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Carbonatite
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Carbonatites are intrusive or extrusive igneous rocks defined by mineralogic composition consisting of greater than 70 volume percent carbonate minerals. Carbonatites may be confused with marble, and may require geochemical verification.
Carbonatites usually occur as small plugs within zoned alkalic intrusive complexes, or as dikes, sills, breccias, and veins. They are, almost exclusively, associated with continental rift-related tectonic settings.
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Encyclopedia
Carbonatites are intrusive or extrusive igneous rocks defined by mineralogic composition consisting of greater than 70 volume percent carbonate minerals. Carbonatites may be confused with marble, and may require geochemical verification.
Carbonatites usually occur as small plugs within zoned alkalic intrusive complexes, or as dikes, sills, breccias, and veins. They are, almost exclusively, associated with continental rift-related tectonic settings. The majority of carbonatites are Proterozoic or Phanerozoic in age. It seems that there has been a steady increase in the carbonatitic igneous activity through the Earth's history, from Archean to present.
Nearly all carbonatite occurrences are intrusives or subvolcanic intrusives. This is because carbonatite lava flows are unstable and react quickly in the atmosphere. Carbonatite lavas may not be as uncommon as thought, but have been poorly preserved throughout Earth's history.
Only one carbonatite volcano is known to have erupted in historical time, Ol Doinyo Lengai in Tanzania. It erupted the lowest temperature lava in the world, at 500-600 °C (930-1,100 °F). The lava is dominated by natrolite and trona, sodic calcite.
Genesis
Carbonatites are rare, peculiar igneous rocks formed by unusual processes and from unusual source rocks. Three models of their formation exist:
- direct generation by very low degree partial melts in the mantle and melt differentiation
- liquid immiscibility between a carbonate melt and a silicate melt
- peculiar, extreme crystal fractionation
Evidence for each process exists, but the key is that these are unusual phenomenon. Historically, carbonatites were thought to form by melting of limestone or marble by intrusion of magma, however geochemical and mineralogical data discount this.
Mineralogy
Primary mineralogy is highly variable, but may include natrolite, sodalite, apatite, magnetite, barite, fluorite, ancylite group minerals, and other rare, peculiar minerals not found in more normal igneous rocks. Recognition of carbonatites may be difficult, especially as their mineralogy and texture may not differ much from marble save for the presence of igneous minerals. They may also be sources of mica or vermiculite.
Carbonatites are classed as calcitic sovite (coarse textured) and alvikite (finer textured) varieties or facies. The two are also distinguished by minor and trace element composition. The terms rauhaugite and beforsite refer to dolomite and ankerite rich occurrences respectively. The alkali-carbonatites are termed lengaite. Examples with 50 - 70% carbonate minerals are termed silico-carbonatites. Additionally carbonatites may be either enriched in magnetite and apatite or rare earth elements, fluorine and barium.
Natrocarbonatite is made up largely of two minerals, nyerereite (named after Julius Nyerere, the first president of independent Tanzania) and gregoryite (named after John Walter Gregory, one of the first geologists to study the Great Rift Valley and author of the book The Great Rift Valley). These minerals are both carbonates in which sodium and potassium are present in significant quantities. Both are anhydrous and when they come into contact with the moisture of the atmosphere, they begin to react extremely quickly. The black or dark brown lava and ash erupted begins to turn white within a few hours.
Geochemistry
Carbonatite is composed predominatly of carbonate minerals and extremely unusual in its major element composition as compared to silicate igneous rocks, obviously because it is composed primarily of Na2O and CaO plus CO2.
Most carbonatites tend to include some silicate mineral fraction; by definition an igneous rock containing >50% carbonate minerals is classified as a carbonatite. Silicate minerals associated with such compositions are pyroxene, olivine, and silica-undersaturated minerals such as nepheline and other feldspathoids.
Geochemically, carbonatites are dominated by incompatible elements (Ba, Cs, Rb) and depletions in compatible elements (Hf, Zr, Ti). This together with their silica-undersaturated composition supports inferences that carbonatites are formed by low degrees of partial melting.
A specific type of hydrothermal alteration termed fenitization is typically associated with carbonatite intrusions. This alteration assemblage produces a unique rock mineralogy termed a fenite after its type locality, the Fen complex in Norway. The alteration consists of metasomatic halos consisting of sodium rich silicates arfvedsonite, barkevikite and glaucophane along with phosphates, hematite and other iron and titanium oxides.
Occurrence
Associated igneous rocks typically include ijolite, melteigite, teschenite, lamprophyres, phonolite, foyaite, shonkinite, silica undersaturated foid-bearing pyroxenite (essexite), and nepheline syenite.
Carbonatites are typically associated with undersaturated (low silica) igneous rocks that are either alkali (Na2O and K2O), ferric iron (Fe2O3) and zirconium-rich agpaitic rocks or alkali-poor, FeO-CaO-MgO-rich and zirconium-poor miaskitic rocks.
The Mount Weld carbonatite is unassociated with a belt or suite of alkaline igneous rocks, although calc-alkaline magmas are known in the region. The genesis of this Archaean carbonatite remains contentious as it is the sole example of an Archaean carbonatite in Australia.
Intrusive morphology
Carbonatite is known to form in association with concentrically zoned complexes of alkaline-igneous rocks, the typical example of this being Phalaborwa, South Africa.
Chilean carbonatites take the form of sills, lopoliths and rare dikes are reported in the Guyana Shield.
The Mud Tank and Mount Weld carbonatites take the form of multi-stage cylindrical intrusive bodies with several distinct phases of carbonatite intrusion. Smaller carbonatite sills and dikes are present in other Proterozoic mobile belts in Australia, typically as dikes and discontinuous pods.
Known examples
Carbonatites are known from Oka and St. Honore, Quebec; Gem Park and Iron Hill, Colorado; Magnet Cove igneous complex, Arkansas;
Mountain Pass, California; the Palabora Complex near Phalaborwa, South Africa;
Jacupiranga, Brazil; Ayopaya, Bolivia; Kovdor, Russia, from India; the Mud Tank and Mount Weld, Australia; the Fen Complex, Norway.
The Ol Doinyo Lengai volcano, in the Great Rift Valley, Africa, is the world's only active carbonatite volcano. Other older carbonatite volcanoes are located in the same region, including Homa Mountain.
Economic importance
Carbonatites may contain economic or anomalous concentrations of rare earth elements, phosphorus, niobium - tantalum, uranium, thorium, copper, iron, titanium, vanadium, barium, fluorine, zirconium, and other rare or incompatible elements. Apatite, barite and vermiculite are among the industrially important minerals associated with some carbonatites.
Vein deposits of thorium, fluorite, or rare earth elements may be associated with carbonatites, and may be hosted internal to or within the metasomatized aureole of a carbonatite.
As an example the Palabora complex of South Africa has produced significant copper (as chalcopyrite, bornite and chalcocite), apatite, vermiculte along with lesser magnetite, linnaeite (cobalt), baddeleyite (zirconium-hafnium), and by-product gold, silver, nickel and platinum.
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