Temporal evolution of the isotopic composition of fumarolic steam in Vulcano Island (Italy).

On the active crater of Vulcano Island (Aeolian Islands, Italy) an intense fumarolic activity is observable, with maximum temperatures of about 600ºC. The figure plots the variations over time (1978-1988) in the isotopic composition of the vapor and in the temperature for two typical fumaroles, F1 (full circles) and F5 (open circles). The arrows indicate local swarms of very shallow earthquakes centered beneath the crater, due to fluid dynamics in the upper structures of the volcanic system. The isotopic data for the magmatic steam of Vulcano island yeild an estimate of original composition in the range of +8.0 to +9.0 delta(18)O and 0 to +10 deltaD (Bolognesi and D'Amore, 1992). The patterns in the figure suggest that, coincident with shallow seismic events, the permeability increased in the direction of the conduits which feed the fumaroles, favoring the introduction of meteoric water with very negative concentrations of delta(18)O and deltaD. Therefore: increase in meteoric water --> increase in flow of steam --> higher steam temperature and lower delta(18)O and deltaD. After the conclusion of periods of local seismic activity, the values of delta(18)O and deltaD gradually become more positive and the temperature values tend to diminish (1981-83, 1986-87). This can be explained by the fact that the scarcity or absence of local seismic activity allows for a gradual reduction in permeability due to the action of self-sealing processes in the access ways to the fumarolic conduits. Therefore, the meteoric component progressively decreases, and the isotopic values of the fumarolic vapor get closer to the delta(18)O and deltaD values characteristic of local magmatic steam. REFERENCES: Bolognesi L. and D'Amore F. (1992) Isotopic evidence of coexisting geothermal and active volcanic systems (Vulcano island, italy). In prep.