The effect of fluorine on mineral-carbonatitic melt partitioning of trace elements – Implications for critical mineral deposits.

Natural carbonatite is associated with various types of deposits, including rare earth elements (REEs) and high-field strength elements (e.g., Nb). Although the spatial relationship between fluorine enrichment and the formation of carbonatite-type deposits is well-established, the underlying mechanism remains unclear. In this study, we investigate the influence of fluorine on the partitioning of trace elements in carbonatite melts (CM) from the mantle to the crust. Elemental partitioning experiments were conducted at 3 GPa and 1100–1400 °C, both with and without fluorine. We determined trace element partition coefficients for the mineral-carbonatitic melt pairs involving the following minerals: clinopyroxene, olivine, spinel, perovskite, magnetite, calcite solid solution, magnesio wüstite. The addition of fluorine influences the types and stability of Ti-rich oxides. Fluorine-bearing systems crystallize magnetite, while fluorine-free systems crystallize perovskite. Compared to fluorine-free compositions, systems with 1.55–2.40 wt% fluorine in the melt exhibited significantly lower (clinopyroxene, spinel, olivine)-melt partition coefficients for REEs and HFSEs by half to one order of magnitude. The decreased uptake of these elements by clinopyroxene, spinel, and olivine may be attributed to changes in their speciation within the melt phase, consistent with the formation of REE-F complexes. Using our partition coefficients, we simulated the differences in trace element characteristics between carbonatite melts derived from the mantle under fluorine-bearing and fluorine-free conditions. The fluorine-bearing carbonatitic melt extracted from mantle peridotite displayed significantly higher trace element compositions compared to fluorine-free carbonatitic melt, with trace element concentrations being 5–10 times higher. We simulated the trace element characteristics of carbonatitic melts released from subducted sediments and ocean crusts and found that these ocean crusts or sediments can release fluorine-rich carbonatite melts that are naturally enriched in REE due to their high fluorine and REE contents. Therefore, the subduction of sediments or the recycling of ocean crusts is highly favorable for the formation of carbonatite-hosted REE deposits. This can explain why the majority of carbonatite-hosted REE deposits are typically found in continental margins with subducted materials signals in source region. Furthermore, we modeled the impact of different mineral crystallization and fractionation in the shallow crust on the trace element compositions of carbonatitic melts. Late-stage magma crystallization and differentiation of silicate and carbonate minerals are favorable for the further enrichment of ore-forming materials (e.g., REEs). [ABSTRACT FROM AUTHOR]