Models of mantle convection: one or several layers

Abstract

Numerical model calculations are used to determine if convection in the Earth’s mantle could be organized in two or more layers with only limited mass exchange in between. The seismic discontinuity at 670 km depth and the top of the D"-layer at the bottom of the mantle are considered as candidates for internal boundaries. If the 670 km discontinuity is caused by an isochemical phase transition, it has to have a Clapeyron slope of dp/dT ⩽ — 6 MPa k -1 to prevent convection currents from crossing; this value is improbably low. If the discontinuity represents a chemical boundary, the intrinsic density difference has to exceed 3 % to prevent subducted lithospheric slabs from penetrating deeply into the lower mantle; also the condition is possibly hard to meet. The least improbable mechanism for a mid-mantle barrier for convection currents would be a combination of endothermic phase transition and chemical change. The boundary between upper and lower mantle would show considerable topography, and a limited material exchange is to be expected at any rate. The possibility of a downward segregation of former oceanic crust, transformed to dense eclogite, is studied in a further model series. It requires a region of low viscosity, as the Delayer probably is, and is faciliated by the decrease of the thermal expansion coefficient with pressure. About 20% of subducted oceanic crust could accumulate at the core-mantle boundary. The dense material would concentrate underneath rising therm al plumes, and some of it is entrained into the plumes, possibly affecting their geochemical signature.