Carbonatites: Classification, Sources, Evolution, and Emplacement

Carbonatites are igneous rocks formed in the crust by fractional crystallization of carbonate-rich parental melts that are mostly mantle derived. They dominantly consist of carbonate minerals such as calcite, dolomite, and ankerite, as well as minor phosphates, oxides, and silicates. They are emplaced in continental intraplate settings such as cratonic interiors and margins, as well as rift zones, and rarely on oceanic islands. Carbonatites are cumulate rocks, which are formed by physical separation and accumulation of crystals that crystallize from a melt, and their parental melts form by either (a) direct partial melting of carbonate-bearing, metasomatized, lithospheric mantle producing alkali-bearing calciodolomitic melts or (b) silicate-carbonate liquid immiscibility following fractional crystallization of carbonate-bearing, silica-undersaturated magmas such as nephelinites, melilitites, or lamprophyres. Their emplacement into the crust is usually accompanied by fenitization, alkali metasomatism of wallrock caused by fluids expelled from the crystallizing carbonatite. Carbonatites are major hosts of deposits of the rare earth elements and niobium, and the vast majority of the global production of these commodities is from carbonatites. Carbonatites are igneous rocks formed from carbonate-rich magmas, which ultimately formed in Earth's upper mantle. Carbonatites are associated with economic deposits ofmetals such as the rare earth elements and niobium, which are essential in high-tech applications. There are more than 600 carbonatites in the geological record but only one currently active carbonatite volcano, Oldoinyo Lengai in Tanzania.