Compressible thermochemical convection and application to lower mantle structures

Abstract

A new finite element code for compressible thermochemical convection is developed to study the stability of a chemical layer at the base of the mantle. Using composition-dependent compressibility and a density difference between compositions at a reference pressure, a composition-dependent density profile is derived. Together with depth-dependent thermal expansion, this combination of parameters yields a wide range of dynamic evolutions for the chemical layer. The chemical structures are classified into five major categories (classical plumes, mushroom-shaped plume, domes, ridges, and continuous layers) and a few abnormal cases, such as hourglass-shaped plumes and columnar plumes. Several models have a chemical structure morphologically similar to the African low V_S structure in the lower mantle, at least at a single time. Guided by our models, several dynamic scenarios are proposed for the dynamic nature of the lower mantle low-velocity structures (a.k.a. superplumes), including plumes at an early stage, plume clusters, ridges, passive piles, sluggish domes, and high-bulk-modulus domes. We predict seismic velocity anomalies from these dynamic models. The thermoelastic parameters used in the conversion are additional constraints. We compare the density structure with normal mode inversion, the predicted seismic signature observations, and the required thermoelastic parameters with mineral physics data. Among the proposed scenarios, only the scenario of high-bulk-modulus domes satisfies all constraints simultaneously. The implication on the geochemistry and mineralogy of lower mantle chemical structures is discussed.