Reversible water weakening of quartz

•We performed pressure stepping experiments on a quartzite and quartz single crystal.•Strengths of both the quartzite and quartz single crystal increase as fH2O decreases.•The strength increase of the quartzite is much greater than that of the single crystal.•The strength increases of both are reversible, confirming quartz fH2O dependence.•The difference in fH2O dependence is related to differing recovery mechanisms.

We have performed looped pressure-stepping deformation experiments at the same temperature (800 °C) and strain rate (2×10−6 s−1) on a milky quartz crystal and a quartzite that deform by dislocation creep. Our results demonstrate that flow strengths at fixed water content depend on fH2O, and that the dependence is reversible. Thus, rheology of quartzite can be predicted if fH2O is known. In addition, the effect of fH2O on strength of quartzite is greater than that of the milky quartz single crystal. The differing sensitivities are due to differing effects that water has on deformation and recovery within grain interiors and at (or near) grain boundaries. These results indicate that, as temperature increases with depth and the dominant recovery mechanism of dislocation creep changes from recrystallization at grain boundaries to intragranular recovery, dependence of quartz creep on fH2O may also decrease.