My name is Sebastian Ritterbex and I am working in the field of theoretical and computational mineral physics, studying the physical properties and deformation processes of materials in the Earth's interior. I particularly focus on using theoretical methods to model the intrinsic properties of Earth materials, crystalline defects and deformation from the atomic to the macroscopic scale. My main interest lies in bridging the gap between geodynamics and the underlying microscopic processes.

Currently, I am a PhD-candidate in the ERC-funded RheoMan project at the University of Lille 1, France. Before this, I studied physics and geophysics at the University of Utrecht in the Netherlands (see CV).

During my PhD, I am using numerical modeling to investigate the intracrystalline plasticity of the transition zone minerals from the atomic up to the grain scale to infer the contribution of dislocation activity in mantle convection. The main methodologies I use are first-principles DFT atomistic simulations, finite element modeling and continuum elasticity modeling.

Some highlights PhD:

– Good agreement between theoretical constitutive equations and experiments for wadsleyite (15 GPa) and ringwoodite (20 GPa) for a wide range of temperatures

– At mantle strain rates, dislocation glide is difficult as a consequence of high lattice friction in both high pressure polymorphs of olivine

– Deformation by pure dislocation glide would result in a high viscosity layer under conditions of the upper and lower transition zone

Critical resolved shear stress (CRSS) versus temperature at a fixed mantle strain rate of 10-16 s1 for thermally actived glide of the [100](010) and ½<111>{101} screw dislocations in wadsleyite at 15 GPa. The shaded area depicts the stability field of wadsleyite in the upper transition zone at 15 GPa.

Comparison with experiments: the critical resolved shear stress (CRSS) versus temperature at a fixed laboratory strain rate of 10-5 s-1 for thermally actived glide of the [100](010) and ½<111>{101} screw dislocations in wadsleyite at 15 GPa.

– See below for further information, where you can find a brief overview of my PhD work.

Wadsleyite at 15 GPa:

Short summary PhD

Quantification of the plastic deformation of the main minerals that constitute the Earth's mantle is a major aim in geophysics as it is the fundamental mechanism by which the Earth expels its internal heat through mantle convection.So far, there is ample evidence, that mass transfer affects the whole convective mantle. However, seismic tomography also shows evidence that some plates tend to stagnate just above or within the transition zone. The viscosity profile across the transition zone is thus an important keystone in mantle convection.

We are using a computational mineral physics approach to

study the intracrystalline plasticity of both Mg2SiO4 end-members of the (Mg,Fe)2SiO4

polymorphs: wadsleyite and ringwoodite, at 15 and 20 GPa, respectively. Atomic

scale modeling of the defect structures is necessary to

quantify their mobility and the subsequent collective

behaviour of the defects which will be used to determine the

viscosity as a function of stress and temperature related to dislocation motion under both laboratory and natural transition zone conditions. The aim is to constrain the role of

dislocation glide activity in the boundary seperating the upper from the lower mantle. The work may as well help to constrain the expected anisotropy in the transition zone as a

consequence of the formation of LPO's during deformation.

Our numerical multiscale model relies essentially on multiphysics operating at different scales in space and time:

Atomic scale study: First-principles density functional theory (DFT)A dislocation is a line defect, characterized by an elementary quantity of shear distortion of the crystal lattice: the Burgers vector. Therefore, we first calculate the response of the crystal structure to a discontinuous shear along a given (slip) plane, which gives us an energy surface: a so called γ-surface. Here, the influence of pressure is accurately taken into account. Hence, calculations are performed based on first-principles (DFT) to explicitly incorporate the pressure-induced changes in the electronic structure. γ-surfaces provide an efficient tool to search for the lowest energy shear paths and form as such the key ingredient for modeling dislocation core structures.

Dislocation structures: Finite element modelingA finite element based (Peierls-Nabarro-Galerking model – Denoual, 2007) implementation of the Peierls-Nabarro model (Peierls, 1940; Nabarro, 1947) has been used to calculate the actual dislocation core structures and to quantify their lattice friction of the potential slip systems in wadsleyite and ringwoodite at the appropriate pressure conditions.

The final dislocation core structures arise by

minimization of the total energy of the system

composed out of the elastic energy and the interplanar

potential across the glide planes. The latter is

described by the γ-surfaces since the gradient of the

energy landscape captures all inelastic restoring forces

across the plane. The easiest dislocations in both

wadsleyite and ringwoodite appear to have complex structures: they show dissociation into partial dislocations.

Dislocation mobility: Elastic interaction modelA dislocation, however, moves as a result of stress-assisted thermal activation, by which a dislocation line overcomes its lattice friction. An elastic interaction model has been constructed which allows to calculate the critical configuration that triggers an elementary displacement of the dislocation, known as a kink-pair, as well as the critical enthalpy associated to this with the extremum of the corresponding total energy change. This model is fully adapted to dissociated dislocations as they occur in wadsleyite and ringwoodite.

Finally, the mobility of dislocations, their collective behaviour and the subsequent constitutive relations (viscosity) have been directly established by the above results. We were able to validate our approach by finding a good agreement between the constitutive relations obtained for wadsleyite and ringwoodite for a wide range of stresses and temperatures with experimental results. To this end, we could calculate the dislocation related viscosity as a function of stress and temperature for typical mantle strain rates of 10-16 s-1 without extrapolations. We show that the sole contribution of dislocation glide in wadsleyite and ringwoodite would result in a high-viscosity layer under conditions of the upper and lower transition zone.

The next step is to study the effect of dislocation climb (vacancy diffusion) using dislocation dynamics which has been already developed for (Mg,Fe)2SiO4 olivine during the RheoMan project.