A POSSIBLE HIGH-TEMPERATURE ORIGIN FOR THE CARBONATES IN THE MARTIAN METEORITE ALH84001

THE meteorite Allan Hills (ALH) 84001, commonly accepted to be of mart ian origin, is unique among known martian meteorites in containing abu ndant, zoned, pre-terrestrial carbonate minerals(1-9). Previous studie s of the oxygen isotope compositions of these minerals(5,6,8) have sug gested that they precipitated from a low-temperature (0-80 degrees C) aqueous fluid in the martian crust-perhaps in a near-surface hydrother mal system, Here we report analyses of the major-element compositions of the carbonates, which provide an independent constraint on the comp osition and temperature of the fluid from which they formed. We argue that the most likely explanation for the observed compositions, and fo r 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 v olatilization of surface carbonates or other CO2 sources) and-by brecc iation-the conduits through which it flowed, Impact metasomatism is al so consistent with the observed oxygen isotope disequilibrium, sequenc e of mineral formation, and carbonate mineral zoning, reflecting carbo nate formation during rapid cooling from high temperatures rather than prolonged exposure to low-temperature fluids(6,8).