Carbonatite () is a type of intrusive or extrusive igneous rock defined by mineralogic composition consisting of greater than 50% 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. It seems that there has been a steady increase in the carbonatitic igneous activity through the Earth's history, from the Archean eon to the present.
Nearly all carbonatite occurrences are intrusives or subvolcanic intrusives. This is because carbonatite lava 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 the Earth's history.
Carbonatite liquid compositions are significantly more alkaline than what is preserved in the fossil carbonatite rock record as composition of the melt inclusions shows.
Only one carbonatite volcano is known to have erupted in historical time, the active Ol Doinyo Lengai volcano in Tanzania. It erupts with the coolest lava in the world, at . The lava is natrocarbonatite dominated by nyerereite and gregoryite.
Genesis
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, barite, 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. Additionally, carbonatites may be either enriched in magnetite and apatite or rare-earth elements, fluorine and barium.
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 East African Rift 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 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.
Geochemistry
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.