Mixtures of crystalline CaCO3, Ca2SiO4, CaSiO3, quartz, and spurrite were reacted between 7 and 27 kbar. The results, combined with other published data, provide a PT projection for the system CaO−SiO2−CO2 from 1 bar to 30 kbar, and a series of isobaric liquidus diagrams giving the changes in composition of eutectic and peritectic liquids in the univariant reactions as a function of pressure. The assemblage calcite + quartz dissociates producing wollastonite + CO2 at pressures below an invariant point at 18.5 kbar, 1,325°C; at this point, the univariant dissociation reaction meets the fusion curve for wollastonite + CO2 = quartz + liquid; at higher pressures, calcite + quartz melts incongruently to liquid + CO2, and there is in addition a eutectic reaction between calcite, wollastonite, and quartz. The thermal barrier on the liquidus associated with the congruent melting of larnite in the system CaO–SiO2 is eliminated by solution of a few percent CO2 at pressures greater than about 1 kbar; the CO2 causes expansion of the liquidus fields for calcite and wollastonite until they meet and exclude both spurrite and larnite from the CO2-saturated liquidus field boundary. The liquidus diagrams show limiting conditions for coprecipitation of calcite and wollastonite in carbonatite magmas. Liquids produced by partial melting of siliceous limestones (±wollastonite) at pressures above about 15 kbar have compositions near 50% CaCO3, 50% CaSiO3. There is a good prospect that some subducted pelagic limestone might escape dissociation and melting and be carried to considerable depths for long-term storage of carbon in the mantle either as aragonite (reacting to dolomite or magnesite), or as diamond if the carbonate is reduced.