Photo-oxidation of hydrated Fe²⁺—significance for banded iron formations

Abstract

The Precambrian banded iron formations (BIFs) are the major iron ore sources on the Earth. They consist of extensive iron-rich and iron-poor layers within siliceous sedimentary rocks^1,2. The banding has been attributed to variations in the conditions for precipitation of Fe²⁺ in ancient seas. The most favoured precipitating agent is oxygen^3–8; this would lead in the first place to insoluble FeOOH. The variations might then arise from fluctuations in low levels of oxygen in the atmosphere⁸. Similar fluctuations could arise through the in situ photosynthetic activity of phytoplankton⁵. Alternatively, oxygen might have been a constant factor, the periodicity arising from a varying supply of iron to the zone of precipitation^8–10. Another suggestion, that UV photons might have been the precipitating agent, without oxygen¹¹, was based on the conversion of Fe²⁺ to Fe³⁺ by UV light (254 nm) in rather strongly acid conditions^12,13. We have now tested this idea by photolysing Fe²⁺ in morerealistic near-neutral conditions. We report here that the presence of Fe(OH)⁺ becomes important, greatly enhancing previous estimates of the iron-precipitating power of early Precambrian sunlight, and suggesting that this sunlight would have been a sufficient precipitating agent for the iron found in BIFs.