The rheology of the slab-mantle interface in subduction zones could potentially be controlled by weak hydrous minerals such as antigorite or talc, formed by reactions involving silica-rich aqueous fluids liberated during dehydration of the subducting slab. To determine the effect of Si-metasomatism on the rheological properties of antigorite, we performed a series of high-shear-strain (γ up to ∼80) frictional sliding experiments on simulated antigorite (70%) plus quartz (30%) gouges at a pore fluid pressure of 200 MPa, an effective normal stress of 200 MPa, temperatures of 20–500 °C, and constant sliding velocities of 0.1–30 µm/s, using a hydrothermal ring shear machine. At temperatures of 300–500 °C, the gouges exhibited a peak friction coefficient (µ) of 0.40–0.62, followed by strain weakening toward a quasi-steady-state strength with µ = 0.25–0.47. In contrast, pure antigorite and pure talc at a temperature of 400 °C showed steady-state shearing at µ = 0.63 and µ = 0.21, respectively. The degree of weakening of the mixtures relative to pure antigorite increased with increasing temperature and decreasing sliding velocity. The increased weakening was associated with increasing amounts of talc reaction product formed in the samples. The weakening was mainly due to the development of throughgoing, talc-bearing boundary (B) shears, which widened until steady-state sliding was attained. Our results demonstrate that in the lowermost part of the fore-arc wedge, where silica-saturated fluids infiltrate from the dehydrating slab, metasomatically produced talc slip surfaces or shear bands will form in the intensely sheared plate interface, causing a much larger weakening effect than expected for antigorite, even if the total amount of talc formed is minor (<10 vol%).