A mantle-to-crust study of magmatic processes in the Main Ethiopian Rift

Continental rifting is the means by which strong continental lithosphere is faulted, weakened, and ruptured to form a new ocean basin. This process evolves temporally and spatially, and is accompanied by significant seismicity and often crustal intrusion of mantle-derived magmas in its penultimate stages, which facilitate further extension through crustal thermo-mechanical weakening. Understanding the relationship between magmatism and extension in rifts is paramount for developing new models of tectonic evolution that account for the effects of magmas during the rifting process.

This thesis investigates the magmatic character of the late-stage Main Ethiopian Rift (MER), the northernmost sector of the East African Rift System. A subject of intense geophysical examination, the MER hosts volcano-tectonic segments that accommodate the bulk of extensional strain. Past literature has highlighted the anomalous nature of the mantle and the presence of both solid and molten intrusions in the rifting crust under these segments. To verify geophysical evidence, new independent petrological observations of melt generation and crustal magmatic storage and transport are necessary.

I explore magmatism in the MER by analysing erupted basalts from scoria cones. Geochemical analyses of these materials, including whole rocks, olivine crystals, and olivine-hosted melt inclusions, are used to explore melt generation, crystal fractionation, and cationic diffusion within crystals.

The three principal studies outlined in this thesis demonstrate that heterogeneous melts, derived from a hot mantle that is geochemically and lithologically enriched relative to ambient mantle, are stalled and stored in a mid-crustal weak layer prior to eruption. Significant degassing of CO2 occurs within this layer. Eruptions are triggered by the intrusion of hot mafic dykes in the months leading up to cone-forming events. These results provide new constraints on the temperature and composition of the sub-MER mantle, the storage conditions of rift magmas, and the timescales of processes that trigger eruptions.