Thesis of Estelle Ledoux

Thèse d'Estelle Ledoux  - laboratoire UMET Transformation and deformation microstructures in the Earth's mantle : application to periclase and wadsleyite  Abstract : The microstructure of rocks depends on the conditions of pressure, temperature and deformation they undergo. In the Earth's mantle, microstructures affect the seismic signals, in the form of seismic anisotropy, for instance. The interpretation of seismic observations in terms of microstructures, however, requires a good knowledge of plastic deformation in mantle minerals.

In this thesis, I am using laboratory experiments to investigate the microstructures that can form in mantle's minerals. I am focusing on three cases: the deformation of periclase at high pressure and high temperature, the transformation of (Mg,Fe)2SiO4 olivine in wadsleyite at conditions relevant for the 410 km depth discontinuity in the mantle, and the deformation of wadsleyite at high-pressure and high-temperature.

I identify microstructures in polycrysals resulting from deformation / transformation experiments using in-situ X-rays diffraction analysis using powder diffraction and multigrain crystallography, and post-mortem scanning and transmission electron microscopy characterization. My results show: i) that an increase of temperature induces a transition of dominant deformation mechanism in polycrystalline periclase, with dislocation creep at low temperatures and grain boundary sliding at 1270 K, ii) that an increase of pressure and temperature induces change of dominant slip systems in periclase, iii) that at conditions of the 410 km depth discontinuity, the transformation from olivine to wadsleyite is not martensitic and then erases the microstructures of the parent rocks, and iv) that the activity of the slip systems in wadsleyite, and so the texture and anisotropy, depend on the temperature and the water content of wadsleyite.

Finally, from the microstructures observed in the deformed wadsleyite, I simulate seismic observables in different scenarii, a subduction zone and a mantle plume, and compare the results to seismic anisotropy from the literature to discuss the predictions of the mineralogy experiments. Keywords : microstructures,deformation,phase transformation