EQUILIBRIUM IRON ISOTOPE FRACTIONATION BETWEEN AQUEOUS FE(II) AND MODEL ARCHEAN OCEAN FE-SI COPRECIPITATES

Iron isotope fractionation was investigated between aqueous Fe(II) (Fe(II)_aq) and two amorphous Fe(III) oxide-Si coprecipitates in a complex solution that simulated Archean marine conditions. The equilibrium isotope fractionation between Fe(II)_aq and Fe(III) coprecipitates in ⁵⁶Fe/⁵⁴Fe, as determined by the three-isotope method, was ‒3.51 ± 0.20(2σ)‰ and ‒3.99 ± 0.17(2σ)‰ for coprecipitates that had Fe:Si molar ratios of 1:1.8 and 1:3.1, respectively. The equilibrium isotope fractionation factor between Fe(II)_aq and Fe(III)-Si coprecipitates corresponds with the local structure of iron, as revealed by prior spectroscopic studies. The kinetics of isotopic exchange was controlled by movement of Fe(II) and Si, where sorption of Fe(II) from aqueous to solid phase facilitates atom exchange, but sorption of Si hinders isotopic exchange through blockage of reactive surface sites. Although Fe(II)-Fe(III) isotopic exchange rates were a function of solid and solution compositions in the current study, in all cases they were much higher than that determined in previous studies of aqueous Fe(III) and ferrihydrite interaction, highlighting the importance of electron exchange in promoting Fe atom exchange. When compared to analogous microbial reduction experiments, isotopic exchange rates are faster in the biological experiments, likely due to promotion of atom exchange by the solid-phase Fe(II) produced in the biological experiments. These results, when mass-balance relations are also considered, provide support for the potential role of microbial iron reduction in producing at least some of the large Fe isotope excursions in Precambrian marine sedimentary rocks.