Application of Fe isotopes to tracing the geochemical and biological cycling of Fe Brian L. Beard a, * , Clark M. Johnson a , Joseph L. Skulan a , Kenneth H. Nealson b , Lea Cox b , Henry Sun b a Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton Street, Madison, WI 53706 USA b Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena CA, USA Received 20 September 2000; accepted 22 January 2002 Abstract Over 100 high-precision Fe isotope analyses of rocks and minerals are now available, which constrain the range in d 56 Fe values (per mil deviations in 56 Fe/ 54 Fe ratios) in nature from  2.50xto + 1.5x . Re-assessment of the range of d 56 Fe values for igneous rocks, using new ultra-high-precision analytical methods discussed here, indicate that igneous Fe is isotopically homogeneous to F 0.05x , which represents an unparalleled baseline with which to interpret Fe isotope variations in nature. All of the isotopic variability in nature lies in fluids, rocks, and minerals that formed at low temperature. Equilibrium (‘‘abiotic’’) isotopic fractionations at low temperatures may explain the range in d 56 Fe values; experimental measurements indicate that there is a large isotopic fractionation between aqueous Fe(III) and Fe(II) (D Fe(III) – Fe(II) = 2.75x ). However, many of the natural samples that have been analyzed have an unquestionable biologic component to their genesis, and the range in d 56 Fe values are also consistent with the experimentally measured isotopic fractionations produced by Fe- reducing bacteria. In this work, we touch on a number of aspects of Fe isotope geochemistry that bear on its application to geochemical problems in general, and biological cycling of metals in particular. We report on new state-of-the-art Fe isotope analytical procedures, which allow precisions of F 0.05x( 56 Fe/ 54 Fe) on samples < 300 ng in size. In addition, we discuss the implications of experimental work on Fe isotope fractionations during metabolic processing of Fe by bacteria and the need to take a ‘‘mechanistic’’ approach to understanding the pathways in which Fe isotopes may be uniquely fractionated by biology. Additionally, we discuss experimental methods, such as the use of enriched isotope tracers that are necessary to evaluate if experimental isotope exchange reactions are transient kinetic fractionations, equilibrium isotopic exchange reactions, or a combination of both, which can be caused by the complexities of multiple isotope exchange reactions taking place in an experimental system. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Fe isotopes; Isotope fractionation; Microbiology; Banded iron formations 1. Introduction The possibility that Fe isotopes may be fractio- nated during geochemical cycling, particularly if 0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2541(02)00390-X * Corresponding author. Tel.: +1-608-262-1806; fax: +1-608- 262-0693. E-mail address: beardb@geology.wisc.edu (B.L. Beard). www.elsevier.com/locate/chemgeo Chemical Geology 195 (2003) 87 – 117