AN ATOMISTIC STUDY OF THE INCORPORATION AND DIFFUSION OF NOBLE GASES IN SILICATE MINERALS*

Trace elements are widely used to unravel magmatic processes and constrain the chemical differentiation of the Earth. Central to this enterprise is understanding the controls on trace element fractionation between solid and liquid phases and thus the energetics of incorporating trace elements into crystals. In this contribution we focus on the incorporation of noble gases into crystals, with implications for the degassing processes in the Earth and the atmosphere. We use ab-initio and force-field methods to study the uptake of noble gases He, Ne and Ar in solid silicates. We calculate atomic defect energies of incorporation both at vacancies and at interstitial positions in the solid. We use these energies to estimate the total uptake of noble gases into the crystal as a function of temperature. Such concentrations are found to be very low (10^-3 and 10^-10 ppm) for He up to Ar respectively with the noble gases incorporated predicted to be more favorable at intrinsic vacancies of Si or Mg sites. We also look at the diffusion of these minerals within the lattice and estimate activation energies for such processes. Our results support the hypothesis that noble gases have very low solubilities in bulk solid minerals.