The system CaO-SiO_2-CO_2-H_2O. II—The petrogenetic model

Abstract

In each of the three isobaric invariant reactions below 900° C on the vapor-saturated liquidus surface of the system CaO-SiO_2-CO_2-H_2O at 1 kilobar pressure, the vapor phase is extremely rich in H_2O. The effect of adding H_2O to the vapor phase on the temperatures of successive decarbonation reactions in the system CaO-SiO_2-CO_2 (involving the formation of wollastonite, spurrite and dicalcium silicate) is illustrated in the petrogenetic model. At 1 kilobar pressure the primary fields of spurrite and calcite on the vapor-saturated liquidus surface are separated by a field boundary extending down to a peritectic at 677° C. Examination of the melting reactions and the decarbonation reactions plotted in the petrogenetic model shows that the latter intersect the former with increasing pressure, and the first result of this at pressures above 1 kilobar is that a primary field for wollastonite replaces the peritectic on the vapor-saturated liquidus surface. Then, calcite and wollastonite can be co-precipitated from a liquid down to temperatures below 700° C. Applications to carbonatites are discussed briefly. If an igneous intrusion gives off enough H_2O during crystallization to flush CO_2 gas from the pores of a siliceous limestone in contact with it, partial melting of the limestone could conceivably occur. This possibility should be considered seriously in studies of thermal metamorphism.