Dehydration Induced Faulting in Eclogite at High Pressure: A Mechanism for Intermediate-Focus Earthquakes

Embrittlement induced by fluid release during dehydration reactions has been proposed to explain both intermediate focus (70-300 km) earthquakes and deep-focus (>300 km) earthquakes in subduction zones. In addition to dehydration of hydrous phases (e.g. lawsonite, phengite, antigorite), in principle hydroxyl-bearing nominally anhydrous minerals (olivine, pyroxene, garnet) within the subducting lithosphere also can provide a source for fluid release in subducting slabs. We have performed deformation experiments at 3 GPa pressure on a reconstituted natural eclogite that contains a significant OH concentration in both pyroxene and garnet. Fluid-free samples of this material exhibit exsolution of H₂O and grain-boundary melting at temperatures between the H₂O-saturated and dry solidi. Deformation of these samples leads to brittle failure under conditions where the total fluid present is very small and the material is strong. At higher temperatures, where the flow stress is significantly reduced by larger amounts of melt, faulting does not occur. At temperatures below the wet solidus, no fluid is produced and faulting does not occur, even though specimens are very strong under such conditions. Within the faulting regime, the failure strength of the eclogite is not temperature sensitive. Microstructural observation of faulted specimens revealed ultra-thin glass films along grain boundaries and within abundant Mode I microcracks. Fault gouge contains small amounts of glass and angular fragments of garnet and pyroxene that appear to have a fractal size distribution. Addition of water to deformation experiments performed is generated by the added H₂O to reduce the flow stress to below the fracture stress. At moderate temperatures in subduction zones, phengite remains stable to very high pressures. Thus, small amounts of phengite stable at lower pressures could provide a source of H₂O that could progressively dissolve into pyroxene and garnet with increasing pressure and thereby be carried to greater depths where it could exsolve and trigger earthquakes or where it could be incorporated into the circulating mantle.