The CaCO3–CaF2 Binary at 6 GPa

The present work aims to study the phase relations along the CaСO3–CaF2 join at 6 GPa. The experiments were performed using a multianvil press in graphite capsules. The system has one intermediate compound, Ca2CO3F2, identified as brenkite by Raman spectroscopy. At 900–1000°C, the presence of brenkite splits the system into two partial binaries: aragonite + brenkite and brenkite + fluorite. An aragonite-brenkite eutectic is situated near 1080°C. An eutectic melt contains 40 mol% CaF2. Brenkite melts incongruently at 1100°C, producing fluorite and peritectic liquid containing 48 mol% CaF2. The presence of fluorine lowers the melting temperature of the calcium carbonate by almost 600°C to a temperature corresponding to a continental geotherm with a surface heat flow of 35 mW/m2. Thus, under mantle conditions, fluorine enable a calcium carbonate-rich melt to remain liquid at a much lower temperature (1080°C) than would be possible without the presence of fluorine (1660°C).