Nuclear tracks and light noble gases in Allan Hills 84001: preatmospheric size, fall characteristics, cosmic-ray exposure duration and formation age

Abstract

Cosmic-ray produced nuclear tracks and noble gases have been studied in the martian orthopyroxenite Allan Hills 84001 to delineate its cosmic-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 and track data. The track data suggest ALH 84001 to be a single fall that has suffered atmospheric mass ablation in excess of 85%, higher than the value deduced for the shergottites, ALHA 77005, EETA 79001, and Shergotty. The formation age, as well as the cosmic-ray exposure duration, 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 cosmogenic ²²Ne/²¹Ne ratio of 1.22 most probably reflects an effect due to non-chondritic composition of ALH 84001 as the track data suggest high shielding (<5cm) for the analysed samples. There are signatures in the noble gas data that indicate the possible presence of trapped Ar and Ne of martian atmospheric origin in ALH 84001.