Magmatic sources to volcanic gas emissions: Insight from the Garibaldi Volcanic Belt, western Canada

Volcanoes are the surficial representation of the complex interplay of magmatic, crustal, and hydrothermal processes operating below the Earth’s surface. Studying volcanic deposits provides a unique snapshot into the composition of the deep mantle source. Melt inclusions, which are small pockets of magma trapped inside growing crystals, preserve multi-faceted records of magma petrogenesis. Additionally, the magma at depth contains a significant proportion of dissolved magmatic gases. Once these gases become saturated in the magma, they exsolve into a separate vapour phase and ascend quickly to the surface, where they are released as volcanic gases. The composition of gases at the surface can provide valuable insight into the composition of the magma at depth, as well as the shallow-level hydrothermal system. Volcanic gases are an important factor controlling whether an eruption is gentle and effusive, or violent and explosive. A greater amount of gas that separates at depth can trigger large, explosive eruptions (Devine et al., 1998; Shinohara, 2008). Therefore, gas detection at the surface is an essential monitoring tool for eruption forecasting. In this thesis, I begin with an in-depth look inside glassy and crystallized olivine-hosted melt inclusions and assess the nature of micron-scale solid phases occupying the vapour bubble. From here, major, volatile and trace element compositions of olivine-hosted melt inclusions from every centre along the Garibaldi Volcanic Belt (GVB) reveals a north to south transition from an enriched mantle signature, to a subduction-modified depleted mantle source. Finally, the recent re-discovery of fumaroles beneath the summit glacier of Mount Meager has prompted the first MultiGAS survey in the GVB. The plumes are H2S, CO2 and H2O rich, and hot enough to melt through the overlying ice. Can the basaltic magma, represented by melt inclusions, produce the volcanic gases currently emitted at the surface? To test this, two different gas modelling software (SolEx and MagmaSat) are used, along with original and recalculated (with the bubble components) as input parameters. The resulting molar ratios are compared with MultiGAS ratios of fumaroles. Overall, SolEx closed-system degassing using recalculated melt inclusions yields the best approximation to MultiGAS ratios. This thesis addresses a compositional knowledge gap within the GVB. By understanding both the distinct magmatic sources underlying the arc, as well as the composition of volcanic gases emanating from summit fumaroles, we gain a broad and comprehensive geochemical overview of the Garibaldi Volcanic Belt.