Evidence for common breakup events of the acapulcoites-lodranites and chondrites

Acapulcoites and lodranites are believed to originate on a common parent bo dy and to represent some of the earliest events in the differentiation of t he chondritic asteroids. We have conducted isotopic studies of the noble ga ses He, Ne, Ar, Kr, and Xe, and determinations of the concentrations of the major elements and of the radionuclides Be-10, Al-26, and Cl-36 in, an att empt to constrain the cosmic-ray exposure history of two members of the aca pulcoite-lodranite dan recovered in Antarctica: Frontier Mountain (FRO) 950 29 and Graves Nunataks (GRA) 95209. From cosmic-ray-produced He-3, Ne-21, a nd Ar-38 and appropriate production rates, we derive parent-body breakup ti mes of 4.59 +/- 0.60 and 6.82 +/- 0.60 Ma for FRO 95029 and GRA 95209, resp ectively. These times are consistent with those obtained from the pairs Be- 10-Ne-21 and Al-26-Ne-21; whereas the times inferred from the pair Cl-36-Ar -36 are slightly longer, perhaps because the Cl-36 activities decreased as a result of decay on Earth. Terrestrial ages up to similar to 50 ka for the two meteorites are consistent with the measured Cl-36 activities of the me tal phases. All acapulcoites and lodranites dated until now show cosmic-ray exposure ages in the range of 4-10 Ma. This is the same range as that foun d for the major exposure age cluster of the H chondrites. As a common paren t body is improbable on the basis of the O-isotopic systematics, a common s et of impactors might have affected the asteroid belt 4-10 Ma ago.