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 facies rocks is crossed once the nominally anhydrous minerals are saturated with hydrogen.

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