Collaborative Research: Helium Diffusion in Lower Mantle Minerals

氦是地球内部组成和演化的最重要的地球化学示踪之一。它由两种同位素组成：3He和4He，3He是原始起源，4He是铀和钍衰变的产物。这两种同位素在来自地幔的样品中的比例是不同的，就像许多其他元素的比例一样。一个长期存在的问题是，观测到的同位素组成的非均质性与通过包括地震成像在内的其他手段观察到的非均质性有何关系。要回答这个问题，有必要知道在地质时期，同位素不均匀能在多大尺度上存在于地幔中，防止扩散和对流混合。由于氦在地质材料中是一种流动性很强的元素，它可以为通过地球化学方法采样的非均质区域提供一个长度尺度的下限。然而，尽管氦的迁移率已经在低气压的浅地幔物质中得到了实验测定，但还没有关于地幔最深处80%的氦的迁移率的信息。这项拟议的工作将提供有关这些深部地幔物质中氦迁移率的第一批数据。这项拟议的研究结合了实验测量和第一性原理计算。实验将在CWRU和哈佛大学进行，以确定He在方镁石中扩散的机制和速度，以及在广泛的温度和压力范围内的溶解度。氦的扩散率将通过哈佛大学的惰性气体质谱仪在氦饱和晶体上的增量放气阶梯加热实验来确定。氦在高压下的扩散系数和溶解度将通过在密封的稀有气体充气胶囊中进行时间序列实验，然后对回收的晶体进行大量分析来确定。威斯康星大学将进行理论第一性原理计算，以确定方镁石和镁钙钛矿中的扩散系数及其机理。这些计算将提供对实验数据的关键交叉检查，并将研究扩展到实验上无法到达的深部地幔条件。这项拟议的研究将对地球深部地幔中可以维持的最小长度的地球化学不均一性提供一个基本的限制。它还可以间接洞察氦在核反应堆材料和拟议的核废料处置材料中的流动性，在核反应堆材料中，氦脆是一个长期存在的问题，其中包括钙钛矿型氧化物。

Helium is one of the most important geochemical tracers of Earth's internal composition and evolution. It consists of two isotopes: 3He, which has a primordial origin, and 4He, which is the product of uranium and thorium decay. The ratio of these two isotopes in samples derived from Earth's mantle is variable, as are the ratios for many other elements. A long-standing question is how the observed heterogeneity in isotopic composition relates to the heterogeneity that is observed by other means, including seismic imaging. To answer this question, it is necessary to know on what length scales isotope heterogeneity can persist in Earth's mantle over geological time, against diffusive and convective mixing. Because helium is a highly mobile element in geological materials, it can provide a lower limit on the length scale of heterogeneous regions that are sampled by geochemical methods. However, while the mobility of helium has been determined experimentally in shallow mantle materials at low pressures, no information is yet available on the mobility of helium in the deepest 80% of Earth's mantle. The proposed work will provide the first data on helium mobility in these deep mantle materials. The proposed research combines both experimental measurements and first-principles calculations. Experiments will be carried out at CWRU and Harvard to determine the mechanism and rate of He diffusion in periclase, as well as its solubility, over a wide range of temperature and pressure. The diffusivity of He will be determined by incremental outgassing step-heating experiments on He-saturated crystals using the noble gas mass spectrometer at Harvard. Helium diffusivity and solubility at high pressures will be determined through time series experiments in sealed noble-gas charged capsules followed by bulk He analysis of the recovered crystals. Theoretical first-principles calculations will be carried out at the University of Wisconsin to determine He diffusivity and its mechanism in both periclase and MgSiO3-perovskite. The calculations will provide a critical cross-check on the experimental data and extend the study to deep lower mantle conditions that are experimentally inaccessible. The proposed research will provide a fundamental constraint on the minimum length scale of geochemical heterogeneity that can be maintained in Earth's deep mantle. It may also provide indirect insight into the helium mobility in nuclear reactor materials, where helium embrittlement is a persistent problem, and in proposed nuclear waste disposal materials, which include perovskite-structured oxides.