Processes of Crustal Carbonatite Formation by Liquid Immiscibility and Differentiation, Elucidated by Model Systems

Abstract

Experimental studies on several silicate–carbonate joins provide a framework in the system CaO–Na_2O–(MgO + FeO)–(SiO_2 + Al_2O_3) (+ CO_2) which illustrates possible processes for the formation of carbonatites. The two key features are the silicate–carbonate liquidus surface, and the miscibility gap liquidus surface. Crystallizing parental carbonated silicate melts may reach a silicate–CO_2 eutectic, a silicate–carbonate field boundary, or a miscibility gap. Some hydrous carbonated silicate melts may bypass the high-temperature miscibility gap and reach the silicate–carbonate field boundary. Immiscible carbonate-rich liquids in model systems simulating magmatic conditions tend to be concentrated near calciocarbonatite compositions (< ∼80% CaCO_3; e.g. nepheline sövite), but may be more alkalic from silicate parents with higher Na/Ca values. An immiscible carbonate-rich liquid separating from the high-temperature parent silicate liquid will cool with the precipitation of silicates only, until it reaches the silicate–carbonate field boundary, where it is capable of precipitating carbonate minerals, which can form carbonatite cumulates. Some parents may reach this boundary by direct crystallization, but most probably traverse the miscibility gap. Along this field boundary, the coprecipitation of calcite drives the liquid toward residual alkali-rich compositions. The carbonate liquidus (>85% CaCO_3) is a ‘forbidden volume’ for magmas. Vapor loss from carbonatite magma can introduce alkalis into country rocks, but this does not cause alkali depletion of magma; calcite precipitates to maintain the magma composition. Hydrous magnesiocarbonatite magmas can precipitate cumulate sövites.