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Helium and lead isotopes reveal the geochemical geometry of the Samoan plume
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|Title:||Helium and lead isotopes reveal the geochemical geometry of the Samoan plume|
show 1 moreFarley, K.A.
|Issue Date:||Oct 2014|
|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.
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