Implications of variable initial water content for mobilising magmatic metals in arc volcanoes

The migration and sequestration of metallic elements within magmatic systems modulates the environmental impacts of volcanic emissions and the development of ore deposits (Edmonds et al., 2022 Tattitch et al., 2021). Many elements classified as "environmental pollutants" by public health agencies are volatile in silicate melts (Ilyinskaya et al., 2021).

Volcanoes in different tectonic settings emit contrasting trace metal "fingerprints", which are linked to differences in the relative timings of vapour and sulfide saturation (Edmonds et al., 2018). Arc volcanoes erupt magmas containing a wide range of primary water contents, spanning less than 1 wt% to more than 7 wt% H2O, inherited from heterogeneities in mantle source and subduction contributions (Plank et al., 2013 Ruscitto et al., 2012).

Magmatic water content influences the physical and chemical properties of silicate melts (e.g., viscosity and crystallisation dynamics) and is consequently a key control on the timing/pressure of vapour saturation - and therefore availability of a hydrous fluid phase for trace elements to partition into according to their melt-fluid partition coefficients - and the conditions under which magmas are stored in the crust (Edmonds et al., 2022). However, the role of primary water content on metal partitioning has yet to be investigated systematically.

This project identifies two arc volcanoes-Fuego and Pacaya, Guatemala-with low and high magmatic water contents, respectively (Lloyd et al., 2014 Walker et al., 2003). The

overarching aim is to reconstruct and compare the evolution of their trace metal cargoes using established petrological strategies. Targeting Fuego and Pacaya for end-member comparisons of magmatic water content avoids introducing additional uncertainties that may influence metal behaviour such as along-arc or inter-arc variations in slab flux or crustal thickness.

This research will involve field sampling of erupted rocks and emitted metal aerosols in the gas plume. Laboratory work will focus on petrological analysis of melt inclusions and glasses, including ion probe analyses of volatile contents, together with geochemical analysis of plume aerosol samples collected on filters. Petrological and thermodynamic modelling will provide a framework to interpret our geochemical data and, in particular, to relate metal behaviour to crystallisation processes and the composition of the exsolved volatile phase in silicate melts.

This project will also explore the potential for developing non-traditional approaches to assess the environmental impact of volcanic emissions: in particular, the feasibility of using trace metals in vegetation, soil, and hydrological samples to assess the delivery mechanisms and pathways of volcanogenic trace metal pollutants to the environment, with consequent implications for ecological science and hazard assessment.