Carbonatites (pronEng|kɑrˈbɒnətaɪt) are
intrusiveor extrusive igneousrocks defined by mineralogy that comprises more than 50 volume-% carbonateminerals. 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 Proterozoicor Phanerozoicin age. It seems that there has been a steady increase in the carbonatitic igneous activity through the Earth's history, from Archeanto 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 Lengaiin Tanzania. It erupted the lowest temperature lavain the world, at 500-600 °C (930-1,100 °F). The lava is dominated by natroliteand trona, sodic calcite.
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
* 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
limestoneor marbleby intrusion of magma, however geochemical and mineralogical data discount this.
Primary mineralogy is highly variable, but may include
natrolite, sodalite, sovite, apatite, magnetite, barite, fluorite, ancylitegroup 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 marblesave for the presence of igneous minerals. They may also be sources of micaor vermiculite. Natrocarbonatiteis 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 Valleyand author of the book "The Great Rift Valley"). These minerals are both carbonates in which sodiumand potassiumare present in significant quantities. Both are anhydrousand 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.
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
nephelineand 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.
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.
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
Proterozoicmobile belts in Australia, typically as dykes and discontinuous pods.
Carbonatites are known from Oka and
St. Honore, Quebec; Gem Parkand Iron Hill, Colorado; Magnet Cove igneous complex, Arkansas; Mountain Pass, California; Phalaborwa, South Africa; Jacupiranga, Brazil; Ayopaya, Bolivia; Kovdor, Russia, from India; the Mud Tankand Mount Weld, Australia; the Fen Complex, Norway.
Ol Doinyo Lengaivolcano, 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.
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.
* Duncan R K, Willett G C (1990) - Mount Weld Carbonatite: in "Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea" The AusIMM, Melbourne Mono 14, v1 pp 591-597
*cite web|title=Carbonatite Deposits|work=
USGSCarbonatite Deposits|url=http://pubs.usgs.gov/of/1995/ofr-95-0831/CHAP6.pdf|accessmonthday=January 31 |accessyear=2005
*cite web|title=Descriptive Model of Carbonatite Deposits|work=USGS Descriptive Model of Carbonatite Deposits|url=http://pubs.usgs.gov/bul/b1693/html/bull5rsj.htm|accessmonthday=January 31 |accessyear=2005
*cite web|title= World's Coolest Lava is in Africa|work=Volcano Watch
April 17, 2003|url=http://hvo.wr.usgs.gov/volcanowatch/2003/03_04_17.html|accessmonthday=January 31 |accessyear=2005
* [http://www.min.tu-clausthal.de/www/lager/mrbl/pdf/Schultz%20et%20al_2004.pdf Bolivian carbonatite occurrences]
* [http://it.stlawu.edu/~cnya/lenweb3b.htm Photos of natrocarbonatite lava]
* [http://volcano.und.nodak.edu/vwdocs/volc_images/africa/ol_doinyo.html Ol Doinyo Lengai, Tanzania]
* [http://www.mtsu.edu/~fbelton/lengai.html Ol Doinyo Lengai - The Mountain of God]
* [http://www.koeln.netsurf.de/~w.steffens/latam.htm List of alkaline rock occurrences in the Americas]
* [http://www.australianminesatlas.gov.au/info/aimr/phosphate.jsp Phosphate deposits of Australia, Mount Weld Carbonatite]
* [http://www.portergeo.com.au/database/mineinfo.asp?mineid=mn770 Description of the Mount Weld Carbonatite]
* [http://www.commerceresources.com/i/pdf/FirResourceEstimate_March62003.pdf Blue River Carbonatites, British Columbia, Canada]
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Look at other dictionaries:
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carbonatite — noun Any intrusive igneous rock having a majority of carbonate minerals … Wiktionary
carbonatite — [kα: bɒnətʌɪt] noun Geology a lava or other igneous rock composed chiefly of carbonates … English new terms dictionary
carbonatite — car·bon·a·tite … English syllables
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