Diffusion kinetics in minerals: Principles and applications to tectono-metamorphic processes
Diffusion is the process by which atoms or ions or ionic species migrate within a medium in the absence of a bulk flow. Diffusion in solids has been a subject of interest in the fields of solid state science (physics, chemistry and metallurgy) for nearly a century, starting with the work of Einstein on the relationship between random atomic movement and diffusion process. The phenomenological study of diffusion began even 50 years earlier, with the empirical formulation of Fick on the relationship between the diffusion flux of a component and its concentration gradient.
The subject of diffusion in the solids may be subdivided into volume, grain boundary and surface diffusion. The last topic has, however, received very little attention in the study of geological processes. The study of grain boundary diffusion is important to the understanding of many metamorphic processes including the problems of mass transport, fluid/rock interactions, thermal history and crystal growth. The interested reader is referred to Joestein (1991) for an excellent review of the subject of grain boundary diffusion and its applications to geological problems.
Diffusion controlled processes within a mineral preserve important records of the thermal and physico-chemical history of the host rocks. Volume diffusion, that is diffusion through the crystal lattices, affects development of compositional zoning in minerals, ordering of atoms in nonequivalent crystallographic sites of a mineral, formation and coarsening of exsolution lamellae, and retention of isotopic characteristics in minerals that can serve as quantitative chronometers in their thermal and growth history.