Brittle versus ductile deformation as the main control of the deep fluid circulation in oceanic crust

The brittle to ductile transition may strongly influence hydraulic properties of rocks at the depth and temperature ranges that hydrothermal fluids circulate. To examine this transition in the context of the oceanic crust, we conducted a series of deformation experiments on a natural basalt sample at in situ oceanic crust conditions. Dilatancy was measured during deformation. The method consisted in monitoring the volume of pore fluid that flows into or out of the sample at constant pore pressure. Mechanical and microstructural observations at experimental constant strain rate of 10(-5)s(-1) indicated that the basalt was brittle and dilatant up to 800 degrees C. At higher temperature, the deformation mode became macroscopically ductile and samples compacted. These observations have important implications on heat transfer and fluid migration in oceanic crust.