Effects of depth-dependent properties on the thermal anomalies produced in flush instabilities from phase transitions

The effects of depth-dependent thermal expansivity, viscosity and thermal conductivity on mantle convection with phase transitions have been examined with two-dimensional finite-element simulations in an aspect-ratio four box. The model includes a core-mantle thermal coupling boundary condition which takes into play the secular cooling of the core by the overlying mantle flow. Initial surface Rayleigh numbers between 2 × 10⁷ and 10⁸ have been considered. With time the surface Rayleigh number decreases to a value upon which a transition takes place from layered to single-cell convection. This tumultuous period is marked by large-scale coherent breakthrough of cold material, trapped in the transition zone, all the way to the base of the mantle and a violent reaction of hot plume in the upper mantle. Both hot and cold anomalies have large magnitudes. Cold anomalies with temperatures exceeding 1000 K are found at the base of the mantle. The magnitude of the cold anomalies is largest with all three depth-dependent properties. The timescales of this catastrophic event are between 20 and 50 Myr, with the longest being produced by the model with depth-dependent expansivity and viscosity. Results from these simple two-dimensional cartesian models represent a lower bound to the larger cold thermal anomalies, potentially capable of being generated in spherical-shell convection models.