Application of Fe isotopes to tracing the geochemical and biological cycling of Fe

Abstract

Over 100 high-precision Fe isotope analyses of rocks and minerals are now available, which constrain the range in δ 56Fe values (per mil deviations in 56Fe/ 54Fe ratios) in nature from −2.50‰ to +1.5‰. Re-assessment of the range of δ 56Fe values for igneous rocks, using new ultra-high-precision analytical methods discussed here, indicate that igneous Fe is isotopically homogeneous to ±0.05‰, 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 δ 56Fe values; experimental measurements indicate that there is a large isotopic fractionation between aqueous Fe(III) and Fe(II) ( Δ Fe(III)–Fe(II)=2.75‰). However, many of the natural samples that have been analyzed have an unquestionable biologic component to their genesis, and the range in δ 56Fe 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 ±0.05‰ ( 56Fe/ 54Fe) 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.