Abstract A closed-system diffusion model has been developed for describing homogenization of spherical crystals by chemical interdiffusion. With the added knowledge of interdiffusion coefficients, it is possible to model temperature-time ( T − t) histories of particular rocks. The model has been used to describe the homogenization of almandine-rich garnets in a polymetamorphic terrane in southwestern New Hampshire. The results of the analysis indicate that the calculated cooling rates are controlled by the deformation history of the area, in particular the folding of isotherms. The proposed model is of general applicability to problems of cooling in metamorphic and subsolidus igneous rocks.