Mechanisms of iron-silica aqueous interaction and the genesis of Precambrian iron formation

Abstract:

Iron formations (IFs), Fe- and Si-rich chemical sediments common in Precambrian successions, preserve key information about the compositional, biological, and oxidative evolution of the Precambrian ocean. Stable Si isotopes (δ³⁰Si) of IF have been used to infer paleo-oceanic composition, and secular variations in δ³⁰Si may reflect ancient biogeochemical cycles. The δ³⁰Si of primary Fe-Si precipitates that formed IF depends not only on the δ³⁰Si of aqueous silica but also on the precipitation mechanism. Multiple formation mechanisms for these primary precipitates are plausible. Aqueous Si may have adsorbed on newly precipitated iron oxyhydroxide surfaces; alternatively, Fe and Si may have coprecipitated as a single phase. Here we explore variations in the Si isotope fractionation factor (ε) with Fe-Si aqueous interaction mechanism (adsorption vs. coprecipitation). In adsorption experiments, sodium silicate solutions (pH 8.1, 125-2000 µM Si) were reacted with iron oxide particles (hematite, ferrihydrite, goethite, and magnetite) for 24 to 72 hours. Resultant solutions had δ³⁰Si between 0 and +6‰. Calculated ε varied significantly with oxide mineralogy and morphology. For ferrihydrite, ε = -1.7‰; for hematite, ε = -2 to -5‰, depending on particle morphology. Apparent ε decreased upon surface site saturation, implying a smaller isotope effect for polymeric Si adsorption than monomeric adsorption. In coprecipitation experiments, solutions of Na-silicate and Fe(II) chloride (0.4-10 mM) were prepared anaerobically, then air-oxidized for 3 days to induce precipitation. At low Si concentrations, magnetite formed; near silica saturation, lepidocrocite and ferrihydrite formed. The Si isotope fractionation factor for coprecipitation was within the range of ε observed for adsorption (ε = -2.3 ± 1.0‰). These results indicate that the mechanism of Fe-Si interaction affects ε, presumably due to varying silicate coordination environments. These isotopic analyses will be paired with Si K-edge and Fe K-edge X-ray absorption spectra of the solids to illustrate how Si bonding environment affects ε. Effective reconstruction of paleo-oceanic δ³⁰Si may require additional constraints on the relative importance of Si adsorption and Fe-Si coprecipitation in the production of IF primary precipitates.