THERMAL AND CHEMICAL CONVECTION IN PLANETARY MANTLES

Melting of the upper mantle and extraction of melt result in the forma tion of a less dense depleted mantle. This paper describes series of t wo-dimensional models that investigate the effects of chemical buoyanc y induced by these density variations. The range of Rayleigh numbers a nd boundary conditions are appropriate to convection in Earth's upper mantle. A tracer particles method has been set up to follow as closely as possible the chemical state of the mantle and to model the chemica l buoyant force at each grid point, Each series of models provides the evolution with time of magma production, crustal thickness, surface h eat flux, and thermal and chemical state of the mantle. First, models that do not take into account the displacement of plates at the surfac e of Earth demonstrate that chemical buoyancy has an important effect on the geometry of convection. A depleted layer similar to 100 lan thi ck forms at the top of the mantle. This light stagnant layer reduces t he heat transfer and widens the aspect ratio of the cells. Consequentl y, the mantle cools down more slowly, The flow field is strongly modif ied by the chemical forces which yield displacements of the partial me lting zones and periodic volcanism. Then models include horizontal mot ion of plates 5000 km wide, Recycling of crust is taken into account. For a sufficiently high plate velocity which depends on the thermal Ra yleigh number, the cell's size is strongly coupled with the plate's si ze, Plate motion forces chemically buoyant material to sink into the m antle. Then the positive chemical buoyancy yields upwelling as deplete d mantle reaches the interface between the upper and the lower mantle. This process is very efficient in mixing the depleted and undepleted mantle at the scale of the grid spacing since these zones of upwelling disrupt the large convective flow. At low spreading rates, zones of u pwelling develop quickly, melting occurs, and the model predicts intra plate volcanism by melting of subducted crust. At fast spreading rates , depleted mantle also favors the formation of these zones of upwellin g, but they are not strong enough to yield partial melting. Their rapi d displacement toward the ridge contributes to faster large-scale homo genization.