ON THE ABILITY OF PHASE-TRANSITIONS AND VISCOSITY LAYERING TO INDUCE LONG-WAVELENGTH HETEROGENEITY IN THE MANTLE

The ability of phase transitions and a viscosity jump at 660 km depth to induce long-wavelength flow in the mantle is systematically investi gated using three-dimensional numerical simulations of internally-heat ed convection at convective vigors (as indicated by the Rayleigh numbe r Ra) ranging from mildly supercritical to greater than the Earth's cu rrent regime. Cases with neither a viscosity jump nor phase changes di splay a steadily decreasing wavelength with increasing Ra, as expected . Increasing the lower mantle viscosity by a factor of 30 induces sign ificantly longer wavelengths, but the ''reddening'' effect decreases w ith increasing Ra; this is explained in large part by the decrease in effective Rayleigh number due to increasing the lower mantle viscosity . In contrast, the effect of phase transitions is minor at low Ra but increases sharply at geodynamically relevant Ra, potentially increasin g the mean wavelength of thermal heterogeneity by factors of greater t han 10. Conclusions regarding the relative importance of the two effec ts are thus highly dependent on Ra.