Carbonate Precipitation Driven by Clay Leachates on Early Mars

Introduction: Secondary carbonate minerals produced by low temperature aqueous processes [1] in the martian meteorite ALH 84001 are evidence of an early active hydrosphere on Mars when they were formed ~3.9 Ga ago [2]. Further evidence for an early hydrosphere includes orbital and surface observations of diverse alteration mineral assemblages [e.g. 3-5] and fluvial geomorphology [6]. While carbonates are observed in ALH 84001, unusually, there is no evidence of secondary phyllosilicates associated to low temperature aqueous alteration of the meteorite, and there is a lack of textural evidence for intensive fluid interaction [7] We have used geochemical modelling techniques to show that the carbonates in ALH 84001 can be produced in a two stage process in which low temperature fluids percolate and leach clay-bearing layers prior to encountering the unweathered host of ALH 84001. Modelling: We carried out thermochemical models with CHIM-XPT [8], limiting the temperature to 15, 20 and 25 °C, consistent with CO2 clumped isotope thermometry of the formation of the carbonates in ALH 84001 [1]. The initial fluid used to alter the meteorite’s host rock was in equilibrium with 1 and 2 bar pCO2, in accordance with estimates of the early martian atmosphere [9]. In contrast to previous models (which then varied the amount of rock relative to the amount of fluid in the system and observed the resulting chemical and mineralogical changes [10, 11]), we first allowed the fluid to percolate through phyllosilicate minerals detected on the surface of Mars (Table 1), and then altered the parent rock of ALH 84001 with these fluids. Finally, we modelled the isothermal evaporation of water from the system and observed the mineralogical changes generated.