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Periodic Table--Iron

Iron has four naturally-occurring stable isotopes, 54Fe, 56Fe, 57Fe and 58Fe. The relative abundances of the Fe isotopes in nature are approximately 54Fe (5.8%), 56Fe (91.7%), 57Fe (2.2%) and 58Fe (0.3%). 60Fe is an extinct radionuclide which had a long half-life (1.5 Myr). Much of the past work on measuring the isotopic composition of Fe has centered on determining 60Fe variations due to processes accompanying nucleosynthesis (i.e., meteorite studies) and ore formation (Volkening and Papanatassiou, 1989; Maeck, 1992). The abundance of 60Ni (daughter product of 60Fe) present in extraterrestrial material may provide insight into the origin of the solar system and its early history. Shukolyukov and Lugmair (1993) found excess 60Ni in meteorites, suggesting that 60Fe was still alive at the time of differentiation. Isochron correlations between 60Ni/58Ni and Fe/Ni also confirm this conclusion.

Recent studies have focused on potential work with Fe isotopes in low-temperature systems. Observed isotopic variations induced by microbial processes suggest that microbially-mediated Fe reduction preferentially favors the lighter iron species (Dixon et al., 1992). In another study, Bullen and McMahon (1998) demonstrated that microbially-mediated Fe reduction behaves as a Rayleigh distillation process, with Fe2+ consistently 5 lighter than the coexisting Fe3+. This suggests that the Fe isotopic composition of water leaving a system can be used to estimate the amount of Fe reduction that has occurred in the system, with increasingly heavy values reflecting greater amounts of Fe reduction, assuming that Fe mobilized inorganically from minerals under either reducing or low-pH conditions will have a distinct isotopic composition than microbially-reduced Fe (Bullen and Kendall, 1998).

Further experimental work is required on both development of analytical procedures for measurement of the Fe isotopes at low concentrations as well as determination of the relative fractionation efficiency of various microbial agents.

Source of text: This review was assembled by Eric Caldwell, primarily from Bullen and Kendall (1998) and Dicken (1995).

References
• Bullen, T.D. and Kendall, C. (1998). "Tracing of Weathering Reactions and Water Flowpaths: A Multi-isotope Approach." In: C. Kendall and J.J. McDonnell (Eds.), Isotope Tracers in Catchment Hydrology. Elsevier, Amsterdam, pp. 611-646.
• Bullen, T.D. and McMahon, P.E. (1997). "Iron isotopes revisited: experimental and field evidence for microbially-mediated Fe reduction." EOS, 78, 17: S-173.
• Dicken, A.P. (1995). Radiogenic Isotope Geology. Cambridge University Press, New York, 452 pp.
• Dixon, P.R., Janecky, D.R., Perrin, R.E., Rokop, D.J., Unkefer, P.L. and Spall, W.D. (1992). "Unconventional stable isotopes: Iron." In: Y.K. Kharaka and A.S. Maest (Eds.), Water-Rock Interaction, Proc. 7th Intl. Symp., Park City, Utah, 13-18 July, 1992. Balkema, Rotterdam, pp. 915-918.
• Faure, G. (1986). Principles of Isotope Geology, Second Edition. John Wiley and Sons, New York. 589 pp.
• Maeck, R. (1992). "The absolute abundance of iron isotopes." Unpublished Ph.D. thesis, Central Bureau of Nuclear Measurements, Geel, Belgium, 324 pp.
• Shukolyukov, A. and Lugmair, G.W. (1993). "Live iron-60 in the early solar system." Science, 259: 1138-1142.
• Volkening, J. and Papanatassiou, D.A. (1989). "Iron isotope anomalies." The Astrophysical Journal, 347: L43-L46.
Related Links
• Periodic Table
• Fundamentals of Stable Isotope Geochemistry
• General References
• Isotope Publications
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