Committee:

• Chair - FLEITOUT Luce - ENS Paris

• Reviewer- BUNGE Hans-Peter - Munich University

• Reviewer - CASTELNAU Olivier - Arts et Métiers Paris Tech

• Member  - DIMANOV Alexandre - Ecole Polytechnique

• Member  - DUPUY Laurent - CEA Saclay DEN/DMN/SRMA

• Supervisor - CORDIER Patrick - Univ Lille

• Supervisor - CARREZ Philippe - Univ Lille

Abstract:

This thesis work addresses the deformation behavior of two major mineral phases of the Earth’s lower mantle: bridgmanite and (Mg, Fe)O. They constitute ~90-95% of the lower mantle and their rheology is of primary importance for a better understanding of mantle convection.
The rheological properties of these phases were modeled through the implementation of numerical and analytical techniques, in order to assess their creep behavior (i.e. steady-state deformation under a constant applied stress).
The relevant deformation agents driving creep are identified and then modeled at the single crystal scale.  In this framework, dislocations are amongst the main carriers of crystal plasticity and the creep behavior of the considered minerals can therefore be assessed by considering dislocation glide and diffusion-driven dislocation climb.
(Mg,Fe)O creep is driven by the interplay between glide and climb and in order to model it, a 2.5-dimensional (2.5D) dislocation dynamics (DD) approach has been deployed. 2.5D-DD is a numerical technique which addresses the collective behavior of dislocations at the mesoscale. It is demonstrated that dislocation glide is responsible for the plastic deformation and climb is the rate-limiting mechanism. From the modeled creep strain rates it was possible to estimate viscosity of (Mg,Fe)O at lowermost mantle conditions.
As for bridgmanite a pure climb mechanism is proposed, and the creep strain rates were evaluated according to a physics-based analytical creep model. The viscosity of bridgmanite along a geotherm is retrieved and compared with the available observables.