Furnas Volcano is a polygenetic quiescent volcano located in the eastern part of São Miguel Island (Azores archipelago, Portugal) and started to be formed at about 100 000 years BP. Geomorphologically Furnas Volcano comprises a summit depression 5 x 8 km wide formed by two nested calderas controlled by NW-SE and NE-SW faults. Since the settlement of the island, in the 15th Century, two intracaldera subplinian volcanic eruptions occurred in this volcano, in 1439-43 and in 1630. Both events started with the formation of a pumice ring and ended with the emplacement of a central crater trachytic dome. At present, Furnas Volcano has a moderate seismic activity and exhibits hydrothermal manifestations characterized by low temperature fumaroles (around 97 – 99 ºC), steaming grounds, thermal and CO2 cold springs and soil diffuse degassing areas.
During the summer months from 2005 to 2008 several soil CO2 flux surveys were carried out at Furnas Volcano applying the accumulation chamber method. A total of 2886 measurements were done and the measured values showed high variability ranging from absence of soil CO2 flux to values higher than 25 000 g m-2 d-1. The data set obtained is positively skewed and soil gas flux was modelled as combination of three overlapping populations, which suggest the existence of multiple sources (biogenic and volcanic-hydrothermal) feeding the soil CO2 diffuse degassing in Furnas Volcano. The δ13C compositions of soil CO2 efflux collected in the volcano caldera varied from -12.28 ‰ to -3.11‰, confirming different sources for the CO2 degassing. The statistical approach combined with the isotopic composition of the CO2 efflux suggested a value of ~ 25 g m-2 d-1 as a reliable limit for the biogenic flux.
Diffuse degassing maps were produced based on one hundred sequential Gaussian simulations. Main diffuse degassing structures observed at Furnas Volcano are associated to the fumarolic fields showing that in these areas CO2 is carried by the steam upflow. Important DDS (diffuse degassing structures) are found out in depressed areas in the inner slopes of some crater rims showing the strong morphostructural control on the gas release. In addition to hydrothermal CO2 degassing measured along previously mapped NW-SE and WNW-ESE tectonic structures, soil CO2 flux patterns allowed to infer new general WNW-ESE trends that probably represent hidden tectonic structures. Luxuriant vegetation and thick pumice deposits observed in this volcanic system difficult the identification of tectonic structures and these degassing maps are therefore useful tools to reveal weak structural zones. Gas geochemical transects performed across historical eruptive centres show low soil CO2 degassing, which is probably explained by the presence of the trachytic domes. Only some residual endogenous degassing was found contouring the domes.
Hydrothermal CO2 output from the whole Furnas Volcano was estimated to be ~ 954 t d-1 (from an area with ~ 5.2 km2). The thermal energy release associated with diffuse degassing at Furnas caldera was estimated to be about 116 MW based on the H2O/CO2 ratio in fumarolic gas.