From Source to Surface: The dynamics of heterogeneous mantle plumes

This thesis is a compilation of three papers investigating the relationship between mantle plume dynamics, Earth's thermo-chemical structure, and geochemical trends recorded by volcanic hotspots, using numerical simulations that are supported by geophysical and geochemical data sets. The results show that hotspot lavas reveal a wide range of dynamical plumes behaviour previously unrecognised, and that the systematic geochemical trends recorded in hotspot lavas have a complex relationship with deep mantle structure and composition. Paper 1 demonstrates that physical property variations across deep-mantle structure influence the structure of mantle plumes, geochemical trends recorded by hotspot lavas are unlikely to reflect large-scale chemical domains in the deep-mantle, and stable, long-lived, chemical `piles' in the deep-mantle will be sampled at the centre of plume conduits. The paper reports results from a suite of numerical models in which the compositional structure in the deep-mantle is tracked through an evolving mantle plume, where the density and rheology of compositional structure in the deep-mantle is varied with respect to the ambient mantle. Results indicate that mantle plumes only preserve deep-mantle composition when such variations in density and rheology are negligible. Paper 2 illustrates how the composition of a mantle plume's head and tail diverges when forming in a thermo-chemical boundary layer, helps explain the compositional differences between large igneous provinces (LIPs) and ocean island basalts (OIBs), provides support for hypotheses that suggest anomalous tungsten isotopic compositions originate from large low-shear wave velocity provinces (LLSVPs) and ultra-low velocity zones (ULVZs), and has implications for the physical and chemical properties of both LLSVPs and ULVZs. The paper presents isotopic calculations that account for the tungsten isotopic compositions of LIPs and OIBs and reports on numerical modelling results for thermo-chemical plumes with reservoir properties that satisfy the calculated relative source contributions to LIPs and OIBs as the respective melt products of plume heads and tails. The results restrict the nature of deep mantle composition to a relatively thin (200 km) layer at the base of the mantle and place hypotheses suggesting that LLSVPs and ULVZs are the likely source of anomalous tungsten isotopic compositions in a dynamically consistent framework. Paper 3 presents a hypothesis for the origin of double-track volcanism that can explain both its physical and chemical features. The paper reports the first evidence that double-track volcanism emerged concurrently across the Pacific plate and simultaneous with an azimuthal change in Pacific plate motion. Numerical modelling results, based on seismological, petrological, and plate-motion data, demonstrate how the azimuthal change in plate motion caused the emergence of double-track volcanism at widespread locations across the Pacific plate, and that the geochemical characteristics of Hawaiian volcanism are accounted for by differential melting of multi-component mantle plume. Results imply that the formation of double-track volcanism is transitory and can be used to identify and place temporal bounds on plate-motion changes