Submarine and inland gas discharges from the Campi Flegrei (southern Italy) and the Pozzuoli Bay: geochemical clues for a common hydrothermal-magmatic source

In this paper the results of a geochemical survey carried out on submarine and subaereal gas discharges located within the caldera of the Campi Flegrei (southern Italy) are presented and discussed. Gas samples were collected inland, i.e. from the Solfatara and Agnano craters and the Pozzuoli Bay, where at least five distinct main submarine fumarolic fields were recognized: Mar Morto, Mercato Ittico, Erculanea, Nisida and Fumose, the latter having the highest temperature (93 degrees C) among the submarine gas discharges. The chemical composition of the steam-dominated gases from the Solfatara crater and Pisciarelli area (western sector of the Agnano crater) is characterized by significant concentrations of H2S, H-2 and CO (up to 18508, 2570 and 3.14 mu mol/mol, respectively), suggesting that these fluids are produced by boiling of a hydrothermal aquifer where magmatic-related gas compounds (i.e. SO2, HCl, HF) are mostly dissolved. Geothermometric calculations based on chemical equilibria of both the CO2-CH4-H-2 and C3H6-C3H8 systems indicate equilibrium temperatures in the range of 340-380 degrees C at redox conditions more oxidizing than those typically dominating hydrothermal reservoirs. The relatively high R/Ra ratios (from 2.9 to 3.1) indicate a clear He contribution from the mantle. The gas chemistry of the bubbling pools located at Agnano Terme, in the centre of the Agnano crater, is significantly different, being characterized by lower (two orders of magnitude) H2S and H-2 concentrations, CO below the detection limit (0.01 mu mol/mol), and a slightly lower (<2.6) R/Ra values. The measured CO2, CH4 and H-2 concentrations of these gases indicate apparent equilibrium temperatures of similar to 200 degrees C.