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：3He是原始起源，而4He是铀和钍衰变的产物。这两种同位素在地幔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非均质性如何与其他方法（包括地震成像）观察到的非均质性联系起来。为了回答这个问题，有必要知道同位素非均质性在地质年代中可以在多大的长度尺度上持续存在于地幔中，以对抗扩散和对流混合。由于氦在地质物质中是一种高度流动的元素，它可以为地球化学方法取样的非均质区域的长度尺度提供一个下限。然而，虽然氦在低压下在浅层地幔物质中的流动性已经通过实验确定，但目前还没有关于地球地幔最深处80%的氦流动性的信息。这项提议的工作将提供氦在这些深地幔物质中流动的第一个数据。提出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在方解石中的扩散机制和速率，以及在广泛的温度和压力范围内的溶解度。利用哈佛大学稀有气体质谱仪对氦饱和晶体进行渐增放气分步加热实验，测定了氦的扩散系数。氦气在高压下的扩散率和溶解度将通过在密封的惰性气体充电胶囊中进行时间序列实验来确定，然后对回收的晶体进行大量He分析。理论第一性原理计算将在威斯康星大学进行，以确定镁方石和mgsio3 -钙钛矿中的He扩散率及其机制。计算将为实验数据提供关键的交叉检查，并将研究扩展到实验无法到达的深下地幔条件。提出的研究将为地球深部地幔地球化学非均质性的最小长度尺度提供基本约束。它还可以间接了解核反应堆材料中的氦迁移率，其中氦脆化是一个持续存在的问题，以及拟议的核废料处理材料，其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.