Processes of crustal carbonatite formation by liquid immiscibility and differentiation, elucidated by model systems

Experimental studies on several silicate-carbonate joins provide a framewor k in the system CaO-Na2O-(MgO + FeO)-(SiO2 + Al2O3) (+ CO2) which illustrat es possible processes for the formation of carbonatites. The two key featur es are the silicates-carbonate liquidus surface, and the miscibility gap li quidus surface. Crystallizing parental carbonated silicate melts may reach a silicate-CO2 eutectic, a silicate-carbonate field boundary, or a miscibil ity gap. Some hydrous carbonated silicate melts may bypass the high-tempera ture miscibility gap and reach the silicate-carbonate field boundary. Immis cible carbonate-rich liquids in model systems simulating magmatic condition s tend to be concentrated near calciocarbonatite compositions (< similar to 80% CaCO3; e.g. nepheline sovite), but may be more alkalic from silicate p arents with higher Na/Ca values. An immiscible carbonate-rich liquid separa ting from the high-temperature parent silicate liquid will cool with the pr ecipitation of silicates only, until it reaches the silicate-carbonate fiel d boundary, where it is capable of precipitating carbonate minerals, which can form carbonate cumulates. Some parents may reach this boundary by direc t crystallization, but most probably traverse the miscibility gap. Along th is field boundary, the coprecipitation of calcite drives the liquid toward residual alkali-rich compositions. The carbonate liquidus (>85% CaCO3) is a 'forbidden volume' for magmas. Vapor loss from carbonatite magma can intro duce alkalis into country rocks, but this does not cause alkali depletion o f magma; calcite precipitates to maintain the magma composition. Hydrous ma gnesiocarbonatite magmas can precipitate cumulate sovites.