The diffuse emission of CO2 from the south east sector of Nisyros caldera (Lakki plain) has been measured during a detailed survey (~1400 soil CO2 flux measurements) performed in October 2018. The gas emissions are fed by hydrothermal sources and, in minor part, by the soil biogenic activity whose mean CO2 flux (4 g m−2 d−1) is here estimated for the first time. The total amount of hydrothermal CO2 reaches 92 ± 8 t/d, a value that is slightly higher than that estimated with the same method between 1999 and 2001 (74 ± 7 t/d). The gas is emitted by different diffuse degassing structures (DDSs), including volcanic-hydrothermal structures (craters and domes) and NE-SW and NW-SE-trending tectonic lineaments. Even if the total CO2 emission is not particularly high at Nisyros (close to the median of CO2 emissions measured in volcanoes worldwide), the process is very energetic. The thermal energy associated with the shallow condensation of the steam in the DDSs reaches ~60 MW, while we estimate at 134–270 MW the total amount of thermal energy involved in the convective rising of the deep geothermal liquids that transport the gas from the depth to- ward the surface. This large flux of energy could dramatically increase during future earthquakes by addition of heat and mass from a deep hydrothermal reservoir, potentially triggering hydrothermal explosions, as it happened several times in the past few centuries.

This study presents the first hydrogeochemical model of the hydrothermal systems of Turrialba and Irazú volcanoes in central Costa Rica, manifested as thermal springs, summit crater lakes, and fumarolic degassing at both volcanoes. Our period of observations (2007–2012) coincides with the pre‐ and early syn‐phreatic eruption stages of Turrialba volcano that resumed volcanic unrest since 2004, after almost 140 years of quiescence. Peculiarly, the generally stable Irazú crater lake dropped its level during this reawakening of Turrialba. The isotopic composition of all the discharged fluids reveals their Caribbean meteoric origin. Four groups of thermal springs drain the northern flanks of Turrialba and Irazú volcanoes into two main rivers. Río Sucio (i.e. “dirty river”) is a major rock remover on the North flank of Irazú, mainly fed by the San Cayetano spring group. Instead, one group of thermal springs discharges towards the south of Irazú. All thermal spring waters are of SO4‐type (i.e. steam‐heated waters), none of the springs has, however, a com‐ mon hydrothermal end‐member. A water mass budget for thermal springs results in an estimated total output fluxof 187 ± 37 L/s, with 100 ± 20 L/s accounted for by the San Cayetano springs. Thermal energy release is estimated at 110 ± 22 MW (83.9 ± 16.8 MW by San Cayetano), whereas the total rock mass removal rate by chemical leaching is ~ 3000 m3/year (~ 2400 m3/year by San Cayetano‐Río Sucio). Despite Irazú being the currently less active volcano, it is a highly efficient rock remover, which, on the long term can have effects on the stability of the volcanic edifice with potentially hazardous consequences (e.g. flank collapse, landslides, phreatic eruptions). Moreover, the vapor output flux from the Turrialba fumaroles after the onset of phreatic eruptions on 5 January 2010 showed an increase of at least ~ 260 L/s above pre‐eruptive background fumarolic vapor fluxes. This extra vapor loss implies that the drying of the summit hydrothermal system of Turrialba could tap deeper than previously thought, and could explain the coinci‐ dental disappearance of Irazú’s crater lake in April 2010.

This short communication aims at providing an updated report on degassing activity and ground deformation variations observed during the ongoing (2012–2019) Campi Flegrei caldera unrest, with a particular focus on Pisciarelli, currently its most active fumarolic field. We show that the CO2 flux from the main Pisciarelli fumarolic vent (referred as “Soffione”) has increased by a factor N 3 since 2012, reaching in 2018–2019 levels (N600 tons/ day) that are comparable to those typical of a medium-sized erupting arc volcano. A substantial widening of the degassing vents and bubbling pools, and a further increase in CO2 concentrations in ambient air (up to 6000 ppm), have also been detected since mid-2018. We interpret this escalating CO2 degassing activity using a multidisciplinary dataset that includes thermodynamically estimated pressures for the source hydrothermal system, seismic and ground deformation data. From this analysis, we show that degassing, deformation and seis- micity have all reached in 2018–2019 levels never observed since the onset of the unrest in 2005, with an overall uplift of ~57 cm and ~448 seismic events in the last year. The calculated pressure of the Campi Flegrei hydrother- mal system has reached ~44 bar and is rapidly increasing. Our results raise concern on the possible evolution of the Campi Flegrei unrest and reinforce the need for careful monitoring of the degassing activity at Pisciarelli, hopefully with the deployment of additional permanent gas monitoring units.

The chemical composition of gases emitted by active volcanoes reflects both magma degassing and shallower processes, such as fluid-rock hydrothermal interaction and mixing with atmospheric-derived fluids. Untangling the magmatic fluid endmember within surface gas emission is therefore challenging, even with the use of well-known magma degassing tracers such as noble gases. Here, we investigate the deep magmatic fluid composition at the Nisyros caldera (Aegean Arc, Greece) by measuring nitrogen and noble gas abundances and isotopes in naturally degassing fumaroles. Gas samples were collected from 32 fumarolic vents at water-boiling temperature between 2018 and 2021. These fumaroles are admixtures of magmatic fluids typical of subduction zones, groundwater (or air saturated water, ASW), and air. The N2, He, and Ar composition of the magmatic endmember is calculated by reverse mixing modeling and shows N2/He = 31.8 ± 4.5, N2/Ar = 281.6, d15N = +7 ± 3 ‰, 3He/4He = 6.2 Ra (where Ra is air 3He/4He), and 40Ar/36Ar = 551.6 ± 19.8. Although N2/He is significantly low with respect to typical val- ues for arc volcanoes (1,000–10,000), the contribution of subducted sediments to the Aegean Arc magma generation is reflected by the positive d15N values of Nisyros fumaroles. The low N2/He ratio indicates N2-depletion due to solubility-controlled differential degassing of an upper-crustal silicic (dacitic/ rhyodacitic) melt in a high-crystallinity reservoir. We compare our 2018–2021 data with N2, He, and Ar values collected from the same fumaroles during a hydrothermal unrest following the seismic crisis in 1996–1997. Results show additions of both magmatic fluid and ASW during this unrest. In the same period, fumarolic vents display an increase in magmatic species relative to hydrothermal gas, such as CO2/CH4 and He/CH4 ratios, an increase of 50°C in the equilibrium temperature of the hydrothermal system (up to 325°C), and greater amounts of vapor separation. These variations reflect an episode of magmatic fluid expulsion during the seismic crisis. The excess of heat and mass supplied by the magmatic fluid injection is then dissipated through boiling of deeper and peripheral parts of the hydrothermal system. Reverse mixing modeling of fumarolic N2-He-Ar has therefore important ramifica- tions not only to disentangle the magmatic signature from gases emitted during periods of dormancy, but also to trace episodes of magmatic outgassing and better understand the state of the upper crustal reservoir.

Fluids supplied by stored magma at depth are causal factors of volcanic unrest, as they can cause pressurization/ heating of hydrothermal systems. However, evidence for links between hydrothermal pressurization, CO2 emission and volcano seismicity have remained elusive. Here, we use recent (2010−2020) observations at Campi Flegrei caldera (CFc) to show hydrothermal pressure, gas emission and seismicity at CFc share common source areas and well-matching temporal evolutions. We interpret the recent escalation in seismicity and surface gas emissions as caused by pressure-temperature increase at the top of a vertically elongated (0.3–2 km deep) gas front. Using mass (steam) balance considerations, we show hydrothermal pressurization is causing energy trans- fer from the fluids to the host rocks, ultimately triggering low magnitude earthquakes within a seismogenetic volume containing the hydrothermal system. This mechanism is probably common to other worldwide calderas in similar hydrothermal activity state.

Earth degassing of CO2-rich fluids has been proven to contribute significantly to the global carbon budget. The presence of ubiquitous outgassing reveals some degree of permeability of the crust that often coincides with seismically active zones. In this study, we took advantage of the most recent global geological datasets to better understand earth degassing and how it correlates with tectonic regimes. Here we use an ad hoc point pattern analysis to show that there is a spatial correlation between CO2 discharges and the presence of active fault systems, in particular with those characterized by a normal slip type. Seismic data demonstrate the existence of a positive spatial correlation between gas discharges and extensional tec- tonic regimes and confirms that such processes would play a key role in creating pathways for the rising gases at micro- and macro-scales, increasing the rock permeability and con- necting the deep crust to the earth surface.

We report here on a UV-camera based field experiment performed on Stromboli volcano during 7 days in 2010 and 2011, aimed at obtaining the very first simultaneous assessment of all the different forms (passive and active) of SO2 release from an open-vent volcano. Using the unprecedented spatial and temporal resolution of the UV camera, we obtained a 0.8 Hz record of the total SO2 flux from Stromboli over a timeframe of 14 h, which ranged between 0.4 and 1.9 kg s-1 around a mean value of 0.7 kg s-1 and we concurrently derived SO2 masses for more than 130 Strombolian explosions and 50 gas puffs. From this, we show erupted SO2 masses have a variability of up to one order of magnitude, and range between 2 and 55 kg (average 20 kg), corresponding to a time integrated flux of 0.0570.01 kg s-1. Our experimental constraints on individual gas puff mass (0.03–0.42 kg of SO2, averaging 0.19 kg) are the first of their kind, equating to an emission rate ranging from 0.02 to 0.27 kg s-1. On this basis, we conclude that puffing is two times more efficient than Strombolian explosions in the magmatic degassing process, and that active degassing (explosionsþpuffing) accounts for 23% (ranging from 10% to 45%) of the volcano’s total SO2 flux, e.g., passive degassing between the explosions contributes the majority (77%) of the released gas. We furthermore integrate our UV camera gas data for the explosions and puffs, with independent geophysical data (infrared radiometer data and very long period seismicity), to offer key and novel insights into the degassing dynamics within the shallow conduit systems of this open-vent volcano.