Dynamic Recrystallization of Olivine During Simple Shear: Evolution of Microstructure and Crystallographic Preferred Orientation From Full-Field Numerical Simulations

Abstract

Upper mantle deformation is mainly controlled by the mechanical behavior of olivine. Crystallographic preferred orientations (CPOs) develop in olivine due to crystal-plastic deformation during mantle flow, where the a-axes of olivine polycrystalline aggregates are aligned with the flow direction. Therefore, the observed CPO in olivine-rich rocks is used as an indicator of the mantle flow direction. Experimental data show that olivine rheology is strongly controlled by the microstructure. While the influence of plastic deformation is in general well characterized, the role of dynamic recrystallization during deformation is not totally understood, limiting our ability to interpret the deformation history of naturally deformed rocks. This contribution presents microdynamic numerical simulations of olivine polycrystalline aggregates with different iron content (i.e., fayalite content) with the aim of exploring the CPO and grain size response to dynamic recrystallization. We use a full-field approach with an explicit simulation of viscoplastic deformation and dynamic recrystallization processes under simple shear boundary conditions up to high strain. The simulations show that the CPOs are similar and practically reach the same maximum regardless of the iron content. CPOs are characterized by a single cluster of a-axis and two-clusters of b-axis, reveling a joint activity of the easy glide [100](010) and the moderate strength [100](010) slip systems. High-strain domains of our models are consistent with experimental results, showing an A-type fabric with double maxima, and where the CPO is aligned with the shear direction. The model provides a deeper understanding of the dynamic recrystallization influence on olivine CPOs resulting from plastic deformation.