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Noble gases in high-pressure silicate liquids: A computer simulation study

The test particle method has been used in conjunction with molecular dynamics simulations to evaluate the solubility of noble gases in silicate melts of various compositions. At low pressure the calculated solubility constants (the inverse of the Henry's constant) are in excellent agreement with data of the literature. In particular it is found that the solubility constant (i) decreases when the size of the noble gas increases, (ii) decreases from silica-rich to silica-poor composition of the melt, and (iii) is positively correlated with the temperature. Moreover it is shown that the solubility is governed primarily by the entropic cost of cavity formation for inserting the noble gas into the melt and secondarily by its solvation energy. Interestingly, the behaviour of these two contributions differ from each other as the entropic cost of cavity formation increases strongly with the size of the solute atom to insert whereas large atoms are better solvated than small ones. With all melt composition investigated here (silica, rhyolite, MORB and olivine), the calculated solubility curves exhibit the same qualitative behaviour with pressure; a steep rise culminating in a broad maximum followed by a gradual decrease of the solubility at higher pressure. At variance with LHDAC experiments (Chamorro et al. (1996, 1998) and Bouhifd et al. (2006, 2008)) where a Ar solubility drop is observed at about 50 kbar in silica and molten olivine and in the pressure range ~100-160 kbar with other melt composition, we do not find such a sudden change of the solubility.