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).

中文翻译：

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氟对微量元素的矿物-碳酸岩熔体分配的影响 – 对关键矿床的影响


天然碳酸岩与各种类型的矿床有关，包括稀土元素（REE）和高场强元素（例如铌）。尽管氟富集与碳酸岩型矿床形成之间的空间关系已明确，但其潜在机制仍不清楚。在这项研究中，我们研究了氟对碳酸岩熔体（CM）中微量元素从地幔到地壳分配的影响。元素分配实验在 3 GPa 和 1100–1400 °C 下进行，有氟和无氟均可。我们确定了涉及以下矿物的矿物-碳酸岩熔体对的微量元素分配系数：单斜辉石、橄榄石、尖晶石、钙钛矿、磁铁矿、方解石固溶体、镁方铁矿。氟的添加影响富钛氧化物的类型和稳定性。含氟系统使磁铁矿结晶，而无氟系统使钙钛矿结晶。与无氟组合物相比，熔体中氟含量为 1.55-2.40 wt% 的体系（单斜辉石、尖晶石、橄榄石）对 REE 和 HFSE 的熔体分配系数显着降低一半到一个数量级。单斜辉石、尖晶石和橄榄石对这些元素的吸收减少可能是由于它们在熔体相内形态的变化，这与 REE-F 复合物的形成一致。利用我们的分配系数，我们模拟了含氟和无氟条件下源自地幔的碳酸岩熔体之间微量元素特征的差异。 从地幔橄榄岩中提取的含氟碳酸岩熔体的微量元素成分明显高于无氟碳酸盐岩熔体，微量元素浓度高出5-10倍。我们模拟了俯冲沉积物和洋壳释放的碳酸盐岩熔体的微量元素特征，发现这些洋壳或沉积物可以释放富氟碳酸盐岩熔体，这些熔体由于氟和稀土元素含量高而自然富集稀土元素。因此，沉积物的俯冲或洋壳的再循环非常有利于碳酸岩稀土矿床的形成。这可以解释为什么大多数碳酸岩稀土矿床通常发现于大陆边缘，源区有俯冲物质信号。此外，我们模拟了浅部地壳中不同矿物结晶和分馏对碳酸盐岩熔体微量元素组成的影响。后期岩浆结晶和硅酸盐、碳酸盐矿物的分异有利于成矿物质（如稀土元素）的进一步富集。