Melt inclusions in olivines from early Iceland plume picrites support high 3He/4He in both enriched and depleted mantle

Abstract

High 3He/4He ratios (up to 50 Ra) are present in near-primary picrite flows from Baffin Island and West Greenland (BIWG) that span a range of compositions from depleted to relatively enriched in incompatible trace elements [Starkey, N.A., Stuart, F.M., Ellam, R.M., Fitton, J.G., Basu, S., Larsen, L.M., 2009. Helium isotopes in early Iceland plume picrites: constraints on the composition of high 3He/4He mantle. Earth and Planetary Science Letters 277, 91–100]. This association has put into question the prevailing models in which high 3He/4He resides in either unprocessed, undegassed mantle or in depleted residues of melting. Whereas helium isotopes are measured in gas extracted from melt inclusions, radiogenic isotopes and incompatible trace elements are measured in whole-rock powders. The possibility exists, therefore, that some helium-rich melts could be sampled in melt volumes that are too small to affect the bulk lithophile element composition of an erupted magma but which are rich enough in helium to dominate its helium-isotope composition. In this case high 3He/4He would appear to reside in a range of mantle compositions.We have tested this possibility by analysis of major and trace elements in olivine-hosted melt inclusions in high-3He/4He BIWG picrites. We find no evidence for the existence of depleted melt inclusions in the enriched picrites, or enriched inclusions in the depleted picrites. REE profiles of melt inclusions in individual samples are parallel to each other and to that of their respective whole-rock. These data support the conclusion that high 3He/4He is present in a wide range of mantle source compositions. No model in which the BIWG picrites originate in a discrete, high-3He/4He mantle source can account for these new data. We propose that the primordial He in the early Iceland plume picrites must originate in a separate reservoir that has a sufficiently high helium concentration to allow it to impose its high 3He/4He equally on the enriched and depleted mantle sources of the BIWG picrites.