Noble Gas Isotopes and the Origin of HIMU

稀有气体同位素和 HIMU 的起源

• 批准号: 9706290
• 负责人: Christopher Ballentine
• 金额: $ 8万
• 依托单位: Regents of the University of Michigan - Ann Arbor
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1997
• 资助国家: 美国
• 起止时间: 1997-06-15 至 1999-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9706290&HistoricalAwards=false
• 关键词: Noble; Gas; Isotopes; Origin; HIMU

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.

9706290 Ballentine这项研究涉及对经典Himu喀麦隆火山链的玄武岩的贵重气和放射性同位素组成的详细表征。地幔衍生材料中的放射同位素组合物（Pb，Sr，ND，HF）被四个最终成员组成所包含。其中之一，HIMU的特征是PB同位素值，其放射原（比时间更高的U/Pb比率）比任何其他地幔终构件要高得多。提议说明HIMU起源的陆地地幔进化模型包括对海洋岩石圈的回收利用，剂量内质体和岩石圈地幔的分层。解决这些模型中的哪一种是正确的，将深刻影响我们对陆地地幔进化的理解。该决议中的一个关键组成部分是HIMU来源的贵重同位素组成。在最简单的情况下，脱气和U富集的3HE/4HE比（相对于上地幔）的回收部分的海洋壳部分将迅速远低于相位学富含U富集的地幔的部分。然而，俯冲的海洋壳和内剂量源之间的区别可能会因将环境上层与Himu玄武岩的混合或HE在原始石材内的扩散平衡而变得模糊。通过将3HE/4HE组成与放射原同位素联系起来，我们希望限制这些过程。因此，喀麦隆线样本将使我们能够表征贵重的气体成分，并因此是该HIMU来源的起源。