Phase boundary deflections at 660‐km depth and episodically layered isochemical convection in the mantle

Abstract

Numerical simulations have been performed using a multiphase, anelastic, axisymmetric spherical, mantle convection model as part of an ongoing effort to explore the ability of the endothermic phase transition at 660 km depth to cause the circulation to assume a layered style. In particular, model solutions have been constructed for a Rayleigh number of 10 7 , internal heating corresponding to 50% heating from within and 50% heating from below, and Clapeyron slopes for the 410‐km and 660‐km phase boundaries set to +3.0 and −2.8 MPa/K, respectively. In this regime the flow exhibits a substantial degree of radial layering wherein the radial mass flux is reduced significantly at 660‐km depth. This layered regime of flow is episodically disrupted by massive localized avalanches of fluid across the 660‐km boundary Uiat recur at intervals separated by hundreds of millions of years. The degree of layering is related to the magnitude of the 660‐km phase boundary deflection away from its average depth. In these Earth‐like simulations we find that the average magnitude of such phase boundary deflections is similar to the average magnitude of seismically observed deflections of this horizon.