CSEDI: Collaborative Research: New Experimental Approaches to He/U-Th Partitioning in the Mantle

CSEDI：合作研究：地幔 He/U-Th 分配的新实验方法

• 批准号: 0215608
• 负责人: Stanley Hart
• 金额: $ 22.26万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-09-01 至 2005-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0215608&HistoricalAwards=false
• 关键词: CSEDI; Collaborative; Research; New; Experimental

It is paradoxical that the mantle sources with the highest 3He/4He and 22Ne/21Ne, and argued to be the least-degassed and therefore the most primitive or primordial, have uniformly depleted (not primitive) Sr, Nd, and Hf isotope signatures. This is an unexplained and long-standing flaw in the "Standard Model" for rare gas evolution in the earth. One of the assumptions underlying the Standard Model is that He is more incompatible than Th and U during upper mantle melting processes, and that it will be preferentially removed during melting and subsequent eruptive degassing. Existing data for mineral-melt partitioning of He ranges over several orders of magnitude, and does not place any real constraints on this most important assumption of the Standard Model. This project will address this issue by methodical experimental measurement of He, Th and U partitioning between olivine, clinopyroxene and silicate melt. The study will provide new solubility data for true structural-bound He in both natural and synthetic olivine and clinopyroxene, and in silicate melt, over a wide range of temperature, pressure, and oxygen fugacity. Helium in single-phase experimental charges will be analyzed first by crushing in vacuo (to release helium from cracks, voids and defects) followed by fusion in an utrahigh vacuum furnace. He in multi-phase charges will be analyzed by in-situ laser-ablation. U and Th partitioning data will also be determined over the same range of conditions for olivine/melt and clinopyroxene/melt equilibrium pairs in simple Fe-free haplobasaltic systems. The experiments will be designed to provide a direct comparison between Th-U partitioning and He partitioning. Ultimately, the resulting data will allow an assessment of a key question in terrestrial helium isotope evolution: is high 3He/4He synonymous with an "undegassed" or "relatively primordial" mantle source? The answer to this question has broad significance to the CSEDI community, as helium is one of the only lines of evidence for relatively undegassed material in the deep earth.

矛盾的是，地幔源具有最高的3 He/4 He和22 Ne/21 Ne，被认为是最少脱气的，因此是最原始的或原始的，具有均匀亏损（不是原始的）Sr，Nd和Hf同位素特征。这是地球中稀有气体演化的“标准模型”中一个无法解释的长期缺陷。标准模型的假设之一是，他是更不兼容的比Th和U在上地幔熔融过程中，它将优先删除熔融和随后的喷发脱气。现有的矿物熔体分配的数据范围超过几个数量级，并没有放置任何真实的约束，这一最重要的假设的标准模型。本项目将通过对橄榄石、单斜辉石和硅酸盐熔体之间的He、Th和U分配进行系统的实验测量来解决这个问题。这项研究将提供新的溶解度数据，真正的结构约束他在天然和合成橄榄石和单斜辉石，并在硅酸盐熔体，在很宽的温度范围内，压力和氧逸度。单相实验装料中的氦将首先在真空中破碎（从裂缝、空隙和缺陷中释放氦），然后在超高真空炉中熔化。多相装药中的He将通过原位激光烧蚀进行分析。U和Th的分配数据也将确定在相同的条件范围内的橄榄石/熔体和单斜辉石/熔体平衡对简单的Fe-自由haplobasaltic系统。实验将被设计为提供Th-U分配和He分配之间的直接比较。最终，由此产生的数据将允许评估的一个关键问题，在陆地氦同位素演化：高3 He/4 He的同义词是“未脱气”或“相对原始”的地幔源？这个问题的答案对CSEDI社区具有广泛的意义，因为氦是地球深处相对未脱气物质的唯一证据之一。