Tracking Unrest from Chemical Mapping of fumarolic fields

Hydrothermal degassing is one of the most evident expressions of the still-ongoing activity of quiescent volcanoes. At hydrothermal volcanoes, degassing takes place in a variety of forms, including fumaroles, steaming grounds, mud pools and degassing soils. These hydrothermal surface manifestations have long been targeted by research because their chemistry contains crucial information for understanding the structure and P-T-X properties of the underlying hydrothermal system that feeds them. Even more importantly, these manifestations exhibit large changes in their chemistry and flux when hydrothermal volcanoes become restless during unrest. In particular, tracking the temporal evolution of the chemistry of hydrothermal fumaroles is key to assessing the dynamics, triggers and evolution of volcanic unrest, some of which can eventually escalate into eruption. Unrest at hydrothermal volcanoes is typically scrutinized by periodic campaigns of direct fumarole sampling. A valuable complement to direct sampling is the use of chemical mapping techniques that, using portable gas sensing units, allow in near-real time to explore and image the chemical diversity of a fumarolic field. These mapping techniques, in contrast to direct sampling that is typically applied to a relatively small number of fumaroles, contribute key information on the spatial heterogeneity of fumarole composition, and on how this heterogeneity varies in time as unrest progresses. In this PhD dissertation, I report on the application of the chemical mapping technique to two quiescent volcanic systems that have recently been in unrest: La Fossa crater of Vulcano island (Sicily, Italy) and Nisyros island (Greece). At Vulcano, periodic gas surveys were conducted at La Fossa between 2021 and 2024, in which a portable Multi-component Gas Analyzer System (Multi-GAS) was used to map the spatial variations in gas composition across the fumarolic field in a period encompassing a major degassing unrest started in fall 2021. Results indicate a significant compositional contrast between rim and inner crater fumaroles. The rim fumaroles exhibited highest CO2, SO2, H2S, and H2 concentrations and relatively low CO2/SO2 ratios (of 20-30), indicative of minor hydrothermal processing. In contrast, a wide range of CO2/SO2 ratios in gas compositions were observed in the inner crater fumaroles, which are interpreted as phases of expansion and contraction of the gas chimney, underneath the 1888-1890 eruption outer crater rim, feeding the unrest. The intermediate, below-background CO2/SO2 ratios observed in crater rim area throughout 2021-2024, and the higher-than-background SO2 fluxes, suggest that hydrothermal activity conditions have not returned to background levels, yet. The mapping technique was also applied at Nisyros, a quiescent volcanic system in the Aegean sea (Greece) that was stuck by a seismic/degassing unrest in 1996-2001, that raised concerns of a possible renewal of volcanic (phreatic) activity. A similar portable Multi-GAS instrument was used to walking traversing the fumarolic fields of Stefanos and Lofos, most active hydrothermal sites, in the attempt to characterize the compositional heterogeneity of fumaroles at high spatial (meters) and temporal (1 Hz) resolution. The obtained spatially resolved gas concentration results allow identifying a cluster of gas-rich spots aligned along a ring-shaped degassing structure, corresponding to the Stefanos inner crater rim (or outer caldera floor margin). We additionally provide a spatially resolved record of gas ratios, demonstrating an overall compositional homogeneity across the fumarolic fields. The results obtained demonstrate a gradual return to background quiescent activity. The potential impact on human health (tourists, visitors and operators) of the atmospheric concentration levels of potentially hazardous gases (CO2 and H2S) at Stefanos is also discussed.