5:15 PM - 7:15 PM
[SGC37-P12] Fumarole geochemistry during the 2021-22 unrest of Vulcano Island (Italy): the magmatic source in the precursory and ongoing phases
★Invited Papers
Keywords:gas geochemistry, magma degassing, volcano
In September 2021, La Fossa volcano at Vulcano island (Italy) started a new phase of unrest. The monitoring system operating in the island since late ‘80s, recorded a great and abrupt variation in fumarole temperatures, soil and plume degassing, seismicity and ground deformation. These variations were interpreted as due to the impressive vaporization and expansion of the hydrothermal system, hypothesized at depth > 1.5 km bsl. At the same time, fumarole chemistry showed a dramatic variation, related to the dominant contribution of the magmatic gas over the hydrothermal one. The CO2 content and the helium isotope composition of the magmatic source revealed the appearance of a more primitive magma, compared to that feeding the fumaroles in the previous period, during the first months of the unrest. The signs of the enhanced contribution of magmatic gases in the fumarole gases was already evident since 2018, so the 2021 unrest appears to have been the outcome of a long lasting preparatory phase. The systematics of gas species together with C and He isotopes, emitted from fumaroles after the first months of the unrest, revealed the appearance of a different magmatic component, poorer in N2, 3He and richer in He, S and 13C.
The chemical characteristics of these fluids, particularly the low relative N2 contents, would be compatible with contamination with fluids coming from deeper levels of the crust. After the first intense phase of the unrest, the decrease in CO2 contents and R/Ra values and even lower N2/He ratios could be ascribed to a source of magma residing at the same depth but partly degassed and contaminated by 4He-rich crustal fluids.
We suggest that, after 2006, the fluids were sourced from magma ponding zones located at greater depth than in the past, probably coincident with the basaltic reservoir, which was supposed to be at 18 km depth. Their pattern of ascent would be focused in the northern part of the La Fossa caldera, probably stimulated by tectonic structures under regional control. They could derive from the Vulcanello plumbing system, which is supposed to be less articulated than at La Fossa.
The chemical characteristics of these fluids, particularly the low relative N2 contents, would be compatible with contamination with fluids coming from deeper levels of the crust. After the first intense phase of the unrest, the decrease in CO2 contents and R/Ra values and even lower N2/He ratios could be ascribed to a source of magma residing at the same depth but partly degassed and contaminated by 4He-rich crustal fluids.
We suggest that, after 2006, the fluids were sourced from magma ponding zones located at greater depth than in the past, probably coincident with the basaltic reservoir, which was supposed to be at 18 km depth. Their pattern of ascent would be focused in the northern part of the La Fossa caldera, probably stimulated by tectonic structures under regional control. They could derive from the Vulcanello plumbing system, which is supposed to be less articulated than at La Fossa.