Rare-earth element systematics of carbonatitic fluorapatites, and their significance for carbonatite magma evolution

Magmatic fluorapatites of five African carbonatite complexes were analyzed for rare-earth (REE) and trace elements by electron microprobe and high-resolution synchrotron micro-XRF to explore the fluorapatite composition during different stages of carbonatite magma evolution. Early crystallized fluorapatites have La concentrations mostly below 1,500 ppm and low Sigma REE. They display convex-upward shaped REE patterns with (La/Nd)(cn)less than or equal to1 and low (La/Yb)(cn) ratios < 100. In contrast, fluorapatites from fractionated carbonatites have straight REE patterns with (La/Nd), and (La/Yb)(cn) generally above 100, and have La up to 1 wt% at a high <Sigma>REE. Model calculations with the fractionating mineral assemblage fluorapatite + calcite clinopyroxene suggest REE distribution coefficients for fluorapatite/carbonatite melt with a positive slope throughout from La to Lu, in order to meet the relationships observed in the natural fluorapatites. The calculations oppose closed system conditions of magma fractionation along the liquid lines of descent, but suggest periods of instantaneous fluorapatite crystallization. Fluorapatite trace element characteristics are therefore thought to be indicative for carbonatite evolution, and can reflect the relative degree of magma fractionation. We suggest that the (Eu/Eu*)(cn) and Y evolution in the fluorapatites is a manifestation of an aqueous fluid immiscibly coexisting with the carbonatite magma from early evolution on, which is able to continuously extract divalent Eu and Y from the carbonatite magma.