First experimental determination of iron isotope fractionation between hematite and aqueous solution at hydrothermal conditions

Abstract

Although iron isotopes provide a new powerful tool for tracing a variety of geochemical processes, the unambiguous interpretation of iron isotope ratios in natural systems and the development of predictive theoretical models require accurate data on equilibrium isotope fractionation between fluids and minerals. We investigated Fe isotope fractionation between hematite (Fe2O3) and aqueous acidic NaCl fluids via hematite dissolution and precipitation experiments at temperatures from 200 to 450°C and pressures from saturated vapor pressure (Psat) to 600bar. Precipitation experiments at 200°C and Psat from aqueous solution, in which Fe aqueous speciation is dominated by ferric iron (FeIII) chloride complexes, show no detectable Fe isotope fractionation between hematite and fluid, Δ57Fefluid–hematite=δ57Fefluid−δ57Fehematite=0.01±0.08‰ (2×standard error, 2SE). In contrast, experiments at 300°C and Psat, where ferrous iron chloride species (FeCl2 and FeCl+) dominate in the fluid, yield significant fluid enrichment in the light isotope, with identical values of Δ57Fefluid–hematite=−0.54±0.15‰ (2SE) both for dissolution and precipitation runs. Hematite dissolution experiments at 450°C and 600bar, in which Fe speciation is also dominated by ferrous chloride species, yield Δ57Fefluid–hematite values close to zero within errors, 0.15±0.17‰ (2SE). In most experiments, chemical, redox, and isotopic equilibrium was attained, as shown by constancy over time of total dissolved Fe concentrations, aqueous FeII and FeIII fractions, and Fe isotope ratios in solution, and identical Δ57Fe values from dissolution and precipitation runs. Our measured equilibrium Δ57Fefluid–hematite values at different temperatures, fluid compositions and iron redox state are within the range of fractionations in the system fluid–hematite estimated using reported theoretical β-factors for hematite and aqueous Fe species and the distribution of Fe aqueous complexes in solution. These theoretical predictions are however affected by large discrepancies among different studies, typically ±1‰ for the Δ57Fe Fe(aq)-hematite value at 200°C. Our data may thus help to refine theoretical models for β-factors of aqueous iron species. This study provides the first experimental calibration of Fe isotope fractionation in the system hematite–saline aqueous fluid at elevated temperatures; it demonstrates the importance of redox control on Fe isotope fractionation at hydrothermal conditions.