| Igneous rock | |
 | |
| Composition | |
|---|---|
| Carbonate minerals (>50%) |
Carbonatite (/kɑːrˈbɒnəˌtaɪt/) is a type ofintrusive orextrusiveigneous rock defined bymineralogic composition consisting of greater than 50%carbonate minerals.[1] Carbonatites may be confused withmarble and may require geochemical verification.
Carbonatites usually occur as smallplugs within zoned alkalic intrusive complexes, or asdikes,sills,breccias, andveins.[2] They are almost exclusively associated with continentalrift-related tectonic settings. It seems that there has been a steady increase in the carbonatiticigneous activity through the Earth's history, from theArcheaneon to the present.
Nearly all carbonatite occurrences are intrusives orsubvolcanic intrusives. This is because carbonatitelava flows, being composed largely of soluble carbonates, are easily weathered and are therefore unlikely to be preserved in the geologic record. Carbonatite eruptions as lava may therefore not be as uncommon as thought, but they have been poorly preserved throughout theEarth's history.[3]
Carbonatite liquid compositions are significantly more alkaline than what is preserved in the fossil carbonatite rock record as composition of the melt inclusions shows.[4]
Only one carbonatitevolcano is known to have erupted in historical time, the activeOl Doinyo Lengai volcano inTanzania. It erupts with the coolest lava in the world, at 500–600 °C (932–1,112 °F). The lava isnatrocarbonatite dominated bynyerereite andgregoryite.
The magmatic origin of carbonatite was argued in detail by Swedish geologistHarry von Eckermann in 1948 based on his study ofAlnö Complex.[5][6] It was however the 1960 eruption ofOl Doinyo Lengai in Tanzania that led to geological investigations that finally confirmed the view that carbonatite is derived frommagma.[5]
Carbonatites arerare,peculiar igneous rocks formed by unusual processes and from unusual source rocks. Three models of their formation exist:
Evidence for each process exists, but the key is that these are unusual phenomena. Historically, carbonatites were thought to form by melting oflimestone ormarble by intrusion ofmagma, but geochemical and mineralogical data discount this. For example, the carbon isotopic composition of carbonatites is mantle-like and not like sedimentary limestone.[7]
The age of carbonatites ranges from Archean to present: the oldest carbonatite, Tupertalik in Greenland, is 3007 Ma old, while Ol Doinyo Lengai volcano in Tanzania is presently active.[8]
Primary mineralogy is highly variable, but may includenatrolite,sodalite,apatite,magnetite,baryte,fluorite,ancylite group minerals, and other rare minerals not found in more common igneous rocks. Recognition of carbonatites may be difficult, especially as their mineralogy and texture may not differ much frommarble except the presence of igneous minerals. They may also be sources ofmica orvermiculite.
Carbonatites are classed ascalciticsovite (coarse textured) andalvikite (finer textured) varieties orfacies. The two are also distinguished by minor andtrace element composition.[9][10] The termsrauhaugite andbeforsite refer todolomite- andankerite-rich occurrences respectively. The alkali-carbonatites are termedlengaite. Examples with 50–70% carbonate minerals are termedsilico-carbonatites.[10] Additionally, carbonatites may be either enriched inmagnetite andapatite orrare-earth elements,fluorine andbarium.[11]
Natrocarbonatite is made up largely of two minerals,nyerereite (named afterJulius Nyerere, the first president of independentTanzania) andgregoryite (named afterJohn Walter Gregory, one of the first geologists to study theEast African Rift and author of the bookThe Great Rift Valley). These minerals are bothcarbonates in whichsodium andpotassium are present in significant quantities. Both areanhydrous, and when they come into contact with the moisture in 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, then grey after a few days, then brown after a few weeks.[12]
Carbonatites are unusual igneous rocks composed predominantly ofcarbonate minerals. 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 arepyroxene,olivine, and silica-undersaturated minerals such asnepheline and otherfeldspathoids.
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 ofpartial melting.
A specific type ofhydrothermal alteration termedfenitization is typically associated with carbonatite intrusions. This alteration assemblage produces a unique rock mineralogy termed afenite after its type locality, theFen Complex inNorway. The alteration consists ofmetasomatic halos consisting of sodium richsilicatesarfvedsonite,barkevikite andglaucophane along withphosphates,hematite and other iron and titanium oxides.[11]
Overall, 527 carbonatite localities are known on Earth, and they are found on all continents and also on oceanic islands. Most of the carbonatites are shallow intrusive bodies of calcite-rich igneous rocks in form of volcanic necks, dykes, and cone-sheets. These generally occur in association with larger intrusions of alkali-rich silicate igneous rocks. The extrusive carbonatites are particularly rare, only 49 are known, and they appear to be restricted to a few continental rift zones, such as the Rhine valley and the East African rift system.[13]
Associated igneous rocks typically includeijolite,melteigite,teschenite,lamprophyres,phonolite,foyaite,shonkinite, silica undersaturated foid-bearingpyroxenite (essexite), andnepheline syenite.
Carbonatites are typically associated with undersaturated (lowsilica) igneous rocks that are either alkali (Na2O and K2O), ferric iron (Fe2O3) andzirconium-richagpaitic rocks or alkali-poor, FeO-CaO-MgO-rich and zirconium-poormiaskitic rocks.[11]
TheMount 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 beingPhalaborwa, South Africa.
Carbonatites in the form of sills,lopoliths and rare dikes are reported in theGuyana Shield.
The Mud Tank andMount 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 otherProterozoic mobile belts in Australia, typically as dikes and discontinuous pods.
Dozens of carbonatites are known including:
In 2017, the discovery of a new carbonatite deposit was confirmed north-west ofPrince George, British Columbia, in a region termed the "Rocky Mountain Rare Metal Belt".[16]
The volcanoOl Doinyo Lengai, in theEast African Rift is the world's only active carbonatite volcano. Other older carbonatite volcanoes are located in the same region, includingMount Homa.
Carbonatites may contain economic or anomalous concentrations ofrare-earth elements (REEs),phosphorus,niobium–tantalum,uranium,thorium,copper,iron,titanium,vanadium,barium,fluorine,zirconium, and other rare or incompatible elements.Apatite,barite andvermiculite are among the industrially important minerals associated with some carbonatites.[11]
Trace elements are extremely enriched in carbonatites, and they have the highest concentration of lanthanides of any known rock type.[17] The largest REE-carbonatite deposits are Bayan Obo,[18] Mountain Pass,[19] Maoniuping,[20] and Mount Weld.[21]
Vein deposits ofthorium,fluorite, orrare-earth elements may be associated with carbonatites and may be hosted internal to or within themetasomatized aureole of a carbonatite.
As an example, the Palabora complex ofSouth Africa has produced significant copper (aschalcopyrite,bornite andchalcocite), apatite, vermiculate along with lesser magnetite,linnaeite (cobalt),baddeleyite (zirconium–hafnium), and by-productgold,silver,nickel andplatinum.[11]
The generic stone found in thevideo gameMinecraft has been theorised to be carbonatite.[22]
