Melting relationships in the systems CaOCO2 and MgOCO2 to 33 kilobars

The melting temperatures of calcite and magnesite in the presence of excess CO₂ have been measured using Ag₂C₂O₄ in sealed capsules m a piston-cylinder apparatus. At 27 kbar, 11.5 wt % CO₂ dissolves in molten CaCO₂, depressing the freezing temperature from 1610 to 1505°C; and 6.5 wt % CO₂ dissolves in molten MgCO₃, depressing the freezing temperature from 1590 to 1510°C. The eutectic between calcite and lime was located at 1385°C at 27 kbar. These and other new results, combined with previously published data, permit completion of PT diagrams for the systems CaO-CO₂ and MgO-CO₂ from 1 bar to 35 kbar. The dissociation curve for each carbonate terminates at an invariant point where melting begins, at 40 bars and 1230°C for CaO-CO₂ and 23 kbar and 1550°C for MgO-CO₂ The differences between the two systems are explained by the different solubilities of CO₂ in the invariant liquids consequent upon the large pressure difference between the locations of these two invariant points. The results show that the temperatures for the beginning of melting of carbonates in the asthenosphere are lowered by about 100°C in the presence of CO₂.