Chemical and isotopic compositions of minerals and waters from the Campi Flegrei volcanic system, Naples, Italy

Abstract

Based on their δ 34 S signature, sulfate minerals and native sulfur around fumaroles and hot water pools from the Campi Flegrei volcanic area derive from supergenic oxidation of volcanic H 2S. Their mean δ 34 S value (−0.2±1.7‰) matches with that of fumarolic H 2S at Solfatara (−0.3±0.3‰), as well as with the δ 34 S of +1.4‰ obtained for total sulfur in fresh trachyte from the area. All δ 34 S values indicate a mostly deep-seated origin for sulfur. Thermal waters were analysed for major and minor chemistry and for oxygen, hydrogen and sulfur isotope compositions. Pools at Pisciarelli are filled with evaporated meteoric water heated by rising (magmatic) gases. The water δ 18 O (+3.8±1.3‰) and δ 2 H (+6.5±2.2‰) values in these steam-heated waters are controlled by mixing and evaporation effects, and the δ 34 S value of dissolved sulfate (−1.3±0.3‰) basically agrees with supergenic oxidation of deep-seated H 2S as the major source of sulfur. Instead, water from thermal springs and wells elsewhere in the Campi Flegrei appears to be a mixture between dilute meteoric and saline marine components. The latter may be local seawater from the bay of Pozzuoli. The δ 18 O and δ 2 H values of waters sampled during 1993–1994 range from −5.6 to +0.3‰ and from −33 to −3.4‰, respectively. The δ 34 S values of dissolved sulfate range between −0.1 and +19.5‰. In general, sulfate is probably derived essentially from two sources, both within the volcanic cover, i.e., oxidation/dissolution of pyrite and anhydrite, and marine water. An occasional source of water and sulfate is represented by (magmatic) gases, which directly interact with shallow meteoric water as in the case of the Hotel Tennis well yielding steam-heated water with δ 18 O =−1.5±0.2‰, δ 2 H =−17±1‰ and δ 34 S =−0.1‰.