A Natural Example of the Kinetic Controls of Compositional and Textural Equilibration

Abstract

Textural and compositional disequilibrium have been documented in both regional metamorphism (Griffin, 1971; Tracy, 1982) and contact metamorphism (Hollister, 1969; Loomis, 1976). However, the significance of kinetics in controlling assemblages, compositions, and textures has been treated by relatively few authors (e.g., Loomis, 1976; Foster, 1982). Among the variables that control the rates of metamorphic reactions are the rate of supply (or removal) of heat, the rate of supply of matter (diffusion control), and the rate of reactant dissolution and/or product absorption during recrystallization (interface control). In the initial high-temperature stages of contact metamorphism (i.e., until cooling begins), reaction rates are more likely to be interface than diffusion controlled because both thermal energy input and diffusion coefficients will be large at the initially higher temperatures and therefore may not be rate limiting. As temperatures fall, however, heat flux and diffusion rates may become increasingly important. The rate-limiting factor will control the nature and extent of compositional and textural equilibrium. Thus, diffusion control will lead to strict local equilibrium (Fisher, 1978) whereas interface control may lead to a partial approach to equilibrium on a larger scale (Loomis, 1976).