GNGTS 2013 - Atti del 32° Convegno Nazionale

sensitivity for the individual gas components must be known (Suer, 2010). Those relative mass spectrometer sensitivities are determined by the measurement and stored as calibration factors. Calibration gas files were prepared via the Quadstar software. After the preparation of the calibration files, the QMS was calibrated with air and the calibration gas. With these calibra- tions, a table containing the gases and their respective calibration factors was generated. During air calibration, the capillary of the QMS was disconnected from the gas line and ex- posed to air. Air was used to calibrate for Oxygen, Nitrogen, and Argon. As internal standard, Argon was used. During calibration with gas standards, the QMS was disconnected from the gas line and then connected to the calibration gas flask. After establishing the connection, the “dead volume”, i.e. the space between the inlet capillary of the QMS and the calibration flask was evacuated by using the QMS for some time until a pressure inside the chamber of <10 -7 mbar was achieved. Then the calibration gas flask was opened to the QMS for measuring. Once calibrated, the QMS was ready to proceed with the quantitative analysis. Geochemical evidence from data set May-June 2010. The best result obtained during the development of the continuous monitoring system has occurred during the months of May- June 2012. In particular from 16 th May to 5 th June have occurred the best conditions for per- forming the continuous extraction of gas 24 hours a day. In the following it will be discussed the geochemical composition trends from the Pisciarelli degassing field as well as the main relationships of good tracer of magmatic fluids injection such us CO 2 /CH 4 and H 2 S/CO 2 . Methane is a gas species which differentiates in hydrothermal systems, where it is present in relatively high concentrations, from high temperature volcanic magmatic fluids where it is normally absent or present in very low concentrations. Measured CO 2 /CH 4 in fumaroles from 23 hydrothermal systems on the world range from 10 to 10 4 roughly in agreement with the theoretical values expected for a gas phase in chemical equilibrium at temperatures from 200°C to 400°C and redox conditions fixed by hydrothermal buffers (Chiodini and Marini, 1998). The CO 2 /CH 4 is a good tracer of magmatic fluids injection because CO 2 concentration increased, due to its higher content of the magmatic component, and CH 4 , a gas species formed within the hydrothermal system, is lowered both by dilution and by the more oxidizing, transient conditions caused by the arrival of SO 2 into the hydrothermal system (Chiodini, 2009, 2012). This opposite behaviour causes rapid increases of the CO 2 /CH 4 ratio in fumarolic fluids like it showed by the following figure. This trend seems to be confirmed by the data of GPS ground deformation that show a general tendency to uplift with an acceleration of the phenomenon in the period spanning from June to August 2012 (25 mm/month in average) and increasing during the last month beginning on De- cember 2012 (10 mm/month). The total lifting from January 2012 is about 8 cm. In general systematically every ground inflation corresponds to an increase of CO 2 /CH 4 , and systematically a decrease of the ratio accompanies any deflation for each of the four minor bradyseisms in the last 25 years (Chiodini, 2009). Therefore, the numerous CO 2 / CH 4 peaks observed at Solfatara fumaroles can be interpreted as the result of the injection of new mag- matic fluids into the hydrothermal system, a process that occurs some time before the geochemical signal is observed at the surface. Fig. 3 – The CO 2 /CH 4 ratio from 16/05/2012 to 05/06/2012 measured by QMS. 249 GNGTS 2013 S essione 1.3