The marine sector surrounding Panarea Island (Aeolian Islands, South Italy) is affected by widespread submarine emissions
of CO
2 -rich gases and thermal water discharges which have been known since the Roman Age. On November 3
rd, 2002 an anomalous degassing event affected the area, probably in response to a submarine explosion. The concentrations of
minor reactive gases (CO, CH
4 and H
2) of samples collected in November and December, 2002 show drastic compositional changes when compared to previous samples
collected from the same area in the 1980s. In particular the samples collected after the November 3
rd phenomenon display relative increases in H
2 and CO and a strong decrease in the CH
4 contents, while other gas species show no significant change. The interaction of the original gas with seawater explains
the variable contents of CO
2, H
2S, N
2, Ar and He which characterize the different samples, but cannot explain the large variations of CO, CH
4 and H
2 which are instead compatible with changes in the redox, temperature and pressure conditions of the system. Two models, both
implying an increasing input of magmatic fluids are compatible with the observed variations of minor reactive species. In
the first one, the input of magmatic fluids drives the hydrothermal system towards atypical (more oxidizing) redox conditions,
slowly pressurizing the system up to a critical state. In the second one, the hydrothermal system is flashed by the rising
high-T volcanic fluid, suddenly released by a magmatic body at depth. The two models have different implications for volcanic
surveillance and risk assessment: In the first case, the November 3
rd event may represent both the culmination of a relatively slow process which caused the overpressurization of the hydrothermal
system and the beginning of a new phase of quiescence. The possible evolution of the second model is unforeseeable because
it is mainly related to the thermal, baric and compositional state of the deep magmatic system that is poorly known.
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