The role of piled-up granite at the transition zone on the evolution of thermal convection pattern beneath a supercontinent

Recent studies on the mechanism of mantle subduction have revealed that not only subducting plate but also wedge of continent are eroded and lost from the surface of the earth. The fate of subducted material is not decided yet, but theoretical and experimental studies suggest that once granite migrates deeper than a depth it becomes the densest component under the upper mantle condition while the density of granite near the surface is much smaller than surrounding mantle components. However, at the lower mantle condition, granite becomes less dense than surrounding mantle components, again. On the other hand granite is known to be enriched in long-lived radio isotopes and play as an internal heat source. Thus even though granite is to be piled-up and stabilized at the transition zone ( at the depth of 410-660km) in terms of density, granite would enhance thermal instability in terms of following thermal evolution. Especially a bunch of granite is to be piled-up when a supercontinent forms, which would invoke a superplume in the following history resulting in break-up of the supercontinent. The growth time of thermal instability due to piled-up granite at the transition zone is estimated to be consistent with formation-break up cycle of supercontinent. However it is not clear yet whether the thermal instability is enough to invoke huge scale superplume or not. Thus, in this study, we carry out two dimensional numerical simulation of thermal convection under wide variety of size and power of internal heat source anomaly to evaluate the time scale of plume formation and the size of the plume.