GENERATION OF MEGA-PLUMES FROM THE CORE-MANTLE BOUNDARY IN A COMPRESSIBLE MANTLE WITH TEMPERATURE-DEPENDENT VISCOSITY

We have investigated the development of axisymmetric mantle plumes in a compressible mantle with temperature-dependent viscosity, using two different values for the thermal activation energy. Thermodynamic para meters were based on realistic Earth values in the shallow mantle. We assumed a compressible mantle with a phase transition and a two layer viscosity structure (mantle 30 times more viscous beneath the phase ch ange). A dramatic transition in plume dynamics occurred as the thermal activation energy was increased from 250 to 500 kJ mol(-1). A ''megap lume'' was formed from the merging of small-scale instabilities swept into the plume's axis, rapidly flushing the entire contents of the hot thermal boundary layer into the upper mantle. Hot plume material with a temperature anomaly of 250 K reached the lithosphere for our assume d temperature drop of 500 K across the base of the mantle. Viscous hea ting over four orders of magnitude greater than chondritic radiogenic sources was observed during the initial plume-lithosphere interaction.