DARK CLASTS IN THE KHOR-TEMIKI AUBRITE - NOT BASALTS

The enstatite achondrite meteorites (aubrites) are ultramafic assembla ges with highly variable bulk rare earth element (REE) compositions. A n enrichment of REE in a dark clast from the Khor Temiki aubrite led W olf et al. (1983) to suggest that such dark clasts could be the basalt ic (i.e., enstatite-plagioclase) complements to the ultramafic aubrite s, with the relatively high REE contents resulting from the presence o f plagioclase, which is a common carrier of the REEs. We have studied several dark clasts from the Khor Temiki aubrite and find no evidence for a basaltic character for such material. The microscopic character of the dark clasts is not significantly different from the main portio ns of Khor Temiki and consists either of highly brecciated material, c ontaining a fine-grained matrix, or of enstatite grains with abundant inclusions. We suggest that the dark clasts are shock-darkened, hetero geneous Khor Temiki material that, by chance, contained variable trace contents of oldhamite (CaS), which has been shown to be a major carri er of REE in aubrites. We find that the REE contents of the clasts ran ge from 0.1 to similar to 20x CI. Most have negative Eu anomalies, but one has a small positive anomaly. Extensive searches have failed to i dentify basaltic material in Khor Temiki and other aubrites. The absen ce of basaltic material is consistent with, but does not prove, the mo del of Wilson and Keil (1991). They calculate that, on an asteroidal p arent body < similar to 100 km in radius, a volatile-rich basaltic par tial melt erupted with a velocity greater than the escape velocity of the asteroid and, thus, was lost into space similar to 4.55 Ga ago.