Deep CO2 emitted at Furnas do Enxofre geothermal area (Terceira Island, Azores archipelago). An approach for determining CO2 sources and total emissions using carbon isotopic data

Quantification of the CO2 released by the volcanoes to the atmosphere is relevant for the evaluation of the balance between deep-derived, biogenic and anthropogenic contributions. The current study estimates the CO2 released from Furnas do Enxofre degassing area (Terceira Island, Azores archipelago) by applying an approach that integrates the flux of CO2 from the soil with the δ13C-CO2 values. A deep-derived CO2 output of 2.54 t d−1 is estimated for an area of ~23,715 m2. High biogenic-derived CO2 flux values (~45 g m−2 d−1) associated with light carbon isotopic content (δ13C = −28‰ ± 1.1‰) are detected and explained by the type of vegetation that characterizes the study site. Carbon isotopic compositions of the CO2 (−6.4‰ ± 1.2‰) measured in olivine-hosted fluid inclusions of the Terceira basalts are presented for the first time and contribute to defining the mantle-CO2 signature. Differences between these values and heavier carbon isotope values from gas in fumaroles at Furnas do Enxofre (−4.66‰ to −4.27‰) are explained by the carbon isotopic fractionation occurring when CO2 reacts to form calcite in the geothermal reservoir at temperatures >180 °C. A clear correlation between the soil temperature and deep CO2 fluxes is observed and the integration of the diffuse degassing information with the composition of the fumarolic emissions allows estimating a thermal energy flux of 1.1 MW.