Helium isotopes in early Iceland plume picrites: Constraints on the composition of high 3He/ 4He mantle

Abstract

A detailed study of the geochemistry of a new suite of early Iceland plume picrites shows that extremely high 3He/ 4He ratios (up to 50 R a) are found in picrites from Baffin Island and West Greenland. High 3He/ 4He picrites display a wide range in 87Sr/ 86Sr (0.70288–0.70403), 143Nd/ 144Nd (0.51288–0.51308) and incompatible trace element ratios (e.g. La/Sm n = 0.5–1.6). These overlap the complete range of compositions of mid-ocean ridge basalts and most northern hemisphere ocean island basalts, including Iceland. Crustal contamination modelling in which high-grade Proterozoic crustal basement rocks for the region are mixed with a depleted parent cannot account for the trend displayed by the Baffin Island and West Greenland picrites. This rules out the possibility that the incompatible trace element, Sr and Nd isotope range of the high 3He/ 4He picrites is due to crustal contamination. The compositional range at high 3He/ 4He is also inconsistent with derivation from a primordial-He-rich reservoir that is a residue of ancient mantle depletion. This implies that the composition of the high 3He/ 4He mantle cannot be determined simply by extrapolating ocean island basalt He–Sr–Nd–Pb–Os isotope data. The apparent decoupling of He from trace element and lithophile radiogenic isotope tracers is difficult to attain by simple mixing of a high-[He], high 3He/ 4He reservoir with various depleted and enriched He-poor mantle reservoirs. The possibility that primordial He has diffused into a reservoir with a composition typical of convecting upper mantle cannot be ruled out. If so, the process must have occurred after the development of existing mantle heterogeneity, and requires the existence of a deep, primordial He-rich reservoir.