Exposures on Holsnoy island (Bergen Arcs, Norway) indicate fluid infiltration through fractures into a dry, metastable granulite, which triggered a kinetically delayed eclogitization, a transient weakening during fluid-rock interaction, and formation of shear zones that widened during shearing. It remains unclear whether the effects of grain boundary-assisted aqueous fluid inflow on the duration of granulite hydration were influenced by a diffusional hydrogen influx accompanying the fluid inflow. To better estimate the fluid infiltration efficiencies and the parameter interdependencies, a 1D numerical model of a viscous shear zone is utilized and validated using measured mineral phase abundance distributions and H2O-contents in nominally anhydrous minerals of the original granulite assemblage to constrain the hydration by aqueous fluid inflow and diffusional hydrogen influx, respectively. Both hydrations are described with a diffusion equation and affect the effective viscosity. Shear zone kinematics are constrained by the observed shear strain and thickness. The model fits the phase abundance and H2O-content profiles if the effective hydrogen diffusivity is approximately one order of magnitude higher than the diffusivity for aqueous fluid inflow. The observed shear zone thickness is reproduced if the viscosity ratio between dry granulite and deforming, reequilibrating eclogite is similar to 10(4) and that between dry granulite and hydrated granulite is similar to 10(2). The results suggest shear velocities <10(-2) cm/a, hydrogen diffusivities of similar to 10(-13 +/- 1) m(2)/s, and a shearing duration of <10 years. This study successfully links and validates field data to a shear zone model and highlights the importance of hydrogen diffusion for shear zone widening and eclogitization. Plain Language Summary At zones of colliding tectonic plates, rocks of the lower continental crust are sometimes buried down into the Earth's mantle. Due to increasing pressures and temperatures during deep burial, the rock's minerals transform. Hence, to maintain a more stable assemblage, the system continuously reequilibrates via metamorphic mineral reactions and the rock properties, such as strength and density, change. Occasionally, dry crustal rocks can resist such processes and are transported to great depths without any transformation and reequilibration. If fluid enters such a metastable system, the rock properties can change, transforming transiently from rigid to weak and enabling ductile deformation. Based on one such example from Western Norway, this work studies the effects of H2O inflow and hydrogen influx on the rock viscosity with the help of a 1D mathematical model, where measurements from the rock were used to constrain and validate the model. The results show the most likely conditions that the rock faced in terms of shear velocity, fluid diffusivity, and the duration of deformation.
