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

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 degrees C and pressures from saturated vapor pressure (P-sat) to 600 bar. Precipitation experiments at 200 degrees C and P-sat from aqueous solution, in which Fe aqueous speciation is dominated by ferric iron (Fe-III) chloride complexes, show no detectable Fe isotope fractionation between hematite and fluid, Delta Fe-57(fluid-hematite) = delta Fe-57(fluid) - delta Fe-57(hematite) = 0.01 perpendicular to 0.08 parts per thousand (2 x standard error, 2SE). In contrast, experiments at 300 degrees C and P-sat, where ferrous iron chloride species (FeCl2 and FeCl+) dominate in the fluid, yield significant fluid enrichment in the light isotope, with identical values of Delta Fe-57(fluid-hematite) = -0.54 + 0.15 parts per thousand (2SE) both for dissolution and precipitation runs. Hematite dissolution experiments at 450 degrees C and 600 bar, in which Fe speciation is also dominated by ferrous chloride species, yield Delta Fe-57(fluid-hematite) values close to zero within errors, 0.15 +/- 0.17 parts per thousand (2SE). In most experiments, chemical, redox, and isotopic equilibrium was attained, as shown by constancy over time of total dissolved Fe concentrations, aqueous Fe-II and Fe-III fractions, and Fe isotope ratios in solution, and identical Delta Fe-57 values from dissolution and precipitation runs. Our measured equilibrium Delta Fe-57(fluid-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 beta-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 parts per thousand for the Delta Fe-57(Fe(aq)-hematite) value at 200 degrees C. Our data may thus help to refine theoretical models for beta-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