Noble Gas Isotopes and the Origin of HIMU

9706290 Ballentine这项研究涉及到从经典的HIMU喀麦隆火山链玄武岩的惰性气体和放射性同位素组成的详细特征。幔源物质的放射性同位素组成（Pb、Sr、Nd、Hf）可分为四个端元组成。其中之一，HIMU，其特点是铅同位素值是更放射性（更高的时间积分U/Pb比）比任何其他地幔端成员。HIMU成因的地幔演化模式包括大洋岩石圈再循环、地幔内交代和岩石圈地幔拆沉。解决这些模型中哪一个是正确的将深刻影响我们对地球地幔演化的理解。这一决议的一个关键组成部分是重离子惯性测量装置源的惰性气体同位素组成。在最简单的情况下，经过脱气和富集U（相对于上地幔）的洋壳再循环部分的3 He/4 He比将迅速变得比通过交代作用富集U的地幔部分低得多。然而，俯冲洋壳和地幔内源之间的区别，可能会变得模糊的混合环境上地幔与HIMU玄武岩，或扩散平衡的原岩内的He。通过将3 He/4 He组成与放射性同位素联系起来，我们希望能够限制这些过程。因此，喀麦隆的管线样品将使我们能够确定惰性气体成分的特征，从而确定这一高密度微量元素来源的来源。

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.