ISOTOPIC VARIATION OF THE HYDROTHERMAL SYSTEM ON VULCANO-ISLAND, ITALY

The isotopic and chemical composition of thermal waters and the isotop ic composition of high-temperature fumarolic steam at Vulcano Island i ndicate an evolution of the hydrothermal system from its magmatic orig in through interaction with rocks and mixing with meteoric water. Seaw ater is not a component in this system. The isotopic values of the fum aroles indicate a local magmatic composition of +7 to +8 parts per tho usand delta O-18 and -5 to +15 parts per thousand deltaD. The high del taD values of this vapor may reflect contamination of the magma by con nate water. The isotopic and chemical composition of water from well W 2 indicates the existence of geothermal fluid formed by mixing of HCI and SO2-bearing condensates of volcanic origin with groundwater, with subsequent evolution via water-rock interaction. The estimated isotopi c composition of the geothermal water is delta O-18 = +1 parts per tho usand and deltaD = -5 96. Local shallow seismic events result in a tem porary increase in fracture permeability, enabling the mixing of magma tic vapor with groundwater or geothermal water. Tectonic activity asso ciated with deep structures appears to increase the contribution of ma gmatic vapor to the fumaroles.