Partitioning of REE between calcite and carbonatitic melt containing P, S, Si at 650\u2013900\xa0\xb0C and 100\xa0MPa

Abstract

Carbonatites host some unique ore deposits, especially REE, and fractional crystallization might be a potentially powerful mechanism for control enrichment of carbonatitic magmas by these metals to economically significant levels. At present, data on distribution coefficients of REE during fractional crystallization of carbonatitic melts at volcanic conditions are extremely scarce. Here we present an experimental study of REE partitioning between carbonatitic melts and calcite in the system CaCO3-Na2CO3 with varying amounts of P2O5, F, Cl, SiO2, SO3 at 650–900 °C and 100 MPa using cold-seal pressure vessels and LA-ICP-MS. The presence of phosphorus in the system generally increases the distribution coefficients but its effect decreases with increasing concentration. The temperature factor is high: at 770–900 °C DREE ≥ 1, while at lower temperatures DREE become below unity. Silicon also promotes the fractionation of REE into calcite, while sulfur contributes to retention of REE in the melt. Our results imply that calcite may impose significant control upon REE fractionation at the early stages of crystallization of carbonatitic magmas and might be a closest proxy for monitoring the REE content in initial melt.