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El Hierro, the smallest, youngest and westernmost island of the Canarian archipelago, with an area of 278 Km2, is settled on an ocean floor 3.5 km deep and reaches 1.5 km above sea level. The island was constructed by rapid constructive and destructive processes in ~ 1.12 Ma. From October 2011 to March 2012 a submarine eruption took place about 2 km south of the small village of La Restinga in the southernmost part of the island. The eruptive process has been the second longest and the second largest volume discharged in the historical volcanic activity of the Canaries (in the last 500 years) and was the first one to be monitored from the beginning. One of the most useful geochemical tools to monitor the volcanic activity of El Hierro is the diffuse degassing studies, since visible volcanic emissions are absent at the surface of El Hierro. Diffuse CO2 emissions have been monitored at El Hierro Island since 1998 in a yearly basis, with higher frequency during the pre and eruptive periods. At each survey, 600 sampling sites are studied and measurements of soil CO2 efflux were performed in situ following the accumulation chamber method (Parkinson, 1981). During pre-eruptive and eruptive periods, the diffuse CO2 emission released by the whole island experienced significant increases before the onset of the submarine eruption and the most energetic seismic events of the volcanic-seismic unrest. In the last survey, performed in the 2018 summer period, soil CO2 efflux values ranged from non-detectable to 31 g•m−2•d−1. Statistical-graphical analysis of the data show three different geochemical populations, background (B), intermediate (I) and peak (P), represented by 90.9%, 7.8 % and 1.3% of the total data respectively, with geometric means of 1.1, 12 and 22 g•m−2•d−1, respectively. To quantify the diffuse CO2 emission for the 2018 survey, 100 sequential Gaussian simulations (sGs) were performed as interpolation method. The estimated 2018 diffuse CO2 output released to atmosphere by El Hierro was at 654 ± 26 t•d-1, value higher than the background average of CO2 emission estimated in 422 t•d-1. The data presented here demonstrate that discrete surveys of diffuse CO2 emission offer important information to optimize the early warning system in volcano monitoring programs and to monitor the evolution of an on-going volcanic eruption. [ABSTRACT FROM AUTHOR]

Tenerife (2,304 km2), the largest of the Canary Islands, has developed a central volcanic complex (Cañadas edifice), that started to grow about 3.5 My ago. Coeval with the construction of the Cañadas edifice, shield basaltic volcanism continued until the present along three rift zones oriented NW–SE, NE–SW and NS (hereinafter referred as NW, NE and NS respectively). Main volcanic historical activity has occurred along de NW and NE rift-zones. Uprising of deep-seated gases occurs along the aforementioned volcanic structures causing diffuse emissions at the surface environment of the rift-zones. In the last 20 years, there has been considerable interest in the study of diffuse degassing as a powerful tool in volcano monitoring programs, an important monitoring task mainly at those volcanic areas where visible manifestations of volcanic gases are absent. Soil gas and diffuse degassing surveys have been focused in CO2 because it is, after water vapor, the most abundant gas dissolved in magma. One of the most inexpensive methods to determine CO2 fluxes is based in the absorption of CO2 through an alkaline medium, followed by titration analysis. In the summer of 2016, a network of 31 closed static chambers was installed, covering the three main structural zones of Tenerife (NE, NW and NS) as well as Cañadas caldera with volcanic surveillance porpoises. 50 cc of 1N KOH solution is placed inside the chamber to absorb the CO2 released from the soil. The solution is replaced weekly and the trapped CO2 is then analyzed at the laboratory by titration. Results are expressed as weekly integrated CO2 efflux values. The CO2 efflux values ranged from 2.7 to 12.5 g•m-2•d-1, with average values of 8.2 g•m-2•d-1 for the NE rift-zone, from 3.1 to 10.9 g•m-2•d-1, with average value of 6.3 g•m-2•d-1 for the NW rift-zone, and from 2.3 to 10.9 g•m-2•d-1, with average value of 6.3 g•m-2•d-1 for NS rift-zone. In the case of Cañadas caldera, values ranged from 1.1 to 8.0 g•m-2•d-1, with average value of 5.0 g•m-2•d-1. The most significant CO2 efflux values were observed at the NE rift-zone, with a maximum value of 12.5 g•m-2•d-1. Soil gas samples were weekly sampled on the head space of the closed chambers to analyze the chemical and isotopic composition of CO2. The gas is considered as CO2-enriched air, showing concentrations of CO2 in the range 355-22,448 ppmV, with average values of 4,959 ppmV, 1,274 ppmV, 1,465 ppmV and 609 ppmV for the NE, NW, NS rift-zones, and Cañadas caldera, respectively. The CO2 isotopic composition indicates that most of the sampling sites exhibited CO2 composed by different mixing degrees between atmospheric and biogenic CO2 with slight inputs of deep-seated CO2, with mean values of -21.4‰, -14.1‰, -16.6‰ and -11.9‰ for the NE, NW, NS rift-zones, and Cañadas caldera, respectively. The methodology presented here represents an inexpensive method that might help to detect early warning signals of future unrest episodes in Tenerife. [ABSTRACT FROM AUTHOR]

La Palma Island is located in the northwest of the Canary Islands. Volcanic activity at La Palma in the last 123 ka has taken place exclusively at Cumbre Vieja volcano located at the southern part, which is characterized by a main north–south rift zone 20 km long and up to 1950 m in elevation. Cumbre Vieja covers an area of 220 km2 with vents located also at the northwest and northeast. Cumbre Vieja is the most active basaltic volcano in the Canaries with 7 historical eruptions being San Juan (1949) and Teneguía (1971) the most recent ones. Since no visible degassing (fumaroles, etc.) at Cumbre Vieja occurs, our geochemical program for the volcanic surveillance of Cumbre Vieja is mainly focused on diffuse degassing monitoring. Diffuse CO2 emission surveys have been yearly performed in summer to minimize the influence of meteorological variations. On 7-9 and 13-14 of October 2017 two intense seismic swarm occurred beneath Cumbre Vieja. In response to this volcanic process, a continuous surveillance diffuse CO2 campaign has been carried out in the area since then. Seismological and geochemical evidences that these swarms were linked to a deep-seated magmatic intrusion were found. Measurements are performed following the accumulation chamber method in about 600 sites. Spatial distribution maps are constructed following the sequential Gaussian simulation (sGs) procedure and, to quantify the total CO2 emission from the studied area, 100 simulations are performed for each survey. Since 2001, the diffuse CO2 output released to the atmosphere from Cumbre Vieja volcano measured on a yearly basis has ranged between 320 to 1,544 t•d-1. After the October 2017 seismic swarms, diffuse CO2 output showed an increasing trend from 788 t•d-1 up to 3,251 t•d-1 in March 2018, to decrease gradually until 852 t•d-1 in September of that same year, and begin to gradually increase again to 2,371 t•d-1 in November 2018. Seismic swarms occurred at Cumbre Vieja were possibly caused by an upward magma migration from an ephemeral magmatic reservoir, located in the upper mantle (about 25 km depth), and toward another reservoir located close to the Moho beneath Cumbre Vieja (12-15 km). The consequent depressurization of the magma batch was the source of the volatiles observed at the surface coherent with the expected geochemical behavior of CO2. [ABSTRACT FROM AUTHOR]