Silicate–Carbonate Liquid Immiscibility and Phase Relations in the System SiO2–Na2O–Al2O3–CaO–CO2 at 0·1–2·5 GPa with Applications to Carbonatite Genesis

Abstract

We present new experiments, combined with a re-evaluation of published data, to characterize the topology of the silicate–carbonate two-liquid solvus in the five-component system SiO2–Na2O–Al2O3–CaO–CO2 (SNAC + CO2). Conjugate liquid compositions have been determined for a wide range of pressures (0·1–2·5 GPa) and temperatures (1225–1700°C) as well as variable degrees of CO2 saturation. The expansion of the two-liquid field with increasing pressure and/or decreasing temperature, and the contraction of the two-liquid field for conditions where PCO2 < Ptotal is accurately presented for the first time. The shape of the two-liquid solvus suggests that alkali-rich carbonatites can have a range of SiO2 + Al2O3 contents down to very low values (<1 wt %), but that low-alkali or alkali-free immiscible carbonatites will always have SiO2 + Al2O3 contents greater than 10–15 wt %. The most commonly observed carbonatite rock compositions observed at the Earth’s surface all tend towards low contents of alkalis SiO2 and Al2O3 and would have fractionated silicate phases from the carbonatite parental melts, possibly associated with alkali loss to coexisting fluids. Our results also show that carbonate liquid exsolution can occur from a CO2-undersaturated (PCO2 < Ptot) silicate melt. Although the expanded high-pressure miscibility gap appears favourable for producing natural silicate melt compositions, a low-pressure (<1·0 GPa) magma chamber in the crust or perhaps in the shallow mantle below a rift provides the most likely environment for immiscibility to arise owing to the lower CO2 demand of the silicate magma. Unusual textures in some experiments, suggestive of a deformable liquid state for the CaCO3 phase, are conclusively shown to be characteristic of a non-quenchable, high-temperature polymorph of solid calcite. Similar calcite globules with this rounded appearance, which are also observed in some nephelinite lavas and mantle xenoliths, must be solid calcite and not immiscible liquids. This is consistent with the high SiO2 + Al2O3 requirement of low-alkali or alkali-free immiscible carbonate liquids.