Effect of mantle depletion buoyancy on plume flow and melting beneath a stationary plate

We analyze a dynamical model of plume-lithosphere interaction in two-dimensional Cartesian geometry that takes into account the buoyancy of the depleted residue produced by melt extraction. The plume enters through the open bottom boundary at 400 km depth and leaves through the right side boundary of the model box. The viscosity is strongly pressure and temperature dependent. We use a large number of Lagrangian tracer particles to monitor progressive melting in the plume head and to track the advection of the depleted residue. The density reduction in the residue enhances small-scale instabilities sinking from the bottom of the lithosphere into the depleted layer. Initially, the melt production rate is slightly enhanced when depletion buoyancy is taken into account. However, in the subsequent evolution, melt production rates are lowered by a factor in the range 0.5 - 0.65, depending on the initial thickness of the lithosphere, compared to cases without density difference of the residue. The buoyancy of the residual mantle opposes its advection away from the top of the plume. A depleted root is formed at the bottom of the lithosphere, which inhibits further thermal erosion of the plate. It forces the plume flow to stagnate at greater depth and hence reduces the melt production rate. The effect is particularly strong for a case where the plume rises beneath locally stretched lithosphere. The results are compared to the evolution of volcanism at the Cape Verde hotspot and the Kenya rift.