GNGTS 2015 - Atti del 34° Convegno Nazionale

172 GNGTS 2015 S essione 1.3 the six peaks, each lasting about one year, which affected the CO 2 /CH 4 and He/CH 4 ratios of the main fumaroles BN and BG in 2007, 2008, 2009-10, 2011, 2012, 2013 (Fig. 2a) correspond to periods of discharge of fluids richer in the magmatic component at the Solfatara fumaroles. Fig. 2b shows the deformation pattern of CFc during the same period, i.e. from 2005 to 2014. The whole CFc uplifted and expanded producing different total displacements, but following a similar accelerating process (continuous GPS data, CGPS, Fig. 2-4 in De Martino et al. , 2014). According to Chiodini et al. (2015), here we refer to the baseline variations between two CGPS stations of the INGVnetwork [ACAE and ARAFE, see Chiodini et al. (2015) for further details]. The deformation curve (Fig. 2b) suggests the overlapping of a general trend of expansion with short periods of dilation (or uplifting) pulses, two of which were particularly important, in 2006-2007, and 2012-2013. Chiodini et al. (2015) fitted the CGPS measurements to a third-order polynomial equation considering only the points less affected by these pulses (i.e., the relative minima of the curve; Fig. 2b, black dots). The residuals of the observed data with respect to the curve (Fig. 2c) clearly repeat the same sequence of seven minima and six maxima, highlighted by the CO 2 / CH 4 and He/CH 4 fumarolic ratios. The main difference is a time lag of about 200 days, with the geochemical signal following the ground deformation (Fig. 2c). Excluding an improbable fortuity, this coincidence between two independent data sets can be interpreted as the consequence of pulsed inputs of magmatic fluids into the hydrothermal system feeding Solfatara fumaroles. The pressurization of the deeper part of the system (magmatic gas zone in Fig. 1), which likely anticipates the degassing event, and the pressure variations within the hydrothermal system during the injection episode cause the deformation. The delay of the geochemical signal represent the transient time of the magmatic fluids from the input zone to the fumarolic discharges. Only the last important deformation event (2012-2013) does not correspond to a geochemical peak of comparable intensity. It is worth to note that recently this deformation episode was attributed to magma intrusion at relatively shallow levels rather than Fig. 2 – a) Measured CO2/CH4 and He/CH4 ratios at fumaroles BG and BN. In order to compare the different signals the measured data were normalized by dividing the difference between each value and the mean by the standard deviation (standardized z-score). The 4 month mobile average of all the data is assumed as the best representation of the geochemical signal; b) 2005-2014 baseline length variation between the ACAE and ARFE CGPS stations (De Martino et al. , 2014). The data used for the derivation of the ‘accelerating trend’ curve are reported as black dots (see the text for further explanations); c) the geochemical signal is compared with the 4 month mobile average of the ground displacement residual (redrawn from Chiodini et al. , 2015).