An isotopically distinct hydrogen reservoir in the South Pacific mantle | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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  An isotopically distinct hydrogen reservoir in the South Pacific mantle

Publication Type:

Journal Article

Source:

Geochimica Et Cosmochimica Acta, Volume 74, Issue 12, p.A183-A183 (2010)

ISBN:

0016-7037

Accession Number:

ISI:000283941400616

Keywords:

UMR 7154 ; Physico-chimie des Fluides Géologiques

Abstract:

We have measured He, Sr, Nd, and Pb isotope ratios and Rb, Sr, Sm, and Nd concentrations in stratigraphically controlled lavas from the Pago shield volcano on Tutuila, American Samoa. We interpret these lavas as products of mixing between two isotopically extreme mantle constituents. The first is a highly enriched component with very high Sr isotope ratios, low3He/4He ratios, and high “Δ(74)” and “Δ(84)” Pb isotopic characteristics. This is probably recently recycled ( < 400 Ma) crustal material. The second component has high3He/4He ratios ( > 24RA) and intermediate Sr, Nd, and Pb isotopic ratios. This material was derived from a largely undegassed mantle source, and, in conjunction with data from several other ocean islands, provides strong evidence for the existence of a high3He/4He ratio mantle end member (primitive helium mantle, PHEM) with consistent Sr, Nd, and Pb isotopic characteristics: near bulk-earth87Sr/86Sr (0.7042–0.7052) and143Nd/144Nd (0.51265–0.51280,εNd = +0.2to+3.2), and radiogenic Pb (206Pb/204Pb∼ 18.5–19.0,207Pb/204Pb∼ 15.5–15.57,208Pb/204Pb∼ 38.4–39.2). Although this material cannot have been derived from a reservoir completely closed to elemental fractionation for the full 4.55 Ga duration of Earth's history, it may indicate the presence of a highly primitive mantle source.