THE ORIGIN OF OLDHAMITE IN UNEQUILIBRATED ENSTATITE CHONDRITES

We report the results of an ion probe study of rare earth elements (RE Es) in six unequilibrated enstatite chondrites. Oldhamite is confirmed as the major but not the only REE carrier in these meteorites. Data a re also provided for other minerals (enstatite, niningerite, and alaba ndite) that carry detectable amounts of REEs. Although the variety of REE patterns in oldhamite is not as extreme as in hibonite, there are seven distinct REE patterns (four of which form a continuum) that char acterize most oldhamite grains from E chondrites. Two of these pattern s (F and G) were each found in only one weathered grain. There is no c orrelation between REE pattern and oldhamite petrologic setting. Our t race element data are used to evaluate earlier work and a number of el ements (Zr, Y, Sc, REEs, Sr, Se, and probably Sb and Cs), including ba th refractory and volatile species under oxidizing conditions, are fou nd enriched in oldhamite. A comparison of REE patterns with the result s of equilibrium condensation calculations indicates that nebular proc esses were important in determining the composition of the oldhamite p recursors, whereas metamorphic processes also affected oldhamite compo sitions. Enrichments of chalcophiles are not necessarily due to intera ction of oldhamite with nebular gas at low temperatures, as previously suggested. Oldhamite, like hibonite, was formed from melts whose trac e element compositions were determined by high temperature transient e vents in the solar nebula.