Chemical and isotopic (C and S) composition of groundwaters from the Mt.Vulture volcanic system

Mt. Vulture volcano is located in the most external part of the Apennine orogene (southern Italy), almost at the edge of the Apulian foreland. The volcanic activity occurred up to 130 Kyrs ago. The volcano is formed by a main feldspar-bearing series of pyroclastic rocks and subordinate lava flows, ranging in composition from basanite and foidite to phonolites (Schiattarella et al., 2005). Rare lava flows and dykes, with distinctly more silica-undersaturated composition melilitites, melilite ankaratrites, and the ‘Melfi’ haüynophyre, were also emplaced. Carbonatite-melilite magmas fed the final phase of volcanism producing maar-type craters. The large Na and S contents of the Vulture magmas (Marini et al., 1994) result in the the widespread presence of sodalite-group phases among the feldespathoids (De Fino et al., 1982; Di Muro et al., 2004). Most groundwaters are characterized by gas bubbling. In order to investigate the origin of solutes a total of 25 springs and wells were analyzed for the major and minor element contents, the isotopic composition of carbon in the total dissolved carbon (TDC) and sulfur in the aqueous sulfate. The chemical data suggest that the effects due to water-rock interaction are largely controlled by the input of CO2 . The dissolution of CO2 in the water causes a substantial increase in its acidity, thus promoting the alteration of the rocks. Good correlations exist among alkalinity, electric conductibility and Na + , Ca 2+ , SO4 2- concentrations. The alkaline-earthbicarbonate and alkaline-bicarbonate compositions of the waters are in keeping with the major role played by CO2 in the alteration processes. The G 13 C values of TDC corroborate the involvement of two carbon sources. The first one is biogenic CO2 , while the second one is magmatic. The G 34 S values of SO4 2- are all positive and similar to those measured by Marini et al. (1994) in the haüynophyre magma, thus supporting for aqueous sulfate a main origin from leaching of volcanites. In few springs, however, some contribution of SO4 2- from dissolution of Triassic evaporite at depth cannot be excluded.