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

作者：Beard BL^*; Johnson CM; Skulan JL; Nealson KH; Cox L; Sun H

来源：Chemical Geology, 2003, 195(1-4): 87-117.

DOI：10.1016/S0009-2541(02)00390-X

• Fe isotopes
• isotope fractionation
• microbiology
• banded iron formations

Over 100 high-precision Fe isotope analyses of rocks and minerals are now available, which constrain the range in delta(56)Fe values (per mil deviations in Fe-56/Fe-54 ratios) in nature from -2.50parts per thousand to +1.5parts per thousand. Re-assessment of the range of delta(56)Fe values for igneous rocks, using new ultra-high-precision analytical methods discussed here, indicate that igneous Fe is isotopically homogeneous to +/-0.05parts per thousand, 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 delta(56)Fe values; experimental measurements indicate that there is a large isotopic fractionation between aqueous Fe(III) and Fe(II) (Delta(Fe(111)-Fe(11)) = 2.75parts per thousand). However, many of the natural samples that have been analyzed have an unquestionable biologic component to their genesis, and the range in delta(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 +/-0.05parts per thousand, Fe-56/Fe-54) 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.

• 出版日期2003-4-15