An experimental and theoretical study of rare-earth-element partitioning between sulfides (FeS, CaS) and silicate and applications to enstatite achondrites

Partition coefficients of the rare-earth-elements (REE) between sulfides (FeS or CaS) and silicate melt were determined experimentally at 1200-1300 °C. The REE sulfide/silicate partition coefficients (D) are ≤1 under the experimental O and S fugacities, which demonstrates that the REE are mainly located in the silicate phase. Rare-earth-element partition coefficients in the FeS/silicate system decrease from light to heavy REE, while the opposite behavior is found for the CaS/silicate system, where partition coefficients increase from light to heavy REE. In both sulfide systems, Eu is preferentially incorporated into the sulfide phases, as also expected from thermodynamic calculations. The Eu sulfide/silicate partition coefficient is about a factor of ten higher than that of neighboring Sm and Gd, in accordance with thermodynamic predictions of REE sulfide/silicate partition coefficients. The low sulfide/silicate partition coefficients indicate that CaS (oldhamite) in enstatite achondrites (aubrites) cannot have gained its high REE concentrations during igneous differentiation processes. The high REE concentrations and the REE patterns in aubritic oldhamite are more plausibly explained by REE condensation into refractory CaS. The refractory nature of CaS prevented major exchange reactions of the oldhamite with other aubritic minerals during the short differentiation and metamorphism period on the aubrite parent body. Thus, oldhamite in aubrites may be relict condensates altered to different degrees during short heating events, as originally suggested by Lodders and Palme (1990).