Effect of normal stress on the friction of brucite: Application to slow earthquake in the mantle wedge

Abstract

Abstract. We report the results of friction experiments on brucite under both dry and water-saturated (wet) conditions under various normal stresses (10–60 MPa). The steady-state friction coefficients of brucite were determined to be 0.40 and 0.26 for the dry and wet cases, respectively, independent of the normal stress. Under dry conditions, velocity-weakening behavior was observed in all experiments at various normal stresses. Under wet conditions, velocity weakening was observed at low normal stress (10 and 20 MPa), whereas velocity strengthening was determined at a higher applied normal stress. The microstructural observations on recovered experimental samples indicate localized deformation within the narrow shear band, implying that a small volume of brucite can control the bulk strength in an ultramafic setting and significantly change the frictional properties. Brucite is found to be the only mineral that has a low friction coefficient and exhibits unstable frictional behavior under hydrated mantle wedge conditions, explaining the occurrence of slow earthquakes in the mantle wedge.