Noble gases in carbonate melts: constraints on the solubility and the surface tension by molecular dynamics simulation

Abstract: Although they are rare elements in the Earth's mantle, noble gases (NG) owe to their strongly varying masses contrasting physical behaviors making them important geochemical tracers. When partial melting occurs at depth, the partitioning of NGs between phases is controlled by a distribution coefficient that can be determined from the solubility of the NGs in each phase. Here we report quantitative calculations of the solubility of He, Ne, Ar and Xe in carbonate melts based on molecular dynamics simulations. The NG solubilities are first calculated in K2CO3-CaCO3 mixtures at 1 bar and favorably compared to the only experimental data available to date. Then we investigate the effect of pressure (up to 6 GPa), focusing on two melt compositions: a dolomitic one and a natrocarbonatitic one (modeling the lava of Ol Doinyo Lengai). The solubility decreases with the amount of alkaline-earth cation in the melt and with the size of the noble gas. In the natrocarbonatitic melt, Henry's law is fulfilled at low pressures (up to ~ 0.1 GPa). At higher pressures the solubility levels off or even starts to diminish smoothly. In contrast, in molten dolomite the effect of pressure is negligible on the studied P range (3-6 GPa). At the pressures of the Earth's mantle, the solubilities of noble gases in carbonate melts are still of the same order of magnitude as the ones in molten silicates (~ 1-10 mol%). This suggests that carbonatitic melts at depth are not preferential carriers of noble gases, even if the dependence with the melt composition is not negligible and has to be evaluated on a case-by-case basis. Finally we evaluate the surface tension at the interface between carbonate melts and noble gases and its evolution with pressure. Whatever the composition of the melt and of the NG phase, the surface tension increases when P increases from 0 to 6 GPa.