Viscoplastic Modeling of MgSiO3 Perovskite and Periclase Aggregates

Abstract:

Plasticity and the subsequent development of crystallographic preferred orientation (CPO) in mantle minerals is important for understanding dynamic processes in the mantle, e.g. the interpretation of seismic anisotropy as a result of CPO developed by mantle convection. Furthermore, plastic behavior of multi-phase polycrystals is still not well understood. A polycrystal plasticity code (VPFFT) with an n-site approach is used to model the development of CPO in a hard magnesium silicate perovskite and a soft periclase aggregate under an applied strain, given certain combinations of active slip systems. Special attention is put to the influence of microstructure and phase hardness contrast on CPO. Preliminary results show that in the case where periclase is surrounding the hard perovskite phase, the deformation concentrates on the soft periclase but the perovskite still develops an appreciable and amount of CPO, in agreement with experimental data. These polycrystal plasticity models are compared to diamond anvil cell experiments in the radial geometry (rDAC) where CPO is recorded as azimuthal variations of intensity along Debye rings in a diffraction pattern. Two different starting materials, enstatite+periclase and olivine, are used for the nucleation of the perovskite and periclase aggregate, microstructures are expected to differ between the two starting materials due to nucleation differences. Preliminary results show that perovskite develops a (001) texture and periclase a stronger (001) maxima. The comparison of VPFFT results with rDAC experiments gives and idea of the relative activities of the various slip systems in this aggregate and help to constrain seismic and geophysical modeling of the lower mantle.