Post-eruptive monitoring of visible volcanic degassing from Tajogaite volcano, La Palma, Canary Islands

Once the Tajogaite eruption was over in December 13, 2021, INVOLCAN continued monitoring visible volcanic emanations. To do so, active in situ/remote monitoring techniques were undertaken based on different methodologies: direct sampling, alkaline traps, Multi-GAS and miniDOAS. These methods are suitable tools to provide as much information as possible on the emitted volcanic gases. Direct volcanic gas sampling was performed at two volcanic degassing sites, F1 and F2, by collecting samples burying a glass funnel at the sampling point and using TEDLAR bags for later chemical (CO₂, H₂, H₂, H₂S and CH₄) and isotopic (δ¹³C-CO₂) analysis. The temperature of F1 and F2 was always measured at each sampling. Three alkaline traps were installed to monitor the chemical composition of acid gases in the crater's atmosphere Traps are made of polythene containers and protected by a metal mesh and a PVC container to prevent contamination by rainwater and installed at a one meter height. The alkaline solution (KOH, 2N) was used to absorb acid gases. Multi-GAS was used to analyze in real-time visible emanations during the year 2024. This instrument comprises an infra-red spectrometer for CO₂ and 2 electrochemical sensors for SO₂ and H₂S. Regarding miniDOAS, we carried out UAV-mounted and ground-based miniDOAS measurements. 

F1 site showed the highest temperature, with a maximum of 613ºC and an average of 455ºC, while F2 showed a maximum of 409ºC and an average of 266ºC. CO₂, He, H₂, CH4 and H₂S concentrations for both degassing vents varied between 0.04-31 mol.%, 4.9-40.8 ppm, 0.3-5,400 ppm, 0-6.2 ppm and 0-4,792 ppm, respectively, with H₂S being detected only during December 2021 and January 2022. The temporal evolution of the He/CO₂ molar ratio shows a continuous decrease throughout the study period (Dec 2021-Sep 2024) indicating an impoverishment of the magmatic component. δ¹³C-CO₂ showed a clear trend in both fumaroles towards lighter values, what was interpreted to be caused by stronger biogenic and air contribution. It should be noted that time series of C/S molar ratio measured with the alkaline traps during the first year showed a significant increase, also suggesting a decrease in the magmatic fraction of volcanic gases. Regardless of the degree of atmospheric contamination, MULTIGAS measurements revealed an increasing trend on CO₂/SO₂ molar ratios ranging 8.7-772, with SO₂ as the major S species. Finally, around 80 SO2 miniDOAS measurements were made between December 15, 2021, and December 17, 2022 (Rodríguez et al., 2023). SO₂ emission rates ​​ranged between 17 and 670 t/d, with a clear decreasing trend observed during this period. These relatively low SO₂ emissions observed during the post-eruptive phase of the Tajogaite eruption appear to be clearly related to the cooling processes of the surface magma within the Tajogaite volcanic edifice. These results show that measuring the changes of visible volcanic degassing improves monitoring of Tajogaite volcano. 

 Rodríguez et al. 2023. EGU General Assembly 2023, Vienna, Austria, EGU23-3620