Complex phase distribution and seismic velocity structure of the transition zone: Convection model predictions for a magnesium-endmember olivine-pyroxene mantle

We have investigated the complex spatial distribution of mineral phases in the Earth's upper mantle, resulting from lateral variations of temperature in a convecting mantle, through numerical modelling of mantle convection. Our convection model includes a self-consistent thermodynamic description for an olivine–pyroxene composition in the SiO2, MgO system. The thermodynamic model is based on lattice vibrations and allows for the calculation of thermophysical properties as well as seismic wavespeeds. Our modelling results show a complex structure in the behavior of physical properties, in particular the seismic shear wavespeed, in a depth range including the mantle transition zone, 400–700 km. We demonstrate that this behavior is related to the distribution of mineral phases in the olivine–pyroxene system. Especially near cold downwelling flows, representing subducting lithospheric plates, our model results show strong lateral variation of mineral phases and associated shear wavespeed. We show that, typically, pockets of contrasting mineral phases smaller than 100 km occur in subduction regions. In line with current developments in seismic imaging of the mantle transition zone we have computed reflectivity profiles from the shear wavespeed distribution obtained from the convection results. We applied frequency filtering to the raw reflectivity data to investigate the requirements for resolving the heterogeneous structure of the transition zone. Our results show that heterogeneous structure from contrasting mineral phase regions is resolved in the reflectivity profiles, for periods below 20 s which may be feasible in seismic imaging applications. This opens a perspective for detailed mapping of mineral phase distribution in the mantle offering new constraints on the thermal conditions of the transition zone region, thus providing valuable constraints for geodynamic models of the upper mantle.