Numerical Study on Rheological Parameters Affecting the Stability of Thermochemical Piles and Plumes

The origin and exact nature of the large low seismic provinces (LLSVPs) located beneath Africa and the Pacific are still open questions and highly debated. As these structures are assumed to be at their respective locations for at least a few hundred million years, a thermochemical nature seems highly likely.
We use a 2D double-diffusive mantle convection model to numerically investigate the temporal and spatial stability of thermochemical piles for various rheological parameters. We compare results of the commonly investigated depth dependence of the viscosity (due to pressure and composition) with the effect of the yield stress and variable thermal expansivity. We find that increasing the top or bottom viscosity yields temporally and spatially more stable piles. Similarly, a decreased thermal expansivity with depth also results in slower entrainment of the high compositional material and thus more stable piles. Additionally, the appropriate combination of parameters can counterbalance destabilizing properties such that, for example, structures containing melt can also be long-lived and spatially stable, which would otherwise be quickly entrained due to the low viscosity of melt.
Furthermore, we studied the effect of rheological parameters on the stability of plumes and investigated the location of plumes with respect to thermochemical piles. Our results show a mutual dependency of the plumes and piles. Typically, large plumes are anchored by piles and located in the pile center. However, strong thermal plumes in the ambient mantle can pull along high compositional material. This can lead to the deformation of piles. During this process, or the merging of piles due to strong slabs, plumes are observed at the edges of piles, existing there for several million years before striving to the center of a pile.