PhD's

Alexandra M. GORYAEVA: « Modeling Defects and Plasticity in MgSiO3 Post-Perovskite at the Atomic Scale »

Defended december 6th 2016 in front of the committee:

  • Chair :

A. Tommasi, Université de Montpellier 2

  • Reviewers :

J.P. Brodholt, University College London

L. Pizzagalli, Université de Poitiers

  • Members :

M.D. Long, Yale University

P.J. Tackley, ETH Zurich

  • Supervisors :

Ph. Carrez, Université de Lille

P. Cordier, Université de Lille

 

Résumé: The D’’ layer, located right above the core-mantle boundary (CMB), 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 in excess of 120 GPa and 2000 K, this region is extremely 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 (e.g. crystal preferred orientation (CPO), oriented inclusions, thermo-chemical heterogeneities etc.). Among them, contribution of CPO in anisotropic silicate post-perovskite phase is commonly considered as substantial. Furthermore, the D’’ layer 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, which, in turn, is controlled by the motion of defects in crystals. However, for the high-pressure post-perovskite phase, 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 [100], [001] and ½<110> dislocations in MgSiO3 post-perovskite, relying on the full atomistic modeling approach, in order to infer the ability of this phase to plastically deform by dislocation glide at D’’ conditions.


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Antoine KRAYCH : « The role of dislocations in bridgmanite deformation: an atomic scale study »

Defended june 20th 2016 in front of the committee:

  • Chair :

B. Reynard, Ecole Normale Supérieure de Lyon

  • Reviewers :

A. Dimanov, Ecole Polytechnique

D. Rodney, Université Claude Bernard Lyon 1

  • Members :

C. Lithgow-Bertelloni, University College London

L. Stixrude, University College London

  • Supervisors :

Ph. Carrez, Université de Lille

P. Cordier, Université de Lille

 

Abstract : Heat transfer through the mantle is carried by convection, which involves plastic flow of the mantle constituents. In this study, we model the mobility of dislocations, and their role in the plastic deformation of bridgmanite, the most abundant constituent of the lower mantle. The dislocation structures at the atomic scale control their mobility, and hence their influence on the material’s deformation. We determine the structure of dislocations at pressure relevant for the lower mantle, by modeling these defects at the atomic scale with molecular static calculations. The thermally-activated mechanism of dislocation glide in bridgmanite, the kink-pair nucleation, is assessed by coupling a continuous model to the fundamental properties of dislocations. These results allow to estimate the glide velocity of dislocations, as a function of pressure and temperature. The model is able to reproduce the yield stress measured in laboratory deformation experiments. The model is also able to estimate the stress level needed to deform bridgmanite by dislocation glide at mantle conditions, and allows us to discuss their role in the deformation of the Earth’s lower mantle.

 

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Sebastian RITTERBEX :  « Modelling the plasticity of wadsleyite and ringwoodite : On the motion of dislocations in the Earth's transition zone »

Defended june 3rd 2016 in front of the committee:

  •  Chair :

B. Romanowicz, Collège de France

  • Reviewers :

S. Jahn, Universität zu Köln

D. Mainprice, Géosciences Montpellier

  • Members :

A. Walker, University of Leeds

B. Schuberth, Ludwig-Maximilians-Universität München

  • Supervisors :

Ph. Carrez, Université de Lille

P. Cordier, Université de Lille

 

Abstract : The transition zone is the region in the Earth's mantle between 410 and 660 km depth that separates the upper from the lower mantle. In spite of its small volume, it may play a role in constraining the style, vigour and scale of global mantle convection through, for instance, the fate of subducting slabs. Mantle convection is governed by plastic flow that occurs through the motion of crystal defects. Line defects or dislocations are considered to be one of the most efficient defects contributing to intracrystalline deformation. That is why in this work, we concentrate on the motion of dislocations in relation to the major phases of the mantle transition zone: wadsleyite and ringwoodite. A theoretical mineral physics approach is used to model the thermally activated glide motion of dislocations at appropriate pressure conditions in both high-pressure polymorphs of olivine. The intrinsic properties of dislocation core structures are modelled and parametrized by atomic scale calculations to take into account the effect of pressure on atomic bonding. Plastic deformation is finally described by taking into account the instrinsic strain rate dependence on the mobility of the defects. Since plastic deformation by the motion of dislocations is associated with creep, we use the above results and a climb mobility law to address the effective creep process in wadsleyite and ringwoodite under natural conditions. We show the inefficiency of dislocation glide as a strain producing deformation mechanism and suggest the potential importance of pure climb creep in the main minerals constituting the Earth's transition zone. This would imply the mantle transition zone to be rheologically distinct from the upper mantle.

 

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