Comparing dry and wet friction of plagioclase: Implication to the mechanism of frictional evolution effect at hydrothermal conditions

To understand mechanism for unstable velocity‐weakening slip behavior of plagioclase at hydrothermal conditions in the lower crust, we employed nominally dry experiments on plagioclase gouge to isolate fluid‐assisted processes that may be activated at hydrothermal conditions and obtained constitutive parameters fitted to rate and state friction laws with temperatures of 99–617°C and confining pressure of 150 MPa, allowing comparison to the hydrothermal counterpart. While the direct effect ( a value) has relatively high values of 0.007–0.01 in the high temperature range above 300–400°C, the evolution effect ( b value) shows a diminishing trend from peak values of 0.0075 and 0.0057, respectively, at 305°C and 400°C down to ~0 at 617°C, in contrast to the increasing trend in both effects at hydrothermal conditions. The evolution effect in the dry experiments may be controlled by structural changes related to degrees of shear localization and evolution of porosity change rate in response to a change in loading rate. The evolution effect in dry friction compared to the thermally enhanced, much stronger one at hydrothermal conditions suggests activation of fluid‐assisted time‐dependent mechanism operating in the wet experiments.