Determination of the Fe(II)aq–magnetite equilibrium iron isotope fractionation factor using the three-isotope method and a multi-direction approach to equilibrium

Abstract

Magnetite is ubiquitous in the Earth's crust and its presence in modern marine sediments has been taken as an indicator of biogeochemical Fe cycling. Magnetite is also the most abundant Fe oxide in banded iron formations (BIFs) that have not been subjected to ore-forming alteration. Magnetite is therefore an important target of stable Fe isotope studies, and yet interpretations are currently difficult because of large uncertainties in the equilibrium stable Fe isotope fractionation factors for magnetite relative to fluids and other minerals. In this study, we utilized the three-isotope method (57Fe–56Fe–54Fe) to explore isotopic exchange via an enriched-57Fe tracer, and natural mass-dependent fractionation using 56Fe/54Fe variations, during reaction of aqueous Fe(II) (Fe(II)aq) with magnetite. Importantly, we employed a multi-direction approach to equilibrium by reacting four 57Fe-enriched Fe(II) solutions that had distinct 56Fe/54Fe ratios, which identifies changes in the instantaneous Fe isotope fractionation factor and hence identifies kinetic isotope effects. We find that isotopic exchange can be described by two 56Fe/54Fe fractionations, where an initial rapid exchange (∼66% isotopic mixing within 1 day) involved a relatively small Fe(II)aq–magnetite 56Fe/54Fe fractionation, followed by slower exchange (∼25% isotopic mixing over 50 days) that was associated with a larger Fe(II)aq–magnetite 56Fe/54Fe fractionation; this later fractionation is interpreted to approach isotopic equilibrium between Fe(II)aq and the total magnetite. All four Fe(II) solutions extrapolate to the same final equilibrium 56Fe/54Fe fractionation for Fe(II)aq–magnetite of −1.56±0.20‰(2σ) at 22 °C. Additional experiments that synthesized magnetite via conversion of ferrihydrite by reaction with aqueous Fe(II) yield final 56Fe/54Fe fractionations that are identical to those of the exchange experiments. Our experimental results agree well with calculated fractionation factors using the reduced partition function ratios for Fe(H2O)62+ from Rustad et al. (2010) and stoichiometric magnetite from Mineev et al. (2007), and these relations may be combined with the experimental constraints to determine the temperature dependence of the Fe(II)aq–magnetite fractionation factor:103lnαFe(II)aq–magnetite=−0.145(±0.002)×106/T2+0.10(±0.02) where T is in K.Part of the reason for large discrepancies in calculated Fe isotope fractionation factors for magnetite likely lies in the stoichiometry of the mineral in specific studies, given the significant effect of octahedral versus tetrahedral Fe isotope fractionation that has been calculated. When our results are applied to BIF genesis, our experimentally determined Fe(II)aq–magnetite fractionation factor indicates that magnetite–siderite mineral pairs in ∼2.5 Ga BIFs did not form in Fe isotope equilibrium with each other, or with ancient seawater. Iron oxides in such BIFs are therefore more likely to have formed through processes that were isolated from equilibrium with the oceans, indicating that such BIF minerals may not be suitable proxies for ancient paleoenvironments.