Genesis of chlorine and sulphur in fumarolic emissions at Vulcano Island (Italy): assessment of pH and redox conditions in the hydrothermal system V. Di Liberto a , P.M. Nuccio a , A. Paonita b;  a Dipartimento di Chimica e Fisica della Terra ed Applicazioni, Universita ' di Palermo, Via Archira¢ 36, 90123 Palermo, Italy b Istituto Nazionale di Geo¢sica e Vulcanologia (INGV) ^ Sezione di Palermo, Via Ugo La Malfa 153, 90146 Palermo, Italy Received 3 September 2001; accepted 26 December 2001 Abstract Chlorine- and sulphur-bearing compounds in fumarole discharges of the La Fossa crater at Vulcano Island (Italy)canbemodelledbyamixingprocessbetweenmagmaticgasesandvapourfromaboilinghydrothermalsystem. This allows estimating the compounds in both endmembers. Magma degassing cannot explain the time variation of sulphurandHClconcentrationsinthedeependmember,whicharemoreprobablylinkedtoreactionsofsolidphases at depth, before mixing with the hydrothermal vapours. Based on the P^T conditions and speciation of the boiling hydrothermalsystembelowLaFossa,theHClandS tot contentsinthehydrothermalvapourswereusedtocompute theredoxconditionsandpHoftheaqueoussolution.Theresultssuggestthatthehaematite^magnetitebuffercontrols thehydrothermal fO 2 values,whilethepHhasincreasedsincetheendofthe1970s.ThemainprocessesaffectingpH values may be linked to Na^Ca exchanges between evolved seawater, feeding the boiling hydrothermal system, and local rocks. While Na is removed from water, calcium enters the solution, undergoes hydrolysis and produces HCl, lowering the pH of the water. The increasing water^rock ratio within the hydrothermal system lowers the Ca availability, so the aqueous solution becomes less acidic. Seawater flowing towards the boiling hydrothermal brine dissolvesalargequantityofpyritealongitspath.Intheboilinghydrothermalsystem,dissolvedsulphurprecipitates as pyrite and anhydrite, and becomes partitioned in vapour phase as H 2 SandSO 2 . These results are in agreement with the paragenesis of hydrothermal alteration minerals recovered in drilled wells at Vulcano and are also in agreement with the isotopic composition of sulphur emitted by the crater fumaroles. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: chlorine; sulphur; hydrothermal system; genetic processes; Vulcano Island 1. Introduction The volcanic activity of Vulcano Island (Aeoli- anIslands,Italy)hasundergonesigni¢cantevolu- tioninthelasttwodecades.Thiscausedsubstan- tial variations in the fumarolic emissions at Spiaggia di Levante beach and at the crater of La Fossa volcano and the several thermal wells on the island (Carapezza et al., 1981; Cioni and D’Amore,1984;PanichiandNoto,1992;Capas- 0377-0273/02/$^seefrontmatterß2002ElsevierScienceB.V.Allrightsreserved. PII:S0377-0273(02)00215-9 * Corresponding author. E-mail address: paonita@pa.ingv.it (A. Paonita). Journal of Volcanology and Geothermal Research 116 (2002) 137^150 www.elsevier.com/locate/jvolgeores