Kinetics metamorphic processes

The first step is to formulate the necessary kinetic equations, and work out values for the appropriate kinetic coefficients from a combination of experiment, theory, and observations on natural rocks. There are several approaches we might use, but the formalism of nonequilibrium thermodynamics is particularly convenient for dealing with metamorphic processes, because it emphasizes the chemical potentials of components, which are often easier to evaluate thcin species concentrations. This paper reviews some ways in which the approach can be used to model the kinetics of metamorphic processes which can be studied in outcrop or thin-section it will not consider regional processes such as heat flow or fluid convection (eg. Reed, 1970). 

TABLE 1. PRINCIPAL KINETIC EQUATIONS IN METAMORPHIC PROCESSES Rate of Diffusion of Component i... 

The relations and coefficients in Tables 1, 2 and 3 can be used to estimate the kinetics of many metamorphic processes, but the job will be much simpler if we make one additional assumption that metamorphic processes closely approximate a steady state. Fisher (1973, p. 910-911 1975, p. 115) showed that diffusion will automatically tend to shift potentials toward values such that the flux differences at every point in a structure just balance the local reactions, establishing a steady state. If this shift is rapid enough relative to growth of the overall structure, most of the diffusion will be driven by the steady-state potentials, and the small amount of mass transfer involved in attaining the steady state may be neglected. 

Another possibility is that one of the reactants is particularly mobile, this is apparent in certain solid—gas reactions, such as the reduction of NiO with hydrogen, which is a well-characterized nucleation and growth process [30,1166]. Attempts have been made to use the kinetic equations developed for interface reactions to elucidate the mechanisms of reactions between the crystalline components of rocks under conditions of natural metamorphism [1167,1168]. 

From the above list, one can see that kinetics of complex heterogeneous reactions are intimately related to important geological processes such as igneous rock formation, volcanic eruptions, and metamorphism. 

Another proof of the importance of temperature is the fact that there is often a strict relationship between the run of isovols and the run of isotherms in deep profiles, both being influenced no doubt by the varying thermal conductivity of the different rocks. The strong influence of temperature on the rank of coal is obvious in the case of contact-metamorphic coals, whose rank increases distinctly when approaching the intrusive body. Apart from these geological observations, all experiments on artificial coalification have shown that temperature is the decisive factor in the coalification process. Thermodynamic and reaction kinetic considerations (9) also support this opinion. 

Factors similar to those in sedimentary processes are involved in metamorphic reactions. The susceptibility of a cation in a mineral to dissolution and recrystallization in a new phase depends on the relative stability of the ion in each crystal structure and the ease of removal of the ion from the structure. Thus, kinetic and thermodynamic factors again determine the fractionation of... 

In analyzing the physicochemical conditions of metamorphism of complex heterogeneous stratified sequences it is necessary to take into account the possible existence of metastable but kinetically stable mineral associations, wide variations in the composition of the fluids in individual parts of the pile being metamorphosed, equilibria of mosaic character, and the presence of systems closed to water or with an insufficiency of it. These particulars require preliminary consideration of several controversial problems of the theory of metamorphism, especially reactions in the case of complex fluids and in the case of different pressures on the solid phases and fluid analysis of the processes in which H2O and COj take part at high pressures, when the properties of these very important volatile components become different, also seems to be very important. 

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