Weakening of Peridotite Accompanying the Initiation of Shear at Hydrothermal Conditions: Effect of Fluid Chemistry

Abstract:

We conducted triaxial friction tests on a peridotite gouge containing ~80% olivine and ~20% orthopyroxene at hydrothermal conditions (25–350 °C, 50 MPa pore pressure, 150 MPa normal stress, 0.001-1 µm/s displacement rates). Pore-fluid chemistry was varied by the use of either peridotite or granite driving blocks housing the gouge layers. Dry peridotite has a coefficient of friction, µ ~ 0.75, and it slides unstably. In contrast, water-saturated samples sheared at slow rates are characterized by an initial increase in strength to a peak value followed by a gradual decrease that is accompanied by a transition from velocity-weakening to velocity-strengthening behavior. The extent of weakening varies with the chemical environment. The strength of peridotite sheared between granite blocks decreases continuously with increasing temperature to µ ~ 0.25 at 350 °C. Strength reductions in the granite-block experiments are the product of solution-transfer processes that are enhanced in the lower-pH fluids associated with silica-saturated crustal rocks. Considerably less weakening occurs in an ultramafic chemical environment. The strength of the peridotite-block experiments reaches a minimum of µ ~ 0.5 at 250 °C, corresponding to the temperature at which olivine hydration rates are near their maximum in ultramafic rocks. These results highlight the important role of fluid chemistry on the strength behavior of faults in ultramafic rocks undergoing hydration. In particular, enhanced weakening can occur when the hydrothermal fluids are derived from crustal rocks, such as in subduction zones or along oceanic transform faults. In these short-duration experiments (<8 days) secondary mineral growth in the peridotite gouge was minor; serpentine crystallized in all samples, along with talc in the granite-block experiments at 300 and 350 °C. As faulting continues, however, the alteration-mineral assemblages will take on an increasingly important role in the frictional behavior.