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

矛盾的是，具有最高的3He/4He和22Ne/21Ne的地幔来源，脱气最少，因此是最原始或最原始的，却均匀地亏损(不是原始的)Sr、ND和Hf同位素特征。这是地球上稀有气体演化的“标准模型”中一个无法解释且长期存在的缺陷。标准模型的基本假设之一是，在上地幔熔融过程中，他比Th和U更不相容，在熔融和随后的喷发脱气过程中，它将优先被去除。现有的He矿物熔体分配的数据范围超过几个数量级，并没有对标准模型这一最重要的假设施加任何真正的限制。该项目将通过对橄榄石、单斜辉石和硅酸盐熔体中He、Th和U的分配进行有条不紊的实验测量来解决这个问题。这项研究将提供在广泛的温度、压力和氧逸度范围内，在天然和合成橄榄石和单斜辉石中，以及在硅酸盐熔体中真正结构结合的He的新的溶解度数据。单相实验装药中的氦将首先在真空中粉碎(从裂缝、空洞和缺陷中释放出氦)，然后在超高真空炉中熔化。用激光原位烧蚀的方法对多相电荷中的氦进行分析。U和Th的分配数据也将在相同的条件范围内确定橄榄石/熔体和单斜辉石/熔体在简单的无Fe单斜辉石系统中的平衡对。这些实验将被设计成提供Th-U分配和He分配之间的直接比较。最终，所得到的数据将使我们能够评估地球氦同位素演化中的一个关键问题：高3He/4He是“未脱气的”或“相对原始的”地幔来源的同义词吗？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的物质的仅有证据之一。

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.--