Iron Isotope Fractionation During Fe(II)-Hematite Interactions

Iron isotopes have the potential to provide useful signatures of biogeochemical Fe transformations in both modern and ancient environments. Recent studies suggest that coupled electron and Fe atom exchange between Fe(II) and Fe(III) oxide surfaces can account for production of low-56Fe/^{54}Fe aqueous Fe(II) during dissimilatory (microbial) iron reduction (DIR). In an effort to further understand the mechanisms of Fe isotope fractionation coupled to DIR, Fe isotope fractionation during abiotic reaction of Fe(II) with hematite surfaces was investigated in reaction systems comparable to those produced in DIR cultures. A series of reactors with an increasing fraction hematite to aqueous Fe(II) (equivalent to 10-80% adsorbed Fe(II)) were equilibrated over 12 days, after which δ56Fe values for aqueous Fe(II), adsorbed Fe(II) (0.05M HCl- extractable) and 'reactive Fe(III)' (0.5M HCl-extractable) at the hematite surface were measured by multi- collector, inductively-coupled-plasma-mass-spectrometry (MC-ICP-MS). Care was taken to verify the recovery of all Fe(II) added to the reactors, including electron equivalents that partitioned deep (relatively) into the oxide mineral structure and were not recovered by stripping of the oxide surface with 0.5M HCl. The results revealed a systematic correlation in Fe isotope compositions between aqueous Fe(II) and the ratio of hematite surface sites to aqueous Fe(II), where the lowest δ56Fe values were associated with highest extent of adsorption. As observed previously in DIR systems, the fractionation between aqueous and sorbed Fe(II) was modest (< 0.5‰) and could not account for the low aqueous Fe(II) δ56Fe values. A simple model of (kinetically-approached) equilibrium Fe atom exchange of (i) nonredox-associated interaction between aqueous Fe(II) and adsorbed Fe(II) (minimal fractionation); and (ii) redox-associated interaction between sorbed Fe(II) and Fe(III) surface sites (the dominant pathway for isotope fractionation) can produce the observed pattern of Fe isotope fractionation during Fe(II)-hematite interaction. The Fe isotope fractionation mechanism defined in this study provides a quantitative basis for inferring Fe isotopic signatures of DIR and other Fe redox transformations. A key observation is that the size of the Fe(III) reservoir in hematite that is in isotopic equilibrium in the abiologic systems is smaller than that of the biologic reduction experiments, producing concomitant shifts in the δ56Fe values for aqueous Fe(II), where the most negative δ56Fe values for aqueous Fe(II) are found in the biologic experiments. When considering the modern and ancient geologic record, production of large volumes of low-δ56Fe aqueous Fe(II) likely requires dissimilatory Fe reduction and units such as banded iron formations cannot be produced by abiologic aqueous Fe(II)-hematite interaction. Additional studies with reaction systems containing dissolved silica at concentrations comparable to those present in Archean oceans are underway to constrain their influence of Fe atom exchange on signatures of DIR in the ancient rock record.