Martian atmospheric and indigenous components of xenon and nitrogen in the Shergotty, Nakhla and Chassigny group meteorites

In a study of the isotopic signatures of trapped Xe in shock-produced glass of shergottites and in ALH 84001 we observe three components: (1) modern Martian atmospheric Xe that is isotopically mass fractionated relative to solar Xe, favoring the heavy isotopes, (2) solar-like Xe as previously observed in Chassigny, and (3) an isotopically fractionated (possibly ancient) component with little or no radiogenic ¹²⁹Xe_rad. In situ-produced fission and spallation components are observed predominantly in the high-temperature steps. Heavy N signatures in ALH 84001, EET 79001 and Zagami reveal Martian atmospheric components. The low-temperature release of ALH 84001 shows evidence for the presence of a light N component (δ¹⁵N ≤ -21‰), which is consistent with the component observed in the other Shergotty, Nakhla and Chassigny (SNC) group meteorites. The highest observed ¹²⁹Xe/¹³⁰Xe ratios in ALH 84001 are lower but appear to reflect varying mixing ratios with other components. The consistently high ¹²⁹Xe/¹³⁰Xe ratios in rocks of different radiometric ages suggest that Martian atmospheric Xe evolved early on. As already concluded in earlier work, only a small fission component is observed in the Martian atmospheric component. Assuming that a chondritic ²⁴⁴Pu/¹²⁹I initial ratio applies to Mars, this implies that either Pu-derived fission Xe is retained in the solid planet (in fact, in situ-produced fission Xe is observed in ALH 84001) or may reflect a very particular degassing history of the planet.