Simulation of dislocation-assisted plastic deformation in olivine polycrystals

Abstract

Studies on mantle convection provide insight into the geodynamics of the Earth. Convection models have relied on flow laws for olivine (e.g. Hager 1984, Christensen 1987), which is believed to be the major constituent of the upper mantle. The mantle is assumed to deform in a régime of steady-state creep in which diffusion and dislocation glide occur. When dislocation glide is present, polycrystalline materials develop anisotropy due to rotations of crystals during straining, which produce a preferred orientation distribution. Strong seismic anisotropy has been observed in the upper mantle underneath oceanic crust (e.g. Hess 1964) and continental crust (e.g. Fuchs 1983), and it is generally accepted that at high pressure where microfractures are closed, seismic anisotropy can be due to crystallographic preferred orientation or texture (e.g. Christensen 1984). Whereas Arrhenius-type flow laws are adequate to describe the diffusion-controlled plastic deformation (e.g. Karato et al. 1986), they do not take account of plastic anisotropy due to slip. We attempt to introduce a generalized description of anisotropic plastic flow of olivine based on polycrystal plasticity theory which takes into account the deformation history. It should be emphasized that all arguments brought forward relate to processes in which dislocation movements are involved and do not apply to recrystallization, which is also important in deformation of olivine (e.g. Avé Lallemant & Carter 1970). Deformation by grain-boundary sliding, such as during superplastic flow, does not produce texture.