GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.3 171 responsible of the heating of the system, which in turn shows the same accelerating trend of ground inflation, thus gaining the role of most likely candidate responsible of the current uplift. The hydrothermal system feeding Solfatara. The total deeply derived CO 2 released from diffuse degassing processes at Solfatara and surrounding (~1.4 km 2 ) is estimated to be 1000 to 1500 t/d in the period 1998-2010 (Chiodini et al. , 2011). In addition, recent (2012-2015) measurements of the gas flux from the three main fumarolic vents, indicate a total CO 2 output ranging from 350 to 850 t/d (Aiuppa et al. , 2013). The total CO 2 flux of 1500-2000 t/d, i.e. the fumarole flux added to the diffuse emission, has to be considered as a minimum estimate of the total hydrothermal CO 2 output because the flux from the numerous minor fumarolic discharges is not taken into account, since it has never been measured. The sketch of Fig. 1 shows the main featuresofthehydrothermalsystemfeeding this large degassing process (Chiodini et al. , 2015). The system consists of a deep zone of magmatic gas accumulation and a shallower hydrothermal reservoir. The first is located at ~4 km depth (Vanorio et al. , 2005) and supplies fluid and heat to the overlying shallower part of the system: it has been hypothesized that it hosts a small batch of magma (De Siena et al., 2010). In the upper part, the hydrothermal reservoir, magmatic fluids mix and vaporize liquid of meteoric origin, forming a gas plume in the subsoil of Solfatara. This scheme, which is derived from geochemical interpretations (e.g. Caliro et al., 2007 and references therein), agrees with the most recent inversion of the ground deformation data observed in the 1982-2013 period (Amoruso et al. , 2014). The measured deformation would be in fact controlled by pressure changes in two sources: a pressurized triaxial ellipsoid (PTE) oriented NW - SE and centred at about 4 km depth in the subsoil of Pozzuoli, and a pressurized spheroid (PS) located at ~ 2 km depth below Solfatara crater. PTE and PS are coincident with the deeper magmatic gas and the shallower part of the hydrothermal system depicted in Fig. 1. Compositional changes of Solfatara fumaroles and the 2005-2013 ground deformation pattern. At Solfatara fumaroles, the proportion of the magmatic component sharply increases during relatively short periods, which can be explained as the results of repeated episodes of magmatic fluid injections into the hydrothermal system (Chiodini et al. , 2012). Such episodes are characterized by the decrease of the methane content of the fumaroles due to the low CH 4 content of magmatic fluids and, possibly, the relatively high and transient oxidizing conditions during the process which prevent the formation of CH 4 in the hydrothermal environment (Chiodini, 2009). On the other hand, since the relative abundances of other gases of prevalent magmatic origin, such as CO 2 and He, may increase, ,the ratio of their contents with CH 4 content is a good indicator of the increased flux of the magmatic component. Allowing that, Fig. 1 – Conceptual model involving the release of magmatic fluids from the deeper part of the hydrothermal system (Magmatic gas, PTE) towards the shallower parts (Hydrothermal reservoir PS) below the Solfatara, where these mix with meteoric fluids (modified from Chiodini et al. , 2015). See the text for further explanations.