EXPOSURE OF ALLAN-HILLS-84001 AND OTHER ACHONDRITES ON THE ANTARCTIC ICE

The enrichment of F on Antarctic meteorites is the result of their exp osure to the atmosphere, and its measurement allows a subdivision of t he terrestrial age into a duration of exposure on the ice and the time a meteorite was enclosed by the ice. In many cases, the periods of su rface exposure are only small fractions of the terrestrial ages of met eorites collected in Antarctica. The enrichment of F on the surfaces o f Antarctic achondrites was investigated by means of nuclear reaction analysis (NRA): scanning proton beams with an energy of 2.7 and 3.4 Me V were used to induce the reactions F-19(p,alpha gamma)160 and F-19(p, p'gamma)F-19, respectively. Gamma signals proportional to the F conten t were measured. The following Antarctic achondrites were investigated : Martian meteorite ALH 84001; diogenite ALHA77256; the eucrites ALHA8 1011 and ALHA78132; and in addition, the H5 chondrite ALHA79025. For A LH 84001, our data indicate a period of exposure on the ice of <500 ye ars. Thus, this specimen was enclosed in the ice >95% of its terrestri al age of 13 000 years.