Reversible water weakening of quartz

We have performed looped pressure-stepping deformation experiments at the same temperature (800 degrees C) and strain rate (2 x 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 f(H2O), and that the dependence is reversible. Thus, rheology of quartzite can be predicted if f(H2O) is known. In addition, the effect of f(H2O) 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 f(H2O) may also decrease. (C) 2013 Elsevier B.V. All rights reserved,