Melting Relationships of Calcite-Dolomite-H2O at 1 GPa and Implications for Partial Melting of Metacarbonate Rocks in Deeply Buried Crust

Abstract Recent studies suggest that hydrous melting of metacarbonate rocks is more common than previously thought and that crustal carbonatites can be thus produced. Metacarbonate rocks mainly consist of calcite (Cal) and/or dolomite (Dol). Thus far, melting relationships in the system Cal-Dol-H2O are poorly constrained at high pressures, which hinders our understanding of the anatexis of such rocks in deeply buried crust. Here, H2O-saturated melting experiments on mixtures of Cal + Dol are presented at 1 GPa and 800-1000 °C. Supplemented by phase equilibria modelling, a T-X pseudosection and compatibility diagrams were constructed for the pseudoternary system Cal-Dol-H2O at both subsolidus and suprasolidus conditions. The results reveal that the solidus and liquidus temperatures of the carbonates are significantly lowered by H2O. The wet solidus temperature along the join Cal-Dol is around 900 °C for pure Cal and at 825 ± 25 °C within the miscibility gap between Mg-Cal and Dol. Melting reactions on the wet solidus are Cal + F (aqueous fluid) = melt (for pure calcite) and Mg-Cal ± Dol + periclase + F = melt (in MgCO3-bearing systems). The melts are (Mg)-calcitic containing Ca(OH)2 and/or Mg(OH)2 components (i.e. the molar ratio of (CaO + MgO)/CO2 is above 1). This study provides an experimental and theoretical foundation for understanding carbonate melting at depth and suggests that metacarbonate rocks can melt in deeply buried crust if free water is present. Feasible geological settings in which hydrous melting of metacarbonate rocks can happen are discussed, but it is also argued that the conditions to form crustal carbonatites are restricted.