Iron Isotope Variations in Natural Aqueous Systems: Further Clues to Fractionation Mechanisms

A number of recent pioneering studies into the potential of iron isotopes to be fractionated by natural processes have revealed a complex web of possible isotope fractionation mechanisms. For example, processes involving microbially-mediated reduction of mineral-bound Fe(III) to Fe(II) have been shown to result in fractionation of the ⁵⁶Fe/⁵⁴Fe ratio of from 0 to 2.5‰ , with the lighter isotopes being concentrated in the Fe(II)-bearing aqueous or solid product. This fractionation clearly results from disequilibrium reactions, although it is as yet unclear whether the cause is strong complexation of iron at the mineral surface that limits back reaction or a true "vital effect". Moreover, field and laboratory studies of abiotic iron oxidation under disequilibrium conditions reveal a fractionation of as much as 2‰ with the heavier iron isotopes being concentrated in the Fe(III)-bearing product, requiring equilibrium fractionation among the coexisting aqueous Fe(II) species prior to oxidation. Furthermore, theoretical considerations predict that under equilibrium conditions large iron isotope fractionations are possible due to contrasts in ligand strength and atomic bonding environment, and that aqueous Fe(III) complexes will concentrate the heavier iron isotopes relative to Fe(II) complexes involving the same ligands. These observations provide important insights into possible isotope fractionation mechanisms, yet beg a more thorough examination of iron isotope fractionation in diverse, relatively simple natural systems. Here we present new iron isotope data that document fractionations accompanying processes such as precipitation of siderite from CO₂-charged springs, of schwertmannite from acid-mine drainage and of ferrihydrite from spring waters by a presumably microbial mechanism. Considered in light of previous studies the new data underscore the importance of aqueous speciation in determining iron isotope fractionations observed in natural and experimental systems.