Influence of Variable Thermal Expansivity and Conductivity on Deep Subduction

Thickening of subducted slabs in transition zone is a robust feature of high-resolution tomographic models, which is generally attributed to the combined effects of the two major phase transitions at 410 and 660 km depth and to a viscosity increase across the upper-lower mantle interface. Numerical models indeed show that phase changes and viscosity jump favour buckling instabilities beneath the transition zone. These ultimately lead to the formation of folded slabs whose lower mantle portion is significantly thicker than the upper mantle one. Such models, however, rely on the assumption of constant thermodynamic properties. Here we present 2D numerical simulations of subduction that include pressure-, temperature-, and phase-dependent thermal expansivity (α) and conductivity (k), and demonstrate that these two parameters exert a dramatic effect on the dynamics of lower mantle slabs. In contrast to simulations in which thermal expansivity and conductivity are held constant, when they vary with both pressure and temperature, significant slab buckling is observed even when neglecting both phase transitions. The decrease of α with pressure causes the slab to lose buoyancy during their descent. This strongly enhances buckling instabilities and reduces the sinking speed, thereby promoting thermal diffusion over advection and inducing a significant spreading of the slab thermal anomaly in the lower mantle, which is also facilitated by the increase of thermal conductivity with pressure. While the temperature dependence of k exerts negligible effects, the temperature dependence of α strongly affects sinking speeds, rendering subduction remarkably faster than in cases with constant expansivity. Pressure- and temperature-dependent thermal expansivity and conductivity exert a first order influence on the dynamics of deep subduction and should be routinely incorporated in mantle simulations. Temperature anomaly for models with constant and variable thermal expansivity and conductivity. All other patameters are the same in the two examples, including a 10-fold viscosity jump across the upper-lower mantle interface.