Volatile metal degassing from volcanoes: source processes, atmospheric transport and deposition
Volcanoes emit metal and metalloid elements in the gas phase at rates that are comparable to industrial emissions of entire countries or regions. These elements (e.g., Cu, Zn, Pb, As, Se), collectively referred to as ‘metal pollutants’ or ‘heavy metals’, can be critical nutrients at low levels, but sustained exposure has been connected with high incidence of diseases such as multiple sclerosis and some cancers in the communities living around volcanoes. This thesis examines the systematics of volcanogenic trace element degassing in multiple tectonic settings (arcs, ocean islands, and continental rifts) and outgassing environments (magmatic vents and lava-seawater interactions), and extends to the compositional evolution of volcanic plumes in the atmosphere. Starting within the magmatic system, in Chapter 3 I use gas and melt compositions, equilibrium chemical speciation modelling, and partitioning data, to interrogate the origins of metal/metalloid enrichments in arc emissions. Chapter 4 moves on to ocean island volcanoes, and compares different, but genetically-related, outgassing environments associated with the 2018 eruption of Kīlauea, Hawai‘i: 1) degassing from the source vent, and 2) lava-seawater interactions at the ocean entry. Speciation modelling of magmatic gases highlights the importance of the S²⁻ ligand in highly volatile trace element degassing (e.g., Se, Te). In contrast, the high availability of seawater-derived Cl⁻ at the ocean entry facilitates enhanced degassing of Cl⁻-complexing elements (e.g., Cu). In Chapter 5 I trace the downwind evolution of atmospheric concentrations of metal and metalloid elements during the 2018 eruption. Specifically, I demonstrate that volatile metal pollutants were rapidly depleted from the atmosphere – up to 100 times faster than refractory species. High rainfall over early stages of downwind plume transport leads to early wet deposition of soluble complexes, with implications for hazard assessment. In Chapter 6 I consider metal transport in the atmosphere during explosive volcanism, using ice core records of the massive halogen-rich ~17.7 ka eruptions of Mt. Takahe, an alkaline volcano on the West Antarctic Rift System. I suggest that the residence time of metals in the atmosphere may be limited by their affinity for chloride, with implications for the utility of metals in ice cores as tracers of volcanism. Overall, this thesis demonstrates the importance of volatility, ligand-availability and oxygen fugacity in both the degassing and downwind transport of volcanogenic metals and metalloids.
Liu, Emma J
Engineering and Physical Sciences Research Council (1943898)