Stability of thermochemical piles of different origins

As the origin of the stable large low shear velocity provinces (LLSVPs) beneath Africa and the Pacific is still unclear, we numerically consider two possible scenarios. Structures can form either from a primordial layer or a growing layer above the core-mantle boundary (CMB). The primordial layer is considered as a remnant of the early magma ocean phase, while the growing layer results from core-mantle interaction. In our 2D Cartesian study we analyze a diffusive influx of iron-rich core material.

We investigate the temporal and spatial stability of thermochemical piles under the influence of rheological parameters. Our model rheology is given by a viscosity depending on temperature, stress, depth and composition. Furthermore, we also investigate the effect of a depth-dependent thermal expansion coefficient. As all these parameters affect the strength of convection, they ultimately also have an impact on the stability of piles. Increasing the ratio between the top and bottom viscosity or expansivity leads to longer pile lifetimes and more stable piles. Therefore, piles can have formed in the Archean mantle but will have broadened and stabilized in time with the cooling of the mantle.

Typically, we find that these piles anchor thermochemical plumes, so that long-lived plumes exist in the center of piles. Less stable plumes occur at the edges of piles for a few million years as piles move and merge. The movement of piles results is a consequence of slabs pushing them around or of thermal plumes attracting dense piles.