Abstract
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.
Highlights
The most prominent feature of the lowermost part of the Earth’s mantle is the two seismically slow domains beneath the Pacific and Africa (e.g., Dziewonski et al, 1993; Garnero et al, 2007a)
We studied the evolution of the convection, and we suggest the introduction of the time-dependent effective buoyancy ratio which characterizes better the dynamics of the thermo-chemical convection (TCC)
The erosion of the dense layer through thermal convection in the overlying mantle reduces the concentration of the dense material in the lower layer (c1) and increases it in the upper one (c0)
Summary
The most prominent feature of the lowermost part of the Earth’s mantle is the two seismically slow domains beneath the Pacific and Africa (e.g., Dziewonski et al, 1993; Garnero et al, 2007a). Deschamps and Tackley (2008, 2009) investigated systematically the influence of some important parameters (depth-, temperature- and concentration-dependent viscosity, internal heating, chemical density contrast, mineralogical phase change at 660 km) on the evolution of the initial dense layer and compared the power spectra of density and thermal anomalies obtained from seismic tomography and numerical models. They mapped the parameter space of the thermochemical convection and suggested the essential ingredients for a successful mantle convection model. We studied the evolution of the convection, and we suggest the introduction of the time-dependent effective buoyancy ratio which characterizes better the dynamics of the TCC
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