Dynamic phase boundary topography by latent heat effects

When radial flow passes through a mantle phase boundary, the interplay of advection and diffusion of latent heat creates "dynamic" topography of this boundary, which means a deflection that is related to the sense and magnitude of radial velocity even though the flow does not advect temperature differences towards the phase boundary. A one-dimensional stationary model is used to quantify the effect, which does not dependent on the sign of the Clapeyron slope. Significant deflection requires slow radial flow and a narrow phase loop. These conditions are most probably met for the phase transition from spinel to perovskite and magnesiowustite at 660-km depth. Up to 4 km of topography (peak-to-peak) are created, with a mass anomaly that is equivalent to 440 m of dynamic topography at the Earth's surface. Other phase transitions of the olivine component also contribute to a degree that is less certain. The time scale of adjustment of phase boundary topography is studied in a two-dimensional model, where a deep mass anomaly in an isentropic mantle drives flow through the 660-km phase boundary. The adjustment time depends on the inverse square of the vertical velocity and is on the order of IO Ma in the Earth, which is shorter than the convective time scare for a high-viscosity lower mantle. Dynamic phase boundary topography created by this mechanism can replace a significant part of the surface topography required in dynamic geoid models and can help to reduce the misfit between predicted dynamic surface topography and the upper limit that observations pose on its amplitude. (C) 1998 Elsevier Science B.V.