MANTLE PHASE-TRANSITIONS AND LAYERED CONVECTION

Numerical simulations with an anelastic, spherical, axisymmetric mantl e convection model have been conducted to address the question of the radial mixing length in the general circulation of the mantle. Continu ing debate centers on the question as to whether or not the 670 km sei smic discontinuity (which we now understand to exist as a consequence of an endothermic phase change of mantle mineralogy from the spinel ph ase to a mixture of perovskite and periclase) in combination with the 400 km discontinuity (associated with the exothermic phase change from olivine to spinel) will impose a sufficient barrier to the circulatio n so as to induce layering. We argue herein that the mantle must curre ntly be convecting in a partially layered style but that the degree of layering is highly time dependent. Moreover, in the perhaps not too d istant past the propensity to layering was greater, possibly to the ex tent that soon after planetary formation mantle mixing occurred in two distinct reservoirs. As the planet cooled and the Rayleigh number fel l, we suggest that the circulation was transformed from the layered st ate to the partially layered state that obtains today.