Quantifying the flux of magma derived CO2 dissipated by fumarolic fields at dormant volcanoes is fundamentalto assess their current state of hydrothermal activity and, therefore, the likelihood of a future phreatic/magmaticeruption. There is, in fact, documented evidence that gas fluxes, and CO2 flux in particular, can increasesubstantially during volcanic unrests and prior to eruption, due to either degassing of new ascending magma orchanges in the hydrothermal system physical regime. Quantifying CO2 emissions is relatively straightforward at open-conduit volcanoes with high-temperature gas venting, which release high enough quantities of SO2 remotelymeasurable with UV spectroscopy and where CO2 flux can then be obtained by scaling to fumarole CO2/SO2ratios. Less actively degassing systems (T less than 300 C), in contrast, release sulphur predominantly as H2S,that is far more difficult to detect optically than SO2, and other procedures are therefore necessary. The main pointis that many dormant volcanic systems worldwide fall in this second category.Here we present the first CO2 budget estimates for two dormant volcanoes displaying intense fumarolic activities:La Soufriere of Guadeloupe (Lesser Antilles arc) and Campi Flegrei (Naples, Italy). Both volcanic systems haveproduced voluminous pyroclastic eruptions in the past and have shown signs of unrest during the last decades(phreatic eruptions at La Soufriere; occasional seismic swarms, extensive degassing and ground uplift at CampiFlegrei). While the CO2 output by diffuse soil degassing is relatively well known at both systems, no accurate CO2budget for their fumarolic exhalations has been obtained so far. For quantifying this budget we used a portableMultiGAS sensor system that allowed us to determine the horizontal/vertical concentration profiles of CO2 (plusH2O, SO2, H2S and H2) along cross-sections of the gas plumes generated by the fumarolic activity, perpendicularto the plume transport direction. Gas sampling data at 0.5 Hz and simultaneous GPS positioning provided us withthe required temporal/spatial resolution to map the chemical heterogeneity of plumes. By integrating over the entireplume section, and scaling to measured plume transport speed (for thermal/UV/video imaging), the CO2 fluxeswere calculated. The validity of this methodology was also tested at Campi Flegrei by comparison with independentCO2 flux estimates which we obtained with a Tuneable IR Laser (good match between the two independent fluxestimates). Our results show that fumarolic CO2 emissions are significant at both volcanoes (e.g. 1000 t/day atCampi Flegrei, about as much as from diffuse soil degassing, and 30 to 60 t/day at La Soufriere). We propose thatthe experimental procedure described above can be applied to many other comparable volcanic targets worldwidein order to improve our knowledge of the budget for global volcanic CO2 emissions.
|Numero di pagine||1|
|Stato di pubblicazione||Published - 2013|