Philippe Carrez

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.

Potassium tantalate KTaO 3 is a cubic, paraelectric perovskite ceramic that exhibits surprising ductility at room temperature as most recently reported. Much like strontium titanate ( SrTiO 3 ), plastic deformation is accommodated by dislocations gliding in { 110 } planes. In this work, we propose a new interatomic potential for KTaO 3 , and apply it to model dislocations with 〈 110 〉 Burgers vector. We demonstrate that dislocations dissociate, and finely characterize their core structure and Peierls potential. Dislocations of edge character can carry a positive or negative electric charge, but we show that charge-neutral configurations are energetically more favorable. We also perform high-resolution electron microscopy to validate our simulation methodology. Comparing our results with other ductile perovskites, we confirm KTaO 3 to be ductile, but stiffer than SrTiO 3 . Published by the American Physical Society 2025

Transport of heat from the interior of the Earth drives convection in the mantle, which involves the deformation of solid rocks over billions of years. The lower mantle of the Earth is mostly composed of iron-bearing bridgmanite MgSiO₃ and approximately 25% volume periclase MgO (also with some iron). It is commonly accepted that ferropericlase is weaker than bridgmanite¹. Considerable progress has been made in recent years to study assemblages representative of the lower mantle under the relevant pressure and temperature conditions^2,3. However, the natural strain rates are 8 to 10 orders of magnitude lower than in the laboratory, and are still inaccessible to us. Once the deformation mechanisms of rocks and their constituent minerals have been identified, it is possible to overcome this limitation thanks to multiscale numerical modelling, and to determine rheological properties for inaccessible strain rates. In this work we use 2.5-dimensional dislocation dynamics to model the low-stress creep of MgO periclase at lower mantle pressures and temperatures. We show that periclase deforms very slowly under these conditions, in particular, much more slowly than bridgmanite deforming by pure climb creep. This is due to slow diffusion of oxygen in periclase under pressure. In the assemblage, this secondary phase hardly participates in the deformation, so that the rheology of the lower mantle is very well described by that of bridgmanite. Our results show that drastic changes in deformation mechanisms can occur as a function of the strain rate.

The ultimate mechanical properties of MgSiO₃ orthoenstatite (OEN), as characterized here by the ideal strengths, have been calculated under tensile and shear loadings using first-principles calculations. Both ideal tensile strength (ITS) and shear strength (ISS) are computed by applying homogeneous strain increments along high-symmetry directions ([100], [010], and [001]) and low index shear planes ((100), (010), and (001)) of the orthorhombic lattice. We show that the ultimate mechanical properties of OEN are highly anisotropic during tensile loading, with ITS ranging from 4.5 GPa along [001] to 8.7 GPa along [100], and quite isotropic during the shear loading with ISS ranging from 7.4 to 8.9 GPa. During tensile test along [100] and [001], a modified structure close to OEN has been found. This modified structure is more stable than OEN under stress (or strain). We have characterized its elastic and ultimate properties under tensile loading. With ITS ranging from 7.6 GPa along [010] to 25.6 GPa along [001], this modified structure appears to be very anisotropic with exceptional strength along [001].

Imaging dislocation microstructures in 3D and monitoring their interactions over time is a major challenge. In this study, we show that enhancing the contrast of dislocation lines prior to reconstruction, allows to optimize an acquisition phase with fewer images and thus to follow the 3D evolution of a microstructure over time. We illustrate this new possibility by studying the first stages of formation of helical dislocations in MgO under electron irradiation. We highlight the role of segment mobility on the initiation of climb and reveal the existence of preferential mixed climb planes.

Imaging dislocation microstructures in 3D and monitoring their interactions over time is a major challenge. In this study, we show that enhancing the contrast of dislocation lines prior to reconstruction, allows to optimize an acquisition phase with fewer images and thus to follow the 3D evolution of a microstructure over time. We illustrate this new possibility by studying the first stages of formation of helical dislocations in MgO under electron irradiation. We highlight the role of segment mobility on the initiation of climb and reveal the existence of preferential mixed climb planes.