Melting Relationships in the System CaO-MgO-CO_2-H_2O, with Petrological Applications

Abstract

Phase relationships on the vapor-saturated liquidus surface in the system CaO-MgO-CO_2-H_2O have been deduced from data in the systems CaO CO_2-H_2O, CaO-MgO-CO_2, and MgO-CO_2-H_2O, and from preliminary experiments in the quaternary system. These are illustrated in composition tetrahedra, and in isobaric sections through the petrogenetic model. The univariant PT curve for the beginning of melting lies between 625° C and 600° C in the pressure range 10 bars to 4 kilobars, in the presence of a vapor phase rich in H_2O. The curve is divided into three sections by two invariant points, each section having a different primary magnesian phase involved in the melting reaction. Periclase occurs on the low-pressure section (less than about 1 kilobar), and with increasing pressure first brucite and then dolomite become stable on the liquidus. The pressure of the second invariant point, above which dolomite is stable on the liquidus, is not known. The effect of FeO as an additional component is considered. Processes of crystallization resulting from changes in temperature, in pressure, and in the composition of the vapor phase are discussed. These processes are applied to the crystallization and differentiation of carbonatite magmas, and the reverse processes involving fusion are applied to the metamorphism of dolomites. Crystallization differentiation of a carbonatite magma could produce the sequence of intrusion observed at some carbonatite complexes: calcitic sövite, followed by ankeritic sövite, and finally sideritic carbonatite. Partial melting may occur during the thermal metamorphism of dolomites, but melting is unlikely during regional metamorphism.

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