Experimental study of iron isotope fractionation at hydrothermal conditions

We have studied experimentally the iron isotope fractionation between hydrothermal fluids and the main Fe-bearing minerals, pyrite and hematite. The experiments consisted of precipitating, from solutions of known chemical composition, pyrite or hematite in a hydrothermal reactor under controlled pressure, temperature, redox and acidity conditions. The results, combined with thermodynamic calculations of Fe chemical speciation and theoretical predictions of Fe isotope fractionation show that the isotope fractionation factors obtained in hematite dissolution and precipitation experiments are in good agreement with theoretical predictions indicating near-zero fractionation between hematite and a fluid dominated by ferric chloride complexes, and slightly negative fractionations between hematite and a ferrous iron dominated fluid (e.g., 57Fefl-hm =-0.5±0.1 ‰ at 300°C/80bar). In pyrite precipitation experiments at 450°C and 600bar, the measured fractionation factors (57Fefl-pyr~0.5 to 0.8‰ ±0.2‰) are by 1.5‰ higher than theoretical predictions. This difference is due to disequilibrium between pyrite and fluid caused by a specific mechanism of pyrite formation via an intermediate phase (mackinawite), which rapidly equilibrates with the fluid and keeps its isotopic composition upon transformation into pyrite. This work reports the first experimental calibration of equilibrium and kinetic Fe isotope fractionation in the systems FeII/FeIII(aqueous)-hematite and FeII(aqueous)-pyrite at elevated temperatures, and provides evidence for the importance of redox control and mineral precipitation mechanisms on Fe isotope fractionation at hydrothermal conditions