The Effect of SiO2 on Trace Element Partitioning between Fluorapatite and Carbonatite melt

Abstract

Carbonatites are considered the primary source of rare earth elements (REE), which are of growing interest due to their essential role in the production of sustainable technologies. To improve our understanding of the evolution of carbonatite melt and the formation of carbonatite-related ore deposits, it is necessary to constrain the partitioning of trace elements (including REE) between minerals and carbonatite melts. As apatite is a common rock-forming mineral in carbonatites, we ran experiments in a piston cylinder apparatus (1250 °C, 1 GPa) and internally heated pressure vessels (1050 °C, 0.2 GPa) to obtain a new set of partition coefficients (Dfap/cbt) for 40 trace elements between fluorapatite (fap) and carbonatite melt (cbt). The starting materials contained varying amounts of SiO2 to investigate its effect on Dfap/cbt. Major elements (including CO2) were measured with EPMA, and trace elements were analyzed using LA-ICP-MS. Adding SiO2 to the system exerts a significant effect on the REE incorporation into fluorapatite. REE are compatible (DREE > 1) when fluorapatites contain > 2.4 wt% SiO2, but are incompatible (DREE < 1) when fluorapatites contain < 0.23 wt% SiO2. REE incorporation is driven by a coupled substitution of Si4+ and REE3+ for P5+ and Ca2+. The presence of SiO2 also enables a coupled substitution of CO32- with SiO44- for two PO43- groups in fluorapatite, and fluorapatites produced in run 181_2 of this study contain up to 7.2 ± 0.2 wt% SiO2 and 10 ± 1 wt% CO2. Dfap/cbt for the tetravalent high field strength elements (e.g., U, Th) also increase when SiO2 is present in the system with DTh > DU throughout all experiments. In contrast, DSr is unaffected by SiO2 in the system with DSr ∼ 0.5 throughout all experiments. The increase in DREE in the presence of SiO2 may serve as a proxy for tracking the major element composition of evolving carbonatite melts, and the carbonate-driven silica incorporation into apatite could represent a potential explorational tool for carbonatite ores.