Search

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.

Atomistic simulations provide a meaningful way to determine the physico-chemical properties of liquids in a consistent theoretical framework. This approach takes on particular usefulness for the study of molten carbonates, in a context where thermodynamic and transport data are crucially needed over a large domain of temperatures and pressures (to ascertain the role of these melts in geochemical processes) but are very scarce in the literature, especially for the calco-magnesian compositions prevailing in the Earth's mantle. Following our work on Li2CO3-Na2CO3-K2CO3 melts, we extend our force field to incorporate Ca and Mg components. The empirical interaction potentials are benchmarked on the density data available in the experimental literature (for the crystals and the K2Ca(CO3)2 melt) and on the liquid structure issued from ab initio molecular dynamics simulations. Molecular dynamics simulations are then performed to study the thermodynamics, the microscopic structure, the diffusion coefficients, the electrical conductivity and the viscosity of molten Ca, Mg-bearing carbonates up to 2073 K and 15 GPa. Additionally, the equation of state of a Na-Ca-K mixture representative of the lavas emitted at Ol Doinyo Lengai (Tanzania) is evaluated. The overall agreement between the MD results and the existing experimental data is very satisfying and provides evidence for the ability of the force field to accurately model any MgCO3-CaCO3-Li2CO3-Na2CO3-K2CO3 melt over a large T - P range. Moreover it is the first report of a force field allowing to study the transport properties of molten magnesite (MgCO3) and molten dolomite (CaMg(CO3)2).

Although molten carbonates only represent, at most, a very minor phase in the Earth's mantle, they are thought to be implied in anomalous high-conductivity zones in its upper part (70-350 km). Besides the high electrical conductivity of these molten salts is also exploitable in fuel cells. Here we report quantitative calculations of their properties, over a large range of thermodynamic conditions and chemical compositions, that are a requisite to develop technological devices and to provide a better understanding of a number of geochemical processes. To model molten carbonates by atomistic simulations, we have developed an optimized classical force field based on experimental data of the literature and on the liquid structure issued from ab initio molecular dynamics simulations performed by ourselves. In implementing this force field into a molecular dynamics simulation code, we have evaluated the thermodynamics (equation of state and surface tension), the microscopic liquid structure and the transport properties (diffusion coefficients, electrical conductivity and viscosity) of molten alkali carbonates (Li2CO3, Na2CO3, K2CO3 and some of their binary and ternary mixtures) from the melting point up to the thermodynamic conditions prevailing in the Earth's upper mantle (~ 1100-2100 K, 0-15 GPa). Our results are in very good agreement with the data available in the literature. To our knowledge a reliable molecular model for molten alkali carbonates covering such a large domain of thermodynamic conditions, chemical compositions and physicochemical properties has never been published yet.