Micromechanical Modelling of the Viscoplastic Behavior of Olivine

Efforts to couple mantle flow models with predictions of mineral deformation typically ignore rheological impacts that texture development may have on the flow evolution. Olivine crystals have only three easy slip systems for dislocation glide, leading to strong mechanical interactions between grains as deformation proceeds. These intergranular interactions are also responsible for very large viscoplastic anisotropy when polycrystals exhibit pronounced Lattice Preferred Orientations (LPO). Using a full field polycrystal plasticity model for creep in dry polycrystalline olivine under thermomechanical conditions prevailing in the upper mantle, it is shown that very large stress and strain rate heterogeneities build up at the grain scale upon deformation. Field heterogeneities increase with the strength of the hard slip system incorporated for the sake of enabling general deformation. Compared with several nonlinear mean field approaches for polycrystal plasticity, all based on the Self- Consistent scheme, only the recent Second Order procedure, which is based on a variational method, really captures the effect of intraphase stress heterogeneities on the effective viscoplastic behavior and on the local stress and strain rate intragranular fluctuations. Results compare very well with those of the full field method, at a significantly reduced computation effort. We anticipate that this model is the best model to date for accurate, from the physical point of view, micromechanical modeling of upper mantle peridotite. The "tangent" model, most commonly used in geophysical studies of the mantle, departs significantly from the full-field reference solutions. Olivine polycrystals are found to be able to deform with only four independent slip systems, the hard system perhaps being provided by dislocation climb (often observed in experimental work) and/or grain boundary mechanisms (sliding, migration). The resistance of this accommodation process may essentially control the flow stress of olivine polycrystals. First attempt to assess the effect of the rheological anisotropy (associated with LPO development) on typical in situ convective flow will also been shown.