Formation of carbonatite-related giant rare earth element deposits by liquid immiscibility

The occurrence of sulfate and fluoride minerals in carbonatite-hosted rare earth element (REE) deposits suggests that sulfur and fluorine play important roles in REE mineralization. However, their influence on the partitioning behavior of REEs during the immiscibility process remains poorly understood. This study performed partitioning experiments to explore the impact of sulfur and fluorine on the liquid immiscibility between carbonatitic melt and alkaline silicate melt at 1000‒1200 °C and 0.5‒2.2 GPa. Surprisingly, the experimental results indicate that the addition of sulfur and fluorine does not significantly change the partition coefficients of trace elements between carbonatitic melt and silicate melt. The key factor determining REE partitioning is the structural difference between the two immiscible melts, which can be characterized by the non-bridging oxygen per tetrahedrally coordinated cation of the silicate melt (NBO/T). Partition coefficients tend to decrease as NBO/T increases. Importantly, REE, SO3, and F exhibit similar behaviors, making sulfate and fluoride minerals useful indicators for exploring carbonatite-hosted REE deposits. Additionally, we used rhyolite-MELTS software to simulate crystallization differentiation and liquid immiscibility in alkaline silicate melts. Modeling results show that the initial CO2 content of silicate melt determines the degree of crystallization at which liquid immiscibility occurs. Lower initial CO2 content enhances the enrichment of REEs in the immiscible carbonatitic melt.