Iron isotopic composition in pyrite: proxy of redox state or of trace element composition of ancient oceans? – PYRISOFE

The elemental and isotopic composition of sedimentary pyrites is commonly used to trace the evolution of ocean chemistry and redox state through geological times. In particular, iron isotopes in pyrite appear to trace the redox state of ancient oceans in response to the progressive rise of atmospheric oxygen during the Archean-Paleoproterozoic. However, the interpretation of these isotopic signatures is highly debated due to a limited understanding of pyrite formation mechanisms and kinetics. In this regard, we have recently demonstrated that the presence of trace metals drastically affects the nucleation kinetic of pyrite, which probably has important consequences on the isotopic signatures. This result is all the more important as the Archean ocean was enriched in many elements (Ni, Co, As, Au,...) compared to the present ocean. The PYRISOFE project aims at quantifying and rationalizing the influence of trace metals on each of the crystal-chemical steps of pyrite formation via various reaction pathways and in particular on Fe isotope fractionation of the resulting pyrite.
To achieve this goal, we will combine measurements of solid/aqueous Fe isotopic fractionation with the determination of the molecular-level structure and crystal chemistry of the corresponding solid and colloidal phases. This will be done on precursors and reaction intermediates during biotic and abiotic experiments of pyrite synthesis in the presence of relevant trace elements. Experimental work will be supported by ab initio calculations that will contribute to the precise identification of the solid and colloidal phases, and their molecular-scale interaction with the studied trace elements. More importantly, the ab initio calculation of isotopic fractionation factors will allow us to understand the relative influence of kinetics and equilibrium on the observed Fe isotope fractionation. The comparison of experimental and theoretical data will also indicate whether a re-equilibration of pyrite Fe isotopic signal occurs with time during diagenetic aging and will help to quantify this re-equilibration. This work will shed a new light on the interpretation of isotopic records in sedimentary pyrites. Particular emphasis will be placed on the remarkable acquisition of very light isotopic signatures just before the Great Oxygenation Event (GOE, ~2.4 Ga), which profoundly affected the evolution of life on Earth.