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.
