The interplay between recycled and primordial heterogeneities: constraints on Earth mantle dynamics via numerical modeling

A quantitative understanding of Earth's deep compositional structure remains elusive. Geophysical and geochemical observations illuminate heterogeneous features on various scales in the lower mantle: however, the origin and interaction of such heterogeneities are not yet fully explained in the context of global mantle dynamics. Conversely, numerical geodynamic models predict a wide range of viable scenarios of mantle convection and heterogeneity preservation. In the "marble cake" end-member mantle model, slabs of Recycled Oceanic Crust (ROC) are subducted and deformed but never fully homogenized in the convecting mantle. In the "plum pudding" model, MgSiO₃-rich primordial material may resist convective entrainment due to its intrinsic strength. Only few geodynamic studies have explored the effects of subducted ROC properties on mantle dynamics while also accounting for the influence of primordial heterogeneity. Furthermore, predictions from numerical models need to be tested against geophysical data. However, current imaging techniques poorly resolve the lower mantle and may be unable to distinguish between both end-member models above.

Here, we use the finite-volume code StagYY to model mantle convection in a 2D spherical-annulus geometry. We investigate the style of heterogeneity preservation as a function of two parameters: the intrinsic density and the intrinsic strength (viscosity) of basalt at lower-mantle conditions.  Additionally, we employ the thermodynamic code Perple_X and the spectral-element code AxiSEM to compute, respectively, seismic velocities and synthetic seismograms from the predictions of our models.

We obtain two main regimes of mantle convection: low-density basalt leads to a well-mixed, "marble cake"-like mantle, while dense basalt aids the preservation of primordial blobs at mid-mantle depths as in a "plum pudding". Intrinsically viscous basalt also promotes the preservation of primordial material. These trends are well explained by smaller convective vigour of the mantle as intrinsically dense (and viscous) piles of basalt shield the core. In order to test these model predictions, we convert model temperatures and compositions to thermoelastic properties for two characteristic models of each regime. These are then used to compute synthetic seismic velocity models, through which we simulate wave propagation using AxiSEM. Finally, we  discriminate between these two end-members by comparing statistical properties of the corresponding ensembles of synthetic seismograms. Our results highlight how the interaction of mantle materials drives the long-term thermochemical evolution of terrestrial planets. Furthermore, they provide a framework for relating the style of heterogeneity preservation in the Earth's lower mantle with specific features of the seismic waveforms.