title: 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 plume. Here we present a relationship between He and Pb isotopes in Samoan lavas that places severe constraints on the distribution of geochemical species within the plume. The Pb-isotopic compositions of the Samoan lavas reveal several distinct geochemical groups, each corresponding to a different geographic lineament of volcanoes. Each group has a signature associated with one of four mantle endmembers with low (3)He/(4)He: EMII (enriched mantle 2), EMI (enriched mantle 1), HIMU (high µ = (238)U/(204)Pb) and DM (depleted mantle). Critically, these four geochemical groups trend towards a common region of Pb-isotopic space with high (3)He/(4)He. This observation is consistent with several low-(3)He/(4)He components in the plume mixing with a common high-(3)He/(4)He 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 geochemical geometry of a mantle plume, and are best explained by a high-(3)He/(4)He plume matrix that hosts, and mixes with, several distinct low-(3)He/(4)He components.
authors.affiliation: Department of Earth Science, University of California Santa Barbara, Santa Barbara, California 93106-9630, USA.
authors.forename: M G
authors.initials: MG
authors.lastname: Jackson
authors.affiliation: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA.
authors.forename: S R
authors.initials: SR
authors.lastname: Hart
authors.affiliation: Department of Geology and Geophysics, School of Earth and Ocean Sciences and Technology (SOEST), University of Hawaii, Manoa, Honolulu, Hawaii 96822, USA.
authors.forename: J G
authors.initials: JG
authors.lastname: Konter
authors.affiliation: Department of Marine Chemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA.
authors.forename: M D
authors.initials: MD
authors.lastname: Kurz
authors.affiliation: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA.
authors.forename: J
authors.initials: J
authors.lastname: Blusztajn
authors.affiliation: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: K A
authors.initials: KA
authors.lastname: Farley