THE meteorite Allan Hills (ALH) 84001, commonly accepted to be of martian origin, is unique among known martian meteorites in containing abundant, zoned, pre-terrestrial carbonate minerals(1-9). Previous studies of the oxygen isotope compositions of these minerals(5,6,8) have suggested that they precipitated from a low-temperature (0-80 degrees C) aqueous fluid in the martian crust-perhaps in a near-surface hydrothermal system, Here we report analyses of the major-element compositions of the carbonates, which provide an independent constraint on the composition and temperature of the fluid from which they formed. We argue that the most likely explanation for the observed compositions, and for the absence of co-existing hydrous minerals, is that the carbonates were formed by reactions between hot (>650 degrees C), CO2-rich fluids and the ultramafic host rock during an impact event, Impact processes on the martian surface can produce both the hot, CO2-rich fluid (by volatilization of surface carbonates or other CO2 sources) and-by brecciation-the conduits through which it flowed, Impact metasomatism is also consistent with the observed oxygen isotope disequilibrium, sequence of mineral formation, and carbonate mineral zoning, reflecting carbonate formation during rapid cooling from high temperatures rather than prolonged exposure to low-temperature fluids(6,8).