Carbonatites sometimes 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. Geochemically, the carbonatites are dominated by incompatible elements (Ba, Cs, Rb) and depletions in compatible elements (Hf, Zr, Ti).
The History Says
Only one carbonatite volcano in historical time is known to have erupted i.e. Ol Doinyo Lengai in Tanzania. It erupted the lowest temperature lava ever in the world, at only 500-600°C (930-1,100°F). The lava is dominated by natrolite and trona, and sodic calcite.
Carbonatites are composed of calcite (or dolomite) of the igneous origin. Minerology defines it as intrusive igneous rocks which is greater than 50% carbonate (CO3-bearing) minerals and less than 10% SiO2. Sometimes carbonatites are confused with marble, and it requires geochemical verification.
Almost all the carbonatite occurrences are either intrusives or subvolcanic intrusives. The reason for it is that the carbonatite lava flows dissolve quickly in the atmosphere. It has been poorly preserved throughout Earth's history.
There are 330 known carbonatite localities on Earth, mostly shallow intrusive bodies of calcite-rich igneous rock which are in the form of valcanic necks, dikes, and cone-sheets. These forms usually occur in association with larger intrusions of alkali-rich silicate igneous rocks.
In historical time, only one carbonatite volcano is known to have erupted. It is Ol Doinyo Lengai in Tanzania. It erupted at the lowest temperature lava in the world, i.e. at 500-600 degrees Celsius (930-1,100 degrees F). The lavas are dominated by natrolite and trona, sodic calcite.
Models of Carbonatites Formation
Carbonatites are rare, and is formed by unusual processes and from unusual source rocks. Few of them are as follows:
- Direct generation at a very low degree partial melts in the mantle and the melt differentiation.
- Liquid immiscibility between carbonate and silicate melt.
- Peculiar, extreme crystal fractionation.
Evidences for the above processes do exists, but the key is that these are unusual phenomenon. In past, it was thought that carbonatites are formed by melting of limestone or marble by the intrusion of magma, however geochemical and mineralogical data discount this.
Deposits of Carbonatites
The deposits of carbonatites exist in the following places:
- Oka, Quebec
- Iron Hill and Gem Park, Colorado
- Magnet Cove, Arkansas
- St. Honore, Quebec
- Mountain Pass, California
- Phalaborwa, South Africa
- Jacupiranga, Brazil
- Kovdor, Russia
- India
- Mount Weld and Mud Tank, Australia