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

具有最高的3He/4He和22Ne/21Ne的地幔源，被认为是脱气最少的，因此是最原始的或最原始的，却均匀地耗尽（不是原始的）Sr、Nd和Hf同位素特征，这是自相矛盾的。这是地球稀有气体演化“标准模型”中一个无法解释且长期存在的缺陷。标准模型的一个基本假设是，在上地幔熔融过程中，He比Th和U更不相容，并且在熔融和随后的喷发脱气过程中，He将优先被移除。关于He的矿物-熔体分配的现有数据范围超过几个数量级，并且没有对标准模型的这个最重要的假设施加任何真正的限制。本项目将通过系统的实验测量橄榄石、斜辉石和硅酸盐熔体之间He、Th和U的分配来解决这一问题。该研究将在很宽的温度、压力和氧逸度范围内，为真正结构结合的He在天然和合成橄榄石、斜辉石以及硅酸盐熔体中的溶解度提供新的数据。单相实验电荷中的氦气将首先通过在真空中粉碎（从裂缝、空洞和缺陷中释放氦气），然后在超高真空炉中熔化来分析。用原位激光烧蚀法分析了多相电荷中的He。U和Th分配数据也将在相同的条件范围内确定简单无铁单玄武岩体系中橄榄石/熔体和斜辉石/熔体平衡对。这些实验旨在提供Th-U划分和He划分之间的直接比较。最终，得到的数据将允许对地球氦同位素演化的一个关键问题进行评估：高3He/4He是否等同于“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

GroveEAR-0215547It 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.--