Violent surface effects such as earthquakes and volcanoes are evidence of the dynamic nature of planet Earth. Understanding the driving forces behind these surface phenomena and the nature of the solid state convection that occurs within the Earth's mantle is a major challenge to geoscientists. In general, we are unable to sample directly the Earth's deep interior, and instead we have to rely on a range of indirect geophysical techniques to observe the Earth's interior. Interpretation of the geophysical results is often heavily reliant on direct experimental measurements of physical and chemical properties conducted at conditions that simulate those of the deep Earth.
In recent years, for example, geophysical studies have provided us with some tantalizing indications of elastic anisotropy at the base of the lower mantle. However, to interpret the results and determine the source of such anisotropy requires knowledge of the elastic and plastic behaviour of relevant phases, predominantly silicate perovskite (Mg,Fe)SiO3 and magnesiowüstite (Mg,Fe)O. At the Geoinstitut dramatic advances have been made in experimental techniques, allowing the measurement of compressional and shear wave velocities of single crystals of (Mg,Fe)O, yielding the complete elasticity tensor for the whole solid solution series. To understand the anisotropic geophysical signature of a rock also requires knowledge of its rheological behaviour and the microstructures and textures that develop during deformation. This information can come from deformation experiments that allow determination of the flow behaviour of (Mg,Fe)O up to very high strains. The textural information combined with elastic properties may be used to determine potential signatures in seismic anisotropy.
The transition zone between ~410 and ~670 km depth is characterized by the formation of the (Mg,Fe)2SiO4 polymorphs wadsleyite and ringwoodite with increasing pressure and temperature. In terms of mantle convection it is unclear whether the transition zone acts as some form of barrier to deformation resulting in split level convection or whether whole mantle convection occurs. Besides the thermodynamics of the phase transitions the relative strengths of these three phases are important for defining the pattern of convection. Within the Geoinstitut in-situ experimental deformation studies such as those reported on olivine and its high-pressure polymorphs under transition zone conditions may provide some insight into their rheological behaviour. Due to the extreme experimental conditions, precise mechanical data are difficult to obtain and the emphasis lies in the analysis of the microstructural features via TEM (e.g. dislocations, slip systems) and high resolution SEM (e.g. grain size and lattice preferred orientation). Knowledge of the active deformation mechanisms derived from microstructural data will potentially allow us to make predictions of the macroscopic deformation behaviour within the transition zone. Predictions of the convective patterns are also reliant on knowledge of the physical conditions, especially temperature, which requires detailed knowledge of the thermal diffusivity of transition zone phases.
Olivine (Mg,Fe)2SiO4 is the major component in the upper mantle and therefore its physical and chemical properties largely determine the behaviour of this region. An important parameter in the olivine solid solution is the Mg-Fe interdiffusion under changing thermodynamic environments, since the chemical composition has an influence on diverse properties such as rheology, melting point, etc. The knowledge of the diffusion coefficient under varying P, T, fO2 conditions is therefore essential.
Although olivine is perhaps one of the most widely studied minerals in terms of deformation behaviour, extrapolation from experimental to natural conditions requires an understanding not only of how physical parameters such as temperature and pressure affect the flow behaviour but also the effect of environmental variables such as water fugacity. The presence of water has a great effect on deformation and phase stability within the mantle, and the question arises, how much water is transported into the mantle via the subduction of oceanic crust. Hydrated minerals, such as talc, could be potential carriers of this water and additionally may have strong effects on seismic properties of the subducting slab.