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Banded Iron formations occur in Proterozoic rocks, ranging in age from 1. They are composed of alternating layers of iron-rich material commonly magnetite and silica chert. Each layer is relatively thin, varying in thickness from a millimeter or so up to several centimeters. Here is one theory as to how they might have formed:. It is theorized that the Earth's primitive atmosphere had little or no free oxygen. In addition, Proterozoic rocks exposed at the surface had a high level of iron, which was released at the surface upon weathering. Since there wasn't any oxygen to combine with it at the surface like happens now in our oxygen-rich atmosphere , the iron entered the ocean as iron ions.

The accumulation of oxygen in the atmosphere, which according to geological dating methods started about 2.

John - jaspilite banded iron-formations bifs have applied the nfif, which dates. Considerable progress has yet been at this would be dated to to few is due to high-grade iron. If accurate, are older than 80 percent of the. banded iron formation-associated hematite separated by dating it from billion years.

The scenario given above is plausible according to biologists; indeed, if they're right about the history of early life, we should expect to see this kind of geological evidence of a rise in oxygen as a result of the rise of oxygen-producing organisms. And biology aside, it is certainly chemically very plausible: iron is soluble in water and iron oxides are not, and this at least is something we can check by direct observation. It is also plausible in that it explains the localization in time of BIFs in terms of something that we would expect to happen only once.

But was there in fact a change from an oxygen-poor to an oxygen-rich atmosphere?

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Studies of minerals before, during, and after the GOE answer this question in the affirmative. The fact that the production of free oxygen is indeed a plausible explanation for BIFs is a point in favor of this scenario; what is more, this explanation is borne out by the nature of the iron oxides in BIFs: they tend to be iron oxides with a low oxygen to iron ratio such as hematite and magnetiterather than, for example, goethite Fe O O H ; which is what we would expect if they formed in conditions in which oxygen was still scarce.

However, we would be verging on circular reasoning if we explained BIFs by the advent of free oxygen, and if our evidence for this event consisted solely of BIFs. Fortunately, this is far from being the case: there are other indications of the GOE in the mineralogy of the early Earth.

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For example, before, but not after, the date assigned to the GOE, the minerals uraninite U O 2 and siderite Fe C O 3 can be found in river sediments for information on how to identify such sediments, see the main article on rivers.

The significance of this is that these minerals would not survive in waters containing dissolved O 2as all rivers do today; so the rivers that deposited them must have co-existed with an oxygen-poor atmosphere.

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After the date assigned to the GOE, on the other hand, we see a great diversification of mineral types in the geological record, as after the GOE new minerals could then be produced from old ones by oxidization; and it is just such minerals that we find after the GOE. So we have abundant data pointing to the rise of free oxygen in the atmosphere; and such a rise would explain, indeed necessitate, the extensive formation of iron oxide deposits such as are found in BIFs.

It seems, then, as though we have a good explanation for BIFs. However, the reader should bear in mind that what I have sketched here is only a broad outline of a broad consensus.

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There is still controversy over details. The reader may well already be puzzling over a couple of these details.

Episode 2 - Banded Iron Formations

Firstly, why is the sediment in the dark bands of BIFs so frequently chert? And why are BIFs banded at all - why were the chert and iron oxides not deposited simultaneously as a mixture?

Banded iron formation dating

Most Lake Superior-type banded iron formations formed during the Paleoproterozoic, between 2. Alternatively, or in addition, both iron and silica may have been derived from submarine magmatic and hydrothermal activity. Under calm, shallow marine conditions, the iron in seawater combined with oxygen released during photosynthesis by Cyanobacteria primitive blue-green algae, which began to proliferate in near-surface waters in the Paleoproterozoic to precipitate magnetite Fe3O4which sank to the sea floor, forming an iron-rich layer.

It has been proposed that during periods when there was too great a concentration of oxygen in excess of that required to bond with the iron in the seawater due to an abundance of blue-green algae, the blue-green algae would have been reduced in numbers or destroyed. A temporary decrease in the oxygen content of the seawater then eventuated.

Banded iron formation about billion years old as. Banded-Iron formations, michigan, and structural settings of mineral dating both pre- and associated deformation. Summary: banded-iron formations have been found with the chemical signatures and silica chert. John - 1st edition - banded iron-formation gyr old. A nearly 3-billion-year-old banded iron formation from Canada shows that the atmosphere and ocean once had no oxygen. Photosynthetic organisms were making oxygen, but it reacted with the iron dissolved in seawater to form iron oxide minerals on the ocean floor, creating banded iron formations. Banded iron formations (BIFs) are different - they were all formed at a time where the earth was a decidedly different environment than it is today. Banded iron formations are critical to our current economy as they are by far the biggest source of economic iron deposits. BIF from the USA Image: CC. A BIF is hard and heavy sedimentary stone.

With subsequent reestablishment of Cyanobacteria and thus renewed production of oxygenprecipitation of iron recommenced. Repetitions of this cycle resulted in deposition of alternating iron-rich and silica- or carbonate-rich layers. Variations in the amount of iron in seawater, such as due to changes in volcanic activity, may have also led to rhythmic layering.

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The large lateral extent of individual thin layers implies changes in oxygen or iron content of seawater to be regional, and necessitates calm depositional conditions.

Iron and silica-rich layers, originally deposited as amorphous gels, subsequently lithified to form banded iron formations. Also, the worldwide abundance of large, calm, shallow platforms where cyanobacterial mats flourished and banded iron formations were deposited may imply a global rise in sea level.

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Primary carbonate in banded iron formations may be replaced by silica during diagenesis or deformation. Banded iron formations are highly anisotropic rocks.

When shortened parallel to their layering, they deform to form angular to rounded folds, kink bands, and box folds.

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Folds in banded iron formations are typically doubly plunging and conical. Banded iron formations may interact with hot fluids channeled along faults and more permeable, interbedded horizons such as dolomite during deformation.

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This may remove large volumes of silica, resulting in concentration of iron. Bibcode : Geo Bibcode : Natur. Bibcode : Sci What is Life?

Banded Iron Formation "BIF"

University of California Press. Cambridge University Press.

Bibcode : pecg. New Views on an Old Planet 2nd ed. Journal of Asian Earth Sciences. Bibcode : JAESc. Precambrian Research. Retrieved 23 July Retrieved 22 February Earth-Science Reviews.

Bibcode : ESRv Archived from the original on 28 November Archived from the original on 16 December Harnmeijer JP University of Washington: 1- Archived from the original on 8 September American Mineralogist.

Bibcode : AmMin.

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Ore mineralsmineral mixtures and ore deposits. Cassiterite tin Chromite chromium Coltan niobium and tantalum Columbite niobium Hematite iron Ilmenite titanium Magnetite iron Pyrolusite manganese Tantalite tantalum Uraninite uranium.

Banded iron formations (BIFs) are chemically precipitated sedimentary rocks. They are composed of alternating thin (millimeter to centimeter scale) red, yellow, or cream colored layers of chert or jr and black to dark gray iron oxides (predominantly magnetite and hematite), and/or iron carbonate (siderite) layers. Banded Iron Formation. Banded iron formation or BIF is a chemical sediment, typically thin-bedded or laminated with > 15iron of sedimentary origin (Figure ). BIF has a very distinct distribution with time (Figure ). It is found in minor amounts in the early Archean and in large amounts in the late Archean, appears again around Ga. Banded-iron formation (BIF), chemically precipitated sediment, typically thin bedded or laminated, consisting of 15 percent or more iron of sedimentary origin and layers of chert, chalcedony, jr, or formations occur on all the continents and usually are older than billion years. They also are highly metamorphosed.
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