Heterogeneous REE patterns in oldhamite from aubrites: Their nature and origin

Abstract

A comprehensive study of the REEs in oldhamite from three aubrites confirms that heterogeneous REE patterns are a general feature of CaS in these meteorites. Ten distinct patterns have been observed in 109 grains analyzed. Four patterns are directly comparable to REE patterns seen in oldhamite from the unequilibrated (E3-4) enstatite chondrites, and many of these exhibit positive Eu and/or Yb anomalies that are best understood in terms of condensation from the solar nebula. Other REE patterns, however, show evidence of igneous fractionation. More than one process is thus required to explain the variety of REE patterns seen in oldhamite from the aubrites. It is proposed that oldhamite was close to the liquidus under the conditions of aubrite formation and that some grains melted while others did not. Thus, some oldhamites have REE patterns that reflect the fractionations of igneous processing, while others have retained REE characteristics established during formation in the solar nebula. In contrast to oldhamite from the aubrites and E3-4 chondrites, CaS from two equilibrated (EL6) enstatite chondrites shows high and uniform REE abundances that result from equilibration of the REEs with neighboring phases during a prolonged metamorphic episode.