INTERACTION OF THE UPWELLING PLUME WITH THE PHASE AND CHEMICAL-BOUNDARY AT THE 670KM DISCONTINUITY - EFFECTS OF TEMPERATURE-DEPENDENT VISCOSITY

Numerical simulations have been performed to investigate the interaction of upwelling plumes with the 670 km discontinuity. Temperature-dependent rheology is employed to introduce the low viscosity and strength of the hot plume. The 670 km discontinuity is taken as a phase and/or chemical boundary. The condition for penetration of the plume into the upper mantle is examined by varying the value of the Clapeyron slope of the phase transition and the compositional density difference of the chemical boundary. Possible styles of the mantle convection are classified into six types depending on the nature of the 670 km discontinuity. The plume in the lower mantle can penetrate into the upper mantle and reach the bottom of the lithosphere when the Clapeyron slope is greater than -3 to -4 MPa/K for the pure phase boundary, and when the compositional density difference is smaller than 0.5-1.5% for the pure chemical boundary. The plume can barely penetrate into the upper mantle when the 670 km discontinuity possesses even a small amount of the compositional density contrast (less than 0.5-1.0%) together with the phase transition of the experimentally determined Clapeyron slope of -3 +/- 1 MPa/K. This result suggests that the mantle may be of uniform composition or at most has a weak compositional layering if the origin of the plume is in the deep mantle.