Solution properties of rare earth elements in silicate melts: Inferences from immiscible liquids

Abstract

The partitioning behavior of rare earth elements (REE) between immiscible silicate liquids was investigated as a function of temperature and total REE oxide concentration. REE preferentially partition into the end-member liquids that are rich in network-modifying cations, with two-liquid partition coefficients greater than any other cation. The two-liquid partition coefficients of REE in a given experiment are essentially identical to one another. No evidence was seen to suggest that the solution mechanisms of REE in high-silica liquids vary with differences in their ionic radii. “Two-lattice” models of trace element solution behavior fail to remove the compositional dependence of REE two-liquid partition coefficients due to the assumption that all network-forming and network-modifying cations mix ideally with one another. As a result, it is probably incorrect to derive physical interpretations from the results of the application of two-lattice models. The results are applied to the origins of the LREE-enriched phenocryst-matrix partition coefficients seen for some ferromagnesian minerals in high-silica rhyolites. Since liquids with extremely high concentrations of T-O-T linkages do not strongly discriminate between the REE on the basis of their ionic radii, the anomalous REE crystal-matrix partition coefficient patterns are best explained as the result of contamination of these minerals by LREE accessory phases.