ALLAN-HILLS-88019 - AN ANTARCTIC H-CHONDRITE WITH A VERY LONG TERRESTRIAL AGE

We have measured the concentrations of the cosmogenic radionuclides Be -10, Al-26 and Cl-36 (half-lifes 1.51 Ma, 716 ka, and 300 ka, respecti vely) in two different laboratories by accelerator mass spectrometry ( AMS) techniques, as well as concentrations and isotopic compositions o f stable He, Ne and Ar in the Antarctic H-chondrite Allan Hills (ALH) 88019. In addition, nuclear track densities were measured. From these results, it is concluded that the meteoroid ALH 88019 had a preatmosph eric radius of (20 +/- 5) cm and a shielding depth for the analyzed sa mples of between 4 and 8 cm. Using calculated and experimentally deter mined production rates of cosmogenic nuclides, an exposure age of simi lar to 40 Ma is obtained from cosmogenic Ne-21 and Ar-38. The extremel y low concentrations of radionuclides are explained by a very long ter restrial age for this meteorite of 2 +/- 0.4 Ma. A similarly long terr estrial age was found so far only for the Antarctic L-chondrite Lewis Cliff (LEW) 86360. Such long ages establish one boundary condition for the history of meteorites in Antarctica.