Experimental determination of equilibrium Fe isotopic fractionation between pyrite and dissolved Fe under hydrothermal conditions

Fe isotope fractionation between pyrite (FeS₂) and dissolved Fe in NaCl- and sulfur-bearing aqueous fluids was determined under hydrothermal conditions (300-350°C, 500bars). The data were collected using two different, but complementary, experimental approaches, one involving classical Fe isotope exchange between Fe in pyrite and dissolved Fe in coexisting fluid, while the other involved homogenous precipitation of pyrite in a redox and pH buffered chemical system. Results from these experiments indicate equilibrium Fe isotopic fractionation between pyrite and fluid, δ⁵⁶Fe_Pyr-Fe(aq), of 0.99±0.29‰ (2σ), in ⁵⁶Fe/⁵⁴Fe. The experimentally determined equilibrium pyrite-fluid Fe isotopic fractionation agrees with theoretical and spectrally-based predictions. The second series of experiments were conducted to better constrain the effect of precipitation rate on the temporal evolution of the Fe isotopic composition of pyrite and Fe bearing fluids in dynamic mixing environments, such as hydrothermal vent sites at mid-ocean ridges. Rapid homogenous precipitation of pyrite at 300 and 350°C indicates that δ⁵⁶Fe of dissolved Fe is significantly greater than pyrite that formed during the earliest stage of the experiment, possibly facilitated by either equilibrium or kinetic isotope effects involving FeS as a reactant during pyrite formation. Subsequent recrystallization of pyrite results in a Fe isotopic fractionation with dissolved Fe that moves towards the experimentally determined equilibrium Fe isotopic fractionation with reaction progress. The experimental data reported here may help to decipher the complex kinetic and thermodynamic processes involved in pyrite formation at deep sea vents, while also providing constraints for the rapidly developing theoretical models used to estimate equilibrium Fe isotope fractionation between pyrite and fluid at elevated temperatures and pressures.