Fumarolic Discharges Research Articles

Pressure increase at the magma-hydrothermal interface at Krafla caldera, North-Iceland, 2018–2020: Magmatic processes or hydrothermal changes?

Following volcano deflation since 1989, the deformation pattern in Krafla caldera, North Iceland, changed in 2018. Geodetic measurements reveal a difference in surface velocity fields for 2015–2018 and 2018–2020 periods, reflecting a change in the deformation pattern. The difference velocity field broadly fits deformation caused by a spherical pressure source within a uniform elastic half-space, with a volume change of 2.6–3.8 × 105 m3/yr and centre depth of 2.1–2.5 km, which is close to the brittle-ductile boundary in the area, at a depth of 1.8 to 2.2 km. Potential processes causing the deformation change are evaluated: magmatic processes such as magma inflow or accumulation of volcanic gas, changes in the geothermal area because of change in geothermal production, or a combination of these. In particular, we evaluate if the change in deformation may relate to about 0.1 MPa/yr pressure increase in the geothermal system as measurements in monitoring well KG-10 indicate, eventually due to changes in the geothermal exploitation strategy at the Krafla power plant. Modelling shows that inferred volume change may be due to a spherical source with 1.4 km radius with 0.1 MPa/yr pressure change if the surrounding crust has a Young's modulus E of about 7 GPa. However, the average regional Young's modulus for the upper crust in Iceland has been estimated to be 30 GPa. We use the Finite Element Method (FEM) to assess the influence on the displacement due to the presence of a local crustal volume 5 × 5 × 4 km (horizontal dimensions × depth), which envelops the source (within the Krafla caldera), with E = 7 GPa in the central area and 30 GPa in the far field, in a three-dimensional model. Such a model can reproduce significant features of the observed deformation. There are no changes in seismicity in 2018. In late 2019, the earthquake rate increases following a slight decrease over few months. The seismic moment release is relatively steady until the end of 2019, when the rate increases and is relatively constant in 2020. Gravity measurements in late 2019, when compared to limited measurements in 2018, are inconclusive regarding the nature of the deformation, but useful for further monitoring. No noticeable changes have been observed in the chemical composition of fumarole discharge in the Krafla field that relate to new intrusions.

Evolution of the magmatic-hydrothermal system at Lastarria volcano (Northern Chile) between 2006 and 2019: Insights from fluid geochemistry

One of the major problems in the volcanic surveillance is how data from several techniques can be correlated and used to discriminate between possible precursors of volcanic eruptions and changes related to non-eruptive processes. Gas chemical surveys and measurements of SO2 emission rates performed in the past (2006–2019) at Lastarria volcano in Northern Chile have revealed a persistent increment of magmatic sourced gas emissions since late November 2012, following a 13 years period of intense ground uplift. In this work, we provide new insights into the gas-chemical evolution of Lastarria’s fumarolic discharges obtained from direct sampling (2006–2019) and SO2 emission rates using UV camera and DOAS instruments (2018–2019) and link these to pre-existing information on ground deformation (1998–2016) in order to determine the origin of observed degassing and ground deformation processes. We revise the four mechanisms originally proposed as alternatives by Lopez et al. (Geosphere, 2018, 14 (3), 983–1007) to explain the changes observed in the fluid geochemistry and ground deformation between 2009 and 2012, in order to explain major changes in gas-geochemistry over an extended period between 1998 and 2019. We hypothesize that a continuous sequence of processes explains the evolution in the fluid geochemistry of fumarolic discharges. Two mechanisms are responsible of the changes in the gas composition during the studied period, corresponding to a 1) deep magma chamber (7–15 km depth) pressurized by volatile exsolution (1998–2020), which is responsible of the large-scale deformation; followed by 2) a crystallization-induced degassing (2001–2020) and pressurization of the hydrothermal system (2003-early November 2012), where the former process induced the changes in the gas composition from hydrothermal-dominated to magmatic-dominated, whereas the last produced the small-scale deformation at Lastarria volcano. The changes in the gas composition since late November 2012, which were strongly dominated by magmatic volatiles, produced two consecutive processes: 1) acidification (late November 2012–2020) and 2) depletion (2019–2020) of the hydrothermal system. In this work we have shown that a long-term surveillance of the chemistry of fluid discharges provides valuable insights into underlying magmatic/volcanic processes, and consequently, for forecasting future eruptions.

Helium isotopes in Plinian and inter-Plinian volcanic products of Vesuvius, Italy

We investigated helium isotopes on gas extracted by crushing from melt and fluid inclusions in minerals from Plinian and inter-Plinian tephra and lavas of Vesuvius, Italy. Erupted products of different ages were considered, from Avellino eruption (1995 BCE) to the last eruption of 1944, with special focus on the 79 AD Plinian eruption. He/He 4 3 ratios between 1.5 and 2.7 RA were measured, with the highest values associated with rocks representative of the roof and the walls of the magma chambers (cumulates). Lowest values occurred in sanidines representative of magma-skarn interfaces. Noteworthy, the highest measured values of the 79 AD pumices were comparable with both lavas and tephra emitted from flank vents and under open-conduit conditions during the Medieval Period and Present Period of Vesuvius activity, and present-day fumarolic discharges. He 3/He 4 values are buffered within an extended, deep-seated reservoir at about 10 km filled with magma rising from the mantle. A fact that might potentially limit the accuracy of future eruption forecasting through monitoring of He/He 4 3 changes in Vesuvius fumaroles. Ageing and interaction with crustal rocks emerged as possible mechanisms that lowered the He/He 4 3 ratio of the melt during its intra-crustal magma chambers stay, with highest values associated with more dynamic conditions.

Decoding water-rock interaction and volatile input at La Soufriere volcano (Guadeloupe) using time-series major and trace element analyses in gas condensates

The geochemical behavior of major and trace elements in fumarolic gas condensate samples collected between April 2017 and January 2021 was investigated at La Soufriere hydrothermal volcano. Samples collected from Cratère Sud and Napoleon Nord fumaroles (temperature varying from 93.9 to 108.4 °C) offer a unique opportunity to reveal the physicochemical processes that affect the deeper magmatic and shallow hydrothermal systems feeding fumaroles. Gas condensate samples exhibit abundant solid particles that suggest strong water-rock interaction and erosive processes in fumarolic conduits at La Soufriere. These particles were incorporated during sampling, except for rare Si-rich spherules that appeared to have precipitated during storage time. Gas condensate samples displayed abundant concentrations of S, halogens (Cl, Br, I), metals/metalloids (B, Ni, Cu, Zn, As, Mo, Ag, Cd, Sb, Pb, and Bi), as well as enhanced concentrations of rock-related lithophile and siderophile elements. The relative abundance of major and trace elements in gas condensates suggests the permanent contribution of volatile-rich magmatic fluids and strong water-rock interaction processes. Based on elemental correlations and the physical properties of chemical elements, it is postulated that most of them are discharged in the aerosol phase, except for the halogens and S, which can be found in the gaseous phase in low-temperature fumarolic emissions. Metals and metalloids (Sb, Bi, Zn, Mo, Ni, Ag, Cd, As, and Pb) enriched in gas condensates were transported within tiny aqueous droplets, while rock-related elements (including REEs) were transported in the aerosol phase. Trace element enrichments in La Soufriere's condensates are comparable with those found in worldwide magmatic-hydrothermal systems such as Kawaj Ijen (Indonesia), Lastarria (Chile), and White Island (New Zealand) volcanoes. The size of the hydrothermal system and the degree of water-rock interaction are thought to be important parameters in controlling the abundance of major and trace elements in low-temperature fumarolic discharge.

Insights from Fumarole Gas Geochemistry on the Recent Volcanic Unrest of Pico do Fogo, Cape Verde

We report the results of the geochemical monitoring of the fumarolic discharges at the Pico do Fogo volcano in Cape Verde from 2007 to 2016. During this period Pico do Fogo experienced a volcanic eruption (November 23, 2014) that lasted 77 days, from a new vent ∼2.5 km from the fumaroles. Two fumaroles were sampled, a low (F1∼100°C) and a medium (F2∼300°C) temperature. The variations observed in the δ18O and δ2H in F1 and F2 suggest different fluid source contributions and/or fractionation processes. Although no significant changes were observed in the outlet fumarole temperatures, two clear increases were observed in the vapor fraction of fumarolic discharges during the periods November 2008–2010 and 2013–2014. Also, two sharp peaks were observed in CO2/CH4ratios at both fumaroles, in November 2008 and November 2013. This confirms that gases with a strong magmatic component rose towards the surface within the Pico do Fogo system during 2008 and 2013. Further, F2 showed two CO2/Stotalpeaks, the first in late 2010 and the second after eruption onset, suggesting the occurrence of magmatic pulses into the volcanic system. Time series of He/CO2, H2/CO2and CO/CO2ratios are low in 2008–2009, and high in 2013–2014 period, supporting the hypothesis of fluid input from a deeper magmatic source. Regarding to the isotopic composition, increases in air-corrected3He/4He ratios are observed in both fumaroles; F1 showed a peak in 2010 from a minimum in 2009 during the first magmatic reactivation onset and another in late 2013, while F2 displayed a slower rise to its maximum in late 2013. The suite of geochemical species analyzed have considerably different reactivities, hence these integrated geochemical time-series can be used to detect the timing of magmatic arrivals to the base of the system, and importantly, indicate the typical time lags between gas release periods at depth and their arrival at the surface. The high3He/4He ratios in both fumaroles in the range observed for mid-ocean ridge basalts, indicating that He is predominantly of upper mantle origin. This work supports that monitoring of the chemical and isotopic composition of the fumaroles of the Pico do Fogo volcano is a very important tool to understand the processes that take place in the magmatic-hydrothermal system and to be able to predict future episodes of volcanic unrest and to mitigate volcanic risk.

Changes in the thermal energy and the diffuse 3He and 4He degassing prior to the 2014–2015 eruption of Pico do Fogo volcano, Cape Verde

We report the temporal evolution of helium isotopes emission and thermal energy release in the period 2007–2018 at the summit crater of Pico do Fogo volcano, Cape Verde. 3He/4He ratio measured in a fumarolic discharge, ranged from 6.69 to 8.63 RA, being the maximum mantle-derived composition measured in November 2013, one year before the eruption onset in 23 November 2014, and confirm the beginning of a possible magmatic intrusion four years and eight months before the 2014–2015 eruption. 3He and mantle 4He emission rates showed similar trends in temporal evolution. Thermal energy release calculated for the study period ranged from 1 up to 72 MW, being the maximum values linked to maximum 3He (1.7 × 10−2 and 1.5 × 10−3 mol d−1 using the first and the second approach respectively), confirming a close relationship between both parameters. After the eruptive event, a simultaneous decrease in RA, 4He and 3He emissions is observed due to the rapid degassing of the magma responsible of the 2014–2015 eruptive process.

Improved environmental monitoring of surface geothermal features through comparisons of thermal infrared, satellite remote sensing and terrestrial calorimetry

Assessing changes in surface heat-loss at geothermally active areas provides insight into the geothermal reservoirs at depth. Surface temperatures and heat output of steam-heated ground can be inferred through processing of thermal infrared images, as well as by direct measurements using terrestrial calorimetry. In February 2014, a high resolution aerial thermal infrared (TIR) survey and a terrestrial heat-loss survey (excluding fumarole discharge) were carried out over Karapiti (ca. 0.35 Km2 in the Wairakei-Tauhara Geothermal System) in the Taupo Volcanic Zone, New Zealand. Estimates of heat-loss are compared to those calculated from TIR data collected by the Landsat-8 satellite of the area. This paper discusses the processing techniques for the datasets and compares inferred surface heat losses. The aerial TIR and terrestrial measurements are in reasonable agreement with calculated surface heat-loss values for Karapiti range from 41 MW (theoretical radiation heat loss using aerial TIR) to 58 MW (total heat loss based on an empirical correlation with boiling-point depths). Satellite images show a large variation between day-time and night-time TIR assessments. The assessment of nocturnal (non-solar), radiation heat-loss is an order of magnitude lower than the total heat-loss determined from the other techniques and approximately 1/3 of the inferred radiated component calculated from the aerial TIR data. This is likely to be a consequence of diffuse, advecting-steam heat-loss, combined with an image resolution issue: the pixel resolution of the satellite image (30 × 30 m) is much larger than the typical size of the active geothermal surface manifestations (<10 m2). Because the radiated heat-loss is a non-linear function of land surface temperature, the in-pixel averaging under-estimates the radiated heat-loss. These and other problems currently restrict the usefulness of repeat low-resolution satellite imagery for monitoring of surface heat-loss changes in steam-heated ground.

Carbon dioxide diffuse emission and thermal energy release from hydrothermal systems at Copahue–Caviahue Volcanic Complex (Argentina)

The north-western sector of Caviahue caldera (Argentina), close to the active volcanic system of Copahue, is characterized by the presence of several hydrothermal sites that host numerous fumarolic emissions, anomalous soil diffuse degassing of CO2 and hot soils. In March 2014, measurements of soil CO2 fluxes in 5 of these sites (namely, Las Máquinas, Las Maquinitas I, Las Maquinitas II, Anfiteatro, and Termas de Copahue) allowed an estimation that ~165t of deeply derived CO2 is daily released. The gas source is likely related to a relatively shallow geothermal reservoir containing a single vapor phase as also suggested by both the geochemical data from the 3 deep wells drilled in the 1980s and gas geoindicators applied to the fumarolic discharges. Gas equilibria within the H–C–O gas system indicate the presence of a large, probably unique, single phase vapor zone at 200–210°C feeding the hydrothermal manifestations of Las Máquinas, Las Maquinitas I and II and Termas de Copahue. A natural thermal release of 107MW was computed by using CO2 as a tracer of the original vapor phase. The magmatic signature of the incondensable fumarolic gases, the wide expanse of the hydrothermal areas and the remarkable high amount of gas and heat released by fluid expulsion seem to be compatible with an active magmatic intrusion beneath this portion of the Caviahue caldera.

Submarine venting of magmatic volatiles in the Eastern Manus Basin, Papua New Guinea

The SuSu Knolls and DESMOS hydrothermal fields are located in the back-arc extensional transform zone of the Eastern Manus Basin. In 2006, highly acidic and ΣSO4-rich vent fluids were collected at both sites and analyzed for the chemical and isotopic composition of major and trace species. Fluids exiting the seafloor have measured temperatures from 48 to 215°C and are milky white in appearance due to precipitation of elemental S0. Vent fluid concentrations of Na, K, and Mg are depleted by as much as 30% relative to seawater, but have the same relative abundance. In contrast, the fluids are highly enriched in dissolved ΣCO2, Cl, SiO2(aq), Fe, and Al relative to seawater. Measured pH (25°C) ranged from 0.95 to 1.87 and aqueous ΣSO4 ranged from 35 to 135mmol/kg. The chemical and isotopic composition points to formation via subsurface mixing of seawater with a Na-, K-, Mg-, and Ca-free, volatile-rich magmatic fluid exsolved from subsurface magma bodies during a process analogous to subaerial fumarole discharge. Estimates of the magmatic end-member composition indicate a fluid phase where H2O>SO2>CO2≈Cl>F. The hydrogen and oxygen isotopic composition of H2O and carbon isotopic composition of ΣCO2 in the vent fluids strongly suggest a contribution of slab-derived H2O and CO2 to melts generated in the mantle beneath the Eastern Manus volcanic zone. Abundant magmatically-derived SO2 undergoes disproportionation during cooling in upflow zones and contributes abundant acidity, SO42−, and S0 to the venting fluids. Interaction of these highly acidic fluids with highly altered mineral assemblages in the upflow zone are responsible for extensive aqueous mobilization of SiO2(aq), Fe, and Al. Temporal variability in the speciation and abundance of aqueous S species between 1995 and 2006 at the DESMOS vent field suggests an increase in the relative abundance of SO2 in the magmatic end-member that has mixed with seawater in the subsurface. Results of this study constrain processes responsible for the formation of hot-spring fluids in magmatically active back-arc environments and the resulting chemical exchange between the lithosphere and water column.

Geogenic and atmospheric sources for volatile organic compounds in fumarolic emissions from Mt. Etna and Vulcano Island (Sicily, Italy)

In this paper, fluid source(s) and processes controlling the chemical composition of volatile organic compounds (VOCs) in gas discharges from Mt. Etna and Vulcano Island (Sicily, Italy) were investigated. The main composition of the Etnean and Volcano gas emissions is produced by mixing, to various degrees, of magmatic and hydrothermal components. VOCs are dominated by alkanes, alkenes and aromatics, with minor, though significant, concentrations of O‐, S‐ and Cl(F)‐substituted compounds. The main mechanism for the production of alkanes is likely related to pyrolysis of organic‐matter‐bearing sediments that interact with the ascending magmatic fluids. Alkanes are then converted to alkene and aromatic compounds via catalytic reactions (dehydrogenation and dehydroaromatization, respectively). Nevertheless, an abiogenic origin for the light hydrocarbons cannot be ruled out. Oxidative processes of hydrocarbons at relatively high temperatures and oxidizing conditions, typical of these volcanic‐hydrothermal fluids, may explain the production of alcohols, esters, aldehydes, as well as O‐ and S‐bearing heterocycles. By comparing the concentrations of hydrochlorofluorocarbons (HCFCs) in the fumarolic discharges with respect to those of background air, it is possible to highlight that they have a geogenic origin likely due to halogenation of both methane and alkenes. Finally, chlorofluorocarbon (CFC) abundances appear to be consistent with background air, although the strong air contamination that affects the Mt. Etna fumaroles may mask a possible geogenic contribution for these compounds. On the other hand, no CFCs were detected in the Vulcano gases, which are characterized by low air contribution. Nevertheless, a geogenic source for these compounds cannot be excluded on the basis of the present data.

Fumarole/plume and diffuse CO2 emission from Sierra Negra caldera, Galapagos archipelago

Measurements of visible and diffuse gas emission were conducted in 2006 at the summit of Sierra Negra volcano, Galapagos, with the aim to better characterize degassing after the 2005 eruption. A total SO2 emission of 11 ± 2 t day−1 was derived from miniature differential optical absorption spectrometer (mini-DOAS) ground-based measurements of the plume emanating from the Mini Azufral fumarolic area, the most important site of visible degassing at Sierra Negra volcano. Using a portable multigas system, the H2S/SO2, CO2/SO2, and H2O/SO2 molar ratios in the Mina Azufral plume emissions were found to be 0.41, 52.2, and 867.9, respectively. The corresponding H2O, CO2, and H2S emission rates were 562, 394, and 3 t day−1, respectively. The total output of diffuse CO2 emissions from the summit of Sierra Negra volcano was 990 ± 85 t day−1, with 605 t day−1 being released by a deep source. The diffuse-to-plume CO2 emission ratio was about 1.5. Mina Azufral fumaroles released gasses containing 73.6 mol% of H2O; the main noncondensable components amounted to 97.4 mol% CO2, 1.5 mol% SO2, 0.6 mol% H2S, and 0.35 mol% N2. The higher H2S/SO2 ratio values found in 2006 as compared to those reported before the 2005 eruption reveal a significant hydrothermal contribution to the fumarolic emissions. 3He/4He ratios measured at Mina Azufral fumarolic discharges showed values of 17.88 ± 0.25 RA, indicating a mid-ocean ridge basalts (MORB) and a Galapagos plume contribution of 53 and 47 %, respectively.

Geochemical and isotopic evidences of magmatic inputs in the hydrothermal reservoir feeding the fumarolic discharges of Tacora volcano (northern Chile)

Tacora volcano is a nearly unknown and understudied 5980 m high volcano located on the northernmost border between Chile and Peru. Tacora is characterized by intense fumarolic activity with extensive, white-colored, hydrothermal areas along the NW and W flanks of the volcanic structure. The chemical and isotopic features of gas emissions indicate that the hydrothermal reservoir is significantly affected by contributions from magmatic degassing, despite relatively low outlet temperatures (82–93 °C). Water and gas isotopic signatures stem from degassing of andesitic magmas produced from the sediment-poor subducting slab. Isotopic data suggests CH 4–CO 2 isotopic equilibration occurs at ~ 400 °C, likely in the deepest portion of the hydrothermal-magmatic fluid reservoir. The H 2–H 2O, CO–CO 2, H 2–Ar, CH 4–CO 2 and C 2–C 3 alkenes/alkanes pairs tend to approach a chemical equilibrium at temperatures between 200 and 320 °C when redox conditions are more oxidizing than those determined by the typical FeO/FeO 1.5 rock buffer system. Boiling occurs at shallow depth at 85 °C within a discontinuous, thin aquifer, enough as thermal buffer but not able to scrub the uprising magmatic-related acidic gases.

Gas discharges from four remote volcanoes in northern Chile (Putana, Olca, Irruputuncu and Alitar): a geochemical survey

We analyzed gas samples collected from fumaroles and bubbling pools at Irruputuncu, Putana, Olca and Alitar volcanoes located in the central Andes volcanic zone (northern Chile). The Irruputuncu and Putana fumarolic discharges showed outlet temperatures ranging from 83 ˚C to 240 ˚C and from 82 ˚C to 88 ˚C, respectively. The chemical and isotopic ( 3 He/ 4 He, d 13 C-CO 2 , d 18 O-H 2 O and dD-H 2 O) compositions of these discharges were similar to medium-to-high temperature volcanic gases from other active volcanoes in this sector of the Andean volcanic chain (e.g. Lascar volcano). Inorganic and organic gas geothermometers for the H 2 O-CO 2 -CO-H 2 , CO 2 -CH 4 and C 2 -C 3 alkenes-alkanes systems indicated equilibrium temperatures that exceed 500 ˚C at the gas sources. These relatively high temperatures are in agreement with the presence of relevantly high concentrations of magmatic gas emissions, including SO 2 . Olca and Alitar volcano fluid chemistries indicated lower amounts of magmatic-derived gas species, while both the helium and the water isotopic compositions suggested significant fractions of shallow, crustal/meteoric-originated fluids. These indicate contributions from a hydrothermal environment with temperatures <400 ˚C. The geochemical and isotopic features derived from the present study show that the Irruputuncu, Putana, Olca and Alitar volcanoes should be considered as active and thus warrant periodic geochemical monitoring to determine the evolution of these systems and their potential hazards.

Geophysical, geochemical and geodetical signals of reawakening at Turrialba volcano (Costa Rica) after almost 150 years of quiescence

Turrialba is a basaltic–andesitic stratovolcano (3340 masl), in the Cordillera Volcánica Central in Costa Rica. After the last eruption (1864–1866), volcanic manifestations were limited to weak fumarolic discharge (continuous since 1980) from the summit. From 1996 onward, the degassing activity has progressively been increasing, reaching its climax after 2005. New fumaroles have appeared in the Central and West summit craters, the latter now being the most active, and in the fracture system in between, showing sulphur deposits and progressively increasing degassing rate. In 2004, fumaroles and new fissures have appeared on the SW outer and SSW distal flanks, the latter being located along a major NE-oriented tectonic lineament. Fumarolic temperatures at the bottom of the West crater have increased from 88 to 282 °C in early 2008. Changes in chemical and isotopic compositions of discharged fluids have shown a progressive enhancing of the magmatic signature since 2001. Since late 2007, SO 2 flux, measured with mini-DOAS, has increased two orders of magnitude (1 t/day in 2002 to 740 t/day in January 2008). The enhanced gas discharge at Turrialba volcano has caused significant interference on tropospheric O 3 measurements at 2–3 km altitude ~ 50 km W from the volcano. Seismic swarms followed an increasing trend consistent with that of the fumaroles. The maximum seismic activity to date, up to thousands of events/day, was recorded in mid 2007. An inflationary trend was observed in the crater area. In this paper we present for the first time all the available data on the activity of Turrialba volcano. New geophysical, geodetical and geochemical data and published geophysical and geochemical data are presented and discussed as a whole. The multidisciplinary approach indicated that from 1996 to 2009 three stages, deriving by the delicate equilibrium between the hydrothermal and the magmatic reservoirs, were recognized. The magmatic-dominated phase is still prevailing as evidenced by the fact that, while completing the present paper, on the 4th of January 2010 at 16.57 (GMT) a loud explosion occurred at the West crater and was followed by three others spaced out every 10 min. These events were interpreted as associated with phreatic eruptions.

Monitoring of fumarole discharge and CO2 soil degassing in the Azores: contribution to volcanic surveillance and public health risk assessment

Fluid geochemistry monitoring in the Azores involves the regular sampling and analysis of gas discharges from fumaroles and measurements of CO2 diffuse soil gas emissions. Main degassing areas under monitoring are associated with hydrothermal systems of active central volcanoes in S. Miguel, Terceira and Graciosa islands. Fumarole discharge analysis since 1991 show that apart from steam these gas emissions are CO2 dominated with H2S, H2, CH4 and N2 in minor amounts. Mapping of CO2 diffuse soil emissions in S. Miguel Island lead to the conclusion that some inhabited areas are located within hazard-zones. At Furnas village, inside Furnas volcano caldera, about 62% of the 896 houses are within the CO2 anomaly, 5% being in areas of moderate to high risk. At Ribeira Seca, on the north flank of Fogo volcano, few family houses were evacuated when CO2 concentrations in the air reached 8 mol%. To assess and analyse the CO2 soil flux emissions, continuous monitoring stations were installed in S. Miguel (2), Terceira and Graciosa islands. The statistical analysis of the data showed that some meteorological parameters influence the CO2 flux. The average of CO2 flux in S. Miguel stations ranges from 250 g/m2/d at Furnas volcano to 530 g/m2/d at Fogo volcano. At Terceira Island it is about 330 g/m2/d and at Graciosa 4400 g/m2/d.

Gas geochemistry of the Cordón Caulle geothermal system, Southern Chile

The Cordón Caulle geothermal system is located in a NW-trending volcano-tectonic depression of the Southern Andean Volcanic Zone of Chile. Outflows of low chloride water were previously interpreted as the surface expression of a shallow steam-heated aquifer, with subsurface temperatures of 150–170 °C. Gas data from fumaroles and hot springs have been used to assess the nature and temperature of the deeper, underlying geothermal reservoir. Fumaroles at the northeastern border of Cordón Caulle have 3He/ 4He ratios typical of subduction margins (6–7 R A) and N 2/Ar ratios of about 40, indicating deep convection of air-saturated groundwater. Fumaroles at the southwestern border have N 2/Ar ratios >300, suggesting the presence of a deep volcanic component. Gas ratios of fumarole discharges yield equilibration temperatures >300 °C, whereas those of hot spring waters suggest temperatures of about 160 °C. Based on these data, and comparisons with well documented liquid and vapor-dominated geothermal systems, a model is proposed of a boiling liquid-dominated geothermal system overlain by a secondary steam-heated aquifer.

Scrubbing process and chemical equilibria controlling the composition of light hydrocarbons in natural gas discharges: An example from the geothermal fields of El Salvador

The compositional features of fluids from both fumarolic discharges and productive geothermal wells of Ahuachapan‐Chipilapa, Berlin‐Chinameca, and San Vicente geothermal systems (El Salvador) are described and discussed in order to investigate the complex geochemical interactions involving geothermal fluids within the shallowest part of the hydrothermal circulation pathways. Our results highlight that secondary processes are able to strongly affect and modify the chemical characteristics of geothermal gases once they discharge to the surface as natural manifestations, mainly in relation to the chemical‐physical properties of each gas species. The effects of both gas dissolution in shallow aquifers and gas‐water‐rock chemical interactions on gas discharge composition make it difficult to get a correct evaluation of the thermodynamic conditions that characterize the geothermal reservoirs by applying the common geoindicators based on the chemical equilibria of the H2O‐CO2‐H2‐CH4‐CO system. Differently, the composition of the C1‐C2‐C3 alkanes and the C3 and C4 alkane‐alkene pair, established within the geothermal reservoirs under the control of chemical reactions, remains stable in samples collected from discharging gas vents. These results suggest that the relative abundances of hydrocarbons characterized by similar structure and molecular size seem to be mainly regulated by the diffusion velocity of gases through the liquid‐dominated system. Therefore the chemical features of the light organic gas fraction of naturally discharging fluids can be successfully utilized for the evaluation of geothermal reservoir temperatures and redox conditions, providing useful indications in terms of geothermal exploration and exploitation. On this basis, the distribution, speciation, and relative abundances of light hydrocarbons can also be considered highly promising in geochemical monitoring of active volcanic systems.

Summary of recent volcanic activity

During late 2002 and early 2003, scientists at Llaima (central Chile) documented increases in seismicity, fumarolic output, a minor eruption, and pronounced glacial disruption. By 8 January 2003, the ice and snow formerly capping the N and S craters had completely melted. During January 2002–April 2003 eruptions remained comparatively modest. Larger eruptions began in April 2003, depositing pyroclastic material, dispersing numerous ballistic blocks, and creating substantial plumes. Llaima’s cover of ice and snow was more complete in October 2002 than in October 1998, so, by comparison, visible signs of thermal activity had diminished significantly. In contrast, seismicity in September 2002 was notably greater than in January 2002. Seismicity continued to increase from December 2002 to mid January 2003 before stabilizing for about 2 months. A flight on 8 January 2003 revealed dense fumarolic emissions from the main crater, no remaining ice or snow on the main crater’s internal walls, exposed rock at both the summit and Pichillaima (the smaller SE-flank cone), and numerous new crevasses in the cone’s glaciers. Although the melting came at the time of the annual thaw, the absence of ice and snow cover on both the shaded and unshaded sides of Llaima was cited as evidence indicating elevated rock temperatures. On the afternoon of 10 April, several ash explosions originated from the main crater. Strong and continuous explosions about 8–9 hours later caused ashfall in the Nslope sector of Captr n. The explosions became more sporadic, and continued into the morning of 11 April, when a column of volcanic gases fed a horizontal lenticular cloud ~ 30 km in diameter, the top of which rose to about 3,900 m altitude. This activity resulted in a thin layer of pyroclastic material spread widely across the flank glaciers, visible from the air out to distances of ~ 4 km. Impact craters in the ice testified to numerous bouncing and rolling projectiles. The crater floor contained a 50-m-diameter vent emitting gases. When photographed on 11 April, the ice and snow at Pichillaima had receded by 1–1.5 km from its topographic high. Many regions of the ice appeared to be in motion and undergoing acute mechanical failure, and numerous crevasses had developed. On 12–13 April, the main crater showed intense pulsing fumarolic degassing, mass wasting, and ballistic discharges. Eruptions began to contain significant tephra. On 16 April, gaseous emissions escaping from the crater reached 200 m above the crater rim. Emissions also came from Pichillaima. During 17 April, scientists witnessed gas emissions to the NE that ascended to 200 m above the crater rim and spawned a plume visible for ~10 km.

Genesis of chlorine and sulphur in fumarolic emissions at Vulcano Island (Italy): assessment of pH and redox conditions in the hydrothermal system

Chlorine- and sulphur-bearing compounds in fumarole discharges of the La Fossa crater at Vulcano Island (Italy) can be modelled by a mixing process between magmatic gases and vapour from a boiling hydrothermal system. This allows estimating the compounds in both endmembers. Magma degassing cannot explain the time variation of sulphur and HCl concentrations in the deep endmember, which are more probably linked to reactions of solid phases at depth, before mixing with the hydrothermal vapours. Based on the P– T conditions and speciation of the boiling hydrothermal system below La Fossa, the HCl and S tot contents in the hydrothermal vapours were used to compute the redox conditions and pH of the aqueous solution. The results suggest that the haematite–magnetite buffer controls the hydrothermal fO 2 values, while the pH has increased since the end of the 1970s. The main processes affecting pH values may be linked to Na–Ca exchanges between evolved seawater, feeding the boiling hydrothermal system, and local rocks. While Na is removed from water, calcium enters the solution, undergoes hydrolysis and produces HCl, lowering the pH of the water. The increasing water–rock ratio within the hydrothermal system lowers the Ca availability, so the aqueous solution becomes less acidic. Seawater flowing towards the boiling hydrothermal brine dissolves a large quantity of pyrite along its path. In the boiling hydrothermal system, dissolved sulphur precipitates as pyrite and anhydrite, and becomes partitioned in vapour phase as H 2S and SO 2. These results are in agreement with the paragenesis of hydrothermal alteration minerals recovered in drilled wells at Vulcano and are also in agreement with the isotopic composition of sulphur emitted by the crater fumaroles.

Degassing activity from Iwodake rhyolitic cone, Satsuma-Iwojima volcano, Japan: Formation of a new degassing vent, 1990–1999

Large changes in the surface manifestation of degassing activity were observed from 1990 to 1999 at the summit crater of Iwodake cone of Satsuma-Iwojima volcano. During this period, a new high-temperature fumarolic area formed in the center of the crater floor and became a degassing vent with a diameter of 40 m. Altered volcanic rocks were ejected during the course of vent formation. Although glass fragments were observed in the ejected ash, the glass comes from altered Iwodake rhyolite that covers the crater floor. The highest fumarolic temperature and equilibrium temperatures of volcanic gases had a maximum of about 900°C at the beginning of the vent formation. The flux of SO2, measured by COSPEC, varied from 300 to 700 ton/day and correlated directly with maximum fumarole temperature. During this period, open fractures formed along the southern rim of the crater almost contemporaneously with the vent formation and changes in the nature of fumarolic discharges. The continuous and intense degassing at Satsuma-Iwojima is likely caused by volatile transport from a deep magma chamber through a convecting magma column. An increase in the magma convection rate might have caused these large changes in surface manifestations, including increase in the SO2 flux and fumarolic temperatures, ground deformation, and the vent formation.