Evaluation of siderite and magnetite formation in BIFs by pressure-temperature experiments of Fe(III) minerals and microbial biomass

- Thermochemical Fe(III) reduction by microbial biomass led to siderite formation.
- Thermochemical magnetite formation during BIF low-grade metamorphism is unlikely.
- Silica preserved primary ferrihydrite against transformation to hematite.
- The presence of silica led to higher Fe(III) reduction yields.
- Silica did not influence the final mineralogy during simulated BIF metamorphism.

Anoxygenic phototrophic Fe(II)-oxidizing bacteria potentially contributed to the deposition of Archean banded iron formations (BIFs), before the evolution of cyanobacterially-generated molecular oxygen (O2), by using sunlight to oxidize aqueous Fe(II) and precipitate Fe(III) (oxyhydr)oxides. Once deposited at the seafloor, diagenetic reduction of the Fe(III) (oxyhydr)oxides by heterotrophic bacteria produced secondary Fe(II)-bearing minerals, such as siderite (FeCO3) and magnetite (Fe3O4), via the oxidation of microbial organic carbon (i.e., cellular biomass). During deeper burial at temperatures above the threshold for life, thermochemical Fe(III) reduction has the potential to form BIF-like minerals. However, the role of thermochemical Fe(III) reduction of primary BIF minerals during metamorphism, and its impact on mineralogy and geochemical signatures in BIFs, is poorly understood. Consequently, we simulated the metamorphism of the precursor and diagenetic iron-rich minerals (ferrihydrite, goethite, hematite) at low-grade metamorphic conditions (170 °C, 1.2 kbar) for 14 days by using (1) mixtures of abiotically synthesized Fe(III) minerals and either microbial biomass or glucose as a proxy for biomass, and (2) using biogenic minerals formed by phototrophic Fe(II)-oxidizing bacteria. Mössbauer spectroscopy and μXRD showed that thermochemical magnetite formation was limited to samples containing ferrihydrite and glucose, or goethite and glucose. No magnetite was formed from Fe(III) minerals when microbial biomass was present as the carbon and electron sources for thermochemical Fe(III) reduction. This could be due to biomass-derived organic molecules binding to the mineral surfaces and preventing solid-state conversion to magnetite. Mössbauer spectroscopy revealed siderite contents of up to 17% after only 14 days of incubation at elevated temperature and pressure for all samples with synthetic Fe(III) minerals and biomass, whereas 6% of the initial Fe(III) was reduced to sideritic Fe(II) in biogenic Fe(III) minerals during incubation. Based on our data, magnetite in BIF is unlikely to be formed by thermochemical Fe(III) reduction in sediments of biogenic ferrihydrite, hematite or goethite-dominated sediments with complex microbial biomass present. Instead, our results suggest that diagenetic magnetite in BIF was either formed by microbial Fe(III) reduction during early diagenesis, i.e., below 120 °C, or by thermochemical Fe(III) reduction with simple organic compounds at higher temperatures, whereas siderite was formed by both microbial, diagenetic Fe(III) reduction and thermogenic Fe(III) reduction with complex biomass. Thermochemical Fe(III) reduction coupled to biomass oxidation during metamorphism provides another origin for BIF siderites and could have led to a significant increase in Fe(II) content in BIF after deposition over geological timescales.