Correspondence and requests for materials should be addressed to C.C. (Email: class@ldeo.columbia.edu).

Abstract

Degassing of the Earth's mantle through magmatism results in the irreversible loss of helium to space, and high ³He/⁴He ratios observed in oceanic basalts have been considered the main evidence for a 'primordial' undegassed deep mantle reservoir. Here we present a new global data compilation of ocean island basalts, representing upwelling 'plumes' from the deep mantle, and show that island groups with the highest primordial signal (high ³He/⁴He ratios) have striking chemical and isotopic similarities to mid-ocean-ridge basalts. We interpret this as indicating a common history of mantle trace element depletion through magmatism. The high ³He/⁴He in plumes may thus reflect incomplete degassing of the deep Earth during continent and ocean crust formation. We infer that differences between plumes and the upper-mantle source of ocean-ridge basalts reflect isolation of plume sources from the convecting mantle for 1–2 Gyr. An undegassed, primordial reservoir in the mantle would therefore not be required, thus reconciling a long-standing contradiction in mantle dynamics.

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