FIGURE 2. Schematic cross-section of the Earth's mantle showing formation of BIFs in the oceans and segregation at the core–mantle boundary.
From the following article:
Subducted banded iron formations as a source of ultralow-velocity zones at the core–mantle boundary
David P. Dobson and John P. Brodholt
Nature 434, 371-374(17 March 2005)
doi:10.1038/nature03430
Left inset, early life in the photic zone of the oceans releases oxygen from the atmosphere by photosynthesis. This reacts with Fe2+ in solution in deep ocean waters to produce insoluble iron oxides, which are precipitated as BIF on the ocean floors. BIF is recycled into the mantle at subduction zones. MOR, mid-ocean ridge. Right inset, BIF ponds at the core–mantle boundary, where it is neutrally buoyant. Convection currents concentrate BIF below regions of mantle upwelling where the ultralow-velocity zone is now observed.
