Carbonatite

The magmatic origin of carbonatite was argued in detail by Swedish geologist Harry von Eckermann in 1948 based on his study of Alnö Complex. It was however the 1960 eruption of Ol Doinyo Lengai in Tanzania that led to geological investigations that finally confirmed the view that carbonatite is derived from magma. 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 phenomena. Historically, carbonatites were thought to form by melting of limestone or marble by intrusion of magma, but geochemical and mineralogical data discount this. For example, the carbon isotopic composition of carbonatites is mantle-like and not like sedimentary limestone. 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. ==Mineralogy==

Primary mineralogy is highly variable, but may include natrolite, 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 from marble except 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. == Geochemistry ==

. Specimen is 75 mm wide. Carbonatites are unusual igneous rocks composed predominantly of carbonate 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 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 ==

found near the carbonatite of the Oka Carbonatite Complex, Oka, Quebec 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. 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 Elk Creek Carbonatite, Nebraska, USA. In 2017, the discovery of a new carbonatite deposit was confirmed north-west of Prince George, British Columbia, in a region termed the "Rocky Mountain Rare Metal Belt". The volcano Ol Doinyo Lengai, in the East African Rift is the world's only active carbonatite volcano. Other older carbonatite volcanoes are located in the same region, including Mount Homa. == Economic importance ==

of apatite-rich carbonatite in cross polarised transmitted light. The sample is from Siilinjärvi apatite mine. Carbonatites may contain economic or anomalous concentrations of rare-earth elements (REEs), 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. The largest REE-carbonatite deposits are Bayan Obo, Mountain Pass, Maoniuping, and Mount Weld. 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, vermiculate along with lesser magnetite, linnaeite (cobalt), baddeleyite (zirconium–hafnium), and by-product gold, silver, nickel and platinum. == Popular culture ==

The generic stone found in the video game Minecraft has been theorised to be carbonatite. ==References==