Interaction of plumes with a compositional boundary at 670 km

The seismic discontinuity observed at a depth of 670 km may result from a phase change, a compositional boundary, or both; likewise, it may mark a change in density, viscosity, or both, between the upper and lower mantle. This paper addresses whether upwelling plumes in the lower mantle penetrate into the upper mantle, and what effect lower mantle plumes have on convection in the upper mantle if there is a density jump at 670 km due to a change in composition with depth. A finite element model of double‐diffusive convection is used to model flow in a spherical, axisymmetric shell heated from below with a Rayleigh number of 106. The ability of plumes to rise into the upper mantle is governed by the buoyancy number, B=Δρc/ραΔT, which is the ratio of the compositional to thermal buoyancy. When B < 1 plumes readily cross the 670 km discontinuity. When B ≥ 1 plumes do not enter the upper layer. When B > 1 flow in the upper layer is shear‐coupled to flow in the lower layer; that is, a plume impinging on the compositional boundary will induce a down‐welling in the overlying layer. When B ≈ 1 the coupling is more complex; strong plumes in the lower layer are shear‐coupled to flow in the overlying layer, but as plumes evolve and weaken the flow in the upper layer switches to thermal coupling with entrainment of lower mantle material.