Sulphur trace components in La Solfatara and Vulcano volcanic gases: searching for suitable new geo-indicators

Volcanic and/or geothermal gases provide essential information on the activity of a volcanic system, on magma degassing and on the origin and evolution of fluids. Their study represents one of the most powerful technique to understand the dynamics of systems and to monitor the volcanic activity (dormant state, but also unrest and/or eruption phase).

Volcanic/geothermal gases are complex mix of chemical elements and compounds. Starting from the study of their chemical composition, many equilibrium reactions in gas phase were introduced in the past as possible geothermometers and geobarometers, and they actually used to estimate the thermodynamic conditions in deep system (both in volcanic or geothermal area) and in volcanic surveillance. However, depending on the system, known geothermal-barometric reactions are not able to accurately describe the thermodynamic conditions of reservoirs, highlighting the need for new geothermometric reactions. Of course, the opportunity to develop new “geothermometers/geobarometers functions” depends to the availability of analytical techniques able to detect and quantify new chemical compounds of interest, often at low to very low concentration levels (ppb).

The GC-ICP-MS (gas chromatography-inductively coupled plasma-mass spectrometry), one of the most useful hyphenated method (Michalski R. et al., 2006; Easter R.N. et al., 2010), combines the high separation capacity of the GC with the high sensitivity and specificity of the ICP-MS. Chemical compounds containing C, S and O are abundant in volcanic/geothermal gases and they can be detected at low levels via GC-ICP-MS technique. The development of new specific analytical methods for volcanic/geothermal gas analysis (in particular for what concern new compounds) may provide the chance to introduce new gas equilibrium as a “key” to better understand thermodynamic and redox conditions at depth.

Dry gas samples from fumaroles of the La Solfatara di Pozzuoli (Bocca Grande, Bocca Nuova and Pisciarelli) and for crater area in the Vulcano island were analysed, studying the distribution of sulphur-bearing species. The results obtained are very satisfactory in terms of chromatographic separation and detection limits, making the GC-ICP-MS method very promising in the study of volcanic/geothermal gases.