Mechanisms and rates of pyrite formation from hydrothermal fluid revealed by iron isotopes

International audienceCritical to interpreting iron isotope signatures in hydrothermal sulfide minerals is the accurate knowledge of Fe isotope fractionation factors, both kinetic and at equilibrium, between the fluid and minerals. Here we report Fe isotope fractionation between pyrite and aqueous saline fluid from in-situ pyrite precipitation experiments performed at 400–450 °C and 400–800 bar in a wide range of pH (2.5–4.8) and salt content (0.05–2.0 mol NaCl/KCl per kg of fluid). Our measurements show that while apparent bulk chemical equilibrium (in terms of total dissolved Fe concentration) between the FeCl2-bearing fluid and crystalline pyrite is attained within a few days, the Fe isotope fractionation values between the fluid and mineral (expressed as Δ57Fefluid-pyrite = δ57Fefluid – δ57Fepyrite) exhibit large variations, from about –0.6 to +1.0‰, depending on the elapsed time (1–30 days) and fluid composition (mostly pH, sulfur solubility, and di- and tri-sulfur radical ion concentration). Yet these values remain significantly higher, by 0.5–2‰, than equilibrium isotope fractionation inferred from β-factors of pyrite and the dominant iron chloride species in solution FeCl2(H2O)20(aq), generated in this study using Density Functional Theory (DFT) modeling (Δ57FeFeCl2(H2O)2-pyrite ≈ –1.25 ± 0.25‰ at 300–450 °C). The lack of isotope equilibrium between fluid and pyrite in this study and other recent work demonstrates that the fluid-pyrite Fe isotope exchange is kinetically controlled even at elevated temperatures (≥300 °C), allowing rate patterns to be identified. Our time-series data, combined with other available measurements at 300–350 °C, provide evidence for two major distinct kinetic regimes of fluid-pyrite Fe isotope exchange: (1) a fast and short (<10–20 days) initial step, likely controlled by Fe (poly)sulfide aqueous precursors that transfer their isotope signature to rapidly precipitating pyrite resulting in apparent isotopic disequilibrium between the FeCl2-dominated fluid and pyrite, (2) followed by a slower and longer second step of pyrite recrystallization, growth, and isotope re-equilibration with the fluid. The derived Fe isotope exchange rate constants suggest that more than 1 year is required to isotopically equilibrate pyrite with hydrothermal fluid at temperatures of 300–450 °C. Our new data provide direct interpretation of the Fe isotope ratios in fluids and sulfide minerals from modern submarine hydrothermal vents, which are characterized by fast (hours to days) pyrite precipitation. Furthermore, our results may enable estimating, using Fe isotope signatures, the dynamics of relatively short (