Chemical and isotopic investigations of fumarolic gases from Ischia island (southern Italy): Evidences of magmatic and crustal contribution

Chemical and isotopic investigations have been carried out on subaerial and submarine fumarolic fluids on the island of Ischia, Gulf of Napoli (Southern Italy). Chemical analyses of gases from the hottest fumarole (117 degrees C) located on Mount Epomeo show a typical geothermal composition. Thermodynamic computations indicate a feeding source temperature of Mt. Epomeo ranging from 300 to 500 degrees C. Isotopic compositions suggest that the island is divided into two parts (East and West). Fumarole gases on the west side of the island indicate a magmatic source with small crustal contamination, and are characterized by carbon isotopic data similar to MORE (delta(13)C ranging from -3.5 to -5.11 parts per thousand) with about 50% of the helium having a magmatic origin (3.6-3.7 R/R(a)). Fumaroles located on the east side of the island have carbon isotopic data ranging from + 0.45 to - 0.76 parts per thousand and a lower helium isotopic ratio (from 2.09 to 2.89 R/R(2)). The east fumaroles seem to be fed by fluids deriving from a reservoir located in or close to a carbonatic source in the crust.