Collaborative Research: Helium Diffusion in Lower Mantle Minerals

合作研究：下地幔矿物中的氦扩散

• 批准号: 1265335
• 负责人: James Van Orman
• 金额: $ 22.09万
• 依托单位: Case Western Reserve University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-05-01 至 2019-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1265335&HistoricalAwards=false
• 关键词: Collaborative; Research; Helium; Diffusion; Lower

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.

氦是地球内部组成和进化的最重要的地球化学示踪剂之一。它由两个同位素组成：3he，具有原始起源，4He是铀和th衰变的产物。 从地球地幔得出的样品中的这两个同位素的比率是可变的，许多其他元素的比率也是如此。一个长期存在的问题是，同位素组成中观察到的异质性与其他方式（包括地震成像）观察到的异质性如何相关。要回答这个问题，有必要在地质时期内，在地质时期内，同位素异质性在什么长度尺度上可以持续存在，而不是扩散和对流的混合。由于氦气是地质材料中高度移动的元素，因此它可以为通过地球化学方法采样的异质区域的长度尺度提供下限。但是，尽管在低压下以浅层地幔材料实验确定了氦气的迁移率，但在最深的80％地球地幔中，尚无有关氦气迁移的信息。拟议的工作将提供有关这些深层地幔材料中氦气流动性的第一批数据。 提出的研究结合了实验测量和第一原理计算。实验将在CWRU和哈佛大学进行，以确定在广泛的温度和压力范围内，在偏心酶及其溶解度中扩散的机理和速率。 HE的扩散率将通过使用哈佛大学的贵重气体质谱仪对HE饱和晶体进行增量超出级数加热实验来确定。高压下的氦扩散性和溶解度将通过密封的贵族带电荷胶囊的时间序列实验确定，然后进行大量分析回收的晶体。理论上的第一原理计算将在威斯康星大学进行，以确定他的扩散率及其在Peniclase和MGSIO3-perovskite中的扩散率及其机制。该计算将对实验数据提供关键的交叉检查，并将研究扩展到实验上无法访问的深层地幔条件。拟议的研究将对可以维持在地球深层地幔中的地球化学异质性的最小长度尺度提供基本限制。它还可以提供对核反应堆材料中氦气迁移率的间接见解，在核反应堆材料中，氦封装是一个持续存在的问题，并且在拟议的核废料处理材料中，其中包括钙钛矿结构氧化物。