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Published March 14, 1997 | public
Journal Article

Paleomagnetic Evidence of a Low-Temperature Origin of Carbonate in the Martian Meteorite ALH84001


Indirect evidence for life on Mars has been reported from the study of meteorite ALH84001. The formation temperature of the carbonates is controversial; some estimates suggest 20° to 80°C, whereas others exceed 650°C. Paleomagnetism can be used to distinguish between these possibilities because heating can remagnetize ferrimagnetic minerals. Study of two adjacent pyroxene grains from the crushed zone of ALH84001 shows that each possesses a stable natural remanent magnetization (NRM), implying that Mars had a substantial magnetic field when the grains cooled. However, NRM directions from these particles differ, implying that the meteorite has not been heated significantly since the formation of the internal crushed zone about 4 billion years ago. The carbonate globules postdate this brecciation, and thus formed at low temperatures.

Additional Information

© 1997 American Association for the Advancement of Science. Received for publication 31 January 1997; accepted for publication 20 February 1997. We thank D. McKay and E. K. Gibson for our sample of ALH84001, P. Carpenter for assistance with the SEM, and G. R. Rossman for help with the delicate sawing operation. H.V. acknowledges financial support from the U.S. National Research Council. We made extensive use of the software provided by C. Jones (cjones@mantle.colorado.edu) for the analysis and presentation of paleomagnetic data. B. C. Murray, J. Eiler, and D. A. Evans made helpful suggestions on the manuscript. This is contribution no. 5897 from the Division of Geological and Planetary Sciences of the California Institute of Technology.

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