The trace element chemistry of CaS in enstatite chondrites and some implications regarding its origin

Abstract

Samples of the mineral oldhamite (CaS) were extracted from enstatite chondrites and analyzed by INAA. Prior to extraction, the petrologic setting of the grains was studied microscopically and their minor element contents determined by microprobe analyses. Minor element contents of CaS are known to vary and correlate with petrologic type, indicating secondary redistribution during metamorphism. For this reason, samples were chosen that displayed a range of minor element contents. The trace element contents determined in this study follow a similar pattern. The most primitive samples of CaS studied, contain virtually the entire inventory of the host meteorite's LREE and Eu plus 30–50% of the HREE inventory. In less primitive samples the LREE are less enriched although Eu remains highly concentrated in CaS. Several other elements, including lithophiles (Ba, Cs, Cr, Hf and Sc) and chalcophiles (Sb and Zn) are most enriched in more primitive CaS. The high concentrations of refractory elements, several of which have a tendency to form sulfides at high temperatures in a gas slightly more reducing than solar, lend support to the suggestion that CaS originated at high temperatures in a reduced region of the nebula. The high concentrations of volatile Cs, Sb and Zn indicate that with decreasing temperature CaS continued to interact with the nebular gas, which therefore must have had a low oxygen fugacity at low temperatures.