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
Source of text: This review was assembled by Eric Caldwell, primarily
from Bullen and Kendall (1998) and Dicken (1995).
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||Bullen, T.D. and McMahon, P.E. (1997). "Iron isotopes revisited:
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||Dicken, A.P. (1995). Radiogenic Isotope Geology. Cambridge University
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||Dixon, P.R., Janecky, D.R., Perrin, R.E., Rokop, D.J., Unkefer, P.L.
and Spall, W.D. (1992). "Unconventional stable isotopes: Iron."
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||Faure, G. (1986). Principles of Isotope Geology, Second Edition.
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||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."
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