Carbonatites

Carbonatites are commonly defined as magmatic rocks with high modal abundance of carbonate minerals (>50 wt%) and geochemistry typified by high abundances of Sr, Ba, P and the light rare-earth elements (LREE) They have been subdivided on the basis of their dominant modal carbonate mineral, such as calcite or dolomite carbonatites and on their corresponding major element geochemistry with Mg Ca, Fe- and REE-carbonatite:

Calcite-carbonatite - where the main carbonate is calcite. If the rock is coarse- grained it may be called sövite; if medium to fine-grained, alvikite
Dolomite-carbonatite - where the main carbonate is dolomite. This may also be called beforsite.
Ferrocarbonatite - where the main carbonate is iron-rich.
Natrocarbonatite - essentially composed of sodium, potassium, and calcium carbonates

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Carbonatite classification diagram; after (Woolley and Kempe 1989). Note: ferrocarbonatite can also be rich in REE.


In parallel, a process-related classification would divide them into two groups: primary carbonatites and Carbothermal residua (Mitchell 2005). In this scheme, primary carbonatites can be further divided into groups of magmatic carbonatites associated with nephelinite, melilitite, kimberlite, and specific mantle-derived silicate magmas, formed by partial melting, whereas carbothermal residua carbonatites form as low-temperature fluids rich in CO2, H2O, and fluorine.

Carbonatites characteristically occur in close association with alkalic silicate igneous rocks, either in individual complexes or in a regional association within magmatic provinces. Associated silicates are usually alkaline ultramafic rocks: pyroxenites or nephelinites (and/or ijolites) generally, but may also include more evolved types such as phonolites and nepheline syenites.

The three main theories for the origin of carbonatites are essentially:

1. Residual melts of fractionated carbonated nephelinite or melilitite
2. Immiscible melt fractions of CO2-saturated silicate melts
3. Primary mantle melts generated through partial melting of CO2-bearing peridotite

Combinations of these three theories are also popular; for example carbonatite liquids generated by deep melting of carbonated eclogite in the upper mantle infiltrate overlying peridotite to produce silica under-saturated carbonate-bearing melts, which then penetrate the crust and evolve or un-mix (Yaxley and Brey 2004).
Carbonatites have also been considered to be generated in the lithospheric mantle as partial melts rising rapidly above a hot ascending mantle plume. If these mantle carbonate melts stall, for example owing to thermal death, they generate carbonate-melt metasomatism in the mantle. As the much hotter center of the plume approaches, melting is induced in the metasomatic horizon and results in generation of the carbonatite melts that are observed on the surface. Although the plume model is quite attractive, recent recognition of strong and repeated lithospheric controls in the compilation of global carbonatite ages are thought to argue against a direct connection to mantle plumes (Woolley and Bailey 2012).

The atomic structures of carbonate melts have been little studied in comparison to the structure of silicate melts, but are fundamental in controlling their physical and chemical behavior in natural systems. Carbonate melts are ionic liquids consisting of carbonate CO32- molecular anions and metal cations that interact principally due to coulombic interactions and are thus very different from silicate melts, which have network structures characterized by polymerization (Mysen 1983). Ionic carbonate melts have been considered to be structureless with no defnite association between metal cations and carbonate molecules. However the combined evidence from phase relations of carbonates, the solubility of metals in carbonate liquids, and the spectroscopy of carbonate glasses and atomic simulations, suggests that carbonate liquids have structure at scales larger than their component molecular groups.


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Honey-yellow Niocalite crystals and dark Perovskite crystals in a sövite from Oka. Quebec, Canada



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Jacupiranga sövite, Brasil. Calcite (withe), Magnetite (black), olivine (green).



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Ca-carbonatite (sövite) from Magnet Cove, Hot Spring County, Arkansas, USA.



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Pyrochlore sövite (dark) from Oka. Quebec, Canada



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Sövite from da Oka. Quebec, Canada



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Carbonatitic veins in a ijolite. Oka. Quebec, Canada





Bibliography



• Geochemistry of oceanic carbonatites compared with continental carbonatites: mantle recycling of oceanic crustal carbonate. Kaj Hoernle (2002), Contrib Mineral Petrol (2002)
• Eric A.K.Middlemost (1985): Magmas and Magmatic Rocks. Longman, London
• Ron H. Vernon (2004): A pratical guide to rock microstructure. Cambridge editore
• K.G.Cox, J.D.Bell & R.J Pankhurst (1979): The interpretetion of igneous rocks. George Allen&Unwin editori.
• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori
• Streckeisen, A., (1979): Classification and nomenclature of volcanic rocks, lamprophyres, carbonatites, and melilitic rocks: Recommendations and suggestions of the IUGS Subcommission on the Systematics of Igneous Rocks. Geology. The Geological Society of America. Boulder, Co. Vol.7, p.331–335.
• Woolley, A.R., kempe, D.R.C., (1989): Carbonatites: nomenclature, average chemical compositions and element distribution. In: K. Bell (Editor), Carbonatite Genesis and Evolution. Unwin Hyman, London. p.1–14.
• Kjarsgaard I.H. (1998): Rare Earth Elements in Sovitic Carbonatites and their Mineral Phases. J.Of Petrology 39, vol11-12. 2105–2121.
• Kaj Hoernle; George Tilton; Mike J. Le Bas; Svend Duggen; D. G. Schoenberg (2002): Geochemistry of oceanic carbonatites compared with continental carbonatites: mantle recycling of oceanic crustal carbonate. Contrib Mineral Petrol 142: 520–542.