The nakhlite hydrothermal brine on Mars

On the basis of ongoing mineralogical characterisation of the nakhlite secondary minerals (Changela and Bridges, 2010; Hicks et al. this meeting) by HR-TEM and X-ray Absorption Spectroscopy we apply thermochemical modelling to determine the fluid composition, temperature and redox conditions of the water. We show that the nakhlite parent rocks on Mars encountered a CO2-rich hydrothermal fluid at 150 ≤ T ≤ 200 °C, pH 6–8 with a water:rock ratio (W/R) ≤300 (Bridges and Schwenzer, 2012). Under these conditions, Fe-rich carbonate was precipitated within brittle fractures. As the fluid cooled to 50 °C, at pH 9 and W/R of 6, Fe-rich phyllosilicate precipitated, followed in turn by rapid precipitation of an amorphous gel. It was enriched in the most soluble species (e.g. K, Na), of alkaline pH, and similar to terrestrial, i.e. not seawater-influenced, dilute brines in basaltic terrains on Earth. Our results show that environments associated with this type of fluid were habitable, unlike those associated with acid-sulphate fluids. Considering the timing of the nakhlite alteration, the most likely cause is impact-generated hydrothermal alteration of the nakhlite pile at the margins of an impact crater. The martian subsurface fluid forming phyllosilicates provided habitable temperatures and many of the nutrients required for life