Abstract: The problems of characterizing inter-granular regions and of estimating rates of intergranular diffusion in metamorphic rocks are discussed. Inter-granular regions can be anhydrous, hydrated but under-saturated with H₂O, or saturated with H₂O, but only in the latter case can a free aqueous fluid phase be present. Estimates of intergranular diffusion coefficients (D^IGR) at 550°C derived from a variety of published experimental work, vary from ∼ 10⁻⁸ m² s⁻¹ for diffusion of species through an intergranular fluid film to ⩽ 4 × 10⁻²⁴ m² s⁻¹ for diffusion of SiO₂ or O in anhydrous grain boundaries in quartzite. Estimates of D^IGR for hydrated grain boundaries vary from ∼ 10⁻¹³ m² s⁻¹ to ∼ 10⁻²¹ m² s⁻¹; the concentration of H₂O in the grain boundaries and the identity of the diffusing species (generally unknown) may be important controlling factors, and there exists the possibility of a spectrum of values between these two extremes.

Using available kinetic data it is shown that a free aqueous fluid could never have been present in parts of the basement terrane of the Sesia Zone (Western Alps) during uplift from the eclogite facies, except possibly late in the cooling history. The breakdown of sodic pyroxene + quartz occurred in response to the localized infiltration of catalytic aqueous fluid, possibly over a time interval as short as 6–6000 a, and possibly under conditions remote from equilibrium. H₂O-present conditions during a dehydration reaction in metapelites of the Adula nappe (central Alps) could also have been of short duration. These examples are consistent with a model in which basement rocks at deep crustal levels are dry for long periods of time and in which the development of equilibrium mineral assemblages and microstructures generally occurs over relatively short periods of time under transitory fluid-present conditions (caused by devolatilization and/or infiltration).