Desiderio, Matteo; (2025) Preservation of Ancient and Recycled Heterogeneities in the Evolving Earth Mantle: Insights from Geodynamic and Seismic Models. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this thesis, numerical models of thermochemical convection, combined with calculations of mantle-materials thermoelastic properties, are used to test hypotheses for Earth's lower-mantle compositional structure. Geodynamic models are used to investigate (a) the style of Recycled Oceanic Crust (ROC) accumulation in the deep mantle; (b) the preservation of large-scale, primordial domains in the mid-mantle, depending on ROC intrinsic density and viscosity at lower-mantle conditions. Seismic tomography reveals low-velocity anomalies in the deep mantle, known as LLSVPs, and often interpreted as thermochemical piles. A moderate density excess is required to form ROC piles; in turn, high intrinsic viscosity promotes stagnation and stratification within them, consistent with geophysical constraints on LLSVPs. Further, only high-viscosity piles can retain ROC within them over billions of years, consistent with geochemical evidence of pristine ancient recycled reservoirs. Mineral physics constraints point to low-viscosity ROC, implying that LLSVPs may be partly formed by early basal-magma-ocean cumulates. Large-scale, primordial mid-mantle anomalies are only preserved when ROC accumulates in the deep mantle. ROC piles trap heat from the core, insulating the mantle and lowering its average temperature. The mantle is thus more viscous and less efficiently mixed, compared to cases with no ROC accumulation. High ROC viscosity enhances primordial material preservation, as stratified piles promote a thicker and therefore more insulating thermochemical boundary layer. Finally, we use Gibbs free energy minimization to convert model snapshots to seismic wavespeed maps. Large-scale primordial domains yield a distinct P-S velocity signature that is preserved even when accounting for the distortions expected from realistic seismic tomography resolution. S-to-bulk-sound-velocity correlation profiles are computed to compare well-mixed and heterogeneous models, with primordial domains significantly lowering correlations compared to a homogeneous mantle. These profiles are compared to the tomographic model SP12-RTS, revealing a complex distribution of chemical heterogeneity across the mantle.

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