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 这项研究涉及对喀麦隆经典 HIMU 火山链玄武岩的稀有气体和放射性同位素组成进行详细表征。幔源材料中的放射性同位素组成（Pb、Sr、Nd、Hf）包含在四种端元组成中。其中之一，HIMU，其特点是 Pb 同位素值比任何其他地幔端元更具放射性（更高的时间积分 U/Pb 比）。为解释 HIMU 起源而提出的陆地幔演化模型包括海洋岩石圈的循环、地幔内交代作用和岩石圈地幔分层。确定这些模型中哪一个是正确的将深刻影响我们对地幔演化的理解。该解决方案的一个关键组成部分是 HIMU 源的稀有气体同位素组成。在最简单的情况下，洋壳的脱气和富铀（相对于上地幔）再循环部分的 3He/4He 比率将迅速变得比通过交代作用富集铀的地幔部分低得多。然而，俯冲洋壳和地幔内来源之间的区别可能会因周围上地幔与 HIMU 玄武岩的混合或原岩内 He 的扩散平衡而变得模糊。通过将 3He/4He 成分与放射性同位素联系起来，我们希望限制这些过程。因此，喀麦隆线样品将使我们能够表征稀有气体成分，从而表征该 HIMU 源的起源。