A coupled model for the evolution of size and chemical composition of crystals undergoing diffusion and its geological implications

The compositional record in minerals formed by element partitioning is the basis of many tools used in geochemistry and petrology such as geothermometry, geobarometry and geochronology. Compositional resetting in response to changes in ambient conditions is considered largely in the context of chemical (such as diffusion, dissolution, precipitation or growth) or mechanical deformation processes. Here we develop a model to show that lattice strain resulting from atomic size mismatch during chemical exchange may lead to recrystallization of grains and thus, erasure of previous records. The model includes the processes of diffusion, solidification and melting, elastic deformation and dislocation motion. Analysis of the parameters of the model shows that crystals undergoing chemical change may be divided into two groups. One group of crystals is stable and grows continuously while the other group of crystals may begin to shrink because the dislocation density, resulting from internal lattice strain due to element partitioning, reaches a critical value. These grains eventually recrystallize. The results of this study have implications for the understanding of closure behavior of geothermometers, geobarometers and geochronometers as well as for the interpretation of results of diffusion chronometry of rocks. Such processes are likely to play an important role in the process of nucleation of minerals. Coupled with models of mechanical evolution, the results of this study carry implications for the evolution of grain size, rheology and ultimately, the behavior of lithospheric plates.