Creep host institution: Géosciences Montpellier, Montpellier, France
Background: I studied at the Institute of Geosciences Jena, Germany, where I completed my Bachelor’s and Master’s, specializing on Geology – in particular structural geology and petrology. With my master thesis I delved into more mineralogical topics, working in a combined public, academic and private sector F&E project (OPTIRISS) dealing with the development of tools for exploration and exploitation of enhanced geothermal systems in Germany. Here I did fission track and U-Pb dating, from sampling over sample preparation to actual dating. A scholarship of the DAAD allowed me to work in Innsbruck (Austria), Dresden (Germany) and Jena (Germany).
While structural geology and field work still have a big place in my heart, I am becoming more and more interested by crystals at the microscale – their structure, how this structure determines their properties and above all, how defects in these in nature never perfect structures influence how the crystals behave.
My project revolves around the very mechanisms of how crystals creep (a very slow and high viscous flow under elevated temperatures). When crystals flow, they do so by moving defects in their crystal lattice – e.g. points where atoms are missing (vacancy), exchanged, where there are additional atoms (interstitial) or lines of misaligned atoms (dislocations). In earths mantle, olivine is the mineral governing the mechanical properties of the mantle rocks due to its high abundance (40-80 %; Bai et al., 1991; Griffin et al., 2009; Mackwell, 1991; Nicolas & Poirier, 1976). In the past, there have been several experiments on the deformation of olivine, however most of them focused on high temperatures (> 1200 °C) and moderate finite strains (10-30 % Bai et al., 1991; Hirth & Kohlstedt, 1996; Mei & Kohlstedt, 2000; Hirth & Kohlstedt, 2003; Faul et al., 2011), achieving steady state deformation or sample failure. However, the early stages of visco-plastic deformation of polycrystalline olivine at low and intermediate temperatures (< 1200 °C) are not well documented. Experiments are needed which investigate the mechanisms of deformation prior to reaching steady state or sample failure. Doing this, one can create a flow law depending on finite strain and hence stress.
To this end, I will take crystals that are not deformed yet and run series of deformation experiments with different values of finite strain (~ 0.5 – 11%) and temperature (1000 °C, 1200 °C), investigating if the density and type of dislocations changes depending on finite strain for a given temperature and how they interact with each other (e.g. entanglement). I will furthermore re-investigate the role of so called disclinations: rotational topological defects, that were known to exist in liquid crystals and later metals, but were recently also found in olivine. Deformation will be performed in a “Paterson Press” (gas medium, internally heated deformation apparatus; Paterson, 1990) using hot pressed, polycrstalline olivine [(Mg0.91Fe0.09Ni0.003)2SiO4; Fournelle, ]. Thin sections of the deformed samples ill be used for several analytical techniques: EBSD (electron backscatter diffraction), TEM (transmission electron microscopy) and the dislocation decoration method (preferred precipitation of iron oxides at dislocation lines during oxidation at high temperatures). Using data gathered on all samples, I will additionally calibrate a tool with which densities of dislocations can be inferred from EBSD data alone (Kernel Average Misorientation, KAM in the MTEX toolbox).
In the end, my data will help to better understand the rheology of the lithospheric mantle. A sound understanding of the rheology of the lithospheric mantle is important both for numerical and experimental modeling and to understand how the mantle convection is related to tectonic plates and their kinematics.
Supervisors: Sylvie Demouchy1, David Mainprice1
1 Geosciences Montpellier, Université Montpellier, France