PHASE-BOUNDARY DEFLECTIONS AT 660-KM DEPTH AND EPISODICALLY LAYERED ISOCHEMICAL CONVECTION IN THE MANTLE

Numerical simulations have been performed using a multiphase, anelasti c, axisymmetric spherical, mantle convection model as part of an ongoi ng effort to explore the ability of the endothermic phase transition a t 660 km depth to cause the circulation to assume a layered style. In particular, model solutions have been constructed for a Rayleigh numbe r of 10(7), internal heating corresponding to 50% heating from within and 50% heating from below, and Clapeyron slopes for the 410-km and 66 0-km phase boundaries set to +3.0 and -2.8 MPa/K, respectively. In thi s regime the flow exhibits a substantial degree of radial layering whe rein the radial mass flux is reduced significantly at 660-km depth. Th is layered regime of flow is episodically disrupted by massive localiz ed avalanches of fluid across the 660-km boundary that recur at interv als separated by hundreds of millions of years. The degree of layering is related to the magnitude of the 660-km phase boundary deflection a way from its average depth. In these Earth-like simulations we find th at the average magnitude of such phase boundary deflections is similar to the average magnitude of seismically observed deflections of this horizon.