Collaborative Research: Helium Diffusion in Lower Mantle Minerals

氦是地球内部组成和演化最重要的地球化学示踪剂之一。它由两种同位素组成：3 He和4 He，3 He是原始来源，4 He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可变的，就像许多其他元素的比例一样。一个长期存在的问题是，观察到的同位素组成的不均匀性如何与通过其他手段（包括地震成像）观察到的不均匀性相关联。为了回答这个问题，有必要知道在地质时期内，同位素的不均匀性在地幔中可以持续多久，以对抗扩散和对流混合。由于氦在地质材料中是一种高度移动的元素，因此它可以提供通过地球化学方法采样的非均质区域的长度尺度的下限。然而，虽然氦的流动性已经在低压下在浅地幔物质中通过实验确定，但还没有关于氦在地球地幔最深80%处的流动性的信息。拟议的工作将提供有关这些深地幔物质中氦流动性的第一批数据。 该研究结合了实验测量和第一原理计算。实验将在CWRU和哈佛进行，以确定在很宽的温度和压力范围内，方镁石中的He扩散的机制和速率，以及其溶解度。He的扩散率将通过使用哈佛的惰性气体质谱仪对He饱和晶体进行的增量除气逐步加热实验来确定。氦的扩散率和溶解度在高压下将通过时间序列实验在密封的惰性气体充电胶囊，然后由散装回收的晶体He分析确定。理论第一性原理计算将在威斯康星州大学进行，以确定他的扩散率和方镁石和MgSiO 3-钙钛矿的机制。计算将提供一个关键的交叉检查的实验数据，并扩大研究，以深下地幔条件下，实验是不可访问的。拟议的研究将提供一个基本的限制地球化学异质性的最小长度尺度，可以保持在地球的深部地幔。它还可以提供对核反应堆材料中氦流动性的间接洞察，其中氦脆化是一个持续存在的问题，以及在拟议的核废物处理材料中，包括钙钛矿结构的氧化物。

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.