Burial diagenesis and metamorphism of precursor minerals in iron formations

Chemical sediments such as Banded Iron Formations (BIFs) are potential recorders of biogeochemical cycles and nutrient availability in ancient oceans. In particular, their metal and nutrient contents have been used to reconstruct concentrations of dissolved elements in seawater. However, chemical sediments represent non-continuous and non-homogeneous archives throughout Earth history, and their formation still remains largely controversial, challenging our ability to reconstruct early geochemical cycles. For instance, the mineralogy of their precursors is highly uncertain, and how such primary mineralogy transformed through diagenesis and metamorphism is equally unclear. Here, we explored the mineralogical transformations occurring upon rising pressure (P) and temperature (T) on key Fe-bearing minerals, commonly reported as ubiquitous in Archean, anoxic and Fe-rich oceans. Specifically, we performed P-T experiments in diamond anvil cells, spanning 25 to 300°C and 0 to 3 GPa to simulate sediment burial and metamorphism, on a range of Fe bearing minerals including ferrihydrite, goethite, hematite, carbonate green rust and greenalite, and used in-situ X-Ray Diffraction to characterize and discuss the evolution of mineralogical assemblages. Our results indicate that, in the absence of organic matter, the entire range of dominant, BIF-forming minerals can be achieved with an initial mixture of carbonate green rust and Fe silicate.