Insights into the acquisition of kinetic iron isotope signatures of pyrite precipitated through the polysulfide pathway

• Strong and systematic kinetic Fe isotope fractionation of precipitated pyrite • Reversal of solid-fluid Fe isotope partitioning at the onset of pyrite formation • Obvious impact of trace Ni and As on the timing of the isotopic reversal • Unification of the H 2 S and polysulfide pathways of pyrite formation • Abiotic experiments may explain the most negative δ 57 Fe of Archean pyrites Sedimentary pyrite (FeS 2 ) chemistry and isotope compositions have often been used to reconstruct the paleo-redox conditions of the ancient Earth’s surface. However, the reaction pathways leading to the formation of this mineral in sedimentary environments remain debated and therefore, its elemental and isotope record is still ambiguous. We thus produced iron isotope time series of pyrite precipitation experiments at ambient temperature. Pyrite minerals were produced through the polysulfide pathway, one of the main mechanisms of pyrite production on Earth. The analysis of precipitated solids and coexisting solutions show that the iron isotope partitioning departs increasingly from equilibrium as the reaction proceeds until the end of our experiments (1510 h). We observe a strong inversion of the isotope partitioning between solid and solution coinciding with the transformation of disordered mackinawite to pyrite. Experiments with added nickel or arsenic, which impact the nucleation kinetics of pyrite, shows that the timing of pyrite nucleation is faithfully recorded by the iron isotope partitioning. However, whatever the timing of the isotope inversion, the final partitioning between pyrite and solution leads to the same ∆ 57 Fe ∼ -3.5 ‰. This apparent fractionation is similar to that previously found in pyrite precipitation experiments through the H 2 S pathway. Combined with new ab initio equilibrium fractionation factors computation and previous X-ray absorption spectroscopic measurements, we propose that the observed fractionation may be explained by isotope exchange with the same key, isotopically light iron molecular intermediate, leading to pyrite nucleation and growth in both the so-called H 2 S and polysulfide pathways. At ambient temperature, this fractionation process corresponds to the lightest δ 57 Fe isotope signatures found in the Archean pyrite sedimentary record and it may sign a specific kinetic and irreversible reaction common to both main abiotic pyrite production pathways.