Rheology of the lower mantle: an alternative review

Understanding the rheology of the Earth's lower mantle is essential for modelling mantle convection and its consequences for the geodynamical, thermal and chemical evolution of the planet. However, the extreme pressure-temperature conditions, the long natural timescales, and the complexity of deep-mantle mineralogy make it difficult to constrain mantle viscosity from experiments alone. In this review, we examine an alternative approach based on mineral physics and multiscale numerical modelling, tracing its theoretical foundations and its development over the last decades. We first outline the key physical mechanisms governing plastic deformation in lower-mantle minerals-point defects, dislocations, diffusion, and grain-boundary processes-and the conditions under which they operate. We then summarize the major experimental advances that provide essential constraints for these models. Building on this foundation, we review the evolution of multiscale modelling strategies, from first-principles calculations of defect energetics and dislocation cores to mesoscale dislocation dynamics and polycrystal-scale formulations. Using ferropericlase and bridgmanite as archetypal examples, we show how successive developments have progressively linked atomic-scale mechanisms with macroscopic rheology under deep-mantle conditions. Finally, we discuss current limitations and outstanding challenges, including the scarcity of diffusion data at high pressures, uncertainties in defect chemistry, and the need to integrate multiphase and multimechanism deformation. Together, these elements provide a coherent view of the present capabilities of mineral-physics-based rheology and outline a roadmap for future efforts to improve our understanding of deep-mantle dynamics.