Rheology of partially molten mantle rocks

Over the past decade, significant progress has been made in understanding the rheological properties of partially molten mantle rocks. Laboratory experiments demonstrate that a few percent of melt can have an unexpectedly large effect on viscosity both in the diffusional creep regime and in the dislocation creep regime. In both cases, the enhancement in creep rate is much larger than anticipated based on deformation models because melt wets at least a fraction of the grain boundaries. For diffusion creep, the wetted interfaces provide a rapid diffusion path that is not included in analyses based on melt distribution in isotropic melt-crystal systems. For dislocation creep, two points require consideration. First, even without a melt phase present, fine-grained samples deformed in the dislocation creep field flow a factor of similar to 10 faster than coarse-grained rocks due to contributions from grain boundary mechanisms to the deformation process. Second, melt has only a small effect on creep rate for coarse-grained rocks but has a relatively large effect for fine-grained samples. Thus, because olivine has only a limited number of slip systems, grain boundaries contribute significantly to deformation of fine-grained rocks in the dislocation creep regime, provided that deformation occurs near the transition between diffusional creep and dislocation creep. Based on laboratory measurements, this transition is expected to occur at a grain size of about 6 mm for a differential stress of 0.1 MPa. Therefore, under mantle conditions, even a few percent melt should reduce the viscosity by as much as a factor of 10. A broad range of problems related to deformation beneath mid-ocean ridges and in the mantle wedge above subducting slabs can now be addressed using experimentally determined rheologies for partially molten mantle rocks.