Low temperature, non-stoichiometric oxygen-isotope exchange coupled to Fe(II)–goethite interactions

Abstract

The oxygen isotope composition of natural iron oxide minerals has been widely used as a paleoclimate proxy. Interpretation of their stable isotope compositions, however, requires accurate knowledge of isotopic fractionation factors and an understanding of their isotopic exchange kinetics, the latter of which informs us how diagenetic processes may alter their isotopic compositions. Prior work has demonstrated that crystalline iron oxides do not significantly exchange oxygen isotopes with pure water at low temperature, which has restricted studies of isotopic fractionation factors to precipitation experiments or theoretical calculations. Using a double three-isotope method (18O–17O–16O and 57Fe–56Fe–54Fe) we compare oxygen and iron isotope exchange kinetics, and demonstrate, for the first time, that oxygen-isotope exchange between structural oxygen in crystalline goethite and water occurs in the presence of aqueous Fe(II) (Fe(II)aq) at ambient temperature (i.e., 22–50°C). The three-isotope method was used to extrapolate partial exchange results to infer the equilibrium, mass-dependent isotope fractionations between goethite and water. In addition, this was combined with a reversal approach to equilibrium by reacting goethite in two unique waters that vary in composition by about 16‰ in 18O/16O ratios. Our results show that interactions between Fe(II)aq and goethite catalyzes oxygen-isotope exchange between the mineral and bulk fluid; no exchange (within error) is observed when goethite is suspended in 17O-enriched water in the absence of Fe(II)aq. In contrast, Fe(II)-catalyzed oxygen-isotope exchange is accompanied by significant changes in 18O/16O ratios. Despite significant oxygen exchange, however, we observed disproportionate amounts of iron versus oxygen exchange, where iron-isotope exchange in goethite was roughly three times that of oxygen. This disparity provides novel insight into the reactivity of oxide minerals in aqueous solutions, but presents a challenge for utilizing such an approach to determine equilibrium isotope fractionation factors. Despite the uncertainty from extrapolation, there is consistency in goethite–water fractionation factors for our reversal approach to equilibrium, with final weighted average fractionation factors of Δ18OGth-water=3.0 (±2.5‰) and 0.2 (±0.9‰) at 22°C and 1.9 (±1.5‰) and −1.6 (±0.8‰) at 50°C for nano-particulate and micron-sized goethite, respectively (errors at 2σ level). This variability of Δ18OGth-water with particle size may reflect differences in the grain boundaries of goethite and ratios of surface area to volume. Reaction of ferrihydrite with Fe(II)aq in two distinct waters resulted in a quantitative conversion to goethite and complete oxygen-isotope exchange in each case, and similar fractionation factors were observed for experiments using the two waters. Comparison of our results with previous studies of oxygen isotope fractionation between goethite and water suggests that particle size may be a contributing factor to the disparity among experimental studies.