Subsolidus and liquidus phase relationships in the system CaO-SiO_2-CO_2 to 30 kbar with geological applications

Abstract

Mixtures of crystalline CaCO_3, Ca_2SiO_4, CaSiO_3, 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-SiO_2-CO_2 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 + CO_2 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 + CO_2 = quartz + liquid; at higher pressures, calcite + quartz melts incongruently to liquid + CO_2, 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 lamite in the system CaO-SiO_2 is eliminated by solution of a few percent CO_2 at pressures greater than about 1 kbar; the CO_2 causes expansion of the liquidus fields for calcite and wollastonite until they meet and exclude both spurrite and lamite from the CO_2-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% CaCO_3, 50% CaSiO_3. 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.

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