Noble Gases in Carbonate Melts: Constraints on the Solubility and the Surface Tension by Molecular Dynamics Simulation

Although they are rare elements in the earth's mantle, noble gases (NGs) are important geochemical tracers because of their chemical inertness, the specificity of their isotopes, and their strongly varying sizes (a factor >50 in mass from He to Rn). 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 NG-s ted calculations of the solubility of He, Ne, Ar, and Xe in carbonate melts based on carbonate melt 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 emitted at OI Doinyo Lengai, Tanzania). 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 similar to 0.1 GPa). At higher pressures (a few GPa), the solubility levels off or even starts to diminish smoothly (for He at P > 2 GPa and Ar at P > 4 GPa). In contrast, in molten dolomite, the effect of pressure is negligible on the studied P range (3-6 GPa). At the pressures in the upper part of the upper mantle, the solubilities of NGs in carbonate melts are still of the same order of magnitude as the ones in molten silicates (10(0) to 10(1) mol %). This suggests that carbonatitic melts at depth are not preferential carriers of NGs, 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 NGs and its evolution with pressure. Whatever the composition of the melt and of the NG phase is, the surface tension increases (by a factor similar to 2) when P increases from 0 to 6 GPa. This behavior is in contrast to the situation occurring when H2O is in contact with silicate melts (then surface tension drops when pressure increases to a few GPa).