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 δ⁵⁶Fe values (per mil deviations in 56Fe/54Fe ratios) in nature from −2.50‰ to +1.5‰. Re-assessment of the range of δ56Fe 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 δ⁵⁶Fe values; experimental measurements indicate that there is a large isotopic fractionation between aqueous Fe(III) and Fe(II) (ΔFe(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 δ56Fe values are also consistent with the experimentally measured isotopic fractionations produced by Fe-reducing bacteria.