Structural control in diffuse degassing areas on the Sete Cidades Volcano (São Miguel, Azores) lava deltas

Le résumé fourni par la source

Sete Cidades is a dormant central volcano in São Miguel Island (Azores archipelago) with the highest rate of volcanic eruptions in the island in the last 5000 years. From a degassing point of view, it is characterized by the presence of thermal springs, and diffuse (subaerial and submarine) degassing areas. This study quantifies for the first time the soil CO 2 emission in the Sete Cidades lava deltas (Mosteiros and Ferraria) and identifies diffuse degassing structures (DDS) that correlate with extensional tectonic structures, essentially NW-SE to W-E oriented. Between 2022 and 2023, 939 soil CO 2 flux measurements, based on the accumulation chamber method, were performed in these deltas, ranging from 0 to 932 and 0.6 to 145 g m -2 d -1 in Ferraria and Mosteiros, respectively. The variations observed together with the application of a graphical statistical approach (GSA) and carbon isotope data suggest the presence of different origins (biogenic and/or volcanic-hydrothermal) for the CO 2 in both areas (biogenic limit values ≈ 21 and 35 g m -2 d -1 , respectively). The small dimension DDS highlight also the lithological control and the need to perform detailed surveys to detect the soil gas anomalous areas. The total emission was estimated at ≈ 2.10 t d -1 and the hydrothermal contribution as ≈ 0.3 t d -1 . Soil temperature was also measured and values varied between 14.7 and 40.2°C. Even if high soil temperature were detected, no clear thermal anomalies were identified in any of the areas. The maximum soil temperature is probably due to the insolation on bare soils over basaltic lavas. The degassing maps are relevant for volcano monitoring as well as for risk assessment, especially at Mosteiros village where incidents associated with hazardous CO 2 already occurred.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.