Carbonatite - Geochemistry

Geochemistry

Carbonatite is composed predominatly of carbonate minerals and extremely unusual in its major element composition as compared to silicate igneous rocks, obviously because it is composed primarily of Na2O and CaO plus CO2.

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.

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