BANDED IRON FORMATIONS
| figure 1: BANDED IRON FORMATION from four locations; The pictures clockwise starting with the upper left corner. A, Neguanee BIF near Ishpeming, Michigan; B, from Son River valley, Uttar Pradesh, India; C, from mine at Krivoi Rog on the Dnepr River, Soviet Union; D, from mine at Bjornevatn, northeastern Norway. Ages range from 2.4-2 Billion Years old.(Cloud, 1988, page; 218) |
WHEN WERE BIFs FORMED?
BIFs were formed from 3.8 Ga to 1.9 Ga. Though the major sources of BIFs come from the older Proterozoic(2.5 -2.1 Ga).
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figure 2: These banded iron formations are from the Isua are of Greenland, and were deposited at least 3.8 billion years ago. It has been weakly metamorphosed. |
WHAT ARE BIFs?
Banded Iron Formations(BIFs) were formed when iron oxides, carbonates, silicates and sulphides settled to the sea floor (Hayes, 2002; page 127). BIFs are characterized by their alternation between Iron rich and Silica rich layers, most commonly in centimeter-scale bands and sub-millimeter-scale bands (Cloud, 1988; page 241). It is beleieved by many that they are formed on isolated submerged platforms (near to the ocean surface) on the continental shelves of Archean cratons. The Iron present in BIFs comes from the output of mid-ocean-ridge settings or from hotspots (Konhauser, 2002; page 1079). Upwelling currents deposit the minerals, from the hydrothermal vents, uniformly onto the shelf. The silica rich bands represent a hydrothermal quiesence or a temporary failure of the mineral rich fluids reaching BIF shelves.
As proposed by Cloud, then tested by Kurt Konhauser, primitive oxygen producing photosynthetic bacteria needed ferrous iron as an oxygen acceptor because the bacteria lacked advanced oxygen mediating enzymes (Konhauser, 2002; page 1079). When the mineral rich fluids would reach these shelves the bacteria would have indirectly precipitated ferric hydroxide to be laid down in the baded iron formations. When the was a hydrothermal quiesence the bacteria population would decline in numbers therefore precipitating less Fe2+. To read about Konhauser's Experiment
Banded Iron Formations cannot exist in the presence of a lot of oxygen, because iron is very readily oxidized by oxygen, this tells us there was not much free oxygen during the time of the BIFs.
A nice site to view how this iron precipitated to form this sedimentary rock is; http://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/Banded_Iron.html
EARTH IN THE ARCHEAN:
By 3.8 billion years ago a solid crust was already present and it was manifestred in small fast moving plates. The atmosphere was rich in Carbon Dioxide and Water vapor at this time, regulating the climate and weathering rocks. There was little to no free oxygen. The land surfaces were mostly composed of volcanics. Because of the Oxygen isotope ratios in the chert of the BIFs it is believed that the surface temperature was between 85 and 110 degrees Celsius (Cloud, 1988; Page 134).
INDUSTRIAL REVOLUTION...
could not have occurred and prospered without the secondarily enriched (metamorphosed to contain more iron) Banded Iron Formations(Cloud, 1988; Page 241). Much of our industrial world's iron comes from BIFs.
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figure 3: Estimated tonnages of BIF in Worlds major deposits (Cloud, 1988; Page 243) |
WHERE CAN YOU FIND BIFs?
| Hamersley Basin of Western Australia |
| Kriviu Rog on Dnepr River in the Ukrain |
| Ishpeming, Michigan |
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Isua, Greenland |
| Norway |
| Carajas Formation of Northern Brazil |
LINKS:
Banded
Iron Formations (BIFs): characteristics, modes of occurrence, and geological
significance
Superior type banded iron formations
REFERENCES
Cloud, Preston. (1988) Oasis in Space. Ontario, Canada: Penguin Books Canada Ltd.
Hayes, J.M. (2002) A lowdown on Oxygen. Nature. vol 417. 127-128
Konhauser, K.O. (2002) Could bacteria have formed the PreCambrian banded iron formations? Geology. vol 30. 1079-1082
Stanley, Steven M. (1999) Eart System History. New York, New York: W.H. Freeman and Co.