The System CaO-CO_2-H_2O and the Origin of Carbonatites

Abstract

The ternary isobaric (TX) prism for the system CaO–CO_2–H_2O was determined at 1,000 bars pressure between 600° C and 1,320° C. At this pressure, calcite melts incongruently at 1,310° C, portlandite (Ca(OH)_2) melts congruently at 835° C, a binary eutectic exists between calcite and portlandite at 685° C, melting begins at 740° C on the join calcite-water and the univariant (isobaric invariant) equilibria lime-calcite-portlandite-liquid and calcite-portlandite-liquid-vapour occur at 683° C and 675° C, respectively. The latter is the minimum liquidus temperature in the TX prism, and the composition of this liquid is 65CaO, 19CO_2, 16H_2O (in weight per cent). PT curves were determined for several univariant equilibria. In the binary system CaO-H_2O, four univariant curves meet at an invariant point, at 810° C and 100 bars pressure. Portlandite dissociates only at pressures below this point. The minimum liquidus temperature in the ternary system varies between 685° C and 640° C in the pressure interval 27 bars to 4, 000 bars. Liquids in the system are regarded as simplified carbonatite magmas in which CaO represents the basic oxides, and CO_2 and H_2O the volatile constituents. The liquids have low viscosity as indicated by the rapid attainment of equilibrium and the observation that crystal settling takes place in 15-min runs. The existence of such liquids at moderate temperatures through a wide pressure range leaves little reason to doubt a magmatic origin for those carbonatites which appear to be intrusive. Differentiation could occur in multicomponent magmas by separation of the successive liquid fractions produced by crystallization of calcite, dolomite, and siderite. The determined phase relations do not favour an origin by gas transfer. The results also suggest that partial melting of limestones is likely at igneous contacts, and that impure limestones may be partially melted during high-grade regional metamorphism.