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

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. Rocks in the upper mantle of the Earth are mainly composed of olivine. This mineral has a significant impact on how the upper mantle behaves mechanically. When the mantle flows, olivine undergoes viscoplastic deformation, leading to the formation of crystallographic preferred orientations (CPO). By studying the observed crystallographic preferred orientations in olivine-rich rocks, scientists can determine the direction of mantle flow. However, how dynamic recrystallization affects the CPO is not well understood. This study conducts microdynamic numerical simulations of olivine aggregates to investigate the impact on the microstructure and CPO under dynamic recrystallization during viscoplastic deformation. The modeling approach is the cutting-edge viscoplastic fast Fourier transform in conjunction with the ELLE modeling platform. The high-strain regions in our models align with the experimental findings, revealing a double-maxima A-type fabric and where the CPO is oriented with the shear direction. This model enhances our comprehension of how dynamic recrystallization influences the deformation of olivine rocks. We present the first full-field numerical model incorporating dynamic recrystallization applied to olivine microstructures 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 Microstructures formed by pure fayalite show higher grain boundary mobility than those composed of forsterite and fayalite