NUCLEAR TRACKS AND LIGHT NOBLE-GASES IN ALLAN-HILLS-84001 - PREATMOSPHERIC SIZE, FAIL CHARACTERISTICS, COSMIC-RAY EXPOSURE DURATION AND FORMATION AGE

Cosmic-ray produced nuclear tracks and noble gases have been studied i n the martian orthopyroxenite Allan Hills 84001 to delineate its cosmi c-ray exposure history, preatmospheric size, and fall characteristics. A K-Ar age of 3.9 Ga, cosmic-ray exposure duration of 16.7 Ma, and a preatmospheric radius of 10 cm have been deduced from the noble gas an d track data. The track data suggest ALH 84001 to be a single fall tha t has suffered atmospheric mass ablation in excess of 85%, higher than the value deduced for the shergottites, ALHA 77005, EETA 79001, and S hergotty. The formation age, as well as the cosmic-ray exposure durati on, determined in this work are in good agreement with values reported earlier and are distinctly different from other shergottite, nakhlite , and chassignite (SNC) meteorites analysed so far. The high cosmogeni c Ne-22/Ne-21 ratio of 1.22 most probably reflects an effect due to no n-chondritic composition of ALH 84001 as the track data suggest high s hielding (greater than or equal to 5cm) for the analysed samples. Ther e are signatures in the noble gas data that indicate the possible pres ence of trapped Ar and Ne of martian atmospheric origin in ALH 84001.