FRACTIONATED MARTIAN ATMOSPHERE IN THE NAKHLITES

Considerable evidence points to a martian origin of the SNC meteorites . Noble gas isotopic compositions have been measured in most SNC meteo rites. The Xe-129/Xe-132 vs. Kr-84/Xe-132 ratios in Chassigny, most sh ergottites, and lithology C of EETA 79001 define a linear array. This array is thought to be a mixing line between martian mantle and martia n atmosphere. One of the SNC meteorites, Nakhla, contains a leachable component that has an elevated Xe-129/Xe-132 ratio relative to its Kr- 84/Xe-132 ratio when compared to this approximately linear array. The leachable component probably consists in part of iddingsite, an altera tion product produced by interaction of olivine with aqueous fluid at temperatures lower than 150 degrees C. The elevated Xe isotopic ratio may represent a distinct reservoir in the martian crust or mantle. Mor e plausibly, it is elementally fractionated martian atmosphere. Format ion of sediments fractionates the noble gases in the correct direction . The range of sediment/atmosphere fractionation factors is consistent with the elevated Xe-129/Xe-132 component in Nakhla being contained i n iddingsite, a low temperature weathering product. The crystallizatio n age of Nakhla is 1.3 Ga. Its low-shock state suggests that it was ej ected from near the surface of Mars. As liquid water is required for t he formation of iddingsite, these observations provide further evidenc e for the near surface existence of aqueous fluids on Mars more recent ly than 1.3 Ga.