The effects of chemical density differences on convective mixing in the Earth's mantle

Abstract

Subducted lithosphere and crust are chemically distinct from ambient mantle, and some have hypothesized that the resulting anomalous buoyancy may affect mixing and, in particular, may cause chemical heterogeneity to persist. To explore this effect, two-dimensional convective mixing calculations have been carried out. Boundary conditions mimicking features of plate kinematics have been imposed and negatively buoyant tracers placed beneath converging plate margins to simulate subduction. The principal parameter of interest is the density anomaly ratio: Δρ/(ρ_o α ΔT), the ratio of the chemical to the thermal buoyancy. For a density anomaly ratio appropriate to complete separation of the oceanic crust from the rest of the lithosphere, it is marginally possible for the tracers to segregate into the warm region near the bottom and neutralize the thermal buoyancy there, for internally heated flows, and to segregate to the base of the uprising limbs of convection, for bottom heated flows. However, for the more likely scenario of oceanic crust and lithosphere maintaining their layering after subduction, there is neither segregation nor neutralization of thermal buoyancy. The average residence time is the average time tracers remain in the box from subduction to sampling beneath ridges and could be increased by 4% from the passive case at the very most if the crust and lithosphere remain layered. Even in the more extreme case of separation of crust from lithosphere, the residence times are only increased by 15% from the passive case.