Origin of supposedly biogenic magnetite in the Martian meteorite Allan Hills 84001

doi:10.1073/pnas.102045799

Origin of supposedly biogenic magnetite in the Martian meteorite Allan Hills 84001

^*Advanced Materials Department, School of Industrial and Manufacturing Science, Cranfield University, Cranfield MK43 0AL, and School of Chemical and Life Sciences, University of Greenwich, London SE18 6PF, United Kingdom; and ^‡Hawai'i Institute of Geophysics and Planetology, University of Hawai'i at Manoa, Honolulu, HI 96822

Edited by John A. Wood, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, and approved March 19, 2002 (received for review January 25, 2002)

Abstract

Crystals of magnetite (Fe₃O₄) and periclase (MgO) in Fe-Mg-Ca carbonate in the Martian meteorite Allan Hills 84001 were studied by using transmission electron microscopy to understand their origin and evaluate claims that the magnetites were made by Martian microorganisms. In magnesian carbonate, periclase occurs as aggregates of crystals (grain size ≈3 nm) that are preferentially oriented with respect to the carbonate lattice. Larger periclase crystals ≈50 nm in size are commonly associated with voids of similar size. Periclase clearly formed by precipitation from carbonate as a result of partial decomposition and loss of CO₂. Magnetite occurs in more ferroan carbonate, and, like periclase, it is associated with voids and microfractures and the two oxides may be intermixed. Magnetite nanocrystals that are commonly euhedral and entirely embedded in carbonate are topotactically oriented with respect to the carbonate lattice, showing that they formed as solid-state precipitates. Magnetites in Fe-rich carbonate rims are not well oriented. These magnetites are generally more irregular in shape and diverse in size than the euhedral variety. All occurrences of magnetite and periclase are entirely consistent with in situ growth by solid-state diffusion as a result of carbonate decomposition during impact heating. Biogenic sources should not be invoked for any magnetites.

Footnotes

↵ † To whom reprint requests should be addressed. E-mail: DavidBarber{at}dbmatcon.demon.co.uk.
This paper was submitted directly (Track II) to the PNAS office.
↵ § Treiman, A. H. (2001) in Proceedings of the 32nd Lunar and Planetary Science Conference [CD-ROM] (Lunar and Planetary Institute, Houston), no. 1304 (abstr.).
↵ ¶ Blake, D., Treiman, A., Cady, S., Nelson, C. & Krishnan, K. (1998) in Proceedings of the 29th Lunar and Planetary Science Conference [CD-ROM] (Lunar and Planetary Institute, Houston), no. 1347 (abstr.).
↵ ‖ Brearley, A. J. (1998) in Proceedings of the 29th Lunar and Planetary Science Conference [CD-ROM] (Lunar and Planetary Institute, Houston), no. 1451 (abstr.).
↵ ** Bell, M. S., Thomas-Keprta, K. L., Wentworth, S. J. & McKay, D. S. (1999) in Proceedings of the 30th Lunar and Planetary Science Conference (Lunar and Planetary Institute, Houston), no. 1951 (abstr.).
↵ ‡‡ Jones, J. H. & Schwandt, C. S. (1998) in Proceedings of the 29th Lunar and Planetary Science Conference [CD-ROM] (Lunar and Planetary Institute, Houston), no. 1425 (abstr.).
Abbreviations:

TEM,

transmission electron microscopy;

ALH,

Allan Hills