Cosmogenic records in Antarctic meteorites

Abstract

Aliquot samples of twenty-nine Antarctic L and H chondrites were analyzed for their nuclear track records and⁵³Mn activities. Track density in the analyzed samples range from 10⁴ to ~ 6×10⁶ cm^-2. An important finding is the observation of track-rich grains in a set of four L3 chondrites ALHA 77215, 77216, 77217 and 77252, suspected to belong to the same fall based on petrographic observations. An additional sample, ALHA 78105, a L6 chondrite, also contains track-rich grains.⁵³Mn activity is at near saturation level in ~65% of the analyzed samples indicating exposure ages > 10 m.y. in these cases. Very few H chondrites from the 7-m.y. exposure age peak are apparently sampled among the ones analyzed in this study. About 6% and 4% respectively of the Antarctic H and L chondrites, analyzed so far for their cosmogenic records, have precompaction irradiation features. Combined analysis of⁵³Mn and nuclear track data allowed us to confirm or rule out the proposed pairing of several sets of Antarctic meteorites, and also to estimate the preatmospheric sizes of some of these meteorites. The results indicate that most of the small Antarctic meteorites (< 1 kg) have suffered high (> 95%) ablation mass-loss.