AR-39-AR-40 AGE, OF THE IBITIRA EUCRITE AND CONSTRAINTS ON THE TIME OF PYROXENE EQUILIBRATION

We determined a Ar-39-Ar-40 age of 4.495 +/- 0.015 Ga for the unbrecci ated Ibitira eucrite. This age ranks among the oldest Ar-39-Ar-40 ages for eucrites and suggests that Ibitira escaped the widespread impact resetting of K-Ar ages similar to 4.1-3.5 Ga ago indicated by other HE D meteorites. Ibitira exhibits a relatively high degree of chemical eq uilibration of its pyroxene caused by prolonged annealing. This equili bration must have occurred subsequent to the reported (207)pb-(206)pb formation age for Ibitira of 4.556-4.560 Ga, but prior to its determin ed Ar-Ar age. If pyroxene equilibration occurred during burial at sign ificant depths or in a large impact deposit, then the Ar-Ar age may da te this event.