IRON ISOTOPE EXCHANGE AND FRACTIONATION BETWEEN AQUEOUS FE(II) AND MAGNETITE: IMPLICATIONS FOR EQUILIBRIUM IRON ISOTOPE FRACTIONATION FACTORS AND INTERPRETING IRON ISOTOPE COMPOSITIONS IN THE ROCK RECORD

Proper interpretation of stable isotope variations in nature requires an understanding of isotopic fractionation mechanisms and accurately knowing equilibrium isotope fractionation factors. Biogeochemical iron cycling initiates secondary abiotic reactions, including electron transfer and atom exchange, that result in isotope fractionation between aqueous Fe(II) and iron oxide minerals. Here, we utilize such reactions to experimentally measure iron isotope fractionation factors at room temperature by reacting ⁵⁷Fe(II) with isotopically “normal” magnetite. We employ the three-isotope method (⁵⁷Fe-⁵⁶Fe-⁵⁴Fe) to monitor isotope exchange (via δ⁵⁷Fe/⁵⁶Fe) and natural mass-dependent fractionation (by variations in ⁵⁶Fe/⁵⁴Fe), and for the first time with iron isotopes, we employ a multi-direction approach to equilibrium by using four isotopically distinct (in ⁵⁶Fe/⁵⁴Fe) Fe(II) solutions to unambiguously demonstrate equilibrium. We report the Fe(II)_aq-magnetite equilibrium iron isotope fractionation factor at room temperature to be -1.56 ± 0.20 ‰ (2σ) in δ⁵⁶Fe. Our findings show that adjacent grains of magnetite and siderite found in well-preserved Precambrian banded iron formations are not in iron isotopic equilibrium and therefore cannot be used as proxies for ancient seawater or atmospheric compositions. On the contrary, these minerals were likely precipitated during a disequilibrium process such as microbial iron reduction.