Carbonatite is a particular type of magmatic rocks enriched in REE relative to the majority of igneous rocks. Among the numerous carbonatites referenced worldwide, only a few of them contain economic REE concentrations widely considered as resulting from late- to post- magmatic processes. However, a magmatic pre-enrichment linked to the processes of carbonatite formation is likely contributing to the REE enrichment. Carbonatites are often spatially associated with alkaline igneous rocks, suggesting a common genetic origin, potentially formed by immiscibility processes. Previous experimental studies have shown that immiscibility between carbonatites and alkaline silicate magmas can occur in a wide range of pressure and temperature conditions, and can lead to both massive and negligible enrichments in carbonatite, making the magmatic processes controlling REE enrichments unclear. Therefore, there is a need to highlight the REE behaviour during the immiscibility processes occurring throughout the differentiation course of alkaline magmas.

The aim of this study is to perform experiments simulating the immiscibility between carbonate and silicate melts during fractionation of alkaline silica-undersaturated magmas, in order to assess the parameter controlling carbonatite REE enrichments all along the alkaline magmatic series.

The experiments were conducted in internally heated pressure vessels and piston-cylinder devices, at crustal and mantle conditions. The experimental charges were analysed for the composition of both liquid and crystal phases, and the REE partition coefficients were calculated for carbonate/silicate melts and crystal/melts.

Our results reveal a REE enrichment optimum in carbonatites during the differentiation of alkaline magmas. In fact, we found that REE and Ca partition coefficients between carbonate and silicate melts are perfectly correlated for all experimental conditions, involving that Ca-dominated carbonatite in coexistence with alkaline magmas are highly enriched in REE. Based on both REE-Ca partitioning correlation and silicate melt composition, we developed a parameterisation predicting the REE composition of carbonatite. This modelling links the carbonatite REE enrichment along the alkaline magmatic series to the precursor magmatic processes.