Articles | Volume 6, issue 1
Research article
28 Jan 2015
Research article |  | 28 Jan 2015

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

A. Galsa, M. Herein, L. Lenkey, M. P. Farkas, and G. Taller

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, Beff was introduced to characterize the evolution of the two-layer thermo-chemical convection in the Earth's mantle. Beff 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 Beff reaches the instability point, Beff = 1, effective thermo-chemical convection starts, and the mantle will be mixed (Beff = 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 Beff ≥ 1 for the mantle.

Short summary
The effective buoyancy ratio was introduced as a diagnostic tool to characterize the evolution of the thermo-chemical mixing in the Earth’s mantle. This parameter tracks the fate of the primordial compositionally dense layer above the core–mantle boundary such as (i) the transition phase of warming dense layer; (ii) the erosion and dilution of the dense layer; (iii) the effective thermo-chemical convection (mixing of layers) and (iv) the homogenization.