Controls on Thermal and Dynamic Characteristics of Descending Slabs in the Lower Mantle

Global seismic tomography studies show that subducted slabs become significantly less "visible" in the lower mantle [Masters and Stixrude, 2006], although they are one of the dominant structures in the upper mantle and mid-mantle. On the other hand, the upwelling plume-like structure, while relatively weak in the upper mantle, becomes significant in the lower mantle, particularly above the CMB. The dominant slab structure in the upper mantle and mid-mantle is often suggested to be consistent with the fact that slabs transfer most of the surface heat flux of the Earth and are the main driving force of plate motion. Recently, Leng and Zhong [2007] have demonstrated that upwelling plumes dominate energy flux in the bottom part of the lower mantle, thus providing an explanation for the increased seismic "visibility" of plumes in the lower mantle. Using 3-D regional spherical finite element code CitcomCU with extended-Boussinesq approximation, we studied the thermal and dynamic characteristics of descending slabs and their implications for mantle geotherm (i.e., subadiabatic temperature) and seismic observations. Our main results can be summarized as following: 1) Slabs are being heated by adiabatic heating and diffusive heating effects as they descend, which lead to the great reduction in slab temperature anomalies, slab mass flux and slab heat flux. 2) Subadiabatic temperature is strongly related with slab deficit temperature and slab heat flux rather than with internal heating rate of the mantle. The reduction of slab heat flux from the surface to the CMB and the slab temperature anomalies near the CMB are both proportional to subadiabatic temperature. Although these results are derived from models with extended Boussinesq approximation, their validity will be verified in fully compressible convection models that we have recently developed.