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Xenon and Argon Isotopes in Irradiated, Etched Nakhla: Characterizing the Host of Martian Atmospheric Xenon

Introduction: A noble gas component with the elevated Xe/Xe ratio characteristic of the Martian atmosphere is found in Nakhla (and the other Nakhlites)[1,2]. It is elementally fractionated with Kr/Xe values depleted by factor of ~5 relative to the Martian atmospheric value (as measured in EET79001 lithology C). We believe this fractionation reflects the processes by which it was derived from the Martian atmosphere and incorporated in the meteorite. If, as has been suggested, aqueous alteration is responsible for this fractionation the xenon component should be associated with ‘iddingsite’[2]. Our aim is to locate the host phase(s) of the Martian atmosphere xenon component and so determine what led to its incorporation and whether the same process can account for the elemental fractionation. We have previously reported analyses of mineral separates from Nakhla that showed that a component with elevated Xe/Xe is associated with olivine and pyroxene but concentrated in mesostasis[3]. Combining our data with literature abundances of the major minerals [4] suggests that around 10% of the Martian atmosphere derived component is found in olivine, approximately half the remainder being associated with pyroxene and half with mesostasis. We have now supplemented these data with analyses of Nakhla samples crushed and etched in water and acid. Experimental: 210 mg of Nakhla was lightly ground to <400 μm and divided into 3 subsamples: Nak1 (61.42 mg), Nak2 (97.99 mg), Nak3 (57.96 mg). A finer grain size was not used because of the anticipated difficulty in handling and analyzing powders after irradiation. Samples Nak2 and Nak3 were derived from the untreated sample by 20 hours’ dissolution in water and 0.3 molar HNO3 respectively. The solutions above samples Nak2 and Nak3 were analyzed for a selection of cations using ICP-OES, while anions from the solution above Nak2 were analyzed by ion chromatography (the ionic strength of the acid solution above Nak3 prevented use of this technique). Aliquots of each sample were prepared and subjected to neutron irradiation. After such irradiation, potassium, chlorine, calcium, iodine, barium and uranium concentrations can be measured as enrichments in the daughter isotopes of argon and xenon produced after neutron capture. Xenon isotopic analysis using RELAX and conventional argon isotopic analysis of these samples are ongoing. Here we report results of laser step heating analyses of aliquots of Nak2 and Nak3. Results: Chemical analysis of the solutions above Nak2 and Nak3 revealed that the major element removed during the water etch was chlorine (20% of the expected budget), traces of sodium and sulphur also being present. Acid treatment removed around 6% of the sample mass, the elemental concentrations being consistent with olivine dissolution and a contribution from iddingsite and phosphate (figure 1). Argon: There is no variation among the Ar/Ar age data from the three separates. The most dramatic difference is a reduction in the chlorine content of Nak2 (as expected from analysis of the solution) and its near-complete removal from Nak3 (figure 2). Xenon: There is no evidence for any depletion in the Martian atmosphere-derived component (as measured by excess Xe) between Nak2 and Nak3, and both are consistent with previously reported bulk values [2]. In Nak2, there is a good correlation between Si Fe Mg Ca Al Na P S .0001 0.001 0.01 0.1 1 10