The concept of a compositional heterogenous reservoir in the deep lower mantle has become increasingly popular in recent years, due to indications from seismology, geochemistry and heat budget calculations. In this study, the dynamic feasibility of this concept was investigated, using numerical modelling techniques.
A thermo-chemical mantle convection model in the extended Boussinesq approximation was applied. The convection equations are solved using a finite element method, where the composition field is described by Lagrangian tracer particles. 2-D cylindrical geometry and a time window of 4.5 Gyr were applied, and the models include the effects of the endothermic phase transition at 660 km depth.
The dynamic evolution of a compositionally distinct layer at the bottom 20% of the mantle has been examined. This anomalous layer was given a positive density contrast and a time dependent internal heating, increased with respect to the background mantle.
The results show a clearly bounded stability domain in a domain-diagram with density contrast and internal heating rate. Also, the parameters of the endothermic phase transition have a large impact on the evolution of the compositional layering. Vigourous mantle convection, once it sets in, controlled by the phase transition, produces cold downwellings, which eventually destroy the layering with (sub)critical density contrasts.