Transition from carbonatitic magmas to hydrothermal brines: Continuous dilution or fluid exsolution?

Carbonatites are the most important primary sources for the rare earth elements (REEs). While fractional crystallization of carbonate minerals results in the enrichment of volatiles, alkalis, and REEs in the remaining melts, the transition from carbonatitic magmas to hydrothermal brines remains unclear. Here, we investigated the pressure-temperature-composition (P-T-X) properties of the Na2CO3-H2O system up to 700°C and 11.0 kbar using a hydrothermal diamond anvil cell and a Raman spectrometer. Our results show that Na2CO3 becomes increasingly soluble under high P-T conditions, leading to the disappearance of melt-fluid immiscibility and the continuous transition from Na2CO3 melts to hydrothermal brines under deep crustal conditions. Given the abundance of Na2CO3 in highly evolved carbonatitic systems, we suggest that the continuous melt-fluid transition in deep-seated carbonatites results in REEs being sufficiently concentrated in the brine-melts to form economic ore bodies, whereas in shallow systems, REEs preferentially partition into carbonatitic magmas over synmagmatic brines and disperse in carbonatite rocks that underwent limited fractionation.