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Carbonatite from Jacupiranga, Brazil. This rock is compound of calcite, magnetite and olivine



Carbonatite is a unique type of intrusive igneous deposit which is rich in calcium carbonate (calcite, CaCO3) and minerals of the carbonate class. Calcium carbonate is typically burned out of igneous materials by intense heat; the preponderance of calcite found in carbonatite therefore makes it a very unusual type of igneous formation. Carbonatite deposits may form as plugs, dikes, sills, or veins when carbonatite magma intrudes into host rock formations. Carbonatites are only observed to occur in continental plates and do not occur in oceanic plates or at plate boundaries. 
      Carbonatite magma differs greatly from the more prevalent silicate magma, and the two different types of magma are immiscible. Carbonatite rarely contains more than 10% silica by mass and may contain much less. Such magmas possess a very low viscosity because the lack of silica prevents extensive silicate polymerization. It has been shown experimentally that at a temperature of around 600° Celsius a magma possessing a high carbon dioxide (CO2) content will divide into separate, immiscible silicate and carbonatite magmas. 


      One possible method of carbonatite formation is that a parent magma originating in the mantle underneath a continental crust rises until it reaches the boundary between crust and mantle. The magma may then be of a higher density than the crustal plate and may be detained. Ferromagnesic silicates may crystallize out at these high temperatures although non ferromagnesic silicates remain liquid. Portions of the crustal plate, which contains plentiful carbonates, may also melt and be incorporated into the magma. When the ferromagnesic silicates are removed from the magma its density decreases due to the newly lessened relative concentration of iron and magnesium. The magma may then rise into the crust until it reaches zones of lower temperature (around 600° Celsius) where it may separate into silicate and carbonatite magmas. 


      Carbonatites contain atypically high concentrations of earth elements such as titanium (Ti), vanadium (V), manganese (Mn), iron (Fe), copper (Cu), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), barium (Ba), lanthanum (La), cesium (Ce), samarium (Sm), europium (Eu), lead (Pb), thorium (Th), and uranium (U). The elements sulfur (S), phosphorous (P), and fluorine (F) are also frequently present. High concentrations of magnetic materials may result in observations of unusual magnetic phenomena; atypically high concentrations of radioactive elements may produce unusual levels of radioactivity in the vicinity of a carbonatite. 


      Calcite (CaCO3), dolomite (CaMg(CO3)2), and ankerite (CaFe(CO3)2) are the most common minerals found in carbonatites. The carbonates strontianite (SrCO3), and magnesite (MgCO3); the sulfides pyrite (FeS2), molybdenite (MoS2), galena (PbS), chalcopyrite (CuFeS2), and sphalerite (ZnS); the oxides ilmenite (FeTiO3), hematite (Fe2O3), rutile (TiO2), and zircon (ZrSiO4); the sulfate barite (BaSO4); the phosphates monazite ((Ce,La)PO4), and fluorapatite (Ca5F(PO4)3); and the halide fluorite (CaF2) are other species which are often present. 


Genesis
Carbonatite lava at Ol Doinyo Lengai volcano, Tanzania.


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.

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