COMMON ASTEROID BREAK-UP EVENTS OF EUCRITES, DIOGENITES, AND HOWARDITES AND COSMIC-RAY PRODUCTION-RATES FOR NOBLE-GASES IN ACHONDRITES

We determined the cosmic-ray produced noble gas abundances in eight di ogenites, four eucrites, and four howardites. Using these and addition al noble gas and chemical data from the literature, composition and sh ielding dependent production rates of achondrites for He-3, Ne-21, Ar- 38, Kr-78, Kr-83, and Xe-126 were derived based on Kr-81-Kr exposure a ges. The cosmogenic ratio Ne-22/Ne-21 is found to be a valid shielding indicator for all howardites, eucrites, and diogenites (HEDs). This r atio correlates with other shielding dependent ratios, such as Kr-78/K r-83, Kr-80/Kr-83, 124Xe/Xe-131, Xe-126/Xe-131, and Xe-128/Xe-131. We show that strong correlations for these ratios also exist for basaltic lunar rocks. The new production rates were used for the calculation o f exposure ages for all HEDs with known noble gas abundances. Two majo r exposure age clusters at 21 Ma and 38 Ma are observed in all three c lasses. We find that five asteroid break-up events, characterized by e xposure ages 6 +/- 1 Ma, 12 +/- 2 Ma, 21 +/- 4 Ma, 38 +/- 8 Ma, and 73 +/- 3 Ma produced all diogenites, fifteen out of seventeen howardites , and twenty-eight out of thirty-nine eucrites. We conclude that aster oidal or cometary impact events at such times on one or several parent bodies are responsible for the ejection of eucrites, howardites, and diogenites. Probable parent bodies are multiple 4 Vesta-derived kilome ter-sized objects of basaltic achondritic composition reaching the 3:1 resonance region, a proposed source area for meteorites. The direct e jection from Vesta and capture by Earth of basaltic meteorites has bee n shown to be dynamically difficult. In such a scenario large events 3 8 Ma and 21 Ma ago are required, liberating simultaneously material re presenting Vesta's surface (eucrites, howardites) and interior regions (diogenites).