Monitoring active volcanoes: The geochemical approach

Shallow magmas located beneath active volcanoes release volatiles both during eruptive activity and during inter-eruptive periods (passive degassing). The fluids in active volcanic areas rise directly from magma, and their composition is characterized mainly by H2O, CO2, SO2, H2S, HF and HCl (condensable gases), and by some non-condensable gases (e.g. He, H2, N2, CO, CH4). The chemical composition of fumarolic gases can reflect the pressure, temperature and oxygen fugacity conditions of the deep magmatic source, provided that during their rise towards the surface, the gases do not undergo re-equilibration processes [Giggenbach 1980, Giggenbach 1996, Nuccio and Paonita 2001, Paonita et al. 2002]. As the equilibrium kinetics of several chemical reactions is much slower than the rising velocities of the gases, the gas molecular compositions often undergo quenching phenomena, so that the gases show temperature and pressure equilibria higher than their outlet values. The concentration of magmatic species or their molecular ratios can be determined by means of direct sampling of fumarole gases or by using telemetric methods of observation. The extensive parameters (mass output) of volcanic fluids, coupled with the intensive parameters described before, provide basic and useful information for the formulation of volcanic fluid degassing models [Italiano et al. 1997, Brusca et al. 2004, Inguaggiato et al. 2011]. The first step in the framework of the geochemical investigation of a volcanic system aimed at surveillance is the chemical and isotopic characterization of the fluids, and the putting forward of a geochemical model [Inguaggiato et al. 2011]; within this geochemical model, it is possible to interpret the observed changes in any single investigated parameter. The geochemical approach is to identify the following topics: • The main end-members involved in the studied system; • The possible type and degree of interaction processes: e.g. water-rock and gas-water interactions; • The mixing among the individual end-members; • The chemical and isotopic characterization of a possible hydrothermal system; • The formulation of a geochemical model

CO2 output discharged from Stromboli Island (Italy)

Total CO2 output from soil gas and plume, discharged from the Stromboli Island, was estimated. The CO2 emission of the plume emitted from the active crater was estimated on the basis of the SO2 crater output and C/S ratio, while CO2 discharged through diffuse soil emission was quantified on the basis of 419 measurements of CO2 fluxes from the soil of the whole island, performed by using the accumulation chamber method. The results indicate an overall output of ≅416 t day−1 of CO2 from the island. The main contribution to the total CO2 output comes from the summit area (396 t day−1), with 370 t/day from the active crater and 26 t day−1 from the Pizzo sopra La Fossa soil degassing area. The release of CO2 from peripheral areas is ≅20 t day−1 by soil degassing (Scari area mainly). The result of the soil degassing survey confirms the persistence of the highest CO2 degassing areas located on the North-East crater side and Scari area

Major and trace element geochemistry of El Chichón volcano-hydrothermal system (Chiapas, México) in 2006-2007: implications for future geochemical monitoring

Isotopic, major and trace element composition studies for the crater lake, the Soap Pool and thermal springs at El Chichón volcano in November 2006-October 2007 confirm the complex relationship between annual rainfall distribution and crater lake volume and chemistry. In 2001, 2004 and 2007 high volume high-Cl lake may be related to reactivation of high discharge (>10 kg/s) saline near-neutral water from the Soap Pool boiling springs into the lake, a few months (~January) after the end of the rainy season (June-October). The peak lake volume occurred in March 2007 (~6 x 105 m3). Agua Tibia 2 thermal springs discharge near the foot of the SW dome but their chemistry suggests a lower temperature regime, an enhanced water-rock interaction and basement contribution (evaporites and carbonates), anhydrite leaching from the 1982 pyroclastic deposits, rather than dome activity. New suggestions of crater lake seepage are evidenced by the Agua Caliente thermal springs. Existing models on the “crater lake-Soap Pool spring” and the deep hydrothermal system are discussed. Chemical changes in the deep geothermal aquifer feeding the thermal springs may predict dome rise. Future volcanic surveillance should focus on spring chemistry variations, as well as crater lake monitoring

Major and trace element geochemistry of El Chichón volcano-hydrothermal system (Chiapas, Mexico) in 2006-2007: implications for future geochemical monitoring

We report a detailed study of isotopic, major and trace element composition in the crater lake, Soap Pool and thermal springs at El Chichón volcano for the period November 2006-October 2007. After two decades of studying the crater lake, it is possible to confirm the complex relationship between the annual rainfall distribution and the crater lake volume and chemistry: during three years (2001, 2004 and 2007) a large volume high-Cl lake can be related to the reactivation of high discharge (>10 kg/s) of saline near-neutral water from the Soap Pool boiling springs towards the lake, only a few months (~January) after the end of the rainy season (June-October). The highest lake volume ever observed occurred in March 2007 (~6x105 m3). Despite the fact that the Agua Tibia 2 thermal springs discharge at the foot of the SW dome, their chemistry indicates a lower temperature regime, an enhanced water-rock interaction and basement contribution (evaporites and carbonates), and anhydrite leaching from the 1982 pyroclastic deposits, rather than dome activity. New suggestions on crater lake seepage are evidenced by the Agua Caliente thermal springs. Existing models on the “crater lake-Soap Pool spring” and the deep hydrothermal system are justified and detailed. We believe that chemical changes in the deep geothermal aquifer feeding the thermal springs will anticipate dome rise. Future volcanic surveillance should focus on the changes in spring chemistry, besides crater lake monitoring

Preliminary estimate of CO2 budget discharged from Vulcano island

Total CO2 output from fumaroles, soil gases, bubbling and water dissolved gases were estimated at Vulcano Island, Italy. The fumaroles output has been estimated from SO2 plume flux, while soil flux emission has been carried out through 730 CO2 fluxes measured on the island surface, performed by means of accumulation chamber method. Vulcano Island, located in the Aeolian Archipelago, is an active volcano that has been in state of solphataric activity, since the last eruption (1888-1890). At present, the main exhalative activity is in the northern part of the island, it is revealed by a wide fumaroles field, on the active edifice of “La Fossa” crater, (100°C <T<450°C); by low temperature fumaroles (T<100°C) and sea-bubbling gases in the Baia Levante area; moreover, strong soil degassing occurs in the Vulcano Porto area and around the volcanic edifice, where the active tectonic discontinuities drive CO2 to the surface. Finally, numerous carbon-rich thermal wells (up to 80°C) in the Vulcano Porto Area, testify the presence of a geothermal system with equilibrium temperature around 200°C. The preliminary results indicate an overall output of 470 T/day of CO2 from the island. The main contribution to the total output is from the summit area of the active cone (450 T/day), where 360 T/day and 90 T/day are from crater fumaroles and crater soil degassing, respectively. Peripheral areas release 8 T/day by soil degassing (Palizzi and Istmo areas mainly), a measure comparable to the contribution of water dissolved CO2 (estimated as 6 T/day) and higher than sea-bubbling CO2 (1 T/day measured in the Istmo area). The presented data (September 2007) refer to a period of moderate solphataric activity, when the highest temperature and gas/water ratio of fumaroles were 457°C and 0.17 respectively. These preliminary data allow the estimation of the background mass release and related thermal energy from the volcanic system. They represent the first complete data set, collected during moderate volcanic activity which can be compared to the new one acquired during subsequent (the next o future) evolution of the activity