Banded Iron Formations (BIF) - Reading

Transcription

1 Banded Iron Formations (BIF) (University of Oregon) Banded Iron Formation (also known as BIF) is a term that is applied to a very unique sedimentary rock of biochemical origin. Unique in their make-up, unique in their age and unique in their origins. They are found all over the world, but only in certain areas of all the major continents. BIF consists of alternating layers of iron oxides and shale, chert, tiger eye or jasper. The alternating layers are generally only a few centimeters thick although the formations themselves can be massively thick. The iron oxide layers are generally composed of the minerals hematite and magnetite but other rarer iron oxides are also found in these formations. The black to gray to silver coloured iron oxide layers contrast with the iron rich chert, jasper and shales which are generally red in colour. The tiger eye versions of BIF are the result of low grade metamorphism creating veins of fibrous or asbestos riebeckite. Asbestos type crystals are replaced by the mineral quartz. The formerly fibrous nature of the crystals causes the play of light that is known as Tiger's Eye and it is quite attractive and used in jewellery.

2 BIF is very old Banded Iron Formations (BIF) - Reading Some of the oldest sedimentary rocks known to scientists have banded iron formations among their constituents date to as old as 3 billion years old, but most are aged at around 2.5 billion and some are as "young" as 1.8 billion. A very young formation of BIF is known to be only 800 to 600 million years old, but this is an exception to the rule. It is believed that banded iron formations occurred at these times in Earth's history due to unique conditions and then never again were the conditions right for the formation of this most unusual rock. Due to the extreme age of these formations, almost all BIF formations have undergone some faulting, fracturing, folding, compaction, veining, intrusions and metamorphism. Although all BIF formations are probably metamorphosed to some degree their general character is still sedimentary. Banded Iron Formations are thought to have formed from the precipitation of iron from the Earth's ancient oceans. Photosynthetic bacteria produced, for perhaps the first time in the young Earth's oceans, free oxygen which oxidized the dissolved iron that existed abundantly at the time. Oxidized iron is not soluble in water and thus it would precipitate out of the oceans and onto the muddy sea floor. For reasons largely unknown, this was a periodic process resulting in the alternating bands of iron oxide and shale. The periodic process might have been due to seasonal fluctuations or storm surges or other hypothesis. Whatever the reason, there never seemed to be a time when the iron layer formation or the shale (mud) formation persisted long enough to produce a layer thicker than 10 centimeters or so. Since the origin of the iron layer is derived from a living organism, the photosynthetic bacteria, BIF is actually a fossil. BIF actually qualifies as a trace fossil. Some of the oldest fossils known to man just predate banded iron formations. Bacteria are believed to be the earliest life forms on Earth and eventually the oxygen producing varieties formed the BIF and helped transform the Earth. BIF is the proof of this transformation when it was under way. The conditions to form BIF, dissolved iron and episodic oxygenation, existed early in Earth's history and then once the Earth's oxygen levels stabilized the conditions for banded iron formation all but ceased to exist When polished, BIF can be very beautiful. The red jasper or Tiger Eye makes a wonderful compliment to the sparkling silver gray of the hematite. The banded layers, sometimes contorted by ages of folding and faulting, make for surreal landscapes of asymmetric bands. BIF can be used for many ornamental purposes from bookends to clock faces to gravestones and monuments. Its only major drawback is its significant weight. Iron is not light. But the solidification of jasper and compaction of the stone make it very durable and capable of being processed into relatively thin slabs. This keeps the weight to usable levels and allows BIF to be used as a popular ornamental stone. The popular polished Tiger Eye stones that are sold in rock shops around the world are usually derived from banded iron formations.

3 As an ore of iron, BIF is king The hematite and magnetite concentrations in BIF are much sought after by mining companies. These two minerals are the best sources of iron and fortunately there is a lot of BIF to supply the world's needs for quite a while. Banded iron formations are found in the continental shield of all continents of the world. The shield areas of continents contain the oldest Precambrian rocks. At one time these rocks may have been together as one continent, but later broke apart and became the core of the modern continents as they exist today. Although each continent has had its own geologic history that had its own impact on their shield rocks, it is somewhat amazing that most BIF rocks are very similar character. This is truly one of the most amazing rocks found on Earth. Banded Iron formations occur in Proterozoic rocks, ranging in age from 1.8 to 2.5 billion years old. Composed of alternating layers of iron-rich material (commonly magnetite) and silica (chert). Each layer is relatively thin, varying in thickness from a millimetre or so up to several centimetres. 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 At the same time, primitive photosynthetic blue/green algae was beginning to proliferate in the near surface waters As the algae would produce O 2 as a waste product of photosynthesis, the free oxygen would combine with the iron ions to form magnetite (Fe 3 O 4 ), an iron oxide This cleansed the algae's environment As the biomass expanded beyond the capacity for the available iron to neutralize the waste O 2 the oxygen content of the sea water rose to toxic levels This eventually resulted in large-scale extinction of the algae population, and led to the accumulation of an iron poor layer of silica on the sea floor. As time passed and algae populations re-established themselves, a new iron-rich layer began to accumulate. The algae were of relatively low intelligence and were unable to learn from their past excesses (this was also before the EPA), so they would again proliferate beyond the capacity of the iron ions to clean up their waste products, and the cycle would repeat. This went on for approximately 800,000,000 years!

4 Banded iron formations are very large bodies of sedimentary rock laid down some 2.5 billion years ago. At that time, the Earth still had its original atmosphere of nitrogen and carbon dioxide. That would be deadly for us but it was hospitable to many different microorganisms in the sea, including the first photosynthesizers. These organisms gave off oxygen as a waste product, which immediately bonded with the abundant dissolved iron to yield minerals like magnetite and hematite. The black parts of this polished slab are thin layers of dark, semi-metallic hematite, and the red layers are jasper, an iron-rich chert. The gray and bright golden areas are part of the same layers, made of tiger-eye quartz. It forms when quartz replaces the fibrous mineral crocidolite (also known as blue asbestos). The dark mineral flashes in golden highlights as you turn it in the light, an effect called chatoyancy. Law Of Uniformitarianism Nowadays, such iron deposits can form only in the airless bottoms of bogs. "Bog ore" was the main source of iron in ancient and medieval times. One way of studying the ancient surface is to look at the chemistry of seawater stored in sediments that s because seawater is the rinsings of the land. Above is a graph of a rare earth element pattern. It is from an iron-silica deposit found in the 2.7 billion year old Abitibi Greenstone Belt in Northern Ontario (data courtesy of Dr. Phil Thurston). It is compared to modern Pacific ocean seawater: First, the ancient seawater had a prominent positive spike at the element Eu And then there is the absence of a negative spike at Ce

5 These two features show that the ancient ocean was starved of oxygen and that more metal entered the deep ocean at hydrothermal vents. Outcrop of the Apex Chert near Chinaman Creek, Pilbara in Western Australia. Microfossil structures occur in a hydrothermal feeder dyke, not in the actual sedimentary portion of the Apex Chert. References Banded Iron Formation, Lecture Material, University of Oregon, Jersey, accessed at