HALOGEN CONTAMINATION IN ANTARCTIC H5 AND H6 CHONDRITES AND RELATION TO SITES OF RECOVERY

The distribution of the elements F, Cl, Br and I was analysed in layer s removed stepwise from Antarctic H5 and H6 chondrites. All meteorites show higher concentrations of these elements on their surfaces than i n their interiors. The degree of halogen enrichment cannot be correlat ed with the degree of visual weathering and is proportional to the tim e the meteorites were residing on the surface of the Antarctic ice. Du ring this period, aerosols, salts and gaseous components are deposited on the surfaces of the meteorites and diffuse into their interior. Th e observed contamination level of the meteorites is influenced by leac hing properties and diffusion behaviour of the individual halogens. Th e major contamination source for F, Cl and probably Br is air-borne se aspray; for I it is the gaseous compound methyl iodide (CH3I) produced by biological processes in the sea. Methyl iodide and its oxidation p roducts formed in the Antarctic atmosphere (e.g., I-2) can be transpor ted over longer distances to the interior of Antarctica than air-borne seaspray. Therefore, the ratio of the halogen contamination is relate d to the collection site of the meteorites. All meteorites that were f ound in the Antarctic interior are contaminated to a lesser degree by F, Cl and Br relative to I than those found near the coast. The measur ed enrichment ratio of I/F is a function of the distance between the c ollection site and the open sea, and increases from the Allan Hills to the Thiel Mountains. By revealing the degree of contamination of a me teorite it is possible to determine its maximum surface residence time on the Antarctic ice.