Halogens in Nakhla and Nwa 998: Evidence of Martian Weathering?

The halogen concentrations of a number of Martian meteorites vary significantly, with larger variations observed in I/Cl compared to Br/Cl. One striking feature is the high I/Cl ratios associated with those meteorites found in Antarctica. Removal of low temperature, superficial contamination does not significantly reduce the I/Cl ratio [1], indicating that processing in Antarctica is responsible for binding iodine into meteorite phases. We suggest that weathering during a meteorite's terrestrial residence acts to increase the I/Cl ratio, whilst not significantly affecting the Br/Cl ratio. Br/Cl and I/Cl ratios in non-antarctic nakhlites and Martian meteorite falls show a wide range which extends down to among the lowest I/Cl values in Martian meteorites [2-3]. In addition, the elevated Br/Cl ratios in Nakhla have been attributed to Martian weathering [4]. We report the halogen concentrations from analyses of Nak-hla and NWA 998 to investigate and assess the effect of Martian weathering on meteorites. Nakhlites are relatively unshocked, and Nakhla, as a fall, should be unaffected by terrestrial weathering , and thus retain strong evidence of Martian weathering. By contrast, NWA 998, a desert find, may have been terrestrially weathered. Mineral separates of both meteorites, including bulk, olivine, pyroxene and feldspar aliquots, were neutron-irradiated and analysed for halogen content using laser step-heating and noble gas mass spectrometry. Our bulk sample of Nakhla has relatively low Br/Cl (~ 3.5 x 10-3) and I/Cl (~ 2.9 x 10-5), whilst olivine has elevated Br/Cl (~ 2.5 x 10-2) and I/Cl (~ 6.7 x 10-4). The elevated olivine Br/Cl ratio is consistent with values reported for Martian weathering by [4]. The highest halogen concentrations are observed within the pyroxene, compared to bulk and olivine aliquots. We therefore suggest that the weathering processes and halo-gen cycles on Earth and Mars are significantly different. Terrestrial iodine contamination is controlled by methyl iodide [5], and is distinct from sources of bromine and chlorine. Terrestrial weathering of meteorites thus leads to elevated I/Cl ratios. On Mars, iodine and bromine are controlled by groundwater/brines acting on the Martian sub-surface. Martian weathering therefore increases both iodine and bromine, at the expense of chlorine, causing elevated Br/Cl and I/Cl. Our preliminary NWA 998 analyses have shown higher halogen concentrations within feld-spar compared to mafic aliquots, a feature observed in other Martian meteorites (e.g [1]). Further analysis of NWA 998, with comparison to Nakhla, will assess the prevalence of Martian weathering.