DEPTH-PROFILES AND SURFACE ENRICHMENT OF THE HALOGENS IN 4 ANTARCTIC H5 CHONDRITES AND IN 2 NON-ANTARCTIC CHONDRITES

Depth-profiles of F, Cl, Br and I concentrations were determined in fo ur different Antarctic H5 chondrites from the Allan Hills and in the t wo chondrites Allende (C3) and Holbrook (L6). Pieces of the meteorites were studied by analysis of stepwise removed layers of 0.5-1.0 mm thi ckness up to a depth of 9 mm. Neutron activation analysis and ion-sele ctive potentiometry were used for the determination of Cl, Br, I and f or F, respectively. The Antarctic meteorites show higher concentration s of the halogens at the surface compared to the interior. The highest enrichment factors are found for I and Cl and the lowest for Br. In c ontrast, F shows the steepest concentration gradient and is only enric hed in the first 2.5 mm below the surface. The other halogens have pen etrated deeper into the meteorites, The measured enrichments at the su rfaces are not correlated to the visible degree of weathering. The ana lysed non-Antarctic meteorites, which were recovered shortly after the ir observed fall, demonstrate similar halogen concentrations at the su rface, including the fusion crust, as in the interior. Based on these results we present a model to estimate the degree of contamination and the relation to the duration of exposure at the surface of the Antarc tic ice.