MORENO, L., VIVEIROS, F., MATIAS, D., SILVA, C., FERREIRA, T., MARCOS, M. (2021) – Characterization of some gas ratios from Azores archipelago fumaroles – application to volcanic monitoring, 1^st CCVG Virtual Workshop, 24-26 de maio.​

​Secondary manifestations of volcanism in the Azores archipelago comprise fumaroles, thermal and cold CO₂-rich springs as well as diffuse degassing areas. The more relevant fumarolic emissions are located in the islands of São Miguel, Terceira and Graciosa and these emissions have been regularly sampled in the last 20 years. Gas data show the typical hydrothermal composition with water vapour as the main component and CO₂ as the most abundant dry gas (more than 96 vol.% for the sampled fumaroles). Different amounts of H₂S, N₂, O₂, Ar, CH₄, He and H₂ have been also quantified. Time series of different gas ratios have been used as a useful tool for seismovolcanic monitoring, and to evaluate potential changes on the reservoirs associated to the geothermal drilling/exploitation. The main gas ratios that have routinely been used in the Azores observatory (IVAR - Instituto de Investigação em Vulcanologia e Avaliação de Riscos) are CO₂/CH₄, H₂S/CO₂, CH₄/H₂ and H₂/Ar. Gas ratios values are specific for each of the fumarolic fields. For instance, CO₂/CH₄ ratio shows higher values for São Miguel and Graciosa islands, in the order of thousands, while in Terceira Island they are in the order of hundreds. These differences reflect the higher CH₄ concentrations measured at Terceira fumaroles. Even in the same volcanic system, gas ratios from fumaroles may show significant differences, what can reflect distinct reservoir conditions, as it seems to be the case of Caldeira Velha and Caldeiras da Ribeira Grande fumaroles at Fogo Volcano (São Miguel Island). Currently, and due to the impossibility to detect CO, and consequently to apply geothermometers with this gas specie, the IVAR team has been applying the H₂/Ar gas geothermometer from Arnórsson et al. (1998) to the São Miguel and Terceira fumaroles. In the case of São Miguel, a range between 229 and 256ºC has been estimated for the reservoirs feeding the fumaroles, while for Terceira fumaroles an average temperature of 252ºC has been estimated. No significant variations of the gas ratios have been observed in the last years, in agreement with the quiescent phase of the Azores volcanic systems.