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We have carried out experimental studies of the rate at which water is consumed by hydration reactions under mid-crustal conditions. Both pelitic and mafic assemblages are susceptible to extensive hydration in the laboratory on a time scale of weeks to months. Quantitative hydration rate determinations were made using enstatite–oligoclase ± diopside powder mixtures and a natural hypersthene hornfels. Under all conditions, the main hydration product was saponite clay with variable amounts of talc according to the initial proportion of enstatite to plagioclase. The experiments yield consistent rates for water consumption of around 10 −8  g H 2 O per m 2  of mineral surface per second at 400°C and 300 MPa (3 kbar). Additional experiments were run at 300°C and 500°C and at lower pressures (40 MPa), as well as with NaCl; rates appear to be faster at higher temperatures and in the presence of salt, but slower at low pressure. Comparison of powder and core experiments on the natural hornfels indicates that it is primarily the outer surface of the rock core that is available for hydration, with only minor infiltration along grain boundaries. The hydration rates reported here appear to be typical for the types of lithology that demonstrate moderate to high degrees of retrogression along joints and deformation zones in crystalline rocks of the upper crust. Assuming that the surface roughness and damage effects in a natural fault zone are comparable with those of the materials used here in the experiments, the measured hydration rates imply that a natural fracture in crystalline rocks of the middle crust that becomes filled with a water film 0·2 mm in thickness will dry out through incorporation of the water into hydrous phases on a time scale of the order of 10–100 years. This clearly implies that free water has only a short residence time in crystalline rocks of the middle crust or deeper, provided they have cooled below their original temperature of formation and therefore have the potential to undergo retrograde hydration. We infer that the strength of retrograde shear zones in the middle to lower crust will fluctuate through time, with episodes of water infiltration resulting in short periods of water weakening before the water is fully consumed and the rocks become stronger once more.

Rates of Retrograde Metamorphism and their Implications for the Rheology of the Crust: an Experimental Study