Xenon isotopes in irradiated ALH84001: Evidence for shock-induced trapping of ancient Martian atmosphere

Abstract

Laser spot fusion and high resolution stepped heating analyses of neutron irradiated and unirradiated samples of ALH84001 have allowed us to identify the site of the major trapped Xe component as orthopyroxene. The apparent depletion of Ar and Kr relative to Xe and the high concentration of trapped Xe suggest the trapping mechanism was adsorption followed by shock incorporation into the major mineral phase. Carbonate, apatite, and feldspathic glass have also been studied and eliminated as major contributors to the trapped xenon budget, although a clear spallation signature was found in analysis of a single apatite grain. Carbonate contains iodine at a concentration of ∼500 ppb, and evidence of an associated minor release of Martian atmosphere suggests that this is not terrestrial contamination. There is no evidence of in situ decay of 129I or 244Pu in the meteorite. However, the 129Xe/ 132Xe ratio of the trapped component (1.95 ± 0.18) is significantly lower than that of the present Martian atmosphere, and differences in the associated 136Xe/ 132Xe ratio from that expected from Martian mantle-Martian atmosphere mixing suggest trapping took place before the atmosphere had evolved to its present state. We believe that trapping was associated with the shock that produced the crush zones and so was no later than resetting of the Ar-Ar system 4 Ga ago.