Noble Gas Isotopes and the Origin of HIMU

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9706290 Ballentine This study involves a detailed characterization of both the noble gas and radiogenic isotopic composition of basalts from the classic HIMU Cameroon volcanic chain. The radiogenic isotopic compositions (Pb, Sr, Nd, Hf) in mantle-derived materials are encompassed by four end member compositions. One of these, HIMU, is characterized by Pb isotopic values which are much more radiogenic (higher time integrated U/Pb ratio) than any other mantle end member. Models of terrestrial mantle evolution proposed to account for the origin of HIMU include recycling of oceanic lithosphere, intramantle metasomatism and delamination of lithospheric mantle. Resolving which of these models is correct will profoundly affect our understanding of the evolution of the terrestrial mantle. A key component in this resolution is the noble gas isotopic composition of the HIMU source. In the simplest case the 3He/4He ratio of a degassed and U enriched (relative to upper mantle) recycled portion of oceanic crust will rapidly become very much lower than portions of the mantle enriched in U by metasomatism. The distinction between subducted oceanic crust and intramantle sources however, may become blurred by either mixing of ambient upper mantle with HIMU basalts, or diffusive equilibration of He within the protolith. By relating the 3He/4He composition with the radiogenic isotopes we hope to constrain these processes. The Cameroon line samples will therefore enable us to characterize the noble gas composition and hence the origin of this HIMU source.