Deep carbon dioxide and volatile degassing in the Lesser Antilles arc

This thesis provides novel constraints on volatile systematics and magmatic plumbing systems at Soufrière Hills Volcano (SHV), Montserrat and La Soufrière volcano, St Vincent, which are two of three volcanoes in the Lesser Antilles arc that have erupted in the last 100 years. In this thesis, SHV melt inclusions in magma erupted across 15 years of activity from 1995-2010 are characterised in order to improve our understanding of the plumbing system, better interpret current unrest, and thus aid risk management. Through the use of total CO₂ and H₂O, solubility models reveal a vertically extensive transcrustal magma mush system beneath SHV. The inferred depth range of 5.7×17 km, spanning the upper to mid crust, far exceeds previously published estimates of 5-6 km, which omitted the 90% of CO₂ hosted within vapour bubbles. The measurements of total CO₂ enabled the pre-eruptive CO₂ budget to be calculated across the studied Phases and, together with published remote sensing-derived fluxes, reveal that the maximum amount of CO₂ that can be fluxed through magma at this volcano is ~1500-1750 tonnes/day, indicating that measured CO₂ fluxes above this threshold is due to CO₂ flushing. To further quantify the release of total CO₂ from Soufrière Hills Volcano, a preliminary soil CO₂ survey shows that diffuse degassing of CO₂ at this volcano amounts to ~340 g m^-2 day^-1 in 2021/2022, a reduction of more than 90% from diffuse CO₂ degassing in 2008. Melt inclusions at La Soufrière are also characterised, providing the first full melt inclusion dataset (H₂O, total CO₂, S, Cl, and F) for the explosive phase of the 2020/2021 eruption, and indeed for any eruption at this volcano, enabling the reconstruction of magma storage characteristics, pre-eruptive volatile budgets, and volatile emissions to the atmosphere. Depths inferred from the H2O and CO₂ contents of melt inclusions are 2.4–8.9 km, and correspond well with independent depth estimates from clinopyroxene-only barometry of 1.1–8.0 km. In contrast to SHV, La Soufrière melt inclusion bubbles are largely empty. However, the presence of carbonates suggests that CO₂ was lost from these bubbles. Additionally, we provide evidence of polybaric crystallisation based on mineral-liquid equilibria, mineral barometry and melt inclusion thermobarometry. Using the improved petrological method, we estimate that a minimum of 2.99 Mt H₂O, 0.14 CO₂, 0.39 Mt SO₂, and 0.18 Mt HCl was emitted during the explosive phase of the 2020/2021 eruption. Preliminary soil CO₂ surveys show that the contribution of diffuse CO₂ degassing to the total CO₂ output is negligible (76 kg day^-1) compared to CO₂ fluxes from passive degassing. Altogether, this thesis provides critical information on CO₂ systematics at Soufrière Hills Volcano and La Soufrière St Vincent in the Lesser Antilles, refines our understanding of the magmatic system at Soufrière Hills Volcano, and provides independent pressure and depth estimates at La Soufrière volcano.