Unravelling the Dynamical Mechanisms Underpinning Intra-Plate Volcanism Within and Around Earth's Continents
This thesis investigates the dynamical mechanisms at the root of Earth's intra-plate volcanism from a computational fluid dynamics perspective. It analyses different convective processes that enable and sustain decompression melting in the uppermost asthenosphere and, subsequently, proposes novel pathways to understanding the distribution, intensity and temporal evolution of volcanism observed within and around Earth's continents. Volcanism within and around Earth's continents displays diverse characteristics and is hard to reconcile with the plate tectonic framework. Where volcanic provinces exhibit a geometrical pattern known as a linear age progression, in which volcanoes grow older in the direction of plate motion, mantle plumes are the preferred driving mechanism for the initiation of mantle melting. Elsewhere, shallower convective instabilities, bound to significant changes in lithospheric thickness, have been gaining popularity in explaining the onset of decompression melting at depth. However, not all volcanic provinces within and around Earth's continents bear such unique geometrical and structural features. In such contexts, the applicability of mantle plumes and edge-driven convective instabilities remains unclear. In this work, I present a systematic series of 2-D and 3-D numerical models that involve sharp transitions in lithospheric thickness within and around continental lithosphere to study the development of shallow convective instabilities and their relation to magmatism. Some simulations incorporate plate motion and an upper-mantle plume to reproduce a range of possible interactions, as documented in the observational record. Model results demonstrate that more intense edge-driven circulations develop in regions where lithospheric geometry allows flow coalescence and where asthenospheric shear, relative to lithospheric motion, promotes ascending flow. However, the activation of decompression melting through edge-driven flows depends substantially on the lithospheric thickness, the potential temperature of the mantle, and the solidus of upper-mantle rocks. In light of this, the results presented suggest that shallow convective instabilities should only be able to sustain shorter-lived, lower-volume volcanism. In addition, simulations incorporating mantle plumes demonstrate that edge-driven circulations are usually negatively impacted by the plume flow field. These simulations highlight that the 3-D structure of lithospheric thickness controls the spatio-temporal occurrence of plume magmatism. A single plume can activate decompression melting simultaneously in separate locations, which can be as far as 1,000 km away from each other. Furthermore, plume magmatism can remain locked for several million years at a lithospheric step, given a specific lithospheric geometry and motion. Overall, findings from the numerical simulations shed light on plausible interactions leading to intricate patterns of magmatic production within and around Earth's continental lithosphere. Finally, this thesis includes collaborative work on determining the concentration of incompatible trace elements within magma generated at depth. The chosen approach relies on incremental batch melting and utilises parameterisations of mineral abundances, element partition coefficients, and melt productivity. The results obtained through this approach closely match a comprehensive collection of mid-ocean ridge observations and reproduce first-order trends from intra-plate volcanic samples, unlocking further constraints in simulating magmatic processes in several geologic settings.
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