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 fractionate d relative to solar Xe, favoring the heavy isotopes, (2) solar-like Xe , as previously observed in Chassigny, and (3) an isotopically fractio nated (possibly ancient) component with little or no radiogenic Xe-129 (rad). In situ-produced fission and spallation components are observed predominantly in the high-temperature steps. Heavy N signatures in AL H 84001, EET 79001 and Zagami reveal Martian atmospheric components. T he low-temperature release of ALH 84001 shows evidence for the presenc e of a light N component (delta(15)N less than or equal to -21 parts p er thousand), which is consistent with the component observed in the o ther Shergotty, Nakhla and Chassigny (SNC) group meteorites. The highe st observed Xe-129/Xe-130 ratio of 15.60 in Zagami and EET 79001 is us ed here to represent the present Martian atmospheric component, and th e isotopic composition of this component is compared with other solar system Xe signatures. The Xe-129/Xe-130 ratios in ALH 84001 are lower but appear to reflect varying mixing ratios with other components. The consistently high Xe-129/Xe-130 ratios in rocks of different radiomet ric ages suggest that Martian atmospheric Xe evolved early on. As alre ady concluded in earlier work, only a small fission component is obser ved in the Martian atmospheric component. Assuming that a chondritic P u-244/I-129 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 pa rticular degassing history of the planet.