Helium and lead isotopes reveal the geochemical geometry of the Samoan plume

Abstract

Hotspot lavas erupted at ocean islands exhibit tremendous isotopic variability, indicating that there are numerous mantle components hosted in upwelling mantle plumes that generate volcanism at hotspots like Hawaii and Samoa. However, it is not known how the surface expression of the various geochemical components observed in hotspot volcanoes relates to their spatial distribution within the plume4-10. Here we present a unique relationship between He and Pb isotopes in Samoan lavas that places severe constraints on the distribution of geochemical species within the plume. In Pb-isotopic space, the Samoan data form several distinct geochemical groups, each corresponding to a different geographic lineament of volcanoes. Each group has signatures associated with one of four mantle endmembers with low 3He/4He: EMII (enriched mantle 2), EMI (enriched mantle 1), HIMU (high μ=238U/204Pb) and DM (depleted mantle). Critically, the four isotopic-geographic groups converge on a common region of Pb-isotopic space with high 3He/4He. This observation is consistent with several low 3He/4He components in the plume mixing with a common high 3He/4He component, but not significantly with each other, otherwise the four isotopic groups would be obscured by mixing. The mixing relationships inferred from the new He and Pb isotopic data paint the clearest picture yet of the geochemical geometry of a mantle plume, and are best explained by a high 3He/4He plume matrix that hosts, and mixes with, several distinct low 3He/4He components.