- Carbonatite
Carbonatites (pronEng|kɑrˈbɒnətaɪt) are
intrusive or extrusiveigneous rocks defined by mineralogy that comprises more than 50 volume-%carbonate minerals. Carbonatites may be confused withmarble , and may require geochemical verification.Carbonatites usually occur as small plugs within zoned alkalic intrusive complexes, or as dikes, sills,
breccia s, and veins. They are, almost exclusively, associated with continental rift-related tectonic settings. The majority of carbonatites areProterozoic orPhanerozoic in age. It seems that there has been a steady increase in the carbonatitic igneous activity through the Earth's history, fromArchean 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 inTanzania . It erupted the lowest temperaturelava in the world, at 500-600 °C (930-1,100 °F). The lava is dominated bynatrolite andtrona , 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 asilicate 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 oflimestone ormarble by intrusion ofmagma , 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 frommarble save for the presence of igneous minerals. They may also be sources ofmica orvermiculite .Natrocarbonatite is made up largely of two minerals, nyerereite (named afterJulius Nyerere , the first president of independentTanzania ) and gregoryite (named afterJohn Walter Gregory , one of the first geologists to study theGreat Rift Valley and author of the book "The Great Rift Valley"). These minerals are bothcarbonate s in whichsodium andpotassium are present in significant quantities. Both areanhydrous 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 ,lamprophyre s,phonolite ,foyaite ,shonkinite , silica undersaturated foid-bearingpyroxenite (essexite ), andnepheline syenite .Carbonatites are typically associated with undersaturated igneous rocks that are
miaskitic (nearlyperalkaline ) rather thanagpaitic (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 andIron Hill, Colorado ;Magnet Cove igneous complex ,Arkansas ;Mountain Pass, California ;Phalaborwa ,South Africa ;Jacupiranga ,Brazil ;Ayopaya, Bolivia ;Kovdor ,Russia , fromIndia ; theMud Tank andMount Weld, Australia ; theFen Complex, Norway .The
Ol Doinyo Lengai volcano, in theGreat Rift Valley , Africa, is the world's only active carbonatite volcano. Other older carbonatite volcanoes are located in the same region, includingHoma 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
* 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=USGS Carbonatite 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 WatchApril 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]External links
* [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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