Real-time quadrupole mass spectrometry of hydrothermal gases from the unstable Pisciarelli fumaroles (Campi Flegrei): Trends, challenges and processes

Abstract

Volcanic gas sampling and post-collection chemical determination in a laboratory may preclude anyreal-time continuous monitoring of volcanic activity. We describe the development, and show the advan-tages, of a system used for the continuous monitoring of fumarolic gases discharged from the Pisciarellisite (Campi Flegrei, Southern Italy) based on a commercial quadrupole mass spectrometer (QMS-301OmnistarTM). Although numerous technical problems were addressed due to the ephemeral nature ofthe emission point and the harsh environment, we also report measurements of the chemical composi-tion of the major gas species emitted from the fumarole for two different periods (in 2009 and 2012). TheCO2/H2S, H2S/H2, He/CO2and CH4/CO2molar ratios were investigated in order to detect magmatic and/orhydrothermal components in the system, while the N2/O2ratio was adopted to infer other non-volcanicprocesses, such as air contamination and mixing with polluted surface waters. The presented method-ology allows continuous gas sampling and provides the first evidence of short-term gas variations notavailable by direct sampling, which is often impractical and hazardous. Compared to the current tech-niques that are used worldwide for the full characterization of gaseous emissions, i.e. chemical analysis oftraditional soda-filled under-vacuum bottles and MultiGAS surveys (laboratory-based and in situ, respec-tively), QMS-based monitoring is complementary and, in prospect, an alternative. With our method, thegeochemical monitoring benefits of the real-time analysis for high sampling rates that can be made com-parable to the continuous measurements of geophysical networks. This allows a better understandingof hydrothermal features, particularly of chemical fluctuations occurring on the very short-term, whichis fundamental for the evaluation of the evolution of unrest episodes at Campi Flegrei, one of the mosthazardous volcanic areas in the world.