The temperature of formation of carbonate in Martian meteorite ALH84001: Constraints from cation diffusion

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 a nd thermal history of carbonates in the Martian meteorite ALH84001. Our dat a have been collected at lower temperatures than in previous studies and pr ovide improved constraints on carbonate formation during relatively low-tem perature processes (less than or equal to 400 degreesC). Measured log D-0 v alues 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 degreesC are substantially fas ter than expected from extrapolation of existing higher-temperature data, s uggesting that different mechanisms may govern diffusion of Mg at temperatu res above and below similar to 550 degreesC. We have used these data to constrain thermal histories which will allow the similar to1 mum variations in Ca-Mg composition in ALH84001 carbonates to survive homogenization by diffusion. Our results are generally consistent w ith models for formation of carbonates in ALH84001 at low temperatures. For initial cooling rates of between similar to 10(-1) and 10(3)degrees /Ma ou r results demonstrate that carbonates formed at temperatures no higher than 400 degreesC and most probably less than 200 degreesC, This range of cooli ng rates is similar to those observed within the terrestrial crust, and sug gests that formation of the carbonates by igneous, metamorphic or hydrother mal (or other) processes in the Martian crust most plausibly occurred at te mperatures below 200 to 400 degreesC. Models that suggest ALH84001 carbonat es formed during a Martian impact event are also constrained by our data. T he thermal histories of terrestrial impact structures suggest that cooling rates sufficiently rapid to allow preservation of the observed carbonate zo ning at formation temperatures in excess of 600 degreesC (>similar to 10(7) degreesC/Ma) occur only within the uppermost, melt-rich portions of an impa ct structure. Material deeper within the impact structure (when cooling is dominated by uplifted crustal material and where much of the formation of h ydrothermal minerals is concentrated) cools much slower, typically at rates of similar to 10(2) to 10(3)degrees /Ma. Carbonates formed within this reg ion would also only preserve similar to1 mum compositional zoning at format ion temperatures of less than similar to 200 to 400 degreesC. Copyright (C) 2001 Elsevier Science Ltd.