GNGTS 2013 - Atti del 32° Convegno Nazionale

Isotopic compositions of water sampled just at the spring (label Sg) diverge from the notional meteoric lines, further suggesting the possible involvement of deep juvenile fluids. This hypothesis is corroborated by the investigation of the dissolved gases, carried out in the laboratories of the Istituto Nazionale di Geofisica e Vulcanologia (INGV, Palermo), according to the method described by Capasso and Inguaggiato (1998) and Liotta and Martelli (2012). Results reported in Tab. 3, indicate that the dissolved CO 2 appears nearly 100 times higher respect to that expected in water equilibrated with the atmosphere (see Fig. 3). The delineated framework is compatible with the carbon isotopic signature (on DIC) measured according to the method of Capas- so et al. (2005) that revealed a δ 13 C (relative to PDB) of -4.2 ‰. Starting from this value, the carbon isotope composition of CO 2 gas phase in equilibrium with the sampled water can be calculated. According to the isotopic fractionations delineated by Grassa et al. (2006) the CO 2 equilibrated with this water at the sampling temperature should have a δ 13 C of -10 ‰. This value is clearly lower than that expected for a carbon originated in mantle horizons in Medi- terranean area (δ 13 CO 2 = -3 to 0 ‰, e.g. Capasso et al. , 1997) and testifies an addition of light carbon, probably of shallow organic origin. It is worth of notice that, in agreement with many other thermal spring of Sicily and surrounding islands, the S. Calogero carbon content and isotopic ratio fall into the two-endmember mixing proposed by Grassa et al. (2006) between a magmatic (-3 to 0 ‰) and an organic (lower than -20 ‰) term. Unfortunately, the 3 He/ 4 He and 4 He/ 20 Ne ratios measured in the gas phase dissolved into San Calogero waters (method of Inguaggiato e Rizzo, 2004) shows a clear atmospheric contamination that covers the eventual presence of deep helium and does not allow to gain significant information from noble gases. Even if effort has been made in the field to collect the samples as close as possible to the water emergence point, the presence of noble gases with atmospheric signature suggests that waters have the possibility to interact with air, possibly shortly before the emergence at surface. Tab. 3 – Composition of gases dissolved in the thermal water of San Calogero ( Sg ), expressed in ml (at Standard Temperature and Pressure, STP) per liter. He O 2 N 2 CH 4 CO 2 6.93E-05 4.84 12.43 2.21E-04 43.9 Conclusions. The upraising of reactive juvenile fluids hypothesized observing the geochemical data is coherent with the pervasive alteration of the volcanic rocks surrounding the spring. The source is probably a magmatic chamber that is plausibly in a cooling stage. Therefore the studied water seems to reflect a mixing between meteoric water and volcanic fluids, whereas the involvement of sea water is not observed. The observed decrease in the spring temperature and discharge is plausibly related to variations on the fluid paths that can occur both for natural or anthropogenic processes. Further investigation is in progress in order to refine and constrain the mentioned hypothesis. Fig. 3 – Gases dissolved in the San Calogero thermal water (Sg; star) compared with the composition of Air Saturated Water (ASW; black circle) and Air Saturated Sea Water (ASSW; white square). Note the predominance of CO 2 that characterize the studied thermal water. 276 GNGTS 2013 S essione 1.3