Influence of two major phase transitions on mantle convection with moving and subducting plates

A series of numerical simulation has been carried out to explore the influence of two major phase transitions at 410 km and 660 km phase boundaries on mantle convection with self-consistently moving and subducting plates, that is, on the “plate-like regime” of mantle convection. The degree of Clausius-Clapeyron slope at the 660 km phase boundary is systematically changed within the range estimated by high pressure experiments. On the plate-like regime where moving plates continuously subduct, as the Clausius-Clapeyron slope is steepened, the upwelling plumes originating from the bottom thermal boundary layer are less buoyant owing to the increase of average mantle temperature, so that the upwelling plumes are hard to penetrate through the 660 km phase boundary. Investigations reveal that three types of small upwelling plumes in the upper mantle are found on the plate-like regime: (1) The secondary plumes directly derived from the upwelling plumes from the lower mantle, (2) the passive upwellings from the shallow parts of the lower mantle due to the diffused return flow by continuously subducting plates, and (3) the secondary plumes originating from the 660 km phase boundary caused by the development of the small-scale convection cells confined in the upper mantle.