Generation of banded iron formations by internal dynamics and leaching of oceanic crust

The chemical signatures and mineralogy of banded iron formations have the potential to provide information about the ocean environment on early Earth(1-7). Their formation requires iron- and silicon-rich fluids, but the mechanisms by which the alternating layers of Si- and Fe-rich rock formed remain controversial(8-11). Here we use thermodynamic calculations to show that Fe- and Si- rich fluids can be generated by hydrothermal leaching of low-Al oceanic crustal rocks such as komatiites. We find that positive feedbacks occur among the chemical reactions when hydrothermal fluids mix with ambient sea water. These feedbacks lead to alternating precipitation of Fe and Si minerals, owing to the formation of complexes between Fe(II) and silicic acid. We suggest that the small-scale (<1 cm) banding was produced by internal dynamics of the geochemical system, rather than any external forcing. As the Archaean eon progressed, the oceanic crust produced was rich in Al-12. When Al-rich crust undergoes hydrothermal alteration, Fe is locked in Al-Fe silicate minerals. This results in iron-depleted hydrothermal fluids, and thus prevents the deposition of Fe- rich minerals. We therefore conclude that the widespread cessation of banded iron formation deposition 1.7 billion years ago reflects the changing composition of the oceanic crust.