Magmatic volatile emissions from arc volcanoes in the southwest Pacific Ring of Fire

<p dir="ltr"><b>Volcanism at subduction zones plays a pivotal role in the geochemical cycling of elements between the Earth’s mantle, crust, and surface. Active volcanoes emit high concentrations of gases and aerosols, which influence the habitability of planet Earth by acting both as absorbers and reflectors of sunlight, as natural fertilizers and hazardous pollutants in the hydro- and biosphere. This thesis compares the current geochemical signatures of magmatic volatiles emitted by three different volcanic systems situated in three different volcanic arcs of the southwest Pacific Ring of Fire using a multi-instrumental sampling and measurement approach that combines ground- and drone-based in-situ sample collection as well as remote sensing measurements (e.g., filter packs, cascade impactors, alkaline traps, multi-GAS and DOAS instruments), as well as petrological techniques. In order to successfully collect near-plume samples by drone, a new deployment configuration for filter-based instruments has been designed, built and successfully used in this work.</b></p><p dir="ltr">The first case study at the open-vent degassing volcano of Mt. Yasur (Republic of Vanuatu) reveals a significant increase in average magmatic gas flux between 2022 (1894 ± 649 t/d SO₂) and 2023 (3183 ± 919 t/d SO₂). This was not detected by satellite remote sensing but is consistent with an increasing state of unrest following the occurrence of in-crater landslides. For the first time, measured plume characteristics reveal signs of high-temperature magmatic gas scavenging within the crater area by the development of ash coatings while measurements at altitude along the plume trajectory outside of the crater highlight an increase and survival of magmatic gas concentrations, especially bromine, up to one kilometre from the vents. Corresponding plume aerosol concentrations fall by an order of magnitude in concentration within one kilometre from their source vents, but elements of magmatic volatile behaviour are still dominant in fine aerosol modes at distance, highlighting how locally complex plume conditions include slow aerosol growth and mixing of the plume with ambient air, as well as phases of strong passive degassing and secondary aerosol formation in-between Strombolian eruptions.</p><p dir="ltr">The second case study at the hyper-acidic Te Wai ā-moe crater lake on the summit plateau of Mt. Ruapehu (New Zealand), in contrast to Mt. Yasur, represents a hydrothermally dominated magmatic system. Here, geochemical measurements of the volcanic plume compositions demonstrate and characterize a period of open-vent-like degassing through the crater lake. Although no visual signs of degassing were observed, two distinct phases of changing magmatic plume compositions including hydrogen halide and secondary metal aerosol emissions are revealed during this prolonged cooling phase of the lake. The multi-instrumental approach thus proved crucial in detecting an otherwise unnoticed phase of magmatic degassing highlighting volatile exsolution of CO₂, SO₂, and metal compounds in the shallow subsurface during an exceptionally long lake heating cycle (> 9 months). The degassing compositions mirror that of the 2006-07 heating cycle, which corresponds to the time period of the last explosive eruptions, but were accompanied by much higher emission rates, providing critical insight to future eruptive forecasting.</p><p dir="ltr">The third case study investigates the role of volatile-rich magma recharge at Ulawun volcano (Papua New Guinea) using petrological techniques to explain the recent (< 6 years) reappearance of basaltic Plinian eruptions. While mineral textures in ash samples highlighted the dominant presence of microlites and thus high crystallinities (> 50%), whole rock geochemistry showed a shift from a previous basaltic-andesitic to a now purely basaltic magma composition, while plagioclase geothermometry revealed high potential melt water contents (> 4 wt.%) and overall mineral-melt disequilibria prior to the eruptions. It is thus inferred that one or more mafic recharge events caused the onset of the 2019 eruptive sequence at Ulawun by causing increased buoyant magma ascent and rapid undercooling of the melt, leading to the explosive release of volatiles within an ultimately very viscous melt.</p><p dir="ltr">This thesis therefore provides insights into magmatic volatile emissions, volcanic plume dynamics, and mafic recharge of magmas from three different volcanic arcs of the southwest Pacific Ring of Fire, highlighting the novel utility of modern technology for collecting unique magmatic signatures even from hydrothermally dominated magmatic systems and remote volcanoes. It involves a data-supported discussion of future geochemical plume monitoring during phases of elevated volcanic unrest and shows new insights on the influence of magmatic aerosols on volcanic plume chemistry as well as the contribution of magmatic processes to strongly explosive basaltic eruptions (VEI ≥ 4).</p>