Plate tectonics caused the demise of banded iron formations

摘要

Early models for the origin of banded iron formations (BIFs) assumed that under a reducing atmosphere the supergene anoxic hydrolysis of mafic silicate minerals would provide an abundance of ferrous iron in solution to be transported to the oceans, there to be oxidised and precipitated by locally produced oxygen in shallow seas. The sparse distribution of BIFs in the Archaean era, its greater abundance during the Palaeoproterozoic era and perceived absence thereafter was considered to be essentially linked to the concentration of O2 in the atmosphere and this was a key factor in hypotheses of atmospheric evolution from anoxic to oxidising. Several assumptions regarding the deposition of BIFs were used to infer atmospheric evolution; however, chemical considerations indicate that the deposition of BIF is independent of atmospheric oxygen levels. Ferrous iron is only soluble in acid solution and in anoxic conditions in natural waters is precipitated as ferrous hydroxide or by carbon dioxide as ferrous carbonate. Silica in solution typically exists in equilibrium between ionic solution and colloidal suspension. Flocculation of colloidal silica is catalysed by the presence of cations and polymerises to a hydrophobic gel, thus removing silica from solution. It is highly unlikely that the oceans.could ever have been the reservoir of iron and silica for the deposition of BIFs.Modern interpretations consider BIFs as deep sea sediments with the source of the iron and silica derived from reactions between circulating sea water and hot mafic to ultramafic rocks producing hydrothermal systems venting onto the sea floor. The solubility of ferrous and ferric iron and silica is greatly increased at elevated temperatures and hydrothermal solutions would immediately precipitate iron hydroxide and iron silicates on quenching by cold seawater, even in the absence of ambient oxygen, to form hydrothermal plumes and mound deposits, subsequently resedimented by turbidity and density currents across the ocean floor. No transportation of ferrous iron in solution at ambient temperatures and no external source of O_2 or Fe~(2+) is necessary and the deposition of BIFs was independent of atmospheric oxygen, biogenic processes and continental sources of dissolved ferrous iron and silica, although any or all of these may have been present during deposition. A similar process occurs today with black smoker hydrothermal vents depositing large quantities of iron hydroxide and iron silicates on the ocean floor that will eventually be subducted beneath the continents by plate tectonic processes. During the Archaean era, shallow oceans and immature continents, preserved sea floor deposits as greenstone belts and in marginal sedimentary basins until the formation of large buoyant continents caused the subduction of deep sea ocean crust including BIF deposits. The temporal distribution of BIFs is thus related to the preservation of deep ocean sedimentary rocks that since the onset of modern style plate tectonics in the late Archaean to Proterozoic eras have largely been destroyed by subduction of the ocean floor beneath the continents.