Isotopic evidence for a Martian regolith component in shergottite meteorites

Abstract

[1] The 80Kr excesses determined in the gas-rich impact-melt (GRIM) glasses in EET79001 and Shergotty correlate poorly with the 129XeM (Martian atmosphere) suggesting that the majority of the neutron-capture 80Krn was not shock-implanted along with 129XeM into these glasses during impact. This inference is consistent with the variations in δ80Krn excesses observed in these samples. The results reported here indicate that the 80Krn excesses in these glasses were produced in the same way as the 149Sm isotopic deficits, i.e., by thermal neutron (n) capture on Br and Sm occurring in the glass-precursor regolith materials on Mars. The thermal neutron fluences calculated from 80Krn excesses (∼0.3–1.0 × 1015 n/cm2) and from 149Sm deficits (1.0 ± 0.4 × 1015 n/cm2) agree with each other confirming that 80Krn was mostly produced in situ. In the Martian regolith, thermal (n) and fast (N) neutrons occur together. Also, in the GRIM glasses, the 83Kr/86Kr ratios correlate positively with 84Kr/86Kr indicating that the cosmogenic Kr contains a fast neutron-produced component in addition to Kr produced by galactic cosmic and solar cosmic irradiation. Using 83KrN and 84KrN excesses produced by fast neutron reactions on Rb and Sr targets in some of these glasses, we determine fast neutron fluences of ∼3–47 × 1015 N/cm2. The integrated fluences of thermal and fast neutrons in GRIM glasses suggest that the glass-precursor materials were irradiated at different depths in the top few meters of the water-ice-bearing regolith near the shergottite source regions on Mars.