3-DIMENSIONAL MODELS OF MARTIAN MANTLE CONVECTION WITH PHASE-TRANSITIONS

We have employed a three-dimensional compressible convection model to study the dynamics of phase transitions in the Martian mantle. A large core model with two exothermic phase transitions. the olivine to beta -spinel and the beta- to gamma-spinel transition, and a small core mod el including also the endothermic spinel to perovskite transition have been considered. The two exothermic transitions create 'thermal barri ers' for small upwellings due to the latent heat consumption from the phase change. Upwelling plumes lose part or all of their buoyancy, whi ch causes the formation of one stable area full of plumes. This tenden cy for the merging of plumes increases with internal heating. This typ e of convective planform is consistent with the relatively few large v olcanic centers. The presence of a 175 km thick perovskite layer above the core-mantle boundary (CMB) yields a similar flow pattern, albeit. with an even smaller number of plumes. However, the excess temperatur es of the plumes and the mantle flow velocities in the lower mantle ar e smaller than those found in models without perovskite layer. The pha se transitions cause an increase of temperature near the CMB, which pr events the lower mantle and the core from extensive cooling. A model w ith a perovskite layer decreasing in thickness with time can account f or a peak in volcanic and magnetic activity early in the Martian histo ry.