Localized Deformation of Lawsonite During Cold Subduction

One of the causes of intermediate depth (50–300 km) earthquakes in cold subduction zones is proposed to be the dehydration of lawsonite, a hydrous mineral that forms a major component of cold subducting oceanic crust. However, experimental studies are limited and the deformation mechanism of lawsonite remains uncertain. Here, we conducted deformation experiments on lawsonite under seismogenic conditions appropriate to cold subduction zones at pressures from 2.5 to 6.0 GPa and temperatures from 300°C to 800°C. Deformation experiments incorporating temperature ramping, in which deformation and heating were synchronous, did not show unstable fault slip associated with dehydration. Instead, we find localized deformation accompanying a rapid stress drop prior to the initiation of dehydration, and then more gradual strain weakening during dehydration. Such microstructures as microcracks and comminution of lawsonite imply a brittle behavior for lawsonite accompanied by the partial amorphization along the fault. Our results suggest that fault slip may occur within the stability field of lawsonite under conditions suitable for the generation of intermediate depth earthquakes.