Partitioning of Rare Earth Elements Between Hibonite and Melt and Implications for Nebular Condensation of the Rare Earth Elements

Abstract

Abstract— The partitioning of the rare earth elements (REE) between hibonite and silicate melt has been investigated at 1470 °C in the system calcium tschermak's molecule‐diopside. Oxygen fugacity was varied between air and the iron‐wustite buffer using conventional gas mixing, and to “infinitely reducing” using Sr2+ as a surrogate for Eu2+. The partition coefficient (D) pattern is light REE enriched with D(La) = 7.15 and D(Yb) = 0.1. D(Eu) varied from about 2 in air to about 0.55 in an “infinitely reducing” atmosphere. Experiments were reversed for Eu and, by implication, a close approach to equilibrium was achieved for other elements. Absolute activity coefficients for REE in hibonite were estimated, yielding γLa = 330, γEu3+ = 1200, and γYb = 24 000. These results suggest that unless the principal phase(s) into which a trace element is dissolving and the activity—composition relationships for that trace element in that phase(s) are known, condensation temperatures based on assumed ideal solution behavior will generally be in error.