Banded-iron formations are a distinctive type of sedimentary rock that formed predominantly dur ing the Precambrian and is the major source of the world's iron reserves. Banded-iron formations (BIFs) are a thinly bedded, chemically precipitated, iron-rich rock, with layers of iron ore minerals typically interbedded with thin layers of chert or microcrystal-line silica. Many are completely devoid of detrital or clastic sedimentary input. Most banded-iron formations formed between 2.6 and 1.8 billion years ago, and only a few very small similar types of deposits have been discovered in younger mountain belts. This observation suggests that the conditions necessary to form the BIFs were present on Earth in early (Precambrian) time, but largely disappeared by 1.8 billion years ago. The chemical composition and reduced state of much of the iron of BIFs suggest that they may have formed in an oxygen-poor atmosphere/ocean system, explaining their disappearance around the time that atmospheric oxygen was on the rise. BIFs may also be intimately associated with early biological activity and may preserve the record of the development of life on Earth. The world's oldest BiF is located in the 3.8 billion-year-old isua belt in southwestern Greenland, and some geologists have suggested that this formation contains chemical signatures that indicate biological activity was involved in its formation.
Two main types of banded-iron formations are classified based on the geometric and mineralogical characteristics of the deposits. Algoma-type BIFs are lens-shaped bodies that are closely associated with volcanic rocks, typically basalts. Most are around a thousand feet to several miles (several hundred meters to kilometers) in scale. in contrast, superior-type BIFs are very large in scale, many initially covering tens of thousands of square miles (kilometers). superior-type BiFs are closely associated with shallow marine shelf types of sedimentary rocks including carbonates, quartzites, and shales.
Banded-iron formations are also divisible into four types based on their mineralogy. oxide-iron formations contain layers of hematite, magnetite, and chert (or cryptocrystalline silica). silicate-iron formations contain hydrous silicate minerals including chlorite, amphibole, greenalite, stilpnomelande, and minnesotaite. Carbonate-iron formations contain siderite, ferrodolomite, and calcite. sulfide-iron formations contain pyrite.
in addition to being rich in iron, BiFs are ubiquitously silica-rich, indicating that the water from which they precipitated was saturated in silica as well as iron. other chemical characteristics of BiFs include low aluminum and titanium, elements that are generally increased by erosion of the continents. Therefore, BiFs are thought to have been deposited in environments away from any detrital sediment input. some BiFs, especially the sulfide-facies Algoma-type iron formation, have chemical signatures compatible with formation near black smoker types of seafloor hydrothermal vents, whereas others may have been deposited on quiet marine platforms. in particular, many of the superior-types of deposits have characteristics of deposition on a shallow shelf, including their association with shallow water sediments, their chemical and mineralogical constituency, and the very thin and laterally continuous nature of their layering. For instance, in the Archean Hammersley Basin of Western Australia, millimeter-thick layers in the BiF can be traced for hundreds of miles.
The environments where BiFs formed and the mechanism responsible for the deposition of the iron and silica in BiFs prior to 1.8 Ga is under current debate. Any model must explain the large-scale transport and deposition of iron and silica in thin layers, in some cases over large areas, for a limited time period of Earth's history. some observations are pertinent. First, to form such thin layers, the iron and silica must have been dissolved in solution. For iron to be in solution, it needs to be in the ferrous (reduced) state, in turn suggesting that the Earth's early oceans and atmosphere had little if any free oxygen, and were reducing. The source of the iron and silica is also problematic; it may have come from weathering of continents or from hydrothermal vents on the seafloor. Evidence supports both ideas for individual and different kinds of BiFs, although the scales seem to be tipped in favor of hydrothermal origins for Algoma-types of deposits, and weathering of continents for superior-type deposits.
The mechanisms responsible for causing dissolved iron to precipitate from the seawater to form the layers in banded-iron formations have also proven elusive and problematic. Changes in pH and acidity of seawater may have induced the iron precipitation, with periods of heavy iron deposition occurring during a steady background rate of silica deposition. Periods of nondeposition of iron would then be marked by deposition of silica layers. Prior to 1.8 Ga the oceans did not have organisms (e.g., diatoms) that removed silica from the oceans to make their shells, so the oceans would have been close to saturated in silica at this time, easing its deposition.
several models have attempted to bring together the observations and requirements for the formation of banded-iron formations, but none are completely satisfactory at present. Perhaps there is no unifying model or environment of deposition, and multiple origins are possible. one model calls on alternating periods of evaporation and recharge to a restricted basin (such as a lake or playa), with changes in pH
A banded iron formation—sample is about 1 inch (2.5 cm) across (Dirk Wiersma/Photo Researchers, Inc.)
and acidity being induced by the evaporation. This would cause deposition of alternating layers of silica and iron. Most BIFs do not appear to have been deposited in lakes. Another model calls on biological activity to induce the precipitation of iron, but fossils and other traces of life are generally rare in BIFs, although present in some. In this model, the layers would represent daily or seasonal variations in biological activity. Another model suggests that the layering was induced by periodic mixing of an early stratified ocean, where a shallow surface layer may have had some free oxygen resulting from near-surface photosynthesis, and a deeper layer would be made of reducing waters, containing dissolved elements produced at hydrothermal seafloor vents. In this model, precipitation and deposition of iron would occur when deep reducing water upwelled onto continental shelves and mixed with oxidized surface waters. The layers in this model would then represent the seasonal (or other cycle) variation in the strength of the coastal upwelling. This last model seems most capable of explaining features of the Superior-types of deposits, such as those of the Hamersley Basin in western Australia. Variations in the exhalations of deep-sea vents may be responsible for the layering in the Algoma-type deposits. Other variations in these environments, such as oxidation, acidity, and amount of organic material, may explain the mineralogical differences between different banded-iron formations. For instance, sul-fide-facies iron formations have high amounts of organic carbon (especially in associated black shales and cherts) and were therefore probably deposited in shallow basins with enhanced biological activity. Carbonate-facies BIFs have lower amounts of organic carbon and sedimentary structures indicative of shallow water deposition, so these probably were deposited on shallow shelves but farther from the sites of major biological activity than the sulfide-facies BIFs. oxide-facies BIFs have low contents of organic carbon but have a range of sedimentary structures indicating deposition in a variety of environments.
The virtual disappearance of banded-iron formation from the geological record at 1.8 billion years ago is thought to represent a major transition on the planet from an essentially reducing atmosphere to an oxygenated atmosphere. The exact amounts and rate of change of oxygen dissolved in the atmosphere and oceans would have changed gradually, but the sudden disappearance of BIFs at 1.8 Ga seems to mark the time when the rate of supply of biologically produced oxygen overwhelmed the ability of chemical reactions in the oceans to oxidize and consume the free oxygen. The end of BIFs therefore marks the new dominance of photosynthesis as one of the main factors controlling the composition of the atmosphere and oceans.
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