When H2O is present along grain boundaries, the deformation processes responsible for plasticity in silicate mineral aggregates can deviate from what may be conventionally expected. Although a necessary component of understanding crustal deformation processes, there is no theoretical framework that incorporates grain boundary processes into polycrystalline quartz rheology. To address this issue, we carried out high-pressure and high-temperature deformation experiments on fine-grained quartz aggregates. Our study illustrates that grain boundary migration (GBM) through dissolution-precipitation (in the presence of an aqueous fluid) and grain boundary sliding (GBS) may act as accommodation mechanisms to prevent hardening from dislocation glide. GBM and GBS can relax incompatibilities resulting from an inadequate number of independent slip systems, plastic anisotropy between neighbouring grains, and non-planar grain boundaries together with grain boundary junctions. As demonstrated earlier in the literature, GBM may act as a recrystallization mechanism counteracting hardening, but also is a potential mechanism that allow H2O to enter in the quartz crystal (hydrolization) at the experimental time-scale. The above serial processes occur over a range of more than two orders of magnitude in grain size (∼3 to 200 μm) and explain a grain-size-insensitive stress exponent (n = 2) and low activation energy (Q = 110 kJ/mol). In the absence of a switch to grain size sensitive deformation mechanisms induced by grain size reduction, our results imply that only a modest weakening (∼5 times the strength of the protolith) is needed (or possible) to localize shear zones in the Earth's crust.