Determining the xenon (Xe) isotope composition of Earth's deep-mantle reservoirs is key to constraining early terrestrial volatile acquisition and subsequent evolution. Here, we employ dynamic mass spectrometry to characterize the Xe isotope composition of the Dos Aguas CO2-rich, cold mineral spring (La Palma, Canary Islands), previously shown to exhibit the highest 3He/4He reported from the Canarian archipelago (10.85 RA). Although the plume mantle-derived Xe contribution is small (<= 0.8%), its signature matches that of the convecting upper mantle (e.g., S129Xe*/S136Xe* approximate to 1; i.e., with comparable 129Xe* and 136Xe* excesses relative to air). This result aligns with the growing evidence that plume mantle- and upper mantle-derived sources are colinear in S129Xe vs. S136Xe and S129Xe*/S136Xe* vs. S130Xe*/136Xe* spaces. For a decade, however, the latter space was considered a powerful means to discriminate between both mantle sources. Here, we use numerical modelling of mantle Xe evolution to answer a simple question: could the distinct evolutions of the highly degassed, convecting upper mantle (primary source of fissiogenic 136Xe*: extant 238U) and the primitive, minimally degassed plume mantle (primary source of 136Xe*: extinct 244Pu) have coincidentally converged towards a unique S129Xe*/S136Xe* approximate to 1. Our model shows that, despite a two orders of magnitude difference in the total extents of degassing, such a coincidence cannot be discarded. An alternative scenario would be that deep, primitive plume mantle sources akin to La Palma may also be dominated by extant 238U fission rather than extinct 244Pu, potentially requiring deep mantle sources to be far more degassed than previously considered.

Tenerife (2,034 km2), the largest of the Canary Islands, is characterized by a complex volcanic structure controlled by a volcano-tectonic rift system with dominant NW, NE and NS trends. The intersection of these rifts hosts the Teide-Pico Viejo volcanic complex, which culminates at 3,718 m a.s.l. The Teide volcano last erupted in 1798 through an adventive vent of the Teide-Pico Viejo system. The summit area of Teide volcano is affected by persistent visible and diffuse activity, with diffuse degassing representing the main pathway for gas release to the atmosphere.Since the late 1990s and until 2025, a long-term monitoring programme has been carried out at the summit crater of Teide volcano, based on repeated diffuse gas emission surveys (more than 250). These surveys were designed to characterise the spatial and temporal variability of diffuse degassing within the summit crater area, providing a robust and consistent dataset for the assessment of changes in the volcanic-hydrothermal system over time.Diffuse CO2 and H2S emission rates were directly estimated from field measurements obtained using the accumulation chamber method. Spatial distribution maps were generated by averaging the results of 100 sequential Gaussian simulations, allowing the estimation of total emission rates and their spatial variability.During the study period, diffuse CO2 emissions ranged between 2 and 1257 t·d-1, while H2S emissions ranged between 0 and 31 kg·d-1. From 2007 until around 2016, diffuse CO2 emissions remained low and relatively stable, with an average of approximately 20 t·d-1. From late 2016 onwards, emissions show a sustained increase, a trend that continues to the present. Since 2021, low emission values are no longer observed, and in September 2023 the maximum value of the series was recorded (1257 t·d-1). H2S shows a nearly synchronous behaviour with CO2. Along with the observed increase in gas emissions, an increase in seismicity has also been recorded, particularly since 2016, suggesting a relationship between seismic activity and the release of diffuse volcanic gases.Temporal variations in diffuse CO2 and H2S emissions provide valuable insights into changes in the activity of the Teide volcanic system and represent an effective tool for tracking unrest processes. Continuous monitoring of diffuse degassing at Teide volcano has proven essential for improving the understanding of volcanic behaviour and contributes significantly to volcanic risk assessment and mitigation on Tenerife.

Tenerife (2,034 km2), the largest and one of the most active islands of the Canarian volcanic archipelago, has registered six volcanic eruptions in the last 500 years. One of the main volcano-structural and geomorphological features of Tenerife is the triple junction-shaped rift system, as a result of inflation produced by the concentration of emission vents and dykes at 120º one to another. The oriented North South Rift Zone (NSRZ; 325 km2) is one of the three active volcanic rift-zones of the island and is characterized mainly by effusive activity of basaltic lavas forming spatter and cinder cones and comprising 139 monogenetic cones representing the most common eruptive activity occurred on the island during the last 1My. The main structural characteristic of the NSRZ is the apparent absence of a distinct ridge and a fan shaped distribution of these monogenetic cones. Since no visible degassing at Tenerife NSRZ surface occurs, a geochemical monitoring program at Tenerife NSRZ was established mainly consisting on performing diffuse CO2 emission surveys to evaluate the temporal and spatial variations of soil CO2 efflux values and the diffuse CO2 emission rate. Ten diffuse CO2 degassing surveys have been carried out at NSRZ of Tenerife since 2002, the last one in the summer period of 2023. Measurements of soil CO2 efflux were performed in situ by means of a portable non-dispersive infrared sensor following the accumulation chamber method at about selected 600 sampling sites to obtain a homogeneous distribution after taking into consideration the local geology, structure and accessibility. During the 2023 survey, soil CO2 efflux values ranged from non-detectable up to 55.5 g m−2 d−1. Statistical-graphical analysis of the 2023 data show three different geochemical populations; background (B), intermediate (I) and peak (P) represented by 93.9%, 5.4% and 0.7% of the total data, respectively. The geometric means of the B, I and P populations are 1.6, 11.4 and 36.6 g m−2 d−1, respectively. Most of the area showed B values while the P values were observed as multiple isolated anomalies in the study area. 100 equiprobable sequential Gaussian simulations were performed to construct the spatial interpolation map and to estimate the diffuse CO2 emission in tons per day released from Tenerife NSRZ in the 2023 survey. The diffuse CO2 output released to atmosphere by the NSRZ of Tenerife estimated in the 2023 survey was 884 ± 27 t d-1. This value overcomes the estimated background range (201 - 760 t d-1), and confirms the clear relationship between the temporal evolution of the CO2 output released by the NSRZ and the seismic activity in and around Tenerife island. These geochemical observations are clear evidence of changes of processes operating deep in the hydrothermal-magmatic system of Tenerife. Monitoring the diffuse CO2 emission contributes to detect early warning signals in the activity of the Tenerife North-South Rift-Zone volcanic system.

Located in the northwest of Canary Islands, La Palma is one of the most volcanically active islands in the archipelago. The island experienced a volcanic eruption from 19 September to 13 December 2021, which had significant social, economic, and scientific impacts. This event serves as a reminder of the island's potential as a host for geothermal resources. Therefore, geochemical prospection of soil gases at Cumbre Vieja can provide valuable information for investigating the presence of permeable areas and potential upflow for degassing of geothermal systems at depth. This study was carried out between July and September 2023 and presents the results of a soil gas study located southwest of the 2021 lava flow. The survey aimed to identify permeable areas by conducting in-situ measurements of diffuse CO2 emissions and sampling and analyzing CO2 concentration and isotopic composition (δ13C-CO2). A total of 766 sampling sites were selected over an area of approximately 25 km2, with an average distance of 100 m between sites. Soil CO2 concentrations ranged from typical atmospheric values (≈ 400 ppm) up to 40,000 ppm. The average CO2 concentration measured was 1,700 ppm. The δ13C-CO2 isotopic composition revealed the presence of three distinct end-members: biogenic, atmospheric and deep-seated CO2, defined by isotopic compositions of 25‰>δ13C-CO2>-15‰, -8‰ and 2.1‰>δ13C-CO2>-8‰ and CO2 concentrations of 100%, 0.04% and 100%, respectively. Results show that, with a mean of -13.7‰, a minimum of -28.9‰ and a maximum of -4.8‰, CO2 at most sampling sites is composed of various mixtures of atmospheric and biogenic CO2, with some contributions from deep-seated CO2. The accumulation chamber method was used to measure soil CO2 efflux at each sampling site using a portable non-dispersive CO2 sensor, model LICOR-Li-820. The measured CO2 efflux values ranged from non-detectable to 160.3 g·m-2·d-1, with an average value of 4.7 g·m-2·d-1. For the estimation of the total diffuse CO2 emission from the study area, we calculated the average of 100 sequential Gaussian simulations. This gave a value of 100.8 ± 2.8 t·d-1, corresponding to a standardized emission rate of 4.1 t·km-2-d-1. The results show a significant correlation between the distribution of 222Rn gas activity anomalies and the highest CO2 efflux values in the eastern part of the study area. Soil gas measurements of CO2 concentration, isotopic ratio and efflux are a valuable and non-invasive technique for surface exploration, helping to define permeable areas and potential upflow zones of potential geothermal system structures and enabling an efficient subsequent subsurface exploration phase.

La Palma Island (708 km2), situated in the northwest of the Canarian Archipelago, stands as one of the youngest (~2.0 My) islands. A new volcanic eruption took place at the Cumbre Vieja volcanic system, located in the southwest flank of the island, on September 19, 2021. Cumbre Vieja is renowned as the most active basaltic volcano in the Canaries. The eruptive event, which lasted for 85 days, featured various volcanic activities, including lava effusion, strombolian activity, lava fountaining, ash venting, and gas jetting, and concluded on December 13, 2021. Regular surface geochemical studies have been conducted focusing on hydrogen (H2) emissions along Cumbre Vieja. H2, being one of the most abundant trace species in volcano-hydrothermal systems, plays a pivotal role in numerous redox reactions occurring in the hydrothermal reservoir gas. This comprehensive study of H2 emissions has been ongoing since 2001, encompassing continuous monitoring of soil gas samples collected at a depth of approximately 40 cm across 600 sites during each survey. H2 concentrations have been meticulously analyzed using a micro-gas chromatograph (Agilent 490 microGC). Spatial distribution maps have been generated using sequential Gaussian simulation (sGs) techniques to quantify the diffuse H2 emissions from the study area. The time series data of the diffuse H2 emissions indicate significant increases before and during the occurrence of seismic swarms observed between 2017 and 2021. Furthermore, during the eruptive phase, substantial spikes in the diffuse H2 emissions were observed, closely correlating with the volcanic tremor escalation. These fluctuations in diffuse H2 emissions were observed preceding the peak of diffuse CO2 emissions, aligning with the anticipated behavior of these gases. Over the last two years following the eruption, the values have reverted to levels like those observed during periods of volcanic calm, reinstating the stability in the diffuse H2 emissions. The absence of visible volcanic gas emissions before the eruption, such as fumaroles or hot springs, on the surface of Cumbre Vieja underscores the importance of such studies in serving as a critical tool for continuous volcanic surveillance and monitoring purposes. This update represents ongoing efforts to comprehensively study and understand the behavior of hydrogen emissions within the volcanic system, providing essential insights into volcanic activity and potential precursor signals for enhanced monitoring and risk assessment.

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. On 19 September 2021, a new eruption began at the west flank of Cumbre Vieja volcano: the 2021 Tajogaite eruption. It resulted in a fissure and powerful strombolian eruption with a magnitude VEI = 3 (Bonadonna et al., 2022), the longest volcanic event on the island during the last 600 years and the most important eruption of Europe during the last 75 years in terms of the significant amount of SO2 released (Burton et al., 2023). Since no visible degassing (fumaroles, etc.) at Cumbre Vieja occurred before the eruption, the geochemical program for the volcanic surveillance has been mainly focused on diffuse degassing monitoring. Diffuse CO2 emission surveys have been yearly performed in summer to minimize the influence of meteorological variations with continuous surveillance diffuse CO2 surveys during periods with anomalous seismic activity and during the eruptive and post-eruptive periods. Diffuse CO2 emission is measured following the accumulation chamber method in about 600 sites and later spatial distribution maps are constructed following the sequential Gaussian simulation (sGs). Important increases in the diffuse CO2 emission rate were observed after the occurrence of several seismic swarms registered in 2017 and 2020, caused by an upward magma migration from an ephemeral magmatic reservoir. During the eruptive period (18 September – 13 December 2021), the diffuse CO2 emission rate showed a sustained increase up to the maximum value of the series: 4,573 ± 284 t/d in 15 December 2021. After the eruption, the time series showed a rapid decline until background values were recovered in March 2022. Diffuse CO2 emission surveys has demonstrated to be an important monitoring tool that contributes to detect early warning signals in the volcanic activity of Cumbre Vieja and to track the depressurization of magma batches beneath the volcanic system during seismo-volcanic unrest and eruptive episodes.Bonadonna et al. (2022). J. Geophys. Res: Solid Earth, 127, e2022JB025302.Burton et al. (2023). Communications Earth & Environment, 4:467.

Geothermal energy has reached the front line on renewable resources assessment in recent years, especially in active volcanic areas such as the Canary Islands (Spain), and precisely in La Palma island, where a recent volcanic eruption occurred in 2021, representing a unique opportunity to carry out in situ exploration. Cost-effective geochemical surveys, like soil radon (222Rn) and thoron (220Rn) gases activities measurements, have demonstrated to provide relevant insights as part of surface geothermal exploration, helping to identify permeable areas and potential up-flow zones and, therefore, defining potential geothermal systems boundaries. Both radon (222Rn) and thoron (220Rn) are radioactive isotopes of radon gas and are derived from the natural decay of uranium (238U) and thorium (232Th) respectively, present in the mineralogical composition of, particularly, igneous rocks. However, 222Rn present a half-life of 3.8 days while 220Rn has a shorter half-life of 55 seconds. High 222Rn activity surface measurements are considered to be associated to deep magmatic sources of gas, providing additional value on defining high porosity and permeable zones. On the contrary, due to its ephemeral half-life, high 220Rn activity is associated to shallow soil gas sources.A detailed and regular surface geochemical survey was carried out in an area of 25 Km2 at the western side of La Palma island and southwards from the recent lava flow of Tajogaite Volcano. A total of 766 soil radon and thoron activities discrete measurements were performed (around 30 sample sites/Km2) using a SARAD radon monitor, model RTM-1688-2, connected to a stainless steel probe inserted at 40 cm depth. Data analysis and treatment showed an average 222Rn value of 1,056 Bq/m3, ranging from 0 to up to 27,000 Bq/m3, and an average 222Rn/220Rn ratio of 0.3, ranging from 0 to a maximum of 49. The spatial distribution maps has enabled to limit areas with higher values of these two variables,which might indicate zones of interest for further investigation. Higher soil 222Rn activity were concentrated along an specific segment of the coast line, which is coincident with the distribution of the well-known anomalous CO2 active diffuse degassing of volcanic origin in Puerto Naos and La Bombilla, which may have played an important role in controlling the migration and transport of these trace gases towards the surface. Radon and thoron gases activities measurements have revealed to be a worthwhile and non-invasive technique for surface exploration in highly environmental-threatened areas, like La Palma, helping to provide the definition of permeable areas and potential up-flow zones of potential geothermal system structures and permitting an efficient posterior subsurface exploration phase.

Geochemical methods are extensively used in geothermal exploration and exploitation phases, played a major role in both the identification and utilization of resources. In regions where geothermal systems are concealed or located at significant depths, soil gas surveys become indispensable. These methods focus on detecting anomalous concentrations of hydrothermal gases within the soil atmosphere, providing key insights into subsurface geothermal activity. Previous studies in the western area of La Palma island (Canary Islands, Spain) identified the highest geothermal potential of the studied areas. Consequently, more detailed investigations were conducted in the zones with the most significant anomalies to better characterize their potential for economic exploitation. A detailed geochemical survey with an average measurement spacing of ~12 m was carried out in an area of 0.11 Km2 at Puerto Naos. A total of 561 sites were sampled at 40 cm depth using a metallic probe. Gas samples were collected with 60 cc hypodermic syringes and stored in 10 cc glass vials for subsequent laboratory analyses. Spatial distribution maps of diffuse He, H2, CH4 and CO2 emission and δ13C-CO2 were constructed to study the presence of enhanced vertical permeability areas related to high temperature hydrothermal activity at depth. The main CO2, H2 and δ13C-CO2 anomalies reveal two well-defined zones located in the southeast and west of the study area. In contrast, He highest values are observed in the northern and southern regions. These patterns may be attributed to secondary processes, including interactions with coastal water and the varying reactivity and mobility of the analyzed species. The spatial distribution of soil gases in Puerto Naos confirms a relative enrichment of and H₂, He, CH4 and CO₂ in the soil gas atmosphere, suggesting a significant contribution from deep-seated sources. These studies aid in identifying permeable zones and potential upflow areas associated with geothermal system structures, thereby facilitating a more efficient subsequent phase of subsurface exploration.

From September 19 to December 12, 2021, a mixed explosive-effusive eruption impacted La Palma Island, in the Canary archipelago, leading to the growth of Tajogaite volcanic cone. Daily Open-Path Fourier Transform Infrared (OP-FTIR) measurements from October 3 until the end of the eruption allowed us to capture the molar proportions of H2O, CO2, SO2, HCl and CO (plus COS occasionally) in magmatic gases emitted from summit and flank vents of the new cone. Our results reveal high CO2/SO2 ratios ranging from 11 to 53 in explosive gas emissions throughout most of the eruption, with a time-averaged value of similar to 30. In contrast, effusive degassing at lower flank vents systematically displayed lower CO2/SO2 ratios (from 3 to 11) and enhanced proportions of H2O, S and Cl. Combined with solubility data and the magma eruption rates, this chemical contrast suggests gas fractionation in a very shallow conduit branching beneath the Tajogaite cone, were most of the pre-exsolved CO2-rich gas phase but a minor fraction of the magma explosively escaped through the main eruptive conduit, while CO2-depleted gas and most of the magma were effusively discharged through lateral branches. The CO/COS/CO2 ratios for explosive degassing are consistent with petrological evidence of oxidized magmatic conditions (FMQ +1.7 +/- 0.3), which enhanced sulfur solubility and late-stage SO2 outgassing. The high oxidation state, as well as low HCl/HF ratios, retrieved from solar occultation across the volcanic plume, are both typical of Ocean Island Basalt (OIB) magmatism. The apparent increase of CO2/SO2 and SO2/HCl ratios at summit vents during the first half of the eruption is consistent with the influx of progressively more mafic, deeper-derived, basanitic magma, though we cannot entirely rule out artefacts due to more challenging measurement of the pure explosive gas in that period. Our study presents the very first detailed data set for gas geochemistry during a subaerial eruption in the Canary archipelago and highlights the remarkable potential of OP-FTIR spectroscopy for real-time monitoring and study of volcanic eruptions.

Tenerife, with 2034 km2, isthe largest active volcanic island of the Canarian archipelago and boasts over 1,000 galleries used for groundwater explotation, enabling access to the aquifer at varying depths and elevations. From mid-2016 to present, we've diligently sampled two important galleries - Fuente del Valle and San Fernando - on a weekly basis for volcanic monitoring. On-site measurements of water's physicochemical parameters such as temperature (ºC), pH, and electrical conductivity (E.C., µS·cm-1) were conducted at each sampling point. Subsequently, the water's chemical/isotopic composition and dissolved gases were analyzed in the laboratory. Noteworthy trends in certain parameters, including increased conductivity, sulfate (SO42-) concentration, chloride, bicarbonate, and the SO42-/Cl- molar ratio, suggest an infiltration of deep-seated gases into the groundwater. Isotopic data further revealed a robust interaction with endogenous gases like CO2, H2S, H2, etc. Additionally, correlations were discerned between specific hydrogeochemical parameters in the gallery groundwaters, correlating with observed seismic activity changes. This study underscores the sensitivity of monitoring the chemical and isotopic composition of groundwater in Fuente del Valle and San Fernando galleries to fluctuations in volcanic activity on Tenerife. Exploring groundwater associated with a volcanic system offers insights into magmatic gas input into the aquifer, models groundwater flow within the edifice, and provides vital geochemical information potentially indicating an imminent eruption.Concurrently, a cost-effective method to gauge CO2 fluxes using alkaline traps has significantly contributed to Tenerife's volcanic surveillance. In the summer of 2016, a network of 31 closed alkaline traps was strategically placed across Tenerife's three volcanic rifts (NE, NW, and NS) and at Cañadas Caldera, persisting until the present. The weekly replacement of alkaline solutions facilitated subsequent laboratory titration analysis of the trapped CO2, expressed as weekly integrated CO2 efflux. Across the study period, the average CO2 efflux stood at 6.41 g·m-2·d-1, with variations across regions: 8.41 g·m-2·d-1 for the NE rift-zone, 5.11 g·m-2·d-1 for Cañadas Caldera, 6.36 g·m-2·d-1 for NW rift-zone, and 6.35 g·m-2·d-1 for NS rift-zone. Notably elevated CO2 effluxes were observed in the NE rift-zone, exhibiting maximum values. While the temporal evolution of CO2 efflux estimated by closed alkaline traps exhibited minimal variation during the study, seasonal fluctuations were noted. The systematic use of closed static chamber alkaline traps proves to be a straightforward and economical method aiding volcanic surveillance, especially in areas lacking visible volcanic gas manifestations.This comprehensive approach using chemical analysis of groundwater and CO2 flux monitoring through alkaline traps showcases their combined efficacy in advancing Tenerife's volcanic surveillance, potentially serving as a crucial precursor to future volcanic activity.

La Palma Island, spanning 708.32 km2, resides at the north-western edge of the Canary Archipelago and is among the youngest within this group. In the last 123,000 years, volcanic activity has been exclusive to Cumbre Vieja in the island's southern region. The last volcanic activity (Tajogaite eruption) took place at the west flank of Cumbre Vieja from 19 September to 13 December 2021. It was a fissure and powerful strombolian eruption with a magnitude VEI = 3 (Bonadonna et al., 2022). Due to the absence of visible geothermal manifestations, recent decades have witnessed a burgeoning interest in studying diffuse degassing as a vital tool for volcano monitoring. With the aim of strengthening the geochemical monitoring of Cumbre Vieja volcanic activity, we have conducted on a weekly basis from October 2017 to the present, two distinct studies for volcano monitoring. Firstly, we monitored physical-chemical parameters and the chemical/isotopic composition and dissolved gases in the groundwater of two galleries (Peña Horeb and Trasvase Oeste) and three water wells (Las Salinas, Charco Verde, and San Isidro) before, during, and after the Tajogaite Volcano eruption at Cumbre Vieja from September 19 to December 13, 2021. We observed significant temporal variations in pH, EC, ion content, pCO2, and δ13C-CO2, correlating with interactions between deep volcanic fluids and groundwater. These changes showed good temporal agreement with the eruption and seismic swarms leading up to it. Simultaneously, we established a network of 21 closed static chambers to measure soil CO2 effluxes. Before the eruption (October 2017 to December 2020), the recorded soil CO2 efflux averaged 7.30 g·m-2·d-1 across Cumbre Vieja (7.48, 7.35, and 7.11 g·m-2·d-1 in the north, east, and west, respectively). Post-eruption (March 2022 to present), it averaged 7.51 g·m-2·d-1 (8.09, 7.58, and 6.99 g·m-2·d-1). The absence of data during this crucial period impedes direct comparisons and assessment of volcanic activity using this specific monitoring tool.These methods underscore an approach to bolster volcanic surveillance on La Palma Island. Our study emphasizes the importance of monitoring the chemical and isotopic composition of groundwaters linked to active volcanic systems. Such evaluations offer critical insights into magmatic gas input within aquifers, despite limitations encountered during eruptive phases, as witnessed in the case of Cumbre Vieja in 2021.Bonadonna et al. (2022). J. Geophys. Res: Solid Earth, 127, e2022JB025302.

La Palma Island (708 km²) is located in the northwestern part of the Canarian Archipelago and represents one of its youngest volcanic structures, with an estimated geological age of around 2.0 million years. On September 19, 2021, a significant volcanic eruption occurred within the Cumbre Vieja volcanic system situated in the southern region of the island. This event, lasting 85 days and 8 hours, is recorded as the longest volcanic episode in La Palma's documented history. The eruption resulted in extensive lava flows that covered an area of approximately 1,219 hectares, causing substantial geological and social impact. Since visible volcanic gas emissions (fumaroles, hot springs, etc.) do not occur at the surface environment of Cumbre Vieja, the geochemical program for the volcanic surveillance has been focused mainly on diffuse (non-visible) degassing studies. This study presents the findings from annual diffuse carbon dioxide (CO₂) emission surveys conducted since 2001, with increased monitoring frequency between 2017 and 2024 to optimize the early warning system for future volcanic eruptions at La Palma island.The measurement of soil CO₂ efflux was performed following the accumulation chamber method across approximately 600 sampling sites distributed throughout the volcanic system. The long-term time series data reveal distinct periods of diffuse CO₂ emissions that provide valuable insights into the system's volcanic activity: (1) A baseline period (2001-2016), when diffuse CO₂ emissions fluctuated between 320 and 1,544 t/d, establishing a reference range for background degassing levels.; (2) A pre-eruptive period (2016-2021), when a marked increase in CO₂ emissions was observed, with values rising from 788 t·d⁻¹ to a peak of 1,870 t·d⁻¹. This last period coincided with the onset of seismic swarm activity, highlighting a clear correlation between increased degassing and evolving magmatic processes beneath the surface; (3) The eruptive period (2021). During the eruption, CO₂ emissions exhibited significant temporal variations. A minimum emission rate was recorded on October 21, followed by a sharp increase that peaked at 4,435 t·d⁻¹ on December 14, aligning with the conclusion of the eruptive phase. This maximum emission rate represents the highest value observed in the entire monitoring serie; and (4) the post-eruptive period (2022-2024), when diffuse CO₂ emissions showed a decreasing trend, stabilizing around 760 t·d⁻¹, reflecting a gradual return to lower degassing levels.These findings underscore the critical importance of continuous diffuse CO₂ monitoring as a key component of volcanic surveillance at Cumbre Vieja. Regular measurements of diffuse gas emissions provide essential early warning indicators of potential volcanic unrest, allowing for improved risk assessment and hazard mitigation strategies. The integration of geochemical monitoring with other geophysical and geological tools enhances the comprehensive understanding of the dynamic behavior of volcanic systems.

Tenerife, the largest and highest island in the Canarian archipelago, houses the active Teide-Pico Viejo volcanic system. Its structure is shaped by a rift-system with various directions intersecting at this volcanic system. The system’s last eruption in 1798 expelled approximately 12 million m³ of lava across a three-month period, resulting in the formation of a distinct black surface that contrasts sharply with the surrounding. While Teide volcano exhibits a faint fumarolic system, the observed volcanic gas emissions mainly consist of diffuse CO2 degassing.Spanning from 1999 to 2024, over 200 surveys meticulously assessed CO2 and H2S emissions across 38 strategic sites within the Teide Volcano's summit crater. Portable fluxmeters, equipped with CO2 and H2S sensors, estimated emission rates via the accumulation chamber method. These rates fluctuated between 2.0 and 1,257 tons per day over a 25-year span. Following a seismic swarm in October 2016, there was a general marked escalation in CO2 and H2S emissions, aligning with heightened seismic activity. This shift led to relatively high CO2 emissions, possibly attributed to fresh magma injection and convective mixing catalyzed by the seismic swarm.It is pertinent to note a distinct event in the latter half of 2023, marked by a notable surge in CO2 and H2S emissions (ranging between 222 and up to 1,257 tons/day for CO2, and 40 to 270 tons/day for H2S), despite a relatively unchanged seismic activity compared to preceding years.This study highlights the value of examining diffuse degassing in understanding volcanic behavior and forecasting potential volcanic activity. Monitoring these emissions has become a crucial tool in predicting seismic and volcanic unrest, contributing significantly to mitigating volcanic risks in Tenerife.

The increasing European demand for high-quality, safe agricultural products have led to the development of stringent control measures to certify product authenticity and geographical origin, protecting both producers and consumers from potential fraud. This study focuses on Aloe Vera, a plant containing around 200 potentially active compounds of interest in the health and wellness industry, including vitamins, minerals, anthraquinones, and polysaccharides. The Canary Islands has a unique climate, which, combined with young volcanic soils, produce exceptionally high-quality Aloe Vera. However, fraudulent Aloe Vera products falsely labelled as having Canarian origin currently represent a 21 million euros market. This situation necessitates the development of reliable scientific protocols for geographical tracing of Canarian Aloe Vera and its derivatives (juices, gels, creams, cosmetics). Chemical profiling of Aloe Vera across the Canary Islands and the Iberian Peninsula includes the determination of strontium isotopic ratios (87Sr/86Sr) by thermal ionization mass spectrometry (TIMS) to trace geographic origin at certified grower plantations, complemented by phytochemical profiling to verify optimal growing conditions and quantitative quality standards. Complete Aloe Vera plants have been analysed, revealing distinct bioactive organic compounds of interest, including phenolic acids, flavonoids, terpenoids, anthraquinones and derivatives, among others. 87Sr/86Sr ratios in Canarian Aloe Vera plants are higher (0.7065-0.7078) than those expected from their dominantly basaltic volcanic soils (0.7032-0.7068), but lower than soil values observed in mainland Spain (0.7089-0.7124). Therefore, development of a full ´fingerprint´ profile of Canarian Aloe Vera must also quantify 87Sr/86Sr contributions from irrigation water sources and additives used in the growing and manufacturing process.

El Hierro, covering an area of 278 km2 is one of the eight islands that make up the Canary Islands archipelago. This oceanic island emerged approximately 1.2 million years ago and is among the most volcanically active in the region. Its most recent volcanic activity was a submarine eruption 2 km off its southern coast, lasting from October 12, 2011, to March 5, 2012. This event was significant as it marked the first eruption in the Canary Islands to be closely monitored. Since 1998, diffuse CO2 emissions across the island have been systematically measured using the accumulation chamber technique. These measurements are taken at 601 sites regularly distributed to cover the island’s surface. During periods of volcanic unrest, such as the 2011-2012 eruption, the frequency of these surveys increases. The island’s CO2 emission rates have varied over time, with the most notable increases occurring during pre-eruptive and eruptive phases (Melián et al., 2014). In the last survey, performed in the summer period of 2024, soil CO2 efflux ranged from levels below detection (

Soil diffuse CO2 efflux and soil radon (222Rn) and thoron (220Rn) gases activities measurements may be useful geochemical indicators of subsurface volcano-hydrothermal processes in geographical areas where visible gas emissions are nearly absent. Both radon (222Rn) and thoron (220Rn) are radioactive isotopes derived from the natural decay of uranium (238U) and thorium (232Th) respectively, present in the mineralogical composition of rocks. The main difference between these two isotopes is their half-life time. While 222Rn presents a half-life of 3.8 days, 220Rn has a shorter half-life of 55 seconds. Therefore, high 222Rn surface activity is considered to be associated with deep magmatic sources of gas while high 220Rn activity is associated with shallow soil gas sources.A total of 968 sampling sites in an area of 25 Km2 have been considered as part of a detailed surface geochemical study at the central-western part of La Palma and southwards from the 2021 volcanic eruption lava flow of Tajogaite Volcano. Both diffuse soil CO2 efflux and radon and thoron activities discrete measurements were executed during field surveys between 2023 and 2024.The diffuse CO2 efflux measurements were determined, based on the non-stationary static accumulation chamber technique, using CO2 sensors contained in a portable flux-meter, and the radon and thoron activities were evaluated using a SARAD radon monitor connected to a stainless steel probe inserted at 40 cm depth. Soil gas samples were also collected and analyzed in the laboratory to obtain the chemical and carbon isotopic composition profile.Data analysis and treatment showed CO2 efflux values up to 106 g*m-2/day, 222Rn values up to 27000 Bq/m3 and 222Rn/220Rn ratio up to a maximum of 49. Both 222Rn versus 222Rn/220Rn ratio plotted together enabled to identify areas with a higher contribution of deeper sourced gas,which might indicate potential zones of interest of geothermal resources.Furthermore, spatial distribution maps of these variables showed that the main CO2 and radon gases anomalies are located along the coastline of the studied area, coincident with anomalous magmatic-hydrothermal origin CO2 diffuse degassing areas. The magmatic-hydrothermal CO2 might have acted as a carrier gas controlling the migration and transport of the radon trace gas towards the surface.In conclusion, surface geochemical surveys might be useful for geothermal resources exploration studies, providing a reasonable definition of potential geothermal system boundaries and permitting an efficient and cost-effective posterior subsurface exploration phase.

The chemical composition of volcanic gases provides essential insights into the activity and dynamics of volcanic systems, as well as the magmatic and hydrothermal processes occurring at depth. These gases, including CO2, H2, CH4, and H2S, are key indicators of physical and chemical processes such as redox reactions and magmatic degassing. Furthermore, the relative concentrations and ratios of specific gas species offer valuable information for interpreting subsurface dynamics and detecting changes in volcanic activity.In recent decades, researchers have made significant efforts to measure gas concentrations and fluxes in volcanic fluids. However, continuous monitoring of gas emissions and their ratios in active volcanoes remains limited. Here, we present results from a continuous monitoring station (CMS) installed in November 2017 on the southeastern flank of Teide volcano. This station monitors the ground gas atmosphere using a device that collects samples at a depth of 10 cm, measuring CO2, H2, He, H2S, CH4, and other gases to analyze their temporal evolution and interrelationships. The data collected spans from its installation to the present day, providing a comprehensive record of gas behaviour over time.The CMS is equipped with an Agilent 490 micro-GC with two channels, capable of analyzing He, Ne, H2, O2, N2, CH4, CO2 and H2S. The system includes an embedded computer with internet connectivity (via WiFi or UMTS router), enabling full remote control of the instrument, automatic data transmission, and automated gas sampling.High concentrations of CO2 (with a moving average exceeding 60% for most of the measurement period), H2 (above 1,200 ppm), He (above 10 ppm), and H2S (above 1,000 ppm) highlight significant temporal trends linked to variations in volcanic and hydrothermal activity. The analysis of gas ratios, such as He/CO2, H2/CO2, and H2S/CO2, shows fluctuations consistent with changes in volcanic activity. Decreases in atmospheric gases like N2 and O2 often coincide with increases in magmatic components, reinforcing the utility of gas ratios in understanding subsurface processes.This CMS constitutes a robust system for volcanic monitoring, capable of detecting low concentrations of key gases and providing critical insights through the analysis of both gas concentrations and their ratios. Such tools are invaluable for advancing volcanic surveillance and risk assessment.

Lanzarote Island (795 km2) is a volcanic island located in the eastern part of the Canary Islands and approximately 100 km from the NW coast of Morocco. The largest historical eruption of the Canary Islands, Timanfaya, took place during 1730-36 in this island when long-term eruptions from a NE-SW-trending fissure formed the Montañas del Fuego. Tinguaton volcano, the last eruption at Lanzarote Island, occurred in 1824 and produced a much smaller lava flow that reached the SW coast. At present, one of the most prominent phenomena at Timanfaya volcanic field is the high maintained superficial temperatures occurring in the area since the 1730 volcanic eruption. The maximum temperatures recorded in this zone are 605ºC, measured in a slightly inclined well 13 m deep. Since fumarolic activity is absent at the surface environment of Lanzarote, to study the diffuse CO2 emission becomes an ideal geochemical tool for monitoring its volcanic activity. We report herein the results of eight soil CO2 efflux surveys performed from 2006 to 2023 at Timanfaya Volcanic Field (TVF) with the aim to evaluate the temporal variations of the diffuse CO2 emission. Approximately 400 sampling sites were selected at each survey to obtain an even distribution of the sampling points over the study area. Soil CO2 efflux was measured following the accumulation chamber method. Soil temperature at 40 cm depth and soil gas samples collected at each sampling site was also measured to evaluate the chemical and isotopic composition of soil gases. Diffuse CO2 emission values have ranged between non detectable values to 34 g·m-2·d-1, with the highest values measured in September 2008. Conditional sequential Gaussian simulations (sGs) were applied to construct soil CO2 efflux distribution maps and to estimate the total CO2 output from the studied area at the TVF. Soil CO2 efflux maps showed a high spatial and temporal variability. Most of the study area have shown relatively low values, around the detection limit of the instrument (~0.5 g·m-2·d-1). Higher soil CO2 diffuse emission values were observed where thermal anomalies occur, indicating a convective mechanism transport of gas from depth at these areas. Diffuse CO2 emission rates ranged between 41 and 519 t·d-1 during the study period (57 t·d-1 for 2023). Long-term temporal variation on total CO2 diffuse emission shows a peak recorded on winter 2011, suggesting a seasonal control on the CO2 emission. These observations along with the results from the eight soil gas surveys performed at TVF indicate that the short and long-term trends in the diffuse CO2 degassing are mainly controlled by environmental factors.

El Hierro (278 Km2), the youngest and westernmost island of the Canarian archipelago, 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. A submarine eruption took place from October 2011 to March 2012 about 2 km south of the small village of La Restinga in the southernmost part of the island. The eruptive process was 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. Since visible volcanic emissions are absent at the surface of El Hierro, one of the most useful geochemical tools to monitor the volcanic activity of El Hierro is the diffuse degassing studies. Diffuse CO2 emissions have been monitored at El Hierro Island since 1998 in a yearly basis, with higher frequency during the pre and eruptive period of 2011-2012. At each survey, 600 sampling sites are studied and measurements of soil CO2 efflux are performed in situ following the accumulation chamber method. During pre-eruptive and eruptive period, 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 2023 summer period, soil CO2 efflux values ranged from non-detectable up to 39 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 97.7%, 1.6 % and 0.7% of the total data respectively, with geometric means of 1.2, 20 and 27 g m−2 d−1, respectively. To quantify the diffuse CO2 emission for the 2023 survey, 100 sequential Gaussian simulations (sGs) were performed as interpolation method. The estimated 2023 diffuse CO2 output released to atmosphere by El Hierro was 528 ± 22 t d-1, value higher than the background average of CO2 emission estimated in 410 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.

During geothermal exploration, the geochemical methods are extensively used and play a major role in both exploration and exploitation phases. Discovery of new geothermal systems at those areas where the resources are either hidden or lie at great depths, the geochemical methods for geothermal exploration must include soil gas surveys, based on the detection of anomalously high concentrations of some hydrothermal gases in the soil atmosphere. We report herein the results of an intensive soil gas study, focused on non-reactive and/or highly mobile gases such as helium (He) and hydrogen (H2), in Cumbre Vieja volcano, the volcanically ac tive part of La Palma island (Canary Islands, Spain). He has unique characteristics as a geochemical tracer: it is chemically inert and radioactively stable, non-biogenic, highly mobile and relatively insoluble in water. H2 is one of the most abundant trace species in volcano-hydrothermal systems and is a key participant in many redox reactions occurring in the hydrothermal reservoir gas.A detailed geochemical survey was carried out in an area of 25 km2 at the western side of La Palma. A total of 766 sites were sampled at 40 cm depth using a metallic probe with 60 cc hypodermic syringes and stored in 10 cc glass vials for later laboratory analyses. Spatial distribution maps of diffuse He and H2 emission were constructed to study the presence of enhanced vertical permeability areas related to high temperature hydrothermal activity at depth. As a result, the main He emission anomalies show different size well-defined concentric-shape structures, distributed along a west to east main direction. On the other hand, H2 highest fluxes are slightly more dispersed in different areas, although some of the main ones are coincident with the distribution of the well-known anomalous volcanic CO2 active diffuse degassing in Puerto Naos and La Bombilla villages. Soil He and H2 surveys have demonstrated to provide meaningful insights of areas that could be acting as preferential zones of vertical permeability that allow deep source gases migration to surface and, therefore, of potential geothermal system structures.