We report a first application of an improved second-order (SO) viscoplastic self-consistent model for multiphase aggregates, applied to an olivine + diopside aggregate as analogue for a dry upper mantle peridotite deformed at 10 15 s 1 shear strain rate along a 20-Ma ocean geotherm. Beside known dislocation slip systems, this SO-model version accounts for an isotropic relaxation mechanism representing 'diffusionrelated' creep mechanisms in olivine. Slip-system critical resolved shear stress (CRSS) are evaluated in both phases - as functions of P, T, oxygen fugacity (fO2) and strain rate - from previously reported experimental data obtained on single crystals and first-principle calculations coupled with the Peierls-Nabarro model for crystal plasticity; and the isotropic-mechanism dependence on T and P matches that of Si selfdiffusion in olivine, while its relative activity is constrained by reported data. The model reproduces well the olivine and diopside lattice preferred orientations (LPO) produced experimentally and observed in naturally deformed rocks, as well as observed sensitivities of multiphase aggregate strength to the volume fraction of the hard phase (here diopside). It shows a significant weakening of olivine LPO with increasing depth, which results from the combined effects of the P-induced [100]/[001] dislocation-slip transition and the increasing activity with T of 'diffusion-related' creep. This work thus provides a first quantification of the respective effects of [100]/[001] slip transition and diffusion creep on the olivine LPO weakening inducing the seismic anisotropy attenuation observed in the upper mantle.