An experimental determination of the liquidus and a thermodynamic melt model in the CaCO3-MgCO3 binary, and modelling of carbonated mantle melting

The binary CaCO3-MgCO3 constitutes the reference model system to reconstruct the petrogenesis of carbonated rocks, and of carbonatite magmas possibly generated in the Earth's mantle. We experimentally determined the melting of aragonite and magnesite to pressures of 12 GPa, and of calcite-magnesite mixtures at 3 and 4.5 GPa, at variable Ca/(Mg + Ca) (XCa). The melting curves of aragonite, and magnesite have similar slopes, the latter melting ≈ 30 °C higher than aragonite. In the Ca-Mg binary, the minimum of the liquidus surface situates at an XCa of 0.65–0.60, at 1200 °C − 3 GPa, and 1275 °C − 4.5 GPa. Together with available data at 1 and 6 GPa, the minimum liquid composition remains approximately constant with pressure. All available experimental data are then fit by the first thermodynamic model for CaCO3-MgCO3 liquids. Surprisingly, although carbonate liquids should behave as relatively simple molten salts, the liquids display large non-ideality and a three-component, pressure dependent, asymmetric liquid solution model is required to model the liquidus surface. Attempts to use only the two end-member components fail, invariably generating a very wide magnesite-liquid loop in disagreement with the experimental evidence. The liquid model is then used to evaluate results of experimentally determined phase relationships for carbonated peridotites in CaO-MgO-SiO2-CO2 (CMS-CO2), and CaO-MgO-Al2O3-SiO2-CO2 (CMAS-CO2). Computations highlight that the liquid composition formed in a model carbonated mantle do not represent “minimum melts” but are more magnesian at high pressure. The pressure–temperature position of the solidus, as well as its dP/dT slope, including the appearance or absence of the “carbonatite ledge”, depend on bulk composition, unless truly invariant assemblages occur.