GEOCHEMICAL CHARACTERISATION OF WATERS AND GASES OF ISCHIA ISLAND (ITALY). Salvatore Inguaggiato 1,2 , Giovannella Pecoraino 2 . 1 Istituto Nazionale di Geofisica. – Via Di Vigna Murata, 605 – Roma (Italy). Poseidon System, Catania. 2 Istituto di Geochimica dei Fluidi C.N.R.-Via U. La Malfa, 153 – Palermo (Italy). Key Words: gas and water geothermometers, fluid geochemistry, Ischia Island, δ 13 C CO2 , gas-water-deep fluids interactions. ABSTRACT Between 1994 and 1995 gas samples from some fumaroles and thermal waters were collected on Ischia Island. The chemical composition of the shallow and deep natural hydrothermal fluids discharged is related to the main hydrological and lithological characteristics of the rock formations present in the reservoir. The calculated mass vapor fraction (y) of the reservoir is always positive suggesting the presence of a biphase system where the dominant liquid has a temperature ranging from 240 to 280°C. The liquid phase is made up of modified seawater, heated fresh seawater and meteoric water. The mixing between these components together with interaction processes with the surrounding rocks is the origin of all of the islands groundwaters. The gaseous phase is mainly carbon dioxide with variable contents of helium that reflect different degrees of interaction with shallow groundwaters. The isotopic composition of the helium and the CO 2 of these gases is characteristic of fluids that originate in a mantle that has undergone crustal contamination. The helium values are from 1 to 3 R/Ra and values of δ 13 CO 2 among -5 and –2 δ‰. The negative δ 13 C value with respect to typical magmas in the Mediterranean (with δ 13 CO 2 ranging from 0 to - 2δ‰) is the result of deep gases and groundwater interaction during their rise towards surface. The equilibrium temperature of the deep reservoir, found by using the chemical composition of the sampled gases, is uniform at 280°± 20°C. The homogeneity of the equilibrium temperatures of the sampled gases in different areas of the island probably indicates the existence of a common geothermal reservoir from which the gases originate. The differences in the equilibrium temperatures, estimated from the water’s chemical composition, are due to mixing processes between modified sea water that represents the deep recharge of the system, and shallow groundwaters recharged by meteoric and marine waters at lower temperatures. The amount of steam in the reservoir is below 2% of total water content. The calculated values of pCO 2 in the reservoir are highly variable and range between 1 and 28 bars. The observed values do not show any correlation with other parameters and do not allow the evaluation of pCO 2 in the geothermal reservoir. 1. INTRODUCTION Ischia, Phlegrean Fields and Vesuvius are part of the classic active volcanic area of the alkali-potassic Quaternary Roman Province of central-southern Italy developed within the Campanian graben, on the central-western margin of the apennine chain. The volcanism is thought to be a consequence of tensional tectonics with NW-SE and NE-SW trends induced by the opening of the Tyrrhenian sea basin. This volcanic activity started about 2.0 Ma ago and has lasted until the present day with the historical eruption of Vesuvius, Phlegrean Fields and Ischia. The NE-SW tectonic trend marks the alignment of the volcanic areas of Phlegrean Fields and Ischia that were formed by many eruptive centers and are characterized by the presence of many thermal springs, fumaroles, frequent earthquakes and bradyseismic phenomena (Santacroce, 1987, Vezzoli, 1988). The island of Ischia lies on the northwest edge (rim) of the Bay of Naples, opposite Capo Miseno. Oblong in shape, Ischia has a circumference of 34 km and a surface area of 46 km 2 . The island is part of a volcanic field that was larger that the present island. Mt. Epomeo, an active volcano-tectonic horst raised by the intrusion of a superficial magmatic chamber, constitutes the highest part of the island (787 m a.s.l.) and the only part involved in the tectonic uplift (Vezzoli, 1988). The last eruption on the island took place in 1301 A.D. at Arso. There have been destructive earthquakes more recentely, the last in 1833 when the entire town of Casamicciola was destroyed. Geothermal research was carried out on the island in the 1940's (Penta and Conforto, 1951). Several wells were drilled there and the hottest temperature (232°C) was measured in the deepest well at 1150 m. Many thermal springs (T ∼90°C) and fumaroles (T∼100°C), and shallow earthquakes (the last one in 1881, Mercalli, 1884) testify to the persistent state of activity of the magmatic system. The aim of this work is to evaluate the thermodynamic features (P, T and vapour fraction) of the island’s deep reservoir feeding shallow fluids. 2. HYDROGEOCHEMICAL FRAMEWORK Between 1994 and 1995 gas samples from some fumaroles and thermal waters were collected on Ischia Island. Figure 1 shows the location of sampled fluids. The chemical composition of water samples from Ischia samples (Table 1) suggests that these are the result of the mixing of seawater and low salinity waters of probable meteoric origin, as hypothesised by De Gennaro et al., (1984), and Panichi et al., (1992). Water-rock interaction processes are particularly evident in many samples. The chemical composition of Ischia thermal waters are due to the mixing of at least three end- members: a “marine” modified component (e.g. S. Lorenzo sample, the most saline, with the highest concentration of B and t=90°); a “fresh” meteoric component that has undergone little rock interaction (e.g. sample 11, the least saline, with the lowest concentration of B, and t=28°C); a “modified” meteoric component due to rock interaction processes (e.g. samples 15 and 18, intermediate salinity and B concentration and t=30 and 53°C respectively). Sample 5 (S. Lorenzo), represents waters of marine origin, subsequently modified and heated. In fact, the salinity of these water samples reaches values sometimes higher then that of seawater. The distribution of sample data points in the ternary diagram in Figure 2 (Giggenbach, 1988) shows that most of the waters fall within the fields of non-equilibrium or partial equilibrium with the rocks. Samples 11 and 5 identified as mixing end members, fall within the fields of immature and partially mature waters, respectively. The intermediate samples tend to lie in positions of increasing maturity towards the saline end member (5). Samples 14, 15 and 16 fall within the field of full equilibrium with equilibrium temperatures between 100 and 160° C. Samples 5, 8 and 18 show Na/K ratios typical of high temperature equilibrium (240 to 280°C) in agreement with temperatures measured in the perforation wells drilled by SAFEN in the 1950’s (Penta and Conforto, 1951). From the application of various geothermometers in mature water samples an estimation of the deep reservoir waters was made. 2623 Proceedings World Geothermal Congress 2000 Kyushu - Tohoku, Japan, May 28 - June 10, 2000