Carbonatite

Carbonatite

[kahr-bon-uh-tahyt]
Carbonatites are intrusive or extrusive igneous rocks defined by mineralogy that comprises more than 50 volume-% 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, sovite, 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.

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, if composed entirely of carbonate minerals, is 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 >20% 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.

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 igneous rocks that are miaskitic (nearly peralkaline) rather than agpaitic (peralkaline).

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 dykes 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 dykes are present in other Proterozoic mobile belts in Australia, typically as dykes 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; 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, uranium, thorium, copper, iron, titanium, barium, fluorine, zirconium, and other rare or incompatible elements.

Vein deposits of thorium, fluorite, or rare earth elements may be associated with carbonatites, and may be hosted internal to or in the aureole of a carbonatite.

References

External links

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