ALBERT

Description: Abstract: The modelling of coupled thermo-kinetic processes associated with non-equilibrium phase change kinetics can be useful for a better understanding of the the time-dependent subduction process. The latent heat release during these non-equilibrium phase changes can dramatically reduce the strength of subducting slabs which would affect their fate. We have investigated the effects of kinetics associated with the olivine-spinel transition in a descending slab. We have laid out the mathematical formulation of a two-dimensional time-dependent model consisting of the kinetic equations, which are cast as a system of nonlinear ordinary differential equations (ODE) at each spatial grid point and the time-dependent partial differential equation (PDE) for the temperature which is coupled to the kinetics by virtue of latent-heat release. This set of ODE-PDE system has been solved by the differential-algebraic method. The structure of the kinetic phase boundary is strongly determined by thermo-kinetic coupling effects during the transition. For slow, warm slabs a localized weakening of the slab could result from the heat production due to the latent heat release. Along the kinetic phase boundary, near the typical depth for equilibrium phase transformations protuberances in the temperature field are obtained caused by the thermal-kinetic feedback inhibiting the transition. Thermal stresses are likely to result from both of these effects. For fast, cold slabs regions with metastable olivine may be pushed down to a depth of about 600 km, while the latent-heat release reduces this effect. Deep focus earthquakes may occur because of the sharp tapering of the metastable phase boundary near 600 km depth. The deep portion of fast subducting slabs may have an anomalously hot and weak region just below the metastable transition wedge. The correlation between slow subducting velocity and the clustering of earthquakes near 400 km depth, e.g. the Izu-Bonin trench, and the fast subducting velocity and the concentration of deep-focus earthquakes at around 600 km depth, as shown for the Tonga-Kermadec trench can be predicted by this 2-D thermal-kinetic model.

Description: The analysis of seismic data represents today the primary tool in the search for the presence of postperovskite in the lowermost mantle (D″). This work aims at testing whether the inversion of gravitational data can also contribute to the detection of postperovskite in D″. We assume that the transition from perovskite to postperovskite is accompanied by a reduction in viscosity and test the effects of such viscosity change on the prediction of the dynamic geoid with a numerical model of subducted lithosphere. Our results show that the long-wavelength component of the geoid is very sensitive to the presence of postperovskite areas in D″, especially if their viscosity is significantly lower than the viscosity of the surrounding perovskite and if these areas are located close to density anomalies, i.e. subducted slabs.

Description: The recent discovery of the postperovskite phase due to mineral physics can have important consequences for understanding the structure of and processes in the lowermost mantle. However, geophysical observables that can confirm the presence and the very existence of this phase in the lower mantle are rare. So far, the search for postperovskite regions has been mainly based on analysing seismic records. In the present study, we investigate whether the analysis of other type of data, namely of the long-wavelength non-hydrostatic geoid, can bring some information about the distribution of postperovskite in the D" layer. We assume that the perovskite-to-postperovskite phase transition is accompanied by a decrease of viscosity, and, for a synthetic model of a subducting slab, we investigate the influence of such viscosity variation on the mantle flow which in turn affects the non-hydrostatic geoid associated with the slab itself. The results of our numerical tests suggest that the geoid is strongly sensitive to the presence of localized low-viscosity regions in the D" layer, especially if their lateral dimension is large (more than 1000 km) and if they are located close to the bottom end of the subducted slab. We also test the robustness of the geoid inversion using synthetic data and demonstrate that the resolution of the inversion can be significantly improved if a priori seismic constraints on geometry of the postperovskite domain are taken into account.