CO2 and H2O are the most abundant volatile components in the Earth’s crust and mantle. The presence of CO2, H2O and mixtures of H2O-CO2 effect the phase relations of mantle lithologies. The H2O-CO2-MgO system, providing the basis for more complex systems, is poorly constrained from some quenched experiments. Here we investigated by in ‑ situ X‑ray diffraction measurements using the large volume press at P61B at PETRA III (DESY) the subsolidus and melting reaction of magnesite mixed with 16 wt% brucite.
We report the melting reaction of magnesite + periclase + vapour à liquid between 6 and 12 GPa and the subsolidus reaction magnesite à periclase + vapour between 2 and 4.8 GPa. These reactions meet at an invariant point at about 5 GPa and 1500 ℃. This invariant point was reported at lower pressure of 3.2 GPa by Ellis  who investigated the topology of subsolidus and melting reactions in the H2O-CO2-MgO system theoretically. Between 5 and 12 GPa and in the presence of a CO2-H2O vapour phase, magnesite melts at a temperature about 90 ℃ lower than under anhydrous conditions. This improves previous results from quenched multi-anvil experiments locating the melting point of hydrous magnesite at 6 GPa 50 ℃ lower than the melting point of anhydrous magnesite .
1 Ellis and Wyllie, Carbonation, hydration, and melting relations in the system MgO-H2O-CO2 at pressures up to 100 kbar. American Mineralogist, 1979. 64(1-2): p. 32-40.
2 Müller, Koch-Müller, Rhede, Wilke, and Wirth, Melting relations in the system CaCO3-MgCO3 at 6 GPa. American Mineralogist, 2017. 102(12): p. 2440-2449.