AVALANCHE EFFECTS IN-PHASE TRANSITION MODULATED THERMAL-CONVECTION - A MODEL OF EARTHS MANTLE

We describe the development and application of an anelastic, multiphas e model of the mantle convection process in axisymmetric spherical she ll geometry. The radial structure of the anelastic reference state has been determined on the basis of elastic wave propagation data, primar ily those used to construct the preliminary reference Earth model (PRE M). The multiphase model is employed to examine the extent to which th e pressure-induced phase transitions in the planetary mantle may consp ire to cause the flow to become radially layered. We find that the end othermic phase transition at 670 km depth profoundly influences the ra dial mixing process in the high Rayleigh number regime. In the Earth-l ike region of parameter space the flow exhibits a low-frequency quasi- periodicity characterized by rather long periods of relative quiescenc e in which the circulation is predominantly layered followed by short periods of intense radial mixing across the endothermic horizon. These ''avalanche'' events are controlled by the periodic instability of th e internal thermal boundary layer that develops on the endothermic hor izon when the flow is layered. This hydrodynamic process appears to ha ve important implications for the understanding of a number of charact eristics of planetary evolution, especially thermal and chemical histo ry.