The standard rim-forming mineral reaction olivine + quartz = orthopyroxene (Ol + Qtz = Opx) has been experimentally performed at very dry conditions, i.e., only 20 wt ppm of water present in the sample container as a fluid, and most "water" was hydrogen dissolved within the solid crystals. Reaction rates and resulting fabrics at Ol-Qtz interfaces mimic reaction features previously known from water-rich conditions (tens of thousands ppm). Our experiment indicates that very small amounts of water (tens of ppm of the entire sample) are highly effective in facilitating mineral reactions where water acts as a catalyst and creates porosity that might start to migrate from the initial centers of self-propagating mineral reaction, if rock deformation allows. The threshold between dry and wet in granulite and eclogite fades rocks is crossed once the nominally anhydrous minerals are saturated with hydrogen.

Orthopyroxene reaction rims were synthesized between polished plates of natural olivine or synthetic forsterite and quartz at 1.9 GPa and temperatures of 750–950°C. The experiments were performed in a piston-cylinder apparatus after drying the samples at 600°C. Each experiment comprised 4 or 7 quartz-olivine contacts that were positioned along a temperature gradient. As a monitor for water content in the samples, the water concentration in the two olivines was determined by FTIR before and after the experiments. The orthopyroxene layers show two different structural variants. Type one (normal layers) has very constant thickness at each contact and formed with equal growth rates at both interfaces. Type two (bulging layers) comprises more irregular areas with 3–5 times thicker rims where porosity provides evidence for the local presence of a fluid. In the bulging layers the growth rate at the olivine-orthopyroxene interface exceeds that at the quartz-orthopyroxene interface. The relative growth rates at the interfaces are in accordance with SiO2-immobile growth of the normal layers and SiO2-mobile growth of the bulging layers. The natural olivine contains about 60 wt-ppm intracrystalline water before and after experiment and took up about 20 wt-ppm water molecular adsorbed to micro- and nanocracks and -pores during the runs. The synthetic forsterite contains about 7 wt-ppm internally adsorbed molecular water before and after experiment, and during the runs took up hydrogen equivalent to 3 wt-ppm adsorbed water. The IR spectra indicate that large parts of the point defects (possibly tetrahedral) were frozen-in at the conditions of the experiments. In both olivines a new band appeared at 3,355 or 3,357 cm−1, respectively, equivalent to about 3 wt-ppm water that at the high pressure of the experiments and opx-buffered aSiO2 of the experiments might already mean water saturation of the olivines. Despite the effective drying before experiment and the absence of porosity, the bulk diffusivity derived from the rim growth rates is perfectly in line with data from water-bearing piston-cylinder experiments at higher temperatures. The bulk diffusivity during rim growth is 4 to 7 orders of magnitude higher than an extrapolation of really dry experiments to the temperature range of this study.