Thermochemical structures within a spherical mantle: Superplumes or piles?

Heterogeneous compositional mantle models are frequently invoked to explain some observations obtained from geochemistry and seismology. In particular, two regions in the Earth's lower mantle, under the Pacific and Africa, are often interpreted as being either piles or superplumes of dense material. We perform numerical modeling of thermochemical convection in a three-dimensional spherical geometry to investigate whether the geometry can focus a dense chemical component into a small number of isolated, rounded piles or superplumes of material. We study the effect of temperature and compositionally dependent viscosity, and we find two different modes of dense layer deformation. Temperature-dependent rheology leads to a low-viscosity dense layer which is passively swept aside by downwellings into linear piles that are spread throughout the entire lower mantle. The addition of compositionally dependent rheology in which the dense layer is more viscous results in active deformation of the dense material, forming large, isolated superplumes. Our results indicate that piles and superplumes are separate features which, in general, do not occur together and, in order for isolated, rounded superplumes to form, an intrinsic compositional viscosity increase is required for the dense material.