Paleomagnetic evidence supports a low-temperature origin of the carbonate in Martian meteorite ALH84001

Indirect evidence for life on Mars has been reported from studies of the carbonate phase on the Allan Hills Antarctica meteorite ALH84001[1]. However, there is at present a controversy concerning the temperature of formation of this material. One line of evidence, based on stable isotope relationships [2] and unstable mineral assemblages[1], argues that it formed at temperatures between 20° and 80° C, whereas another [3] suggests formation temperatures above 650° C. Paleomagnetism has the ability to distinguish between these divergent end members, as the Neél (Curie) temperature of magnetite is 580° C, and blocking temperatures for grains of single magnetic domain size extend well below this. As the Neél (~Curie) temperature of pure magnetite is 580° C, a high-temperature origin of the carbonate would imply that both the pyroxenite ground mass and the carbonate veins had cooled together, and hence they would preserve the same direction through all parts of their blocking temperature and coercivity spectra. However, if the carbonate formed at low temperatures long after the pyroxenite had cooled, its NRM ought to be either a chemical remanent magnetization or a detrital remanent magnetization, and there would be ample opportunity for the vector direction of the Martian magnetic field to have changed relative to the pyroxenite ground mass of the rock. An intermediate level of heating in the presence of a magnetic field after formation of the carbonate could impart a secondary magnetic component in the lower portion of the blocking temperature spectrum. Courtesy of Dr. D. McKay, we studied a single 20 mg chip of ALH84001, sub-sample 190 from its central breccia zone, which contained a fragment of the carbonate vein materials on one surface. Due to the unusually small size of the sample, we developed a clean-lab technique using acidwashed quartz-glass fibers for holding it and its sub-samples oriented stably during the paleomagnetic. Using this system and a DC-SQuID moment magnetometer system, we were able to obtain replicate measurements to better than 1% intensity, and 0.5° in direction, on magnetic moments as weak as 10 Am, equivalent to the saturation remanence produced by ~ 20 picograms of uniformly magnetized (SD) magnetite. After initial measurements of the natural remanent magnetization (NRM) of the sample, we were able to separate the carbonate from the pyroxenite using a thin diamond-impregnated copper wafering saw, all while maintaining accurate relative orientations. We found that both the carbonate layer and the pyroxenite ground mass possess a stable natural remanent magnetization (NRM), implying that Mars had a substantial magnetic field during formation of the rock. Principal component analysis reveals that the carbonate has only one stable magnetic direction, whereas several components are present in the pyroxenite. However, the NRM direction from the carbonate layer differs by 50 to 80° from those of the pyroxenite, which rules out a high temperature origin for the carbonate. Analysis of the coercivity spectra, coupled with detailed measurements of the size and shape of individual single-domain magnetite crystals, places an upper limit of about 150° C for the formation temperature of the carbonate. Our results are compatible with the presence of ancient bacterial life on Mars.