OXYGEN ISOTOPIC CONSTRAINTS ON THE GENESIS OF CARBONATES FROM MARTIANMETEORITE ALH84001

Ion microprobe oxygen isotopic measurements of a chemically diverse su ite of carbonates from Martian meteorite ALH84001 are reported. The de lta(18)O values are highly variable, ranging from +5.4 to +25.3 parts per thousand, and are correlated with major;element compositions of th e carbonate; The earliest-forming (Ca-rich) carbonates have the lowest delta(18)O values and the late-forming (Mg-rich) carbonates have the highest delta(18)O values. Two models are presented which can explain the isotopic variations. The carbonates could have formed in a water r ich environment at relatively low, but highly variable temperatures. I n this open-system case the lower limit to the temperature variation i s similar to 125 degrees C, with fluctuations of over 250 degrees C po ssible within the constraints of the model. Alternatively, the data ca n be explained by a closed-system model in which the carbonates precip itated from a limited amount of CO2-rich fluid. This scenario can repr oduce the isotopic variations observed at a range of temperatures, inc luding relatively high temperatures (> 500 degrees C). Thus the oxygen isotopic compositions do not provide unequivocal evidence for formati on of the carbonates at low temperature. Although more information is needed in order to distinguish between the models, neither of the impl ied environments is: consistent with biological activity. Thus, we sug gest that features associated with the carbonates which have been inte rpreted to be the result of biological activity were most probably for med by inorganic processes. Copyright (C) 1998 Elsevier Science Ltd.