First-Principles Simulations of Noble Gases Dissolved in Liquid Silica

Noble gases are widely used in geochemistry to gain insight into the structure of the deep Earth, its formation and differentiation, and the efficiency of large-scale processes such as mantle convection, These studies depend in part on understanding the solubility and local structure of noble gases in Earth materials, particularly silicate liquids, under high pressures and temperatures. Silicate liquids undergo profound changes in structure with increasing pressure, and it is not well known how dissolved noble gases respond to these modifications.

First principles molecular simulations were performed, using VASP, on five different noble gases in liquid silica at 4000 K, under pressures from ambient pressure to around lower mantle pressure. The sizes of noble gas atoms were estimated based on radial distribution function results, and were found to vary linearly with the volume ratio of the simulated liquid to its ambient melt. The VASP results agree well with available experimental results for Kr and should be more accurate than former available estimations.

The number of nearest-neighbor oxygen atoms surrounding a noble gas atom was also investigated. At high pressures, where silica adopts a more close-packed structure, the coordination number was found to be in excellent agreement with theoretical results on the number of hard spheres of different size that can be packed around a central sphere.