Metamorphic Reactions in Carbonate Rocks

Abstract

The metamorphism of quartz-bearing carbonate rocks provides illuminating and readily understood examples of metamorphic reactions. Siliceous dolomites and limestones are very widespread rocks. BOWEN (1940) made a systematic study of the sequence of reactions occurring in such rocks at some given pressure in response to rising temperature. These reactions liberate CO2 but, since water is present in the pores before metamorphism, it is not permissible to regard temperature and pressure of CO2 as the only factors of metamorphism. The pressure of the fluid phase present during metamorphism is equal to the sum of the partial pressures of CO2 and H2O. Besides the fluid pressure, the ratio of the two partial pressures, or the mole fraction of either CO2 or H2O, has to be taken into account. Therefore, all reaction equilibria involving carbonates are bivariant. This is true, even if H2O does not appear in the reaction equation, because H2O is always present in the fluid phase occurring in the rocks. In this chapter, reactions in rocks composed of quartz, dolomite, calcite or magnesite are discussed, as they proceed in order of increasing grade of metamorphism.