Long-term variations of diffuse CO2, He and H2 at the summit crater of Teide volcano, Tenerife, Canary Islands

<p>Tenerife (2,034 km<sup>2</sup>) is the largest of the Canary Islands. Its structure is controlled by a volcano-tectonic rift-system with NW, NE and NS directions, with the volcanic system Teide-Pico Viejo located in the intersection. Teide is 3,718 m.a.s.l. high and its last eruption occurred in 1798 through an adventive cone of Teide-Pico Viejo volcanic complex. Persistent degassing activity, both visible and diffuse, takes place at the summit cone of the volcano, being the diffuse degassing the principle mechanism.</p><p>During the period 1999-2020, more than 200 diffuse CO<sub>2</sub> efflux surveys have been performed in the summit crater of Teide Volcano. For each survey, 38 sampling sites homogeneously distributed inside the crater covering an area of 6,972 m<sup>2</sup> were selected. Diffuse CO<sub>2</sub> emission was estimated in each point by means of a portable non dispersive infrared (NDIR) CO<sub>2</sub> fluxmeter using the accumulation chamber method. Additionally, soil gas samples were taken at 40 cm depth and analyzed later in the lab for the He and H<sub>2</sub> content by means of quadrupole mass spectrometry and micro-gas chromatography, respectively. To estimate the He and H<sub>2</sub> emission rates at each sampling point, the diffusive component was estimated following the Fick’s law and the convective emission component model was estimated following the Darcy’s law. In all cases, spatial distribution maps were constructed averaging the results of 100 simulations following the sequential Gaussian simulation (sGs) algorithm, in order to determine CO<sub>2</sub>, He and H<sub>2</sub> emission rates.</p><p>During the study period, CO<sub>2</sub> emissions ranged from 2.2 to 176.1 t/d, He emissions between 0.013 and 4.1 kg/d and H<sub>2</sub> between 1.3 and 35.6 kg/d. On October 2, 2016, a seismic swarm of long-period events was recorded on Tenerife followed by a general increase of the seismic activity in and around the island (D’Auria et al., 2019). Since then, relatively high values have been obtained in the diffuse CO<sub>2</sub>, He and H<sub>2</sub> emission rate the crater of Teide. This increase reflects a process of pressurization of the volcanic-hydrothermal system.</p><p>The variations in CO<sub>2</sub>, He and H<sub>2</sub> emissions indicate changes in the activity of the system and can be useful to understand the behaviour of the volcanic system and to forecast future volcanic activity. Monitoring the diffuse degassing rates at Teide volcano has demonstrated to be an essential tool for predicting future seismic–volcanic unrest, and has become important to reduce volcanic risk in Tenerife (Melián et al., 2012; Pérez et al., 2013).</p><p>D'Auria .L, Barrancos J., Padilla G.D., Pérez N.M., Hernández P.A., Melián G., Padron E., Asensio-Ramos M., García‐Hernández R. (2019). J. Geophys. Res. 124, 8739-8752</p><p>Pérez N. M., Hernández P. A., Padrón E., Melián G., Nolasco D., Barrancos J., Padilla G., Calvo D., Rodríguez F., Dionis S. and Chiodini G. (2013). J. Geol. Soc., 170(4), 585-592.</p><p>Melián G., Tassi F., Pérez N. M., Hernández P., Sortino F., Vaselli O., Padrón E., Nolasco D., Barrancos J., Padilla G., Rodriguez F., Dionis S., Calvo D., Notsu K., Sumino H. (2012).  Bull. Volcanol, 74(6), 1465-1483.</p><p> </p>

Soil gas CO2 concentration, isotopic ratio and efflux measurements for geothermal exploration at Cumbre Vieja volcano, La Palma, Canary Islands

<p>The exploration of geothermal resources on the island of La Palma, Canary Islands, was first conducted by the Spanish Geological Survey (IGME) from 1982 to 1984. These studies were focused exclusively on the southern part, where the last historical eruption, Teneguía, took place in 1971. This area still shows some geothermal features such us relatively high ground water temperatures (about 40ºC) and soil CO2 efflux values. Recent studies carried out at Cumbre Vieja volcano, the southern part of the island, on diffuse degassing, 3D gravimetry and Audio-MT probes point to promising results, although more studies are needed. We continue applying a multidisciplinary approach to obtain additional information about the geothermal system underlying at Palma island using novel techniques as well as tools which are appropriate to evaluate this system. For this reason, during summer 2019 a soil diffuse degassing research started at Cumbre Vieja volcano (220 km<sup>2</sup>) for geothermal exploration purposes. In this first phase of the diffuse degassing study about 1,200 sampling sites, with an average distance between sites of approximately 250 m were selected after taking into consideration the volcano-structural features and accessibility. In each sampling site in-situ soil CO2 efflux measurements were performed, and soil gas samples were collected at 40 cm depth for chemical and isotopic analysis. Spatial distribution of CO<sub>2</sub> efflux, statistical-graphical analysis of CO<sub>2</sub> efflux, and δ<sup>13</sup>C-CO<sub>2</sub> isotopic data to calculate and map the volcano-hydrothermal contribution of CO<sub>2</sub> were combined and used for geothermal exploration. The statistical-graphic analysis of the diffuse CO<sub>2</sub> efflux values confirms the existence of different geochemical populations showing two log-normal geochemical populations, a fact that suggests the addition of deep-seated CO<sub>2</sub>. Relatively low CO<sub>2</sub> efflux values were measured ranging from non-detected up to 72.8 g m<sup>-2 </sup>d<sup>-1</sup>, with an average value of 4.6 g m<sup>-2 </sup>d<sup>-1</sup>. The highest CO<sub>2</sub> efflux values were measured at the north end of Cumbre Vieja, around the surface contact with Cumbre Nueva ridge. The CO<sub>2</sub> isotopic composition, expressed as δ<sup>13</sup>C- CO<sub>2</sub> showed the contribution of three different end-members: biogenic, atmospheric and deep-seated CO<sub>2</sub>. The results indicate that most of the sampling sites exhibited CO<sub>2</sub> composed by different mixtures between atmospheric and biogenic CO<sub>2</sub> with slight inputs of deep-seated CO<sub>2</sub>, with a mean value of -15.3‰, being the maximum and the minimum -2.8‰ and -25.4‰ respectively. The results showed here can help to identify the existence of zones where deep-seated actively degassing from geothermal reservoirs occurs, particularly where the interpretation and application of geophysical data might be difficult.</p>

Heat and Helium-3 Fluxes from Teide Volcano, Canary Islands, Spain

During July 2016, the first integrated heat flow, CO2, and 3He emission survey was conducted across 0.5 km2 of the summit cone and crater of Teide volcano, Tenerife, Canary Islands, Spain. The thermal energy released from Teide summit cone by diffuse degassing was 2.2 MW, and the heat flux calculated through Dawson’s method was 8.1 MW, difference due to the comparison of purely convective areas as the crater with diffusive areas as the flanks of the volcano. Diffuse CO2 output was 211±20 t d-1, and 3He emission was estimated to be within a range between 0.35 and 0.89 mol y-1. The obtained values of diffuse degassing and heat fluxes are close to others obtained for similar volcanic areas. The calculation of 3He/heat ratio for the first time in this volcanic system supports the presence of an important mantle source for the degassing of Teide volcano.