VARIOUS INFLUENCES ON 3-DIMENSIONAL MANTLE CONVECTION WITH PHASE-TRANSITIONS

Three-dimensional numerical calculations of mantle convection with the two major phase transitions have been carried out in a 5 x 5 x 1 conf iguration to study the effects of increasing vigor of convection, inte rnal heating and extremely negative Clapeyron slope of the spinel-pero vskite phase transition. Depth-dependent properties of thermal expansi vity, viscosity and thermal conductivity have been incorporated. Three -dimensional solutions for surface Rayleigh number (Ra-s) between 2 x 10(6) and 4 x 10(8) show that there is a distinct transition between R a-s = 4 x 10(7) and 10(8) in which the system changes from single-laye red to layered convection with the mass flux decreasing to below 10% a t Ra-s = 10(8). Surface heat flux does not decrease with increasing Ra -s and with the accompanying decrease in the mass flux at the transiti on zone. The effects of internal heating are to reduce the wavelengths of the planforms which cause greater degree of layering in the system . Comparison between 2D and 3D results shows that there is a greater m ass flu passing through the transition zone in the 2D models for the 5 x 5 x 1 box, but for larger aspect ratio (8 X 8 X 1) the 3D flows bec ome more layered than the corresponding 2D solution. Increasing the ma gnitude of the negative Clapeyron slope by three times the experimenta l value can bring about a dramatic reduction in the amount of mass flu x across the 670 km discontinuity. Results from a 8 x 8 x 1 box show t hat a greater amount of layering is produced in the larger aspect-rati o configuration because of the shorter wavelengths of the developed pl anforms. Increasing the degrees of freedom in a 3D system by either gr eater amounts of convective vigor or larger domains may give rise to a greater tendency for layered convection. Three-dimensional spherical shell models may produce a greater degree of layering than the corresp onding axisymmetric models.