Effective buoyancy ratio: a new parameter for characterizing thermo-chemical mixing in the Earth's mantle

Abstract. Numerical modeling has been carried out in a 2-D cylindrical shell domain to quantify the evolution of a primordial dense layer around the core–mantle boundary. Effective buoyancy ratio, B_eff was introduced to characterize the evolution of the two-layer thermo-chemical convection in the Earth's mantle. B_eff decreases with time due to (1) warming of the compositionally dense layer, (2) cooling of the overlying mantle, (3) eroding of the dense layer through thermal convection in the overlying mantle and (4) diluting of the dense layer through inner convection. When B_eff reaches the instability point, B_eff = 1, effective thermo-chemical convection starts, and the mantle will be mixed (B_eff = 0) over a short time period. A parabolic relationship was revealed between the initial density difference of the layers and the mixing time. Morphology of large low-shear-velocity provinces and results from seismic tomography and normal mode data suggest a value of B_eff ≥ 1 for the mantle.