EXPERIMENTAL INSIGHTS INTO THE FORMATION OF IRON SILICATES IN BANDED IRON FORMATIONS (Invited Presentation)

In deciphering the Precambrian world, key targets to understand the geobiology of early Earth are marine iron- and silica-rich chemical precipitates that form the Banded Iron Formations (BIFs) dispersed across Earth today. Recently, iron silicate inclusions were discovered within early-silicifying chert in 2.45 to 2.63 billion-year-old BIFs, suggesting that primary minerals forming from the Archean oceans were iron silicates (Rasmussen et al., 2015). We previously characterized the chemistry of these inclusions and determined that they are dominantly low-Fe(III) greenalite (Johnson et al., 2018). In complementary laboratory-based experiments, we are now working to replicate Archean-like oceanic conditions to explore how low-Fe(III) greenalite can form under both abiotic conditions and in the presence of iron-oxidizing phototrophs. We have characterized the crystal structure and iron redox state of our raw and aged experimental iron-silica precipitates using a suite of methods including X-ray absorption spectroscopy, X-ray diffraction, and Raman spectroscopy. As our investigation into the mechanism of Fe(II/III)-silicate formation continues, this research will provide new constraints on the activity of silica, pH, and pO₂ in the Archean ocean and ascertain whether microbes could have mediated the precipitation of the iron silicates observed in ancient Banded Iron Formations.

Johnson, J.E., Muhling, J.R., Cosmidis, J., Rasmussen, B., and Templeton, A.S., 2018, Low-Fe(III) Greenalite Was a Primary Mineral From Neoarchean Oceans: Geophysical Research Letters, v. 45, p. 3182–3192, doi: 10.1002/2017GL076311.

Rasmussen, B., Krapež, B., Muhling, J.R., and Suvorova, A., 2015, Precipitation of iron silicate nanoparticles in early Precambrian oceans marks Earth’s first iron age: Geology, v. 43, p. 303–306, doi: 10.1130/G36309.1.