The temperature of formation of carbonate in martian meteorite ALH84001: constraints from cation diffusion

Abstract

We have measured the rates of chemical diffusion of Mg in calcite and Ca in magnesite and used these new data to constrain the formation temperature and thermal history of carbonates in the Martian meteorite ALH84001. Our data have been collected at lower temperatures than in previous studies and provide improved constraints on carbonate formation during relatively low-temperature processes (≤400°C). Measured log D 0 values for chemical diffusion of Mg in calcite and Ca in magnesite are −16.0 ± 1.1 and −7.8 ± 4.3 m 2/s and the activation energies ( E A ) are 76 ± 16 and 214 ± 60 kJ/mol, respectively. Measured diffusion rates of Mg in calcite at temperatures between 400 and 550°C are substantially faster than expected from extrapolation of existing higher-temperature data, suggesting that different mechanisms may govern diffusion of Mg at temperatures above and below ∼550°C. We have used these data to constrain thermal histories which will allow the ∼1 μm variations in Ca-Mg composition in ALH84001 carbonates to survive homogenization by diffusion. Our results are generally consistent with models for formation of carbonates in ALH84001 at low temperatures. For initial cooling rates of between ∼10 −1 and 10 3°/Ma our results demonstrate that carbonates formed at temperatures no higher than 400°C and most probably less than 200°C. This range of cooling rates is similar to those observed within the terrestrial crust, and suggests that formation of the carbonates by igneous, metamorphic or hydrothermal (or other) processes in the Martian crust most plausibly occurred at temperatures below 200 to 400°C. Models that suggest ALH84001 carbonates formed during a Martian impact event are also constrained by our data. The thermal histories of terrestrial impact structures suggest that cooling rates sufficiently rapid to allow preservation of the observed carbonate zoning at formation temperatures in excess of 600°C (>∼10 7°C/Ma) occur only within the uppermost, melt-rich portions of an impact structure. Material deeper within the impact structure (where cooling is dominated by uplifted crustal material and where much of the formation of hydrothermal minerals is concentrated) cools much slower, typically at rates of ∼10 2 to 10 3°/Ma. Carbonates formed within this region would also only preserve ∼1 μm compositional zoning at formation temperatures of less than ∼200 to 400°C.