Near-equilibrium kinetics in the Fe(II)-silicate system and the significance of nanoparticle greenalite in Archaean Iron Formations

Abstract

As the products of chemical sedimentation in the Archean oceans, Banded Iron Formations (BIFs) have been interpreted to record (bio)geochemical transitions in Earth’s ancient biosphere. Nonetheless, the effects of diagenesis and metamorphism over the long history of these rocks make it difficult to identify the minerals involved in the earliest stages of BIF formation. A series of recent studies has suggested that greenalite (Fe2+3Si2O5(OH)4), formed through hydrothermal fluid-seawater interactions, was among the primary mineral components of BIFs. However, the reactivity of greenalite as a function of relevant environmental parameters has not yet been mechanistically studied. The plausibility of its role in forming BIF deposits therefore remains speculative. Here, we fill this knowledge gap by conducting a series of kinetic experiments using a novel Si isotope doping method with hydrated, amorphous Fe(II)-silicate (a precursor to crystalline greenalite). The advantage of this technique is that it permits simultaneous determination of near-equilibrium forward and reverse reaction rates of Fe(II)-silicate-fluid interaction in plausible Archean ocean compositions. Reaction rate calculations indicate that the system’s behavior is governed by Fe(II)-silicate saturation state, with SiO2 sorption becoming dominant once a saturation threshold is exceeded. Combining kinetic data and thermodynamic calculations for the Fe-silicate-seawater system permits determination of a new solubility product for amorphous Fe(II)-silicate as log(K) = 24.9 ± 0.25. This value indicates maximum Fe2+ concentrations in Archean ocean waters at 25 °C would range from ∼ 1 mmol/kg at pH 7 to ∼ 10 µmol/kg at pH 8. Combining these observations with calculations of Stokes’ settling velocity implies that long-distance transport of greenalite nanoparticles – e. g., from deep-ocean hydrothermal vent sources to loci of BIF deposition – would have been feasible. Coupled with SiO2 sorption behavior on greenalite surfaces and the background SiO2 flux associated with the unique styles of Archean chert deposition, these results suggest that periodic waxing and waning of greenalite nanoparticle transport to BIF depositional environments can help to explain the Fe- and Si-enriched layers preserved in BIFs. Our results also provide a mechanistic underpinning for the exceptional preservation of greenalite in Archean sediments and its frequent association with chert. Ultimately, the readiness with which greenalite would have precipitated from Archean seawater and its apparent ability to be preserved despite transport across ocean basins suggests that it is time to reassess the traces of Earth’s early oceans recorded in BIFs and the ways in which these may be interpreted in light of new depositional models.