The Nakhlite Hydrothermal Brine

Introduction: Mineralogical studies have demonstrated that the nakhlite SNC meteorites have preserved the effects of water-rock interaction between a hydrothermal brine and the nakhlite parent rocks on Mars. Brittle fracturing and corrosion of olivine grains and mesostasis associated with the hydrothermal event have left veins of zoned Fe-carbonate, Fe-smectite (with minor serpentine), and an amorphous gel of smectite composition [1] (Fig. 1). In addition the fluid was trapped in fluid inclusions within the cumulus pyroxene grains, though most of these have since been split, perhaps during the impact ejection [2]. The most complete assemblage is preserved in the Lafayette meteorite, which on Mars was closest to the heat source that we envisage as an impact prior to the one which led to the ejection of the nakhlites from Mars. Final evaporation of the residual brine left soluble salts. A terrestrial overprint can be distinguished in some of the nakhlites, e.g. jarosite in Y000749 [1]. This detailed characterization now allows us to accurately determine the T, pH and composition of the martian fluid.