Isotopic variation of the hydrothermal system on Vulcano Island, Italy.

The isotopic and chemical composition of thermal waters and the isotopic composition of high-temperature fumarolic steam at Vulcano Island indicate an evolution of the hydrothermal system from its magmatic origin through interaction with rocks and mixing with meteoric water. Seawater is not a component in this system. The isotopic values of the fumaroles indicate a local magmatic composition of +7 to +8 parts per thousand delta(18)O and -5 to +15 parts per thousand deltaD. The high deltaD values of this vapor may reflect contamination of the magma by connate water. The isotopic and chemical composition of water from well W2 indicates the existence of geothermal fluid formed by mixing of HCl and SO2-bearing condensates of volcanic origin with groundwater, with subsequent evolution via water-rock interaction. The estimated isotopic composition of the geothermal water is delta(18)O = +1 parts per thousand and deltaD = -5 parts per thousand. Local shallow seismic events result in a temporary increase in fracture permeability, enabling the mixing of magmatic vapor with groundwater or geothermal water. Tectonic activity associated with deep structures appears to increase the contribution of magmatic vapor to the fumaroles.