Even small amounts of water can profoundly effect the physical properties of minerals. In olivine 2 eV more favourable to put the first proton into the Si vacancy than the magnesium site, the presence of water will certainly act to increase the population of silicon vacancies. In fact, in the presence of water the energy required to form a Si vacancy is perhaps less than that to form an Mg vacancy. This is in stark contrast to dry olivine where Si vacancies are many eV less favourable. If creep is rate limited by the diffusion of the slowest species, silicon in olivine, then increasing the Si vacancy concentration could provide a mechanism for hydrolytic weakening. ¿ 2000 American Geophysical Union