The significance of grain-scale stresses in the kinetics of metamorphism

Abstract

During metamorphism phases grow and dissolve dominantly along grain boundaries. Bulk-diffusion models for the development of metamorphic textures often assume that diffusion is homogeneous in regions of each phase, rather than being localised along phase boundaries. Under isotropic stress, chemical potentials of components involved in reactions are likely to be buffered along grain boundaries between phases, thus inhibiting diffusion and growth. Motive forces must exist which overcome this and it is proposed here that local heterogeneous stresses will be important. A mathematical theory for stress-controlled growth is derived. This theory predicts that the growth rates of two phases along their mutual interface will be linked in a quantifiable way. It forms a basis for considering the important effects of stresses induced by local volumetric changes during metamorphism, and it provides a link between these effects and those related to pressure-solution in deforming rocks.