Phase-transition modulated mixing in the mantle of the Earth

Abstract

Recently published a priori analyses of the mantle convection process demonstrate that the influence of the endothermic phase transformation at 660 km depth could be profound and such as to enforce a strongly intermittent style of circulation. In this phase-transition modulated regime, mass transfer across the 660 km horizon is inhibited, leading to the formation of an internal thermal boundary layer that straddles the phase change interface. This thermal boundary layer is itself episodically disrupted by convective instability, leading to the development of intense ‘avalanches’ of cold material from the transition zone into the lower mantle. The consequences of such a circulation regime to the understanding of trace element geochemistry and the supercontinent cycle may be extremely important. Equally important are the implications for mantle viscosity, since a circulation of this type would appear to allow reconciliation of a wide range of geodynamic data with a single viscosity model. The relevant data include convection related observables such as the aspherical geoid and postglacial rebound related observables such as relative sea level histories and certain anomalies in Earth rotation, including the so-called ‘non-tidal’ acceleration of rotation and the ongoing ‘wander’ of the rotation pole towards Greenland at a rate near 0.95 degrees per million years.