Author name: Jay J. Ague

Garnet bears witness to the importance of kinetics during metamorphism in its microstructural features, compositional zoning, and diffusional response to thermal events. Porphyroblastic textures carry quantitative signals of protracted nucleation and sluggish intergranular diffusion, key impediments to reaction progress that may result in crystallization under conditions well removed from equilibrium. Growth zoning in garnet reveals partial chemical equilibration with matrix minerals: intergranular transport keeps pace with garnet growth for some elements but not for others, leading to variable degrees and length scales of chemical equilibration. Partial relaxation of compositional zoning by intracrystalline diffusion is a sensitive and quantitative indicator of thermal history, constraining rates and timescales of peak metamorphic heating, processes of burial and exhumation, and retrogression on cooling.