COSMIC-RAY EXPOSURE AGES OF DIOGENITES AND THE RECENT COLLISIONAL HISTORY OF THE HOWARDITE, EUCRITE AND DIOGENITE PARENT BODY BODIES/

We determined the cosmic-ray exposure age of 20 diogenites from measur ed cosmogenic noble gas isotopes and calculated production rates of He -3, Ne-21 and Ar-38. The production rates were calculated on the basis of the measured chemical composition and the cosmogenic Ne-22/Ne-21 r atio of each sample. The shielding conditions of each sample were also checked on the basis of the measured Be-10 and Al-26 concentrations. The exposure ages range from 6 to 50 Ma but do not form a continuous d istribution: ten ages cluster at 21-25 Ma and four at 35-42 Ma. The tw o diogenite clusters coincide with the 22 Ma and 38 Ma peaks in the ex posure age distribution of eucrites and howardites. After the selectio n from literature data of 32 eucrites and 11 howardites with reliable ages, we find a total of 23 howardite, eucrite and diogenite (HED) gro up meteorites at 20-25 Ma and 10 at 35-42 Ma. The shape of the two pea ks is consistent with single impact events, and random number statisti cs show that they are statistically significant at the 99% level. Alto gether, this provides strong evidence for two major impact events 22 M a and 39 Ma ago. Although these two events can explain more than half of all HED exposure ages, it takes at least five impact events to expl ain all ages <50 Ma. An impact frequency of one per 10 Ma corresponds to projectiles of at least 2-4 km in diameter for Vesta and of 60-300 m for the 100 x smaller Vesta-derived ''vestoids.'' Based on the HED e xposure-age distribution, the size distribution of the main-belt aster oids and the difference in size between Vesta and the kilometer size v estoids, we favor Vesta as the major source of HED meteorites, althoug h some of the meteorites may have been ejected from the vestoids rathe r than directly from Vesta.