Low Temperature Near-surface Thermochemical Modelling of the Alteration Assemblage in Martian Meteorite Alh84001

Introduction: The origin of secondary carbonate minerals in the oldest known martian meteorite ALH84001 have been intensely debated in the literature, with widely different thermal origins considered: ranging from hydrothermal alteration (possibly impactinduced) [e.g. 1, 2, 3] to low temperature alteration conditions [e.g. 4, 5, 6]. Recent isotopic evidence suggests the carbonates precipitated from evaporating water at 18±4 oC under near-surface conditions [7]. Here, we aim to test this latter scenario with thermochemical modelling. Modelling method and conditions: CHIM-XPT, a modelling software to compute reaction processes at 0.01 – 600 oC [8], was used here to carry out the aqueous alteration scenarios at 15, 20 and 25 oC; and pressures of 1 and 2 bar of atmospheric CO2. The water to rock ratio (W/R) was varied from 0.1 to 100,000 to obtain various alteration mineral precipitates. Isothermal evaporation scenarios where only water was evaporated were also modelled starting from W/R of 0.1, 0.5, 1 and 5. The composition of the ALH84001 host rock was obtained from [9] and the secondary carbonate phases were not included in the initial unaltered rock composition. The composition of the starting fluid consisted of pure water equilibrated with 1 or 2 bar of atmospheric CO2. We chose these pressures, because they have been used as surface pressures in early martian climate models [10, 11]. Results: 15 oC. At both 1 and 2 bar of CO2 pressure, the alteration assemblage as W/R decreases consists mostly of amphibole group minerals (ferroanthophyllite, tremolite, ferroactinolite) and phyllosilicates (serpentines, smectites, talc). A small amount of K-rich nontronite is formed in evaporation scenarios at W/R<1.2. No carbonates were formed. 20 oC. At 1 bar CO2, a similar assemblage of phyllosilicates as those at 15 oC are found. However, at 2 bar CO2, a series of carbonate minerals precipitate at varying W/R. Calcite (CaCO3) precipitates at W/R>60, ankerite (CaMg(CO3)2) at W/R>300 and siderite (FeCO3) at W/R>500. Rhodochrosite (MnCO3) is present at W/R<50, forming ~1% of the alteration assemblage. Fe-rich celadonite is formed both at 1 and 2 bar CO2 at W/R<0.5. In evaporation models beginning at W/R≥1, celadonite is not formed. Figure 1. Mineral abundance after alteration of ALH84001 at W/R=0.1. Phases with (*) are shown enhanced by a factor of 10.