EFFECTS OF MULTIPLE PHASE-TRANSITIONS IN A 3-DIMENSIONAL SPHERICAL MODEL OF CONVECTION IN EARTHS MANTLE

Numerical models of mantle convection that incorporate the major mantl e phase changes of the transition zone reveal an inherently three-dime nsional flow pattern, with cylindrical features and linear features th at behave differently in their ability to penetrate the 670-km discont inuity. The dynamics are dominated by accumulation of cold linear down wellings into rounded pools above the endothermic phase change at 670 km depth, resulting in frequent ''avalanches'' of upper mantle materia l into the lower mantle. The effect of the exothermic phase transition at 400 km depth is to reduce the overall degree of layering by pushin g material through the 670-km phase change, resulting in smaller and m ore frequent avalanches, and a wider range of morphologies. Large quan tities of avalanched cold material accumulate above the core-mantle bo undary (CMB), resulting in a region of strongly depressed mean tempera ture at the base of the mantle. The 670-km phase change has a strong e ffect on the temperature field, with three distinct regions being visi ble: (1) the upper mantle, containing linear downwellings arid pools o f cold material in the transition zone and characterized by a high amp litude long-wavelength spectrum; (2) the midmantle, containing quasi-c ylindrical avalanche conduits and characterized by a low amplitude, br oad spectrum; and (3) the deep mantle, containing large pools of cold, avalanched material and characterized by a high amplitude, ultra-red (i.e., long wavelength) spectrum. The effect on the velocity field is very different. Flow penetration across the 670-km phase change is str ongly wavelength-dependent, with easy penetration at long wavelengths but strong inhibition at short wavelengths. Thus, when comparing numer ical models with long-wavelength seismic tomography, diagnostics based on the density field, such as the radial correlation function, are mu ch more sensitive to the effects of phase transitions than those based on the velocity field. The amplitude of the geoid is not significantl y affected by the partial layering, because the contribution from the strong heterogeneity in the transition zone is almost perfectly balanc ed by the contribution from deflection of the 670-km discontinuity. Av alanches are associated with geoid lows. However, a more complex visco sity structure is required to correctly match the sign of the geoid ov er slabs in Earth.