The viscosity (η) of a geofluid dictates its roles within the Earth, such as migration in subduction zones and volcanism. High pressures and temperatures at depth influence visocity (η). The falling sphere method is effective to measure viscosity at high-pressures for geofluids that form solids at ambient temperature. A typically metallic sphere is placed atop a solid sample, which is then compressed to high-pressure and later melted by heating. This method is more challenging for geofluids that do not form solids at ambient temperature. A diamond-anvil-cell (DAC) is often used to contain such geofluids, but requires a small sample chamber and that the sphere either falls parallel to the diamond culet faces or rolls along one face. This geometry produces complicated drag effects on a sphere fall and additional frictional forces for a roll. The sphere may also adhere to chamber surfaces, preventing its fall/roll. To circumvent these issues, in this study, we quantify the viscosity of a geofluid (H2O) at pressures <2.5 GPa using the Brownian motions of suspended particles in DAC. Previous high-pressure efforts used particles of polystyrene, which are unstable at ≥300°C, or silica, but only at ambient temperatures. Such temperatures are relatively low for hydrothermal to supercritical geofluids. We tested quartz particles (∼1–2 µm diameter) with heating, as quartz does not significantly dissolve/melt until ≥600°C. Although three times denser than water, the particles remained suspended and displayed Brownian motions for long timescales at temperatures ≤200°C. The measured viscosities are relatively high due to drag from the culets and particle–particle interactions. Regardless, our measured pressure-effect on viscosity shows excellent agreement with the standard reference for water. After correcting for the drag, the η are very low (< 2 mPa s) highlighting that H2O-rich geofluids should be highly mobile at depth.

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).