Abstract
Creep of silicate polycrystals is governed by the mobility of lattice defects. The relative importance of bulk diffusivity, grain-boundary diffusivity, dislocation motion, grain-boundary sliding, and liquid diffusivity (in two-phase systems) is a function of temperature, pressure and grain size. Assuming an olivine (Fo90) composition for the upper mantle, applied deformation maps are constructed that allow the prediction of creep laws, strain rates and effective viscosities as a function of depth for various geothermal gradients. Similar calculations for the lower mantle require an estimate of the variations of activation energy and activation volume with depth, and an evaluation of the effect of polymorphic phase transformations on rheological parameters. Using the best available estimates of these quantities, it is concluded that the whole mantle, with the possible exception of local low-viscosity zones within the asthenosphere, flows by power-law creep with effective viscosities that do not vary with depth by more than two orders of magnitude.
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