La couche D" située à la frontière manteau-noyau est une région complexe caractérisée par une forte anisotropie à différentes échelles. Inaccessible de par sa profondeur et caractérisée par des conditions P-T extrêmes, l’étude de cette région de la Terre représente un défi majeur qui ne peut être abordé qu’au travers d’observables géophysiques et d’expériences de hautes pressions. Les causes de l’anisotropie sismique de D" sont toujours l’objet de débats. La contribution de l’orientation préférentielle des cristaux reste cependant une piste privilégiée compte-tenu de la structure très anisotrope de la post-perovskite (ppv). De plus, D" est une couche limite thermique à l’interface entre le noyau constitué d’un alliage de fer liquide et le manteau inférieur constitué de silicates visqueux. Les propriétés physiques de D" sont donc particulièrement importantes pour comprendre les transferts thermiques en provenance du noyau et leur contribution à la convection mantellique. Ce phénomène implique l’écoulement plastique de roches contrôlé par le déplacement de défauts cristallins. Cependant, pour la ppv, les informations concernant les systèmes de glissement majeurs ou les défauts sont extrêmement parcellaires. Pour les phases de hautes pressions, la modélisation numérique représente une approche de choix pour obtenir des informations sur les mécanismes de déformations élémentaires difficiles à obtenir par voie expérimentale. Le but de ce travail est d ‘étudier à l’échelle atomique les défauts majeurs de la ppv MgSiO3 ainsi que leurs mobilités afin d’évaluer la capacité de cette phase à se déformer plastiquement par glissement de dislocations dans les conditions de D". / The D’’ layer, located right above the core-mantle boundary, represents a very complex region with significant seismic anisotropy both at the global and local scale. Being a part of inaccessible deep Earth interior, characterized by extreme P-T conditions (>120 GPa, 2500 K), this region is very challenging for interpretation relying only on the direct geophysical observations and high-pressure experiments, leading often to contradictory results. Thus, the reasons of the pronounced anisotropy in D’’ are still debated. Among them, contribution of the crystal preferred orientation in anisotropic silicate post-perovskite (ppv) phase is commonly considered as substantial. Furthermore, the D’’ is a thermal boundary layer located at the interface between liquid iron alloy, constituting the outer core, and solid although viscous silicates of the lowermost mantle. As such, its physical properties are critical for our understanding of the heat transfer from the core, driving mantle convection. The latter is governed by plastic flow, controlled by the motion of defects in crystals. However, for the ppv, information about mechanical properties, easy slip systems, dislocations and their behavior under stress is still scarce. For high pressure phases, numerical modelling represents a powerful tool able to provide the intrinsic properties and the elementary deformation mechanisms, not available for direct observations during high-pressure experiments. The aim of this study is to access the structure and mobility of dislocations in MgSiO3 ppv, relying on the atomic-scale modeling, in order to infer the ability of this phase to plastically deform by dislocation glide at D’’ conditions.

Our understanding of the nature of Earth’s D” region was changed significantly by a recent finding by Murakami et al. (2004), who revealed a phase transition from perovskite to post-perovskite structure in MgSiO3 at about 125 GPa and 2500 K, corresponding to conditions of the lowermost mantle. A perovskite to post-perovskite phase transition accounts for many unusual features of the D” region, including its notable seismic anisotropy, and also accounts for the unusual topology of the D” discontinuity. However, the experimentally synthesised post-perovskite phase of MgSiO3 is not quenchable to ambient conditions, which means that many of its physical properties remain difficult to determine. On the other hand, there are several post-perovskite oxides, CaIrO3, CaPtO3, CaRhO3 and CaRuO3, which can be quenched to ambient conditions, maintaining their structure. High pressure synthesis of CaIr1-xPtxO3 solid solutions (x = 0, 0.3, 0.5, 0.7) and CaIr0.5Rh0.5O3 was conducted at the University of Edinburgh and Geodynamics Research Center, Ehime University, and structures and physical properties of these novel post-perovskite materials determined. Substantial [100] grain growth was observed in all solid solutions leading to pronounced texture even in powdered materials. Temperature-independent paramagnetism above 150 K and small magnetic entropy observed in heat capacity measurements suggest that CaIrO3 is an intrinsically weak itinerant ferromagnetic metal, while electrical resistivity measurements show that it is a narrow bandgap semiconductor, possibly due to grain boundary effects. CaIrO3 undergoes a magnetic transition at 108K and possesses a saturated magnetic moment of 0.04 μB. Doping with Pt or Rh induces Curie-Weiss paramagnetism and suppresses the magnetic transition. The anisotropic structure and morphology of CaIrO3 combined with the Ir4+ spin-orbit coupling results in a large magnetic anisotropy constant of 1.77 x 106 Jm-3, comparable to values for permanent magnet materials. A new high-pressure phase of CaIr0.5Pt0.5O3 was synthesised at 60GPa, 1900K using a laser-heated DAC (diamond anvil cell) at GRC, Ehime University. Its Raman spectra resemble those of perovskite phases of CaIrO3 and CaMnO3, implying that CaIr0.5Pt0.5O3 undergoes a post-perovskite to perovskite phase transition with increasing pressure. I estimate an increase in thermodynamic Grüneisen parameter γth across the post-perovskite to perovskite transition of 34 %, with similar magnitude to (Mg,Fe)SiO3 and MgGeO3, suggesting that CaIr0.5Pt0.5O3 is a promising analogue for experimentally simulating the competitive stability between perovskite and post-perovskite phase of magnesium silicates in Earth’s lowermost mantle. Such estimation is reliable since the estimated and directly calculated thermodynamic Grüneisen parameter γth from heat capacity show consistent values. The marked effect that Pt has on stabilising the post-perovskite structure in CaIr1-xPtxO3 solid solutions explains why the post-perovskite to perovskite phase transition has not been observed for CaPtO3 in contrast to other quenchable post-perovskite oxides: CaIrO3, CaRhO3 and CaRuO3.Work presented here demonstrates that CaIrO3 solid solutions can be used to provide new insight into factors stabilising post-perovskite structures in Earth’s lowermost mantle.

La plupart des processus géologiques affectant la surface de la Terre sont le reflet des mouvements de convection au sein du manteau terrestre. Ces mouvements sont essentiellement gouvernés par le fluage par dislocations des silicates du manteau et sont à l'origine d'une anisotropie des vitesses des ondes sismiques. Cependant, les mécanismes de déformation de ces minéraux sont mal connus. Les conditions dans les couches les plus profondes sont extrêmes; la température y atteint plusieurs milliers de degrés et la pression est plus d’un million de fois supérieure à la pression atmosphérique. La détermination expérimentale de la plasticité de ces minéraux nécessite des expériences de déformation sous hautes pression et température. Les mécanismes de déformation sont généralement déterminés à partir d'expériences en cellule à enclumes diamants permettant d'atteindre les conditions de pression et de température du manteau. L'objectif de cette thèse visait à développer une nouvelle technique permettant d'étudier les dislocations in situ dans les grains d’un polycristal sous haute pression, directement à partir de leur effet sur les raies de diffraction X. De ce fait, nous avons combiné la diffraction X tridimensionnelle (3D-XRD) à la méthode d'analyse des profils de pics de diffraction (XLPA, X-ray Line Profile Analysis). Les travaux de cette thèse ont été appliqués à la post-perovskite, présente dans la couche D'' à l'interface noyau-manteau et à la stishovite, présente principalement dans les plaques en subduction. Les résultats obtenus sont utiles à la compréhension et la modélisation des mouvements de convection et du développement d'anisotropie sismique dans le manteau. / The Earth mantle and inner core are submitted to large scale movements of solid materials. The physical process allowing the flow of solid materials is connected to plastic properties and, in particular, dislocations. It is the source of seismic wave velocities anisotropy. However, the deformation mechanisms of deep Earth minerals are poorly understood. Deep in the Earth’s interior, minerals are under extreme conditions; the temperature reaches several thousand degrees and the pressure is more than one million times the atmospheric pressure. The experimental study of the plasticity of those minerals requires deformation experiments under high pressure and temperature. High pressure phenomena are often determined from experiments using diamond anvil cell to reach the conditions of pressure and temperature of the mantle. The objective of this thesis was to develop a new technique for studying dislocations in situ in grains inside a polycrystal under high pressure, directly from their effect on the X ray diffraction profiles. To do so, we combine three-dimensional X-ray diffraction (3D-XRD) to X ray Line Profile Analysis method (XLPA). The development done in this thesis was applied to post-perovskite, the main constituent of the D'' layer at the core-mantle boundary and to stishovite, present mainly in subducting slabs. The results obtained are useful for understanding and modeling of convection and the development of seismic anisotropy in the mantle.

In this thesis, we present results of a numerical modelling study focused on the thermal evolution of the Earth and terrestrial planets. We focus particularly on two problems: I) constraining the internal structure of Venus and Mercury using their geoid and surface topography data and II) evaluating the effects of a rhe- ologically distinct post-perovskite on the secular cooling of the Earth. In part I, we performed simulations in a broad group of models of the Venusian man- tle, characterised by different rheological descriptions, and we compared spectra of their geoid and their surface topography with the observed quantities. Our analysis suggested that the geoid and the surface topography of Venus are con- sistent with a radially symmetric viscosity model with a strong 200 km thick lithosphere, without an asthenosphere and with a gradual viscosity increase in the underlying mantle. In the case of Mercury, none of our models was able to predict observed data, thus suggesting other than a dynamic origin of observed geoid and topography. In part II, we investigated style of Earth's mantle con- vection and its long-term evolution in the models that take into account a weak post-perovskite. We conclude that the presence of the weak post-perovskite en- hances the core cooling. This effect is comparable in...