EXPERIMENTAL DETERMINATION OF IRON ISOTOPE FRACTIONATIONS AMONG FE(II)aq-FeSaq-MACKINAWITE AT LOW TEMPERATURES

This study provides the first rigorously determined equilibrium Fe isotope fractionation factors among aqueous iron, aqueous FeS clusters (FeS_aq), and nano mackinawite using a three-isotope method. A key component to our work has been use of dialysis membranes to allow analysis of aqueous and solid components that are not otherwise easily separable. Aqueous speciation determined using voltammetry indicates that the major species for the experiments are Fe(H₂O)₆²⁺ and FeS_aq at pH 7. The equilibrium fractionation factor at 20^oC and pH 7 has been determined to be -1.59±0.28 (2σ)‰ between Fe(II)_aq (minor FeS_aq also present in the experiment) and mackinawite. Conditions were changed to vary Fe(II)_aq/FeS_aq, and at pH=7, no significant change in fractionation occurred. The magnitude of the fractionation decreased when temperature was increased to 35^oC. Isotopic exchange kinetics in the experiments reflects fast exchange across the membrane and slower exchange between aqueous Fe and mackinawite. Precipitation experiments similar to those of Butler et al. (2005, EPSL) were also done, exploring the effects of pH, initial Fe(II)_aq, and time. Our replicate of Butler et al.’s experiments at pH 4, but extended to longer times, show broadly consistent results: over time, the aqueous Fe-mackinawite fractionation decreased and changed from a positive value to more negative values, moving towards the equilibrium fractionation factor. The positive fractionation factor measured by Butler et al. (2005), which is opposite to the equilibrium value, reflects the shorter time of their experiments, which did not erase initial kinetic effects. Experiments at pH 7 show more rapid exchange than those at pH 4 or 8, yet remained far from equilibrium even after 38 days. Experiments with lower initial Fe(II)_aq concentrations show slower exchange, reflecting coarsening of particles over time. The slow exchange kinetics for mackinawite bears on the Fe isotope compositions expected in nature – once formed, mackinawite appears to be quite resistant to later isotopic exchange with ambient fluid or sediment pore water. This in turn has important implications for determining the Fe isotope compositions of sedimentary pyrite, which is often considered to form from FeS precursors.