Isotopic evidence for a Martian regolith component in shergottite meteorites

摘要

The ~(80)Kr excesses determined in the gas‐rich impact‐melt (GRIM) glasses in EET79001 and Shergotty correlate poorly with the ~(129)XeM (Martian atmosphere) suggesting that the majority of the neutron‐capture ~(80)Krn was not shock‐implanted along with ~(129)XeM into these glasses during impact. This inference is consistent with the variations in d ~(80)Krn excesses observed in these samples. The results reported here indicate that the ~(80)Krn 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 ~(80)Krn excesses (～0.3–1.0 × 10~(15) n/cm~2) and from ~(149)Sm deficits (1.0 ± 0.4 × 1015 n/cm~2) agree with each other confirming that ~(80)Krn was mostly produced in situ. In the Martian regolith, thermal (n) and fast (N) neutrons occur together. Also, in the GRIM glasses, the ~(83)Kr/~(86)Kr ratios correlate positively with 84Kr/~(86)Kr indicating that the cosmogenic Kr contains a fast neutron‐produced component in addition to Kr produced by galactic cosmic and solar cosmic irradiation. Using ~(83)KrN 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/cm~2. 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.