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

Jackson, M; Hart, S; Konter, J; Kurz, M; Blusztajn, J; Farley, K

Hotspot lavas erupted at ocean islands exhibit tremendous isotopic variability, indicating that there are numerousmantle components1,2 hosted in upwelling mantle plumes that generate volcanism at hotspots likeHawaii andSamoa3.However, it is not known howthe surface expression of the various geochemical components observed in hotspot volcanoes relates to their spatial distribution within the plume4–10.Herewe present a relationshipbetweenHeandPb isotopes in Samoan lavas that places severe constraints on the distribution of geochemical species within the plume.ThePb-isotopic compositions of the Samoan lavas reveal several distinct geochemical groups, each corresponding to a different geographic lineamentof volcanoes.Each group has a signature associated withoneof fourmantleendmembers with low 3He/4He:EMII(enrichedmantle 2),EMI(enrichedmantle 1), HIMU(highm 5238U/204Pb) andDM(depleted mantle). Critically, these four geochemical groups trend towards 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 commonhigh-3He/4He component, but not mixing much with each other. The mixing relationships inferred from the new He and Pb isotopic data provide the clearest picture yet of the geochemicalgeometry of a mantle plume, and are best explained by a high-3He/4He plumematrix thathosts, and mixeswith, several distinct low-3He/4He components.