Noble gas migration in silica polymorphs at Earth's mantle conditions

The diffusion of noble gases in SiO2 phases is studied using ab initio molecular dynamics based on the density functional theory, covering pressure and temperature conditions from the crust to the core. Our results show that the diffusion of noble gases in SiO2 minerals is not only controlled by external conditions such as temperature and pressure but is also highly sensitive to the structure of the host mineral as well as the size of the noble gas. We show that the diffusion coefficient of He in quartz at 1700 K is two orders of magnitude larger than that of He in seifertite at 5000 K. In quartz, the larger the noble gas, the slower the diffusion. Nudged elastic band (NEB) computations in quartz also give Ea(Ar) < Ea(Kr) < Ea(Xe). Interestingly, we predict that Ne diffuses faster than He in dense SiO2 phases at a given temperature, in apparent contradiction with the common assumption that heavier atoms diffuse slower. We explain this phenomenon in seifertite by greater repulsive interactions between Ne and its local environment than for He in the minimum energy configuration, triggering the rapid jumps of Ne toward the lower-coordinated transition site. In agreement with other studies, we predict He and to a lesser extent Ne are quickly released from the continental crust with limited storage in the deepest part of the mantle. However, Ar can be transported deeper than lighter noble gases, potentially trapped in stishovite, CaCl2-type and seifertite in the deep lower mantle. Given the abundance of these minerals in mantle basaltic compositions and the continental crust, their contribution to the volatile cycle of noble gases is far from negligible.