HETEROGENEOUS REE PATTERNS IN OLDHAMITE FROM AUBRITES - THEIR NATURE AND ORIGIN

A comprehensive study of the REEs in oldhamite from three aubrites con firms that heterogeneous REE patterns are a general feature of CaS in these meteorites. Ten distinct patterns have been observed in 109 grai ns analyzed. Four patterns are directly comparable to REE patterns see n in oldhamite from the unequilibrated (E3-4) enstatite chondrites, an d many of these exhibit positive Eu and/or Yb anomalies that are best understood in terms of condensation from the solar nebula. Other REE p atterns, however, show evidence of igneous fractionation. More than on e process is thus required to explain the variety of REE patterns seen in oldhamite from the aubrites. It is proposed that oldhamite was clo se to the liquidus under the conditions of aubrite formation and that some grains melted while others did not. Thus, some oldhamites have RE E patterns that reflect the fractionations of igneous processing, whil e others have retained REE characteristics established during formatio n 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 o f the REEs with neighboring phases during a prolonged metamorphic epis ode.