Phase relations of a Carbonated High-CaO Nephelinite at 0.2 and 0.5 GPa

Experimental results for a carbonated, high-CaO evolved nephelinite at 0.2 and 0.5 GPa at 900–1040°C are presented. Immiscible carbonate and silicate liquids are in equilibrium with a variety of crystal phases (clinopyroxene, nepheline, melilite, wollastonite, perovskite, melanite garnet, titanite, Fe-rich spinel, apatite and calcite). Immiscible carbonate liquids produced are high CaO and low alkali in composition. They contain significant Sr, Ba, F, Cl, P and are considered good analogues for natural sövite parent magmas. This study documents for the first time two-liquid + melilite phase assemblages in the 0.2 GPa runs, suggesting that silicate-carbonate liquid immiscibility could be important in generating sövites associated with evolved nephelinites, and possibly melilite nephelinites. The two-liquid field at 0.5 GPa is significantly wider than is observed in synthetic model systems at 0.5 GPa, being similar in size to the 1.0 GPa model system solvus. Results from the experiments are consistent with and confirm the geometrical arrangement of pseudoternary liquidus phase field boundaries described previously for model systems. The recognition of primary calcite as a sub-liquidus phase in the experiments has important relevance to calcite-bearing, evolved nephelinites and melilitites, which are commonly observed in silicate–carbonatite complexes.