Despite carbonatites being the principal host-rocks for important critical resources such as REE-(Nb), the conditions of their formation, magmatic evolution and final crystallization at upper crustal conditions remain poorly understood. While the majority of carbonatites are calcite dominated cumulate rocks, about 1/3 of all carbonatite occurrences also have dolomite-dominated rocks. Although dolomite is stable at lithospheric mantle conditions, at <0.8 GPa, dolomite decarbonation (Dolomite → Calcite + Periclase + CO2) lies below the eutectic temperature of the CaCO3 – MgCO3 binary, such that dolomite could not be a magmatic phase in the (Ca,Mg)CO3-system. It is suggested that, fluxing agents, such as alkalis and/or H2O suppress the liquidus temperature of the melts leading to dolomite crystallization.

This study determines the liquidus surface of the CaCO3 – MgCO3 – Na2CO3 ternary system at 0.2 GPa. Experiments are conducted in externally heated pressure vessels using Au90-80Pd10-20-capsules. Recovered capsules are prepared and dry-polished in a glovebox (humidity <3 %) to prevent the otherwise almost instant dissolution of hygroscopic alkali-carbonates.

At T ≥800 °C, melt compositions equivalent to primary carbonatitic melts fractionate towards a calcite + periclase cotectic, dolomite is not yet a liquidus phase. Co-precipitation of calcite and periclase leads to melt compositions with higher alkalinity. Only at molar fractions of 0.35-0.4 Na2CO3 (XNa2CO3) and T <775 °C, a peritectic yields dolomite at the expense of periclase. On the CaCO3 – Na2CO3 side, nyerereite (Na2Ca[CO3]2) appears with calcite along a second cotectic at 750 °C and ~0.45 XNa2CO3. The two cotectics intersect in a peritectic at T <700 °C and 0.5 XNa2CO3 where calcite is replaced by nyerereite and dolomite. The results show that a sufficient alkalinity is critical to the crystallization of dolomite-bearing carbonatites at crustal conditions. Further, the experiment suggest that calcite could react out at the dolomite – calcite – nyerereite peritectic and may not remain a primary phase in late-magmatic carbonatites.