METAMORPHIC REACTIONS AND METAMORPHIC FACIES
This book presents a detailed discussion of a few selected topics in metamorphism. Chapter I (Turner) is a historical account of the development of the facies concept. A new definition of metamorphic facies is formulated.
Chapters II to IV (Fyfe and Verhoogen), each of which ends with a summary in nontechnical language, deal respectively with certain thermodynamic and kinetic aspects of metamorphic-reactions. In Chapter II, the authors consider successively the magnitude of the free energy of metamorphic reactions, methods for computing approximate values of the entropies of silicates, a definition of the pressure variables in metamorphism, the role of surface films, and structural aspects of metamorphic mineralogy. The conclusion of Chapter III, which is devoted to a study of kinetics, is that reaction rates in systems containing water are generally such as not to lead to difficulties or inconsistencies in the interpretation of metamorphic facies. The role of water in metamorphic reactions is stressed again. This leads to a closer study of systems containing water, which are discussed in Chapter IV. Some properties of water are evaluated, such as its dielectric constant and ionic product, at high pressures and temperatures. A model is set up by which thermodynamic functions for hydration-dehydration reactions can be evaluated. There follows a discussion of solubility in aqueous systems and of variations of pH under natural conditions.
In Chapter V, Fyfe and Turner correlate field and experimental data on the stability of critical mineral assemblages in metamorphic rocks. They conclude that there are at present no major inconsistencies between field and experimental data. Metamorphic facies, defined by mineralogical and field criteria, can still be interpreted in terms of a few intensive physical variables.
The role of water and heat in metamorphism is considered in Chapter VI (Fyfe and Verhoogen). Regional metamorphism is not a normal phenomenon, in the sense that it would occur wherever rocks are buried to sufficient depth. On the contrary, it appears that regional metamorphism can occur only where the heat flow is notably increased and where water, probably of juvenile origin, is abundant.
Chapter VII (Turner) is a revision of individual metamorphic facies. In the light of new experimental and mineralogical data the limits of some facies and subfacies are redefined, and a few new divisions are proposed.