Composition Of Fumarolic Gases Research Articles

Quantification of volcanic degassing and analysis of uncertainties using numerical modeling: the case of Stephanos crater (Nisyros Island, Greece)

Nisyros Island (Greece) is affected by widespread gas emissions from fumarolic fields located at the bottom of hydrothermal craters in the southern part of its caldera. This morphology and the current low gas fluxes make Nisyros an ideal site for testing the limits of physics-based gas dispersal models in confined and low-emission conditions. Here, we focused our attention on the local scale volcanic gas dispersion from the Stephanos hydrothermal crater. In April 2023, a 1-week survey was carried out to measure weather data, CO2 and H2S gas fluxes, air concentrations from portable gas stations, and chemical composition of fumarolic gases and to acquire thermal images of the crater floor. These data were used as inputs and boundary conditions for numerical simulations using a DISGAS-2.6.0 model in order to quantify the present-day volcanic degassing and its associated uncertainties, accounting for the meteorological variability. Model results are provided in terms of H2S probabilistic exceedance and persistence maps, showing gas concentrations within the crater that fall below the thresholds indicated for the occurrence of serious respiratory problems. Since DISGAS-2.6.0 does not account for chemical reactions, this study represents a good opportunity to discuss the methodological limits of simulating the dispersion of H2S which is challenging due to its rapid degradation and dilution in the atmosphere. In this regard, we also provided an empirical law of the H2S depletion in low-emission conditions that takes into account the uncertainties related to the field measurements.

Monitoring of magmatic–hydrothermal system by noble gas and carbon isotopic compositions of fumarolic gases

We repeatedly measured isotopic compositions of noble gases and CO2 in volcanic gases sampled at six fumaroles around the Kusatsu-Shirane volcano (Japan) between 2014 and 2021 to detect variations reflecting recent volcanic activity. The synchronous increases in 3He/4He at some fumaroles suggest an increase in magmatic gas supply since 2018. The increase in magmatic gas supply is also supported by the temporal variations in 3He/CO2 ratios and carbon isotopic ratios of CO2. The 3He/40Ar* ratios (40Ar*: magmatic 40Ar) show significant increases in the period of high 3He/4He ratios. The temporal variation in 3He/40Ar* ratios may reflect changes in magma vesicularity. Therefore, the 3He/40Ar* ratio of fumarolic gases is a useful parameter to monitor the current state of degassing magma, which is essential for understanding the deep process of volcanic unrest and may contribute to identifying precursors of a future eruption. These results provide additional validation for the use of noble gas and carbon isotopic compositions of fumarolic gases for monitoring magmatic–hydrothermal systems.

Time variations in the chemical and isotopic composition of fumarolic gases at Hakone volcano, Honshu Island, Japan, over the earthquake swarm and eruption in 2015, interpreted by magma sealing model

Definite increases in the components ratios of CO2/H2O, CO2/H2S, CO2/CH4 and He/CH4 were observed at the fumarolic gases from Owakudani geothermal area located at the center of Hakone volcanic caldera (Honshu Island, Japan), synchronized with the earthquake swarm in 2015. Such variations were due to the dominance of a magmatic component over a hydrothermal component, suggesting the earthquake swarm was produced by the injection of magmatic gases into the hydrothermal system. The CO2/H2O ratio of magmatic gas was estimated to be 0.0045 before the earthquake swarm, which increased up to 0.013 during the earthquake swarm, likely produced by the pressurization of magma as a result of magma sealing where the pressure increment in magma was estimated to be 3% to the lithostatic pressure. The H2O and CO2 concentration in magma were estimated to be 6.3 wt% and 20 wt ppm, respectively, assuming a temperature 900 °C and a rhyolitic composition. In May 2015, a few months prior to the earthquake swarm in May 2015, a sharp increase in the Ar/CO2 and N2/He ratios and a decrease in the isotopic ratio of H2O were observed at the fumarolic gas. The invasion of air into the hydrothermal system increased the Ar/CO2 and N2/He ratios. The decrease in the isotopic ratio of H2O was induced by partial condensation of H2O vapor.

Chemical and Isotopic Compositions of Fumarolic Gases from Yakedake Volcano, Japan

Chemical and Isotopic Compositions of Fumarolic Gases from Yakedake Volcano, Japan

Hydrothermal system of the Kirishima volcanic area inferred from the chemical and isotopic composition of spring waters and fumarolic gases

The chemical and isotopic composition of hot spring waters and fumarolic gases from the Kirishima volcanic area were investigated in order to delineate the geochemical structure of the hydrothermal systems. There are active volcanoes, Iwoyama, Shinmoe and Ohachi locate in the area, and each of them has characteristic hydrothermal systems. At Iwoyama volcano, chloride-sulfate type waters are discharging from central part, and acid sulfate type waters are located on its north and west flanks. On the south west flank of Shinmoe volcano, CO 2 type waters are located. At a deep part on the west Sank of both volcanoes chloride type waters are distributed. This variety of different type of waters can be derived from differentiations of a hypothetical parent fluid. The chloride type water seems to be a liquid phase separated from the mixture of the parental fluid and ground water. The vapor phase separated from the liquid phase heats ground water and produces the sulfate type and the CO 2 type waters. In the hydrothermal systems beneath Iwoyama volcano, a parental fluid is differentiated to a chloride-sulfate type water and a H 2 S rich fumarolic gas. The parent fluid in the hydrothermal system of Shinmoe volcano seems to be depleted in sulfur species than the parental fluid for Iwoyama. Ohachi volcano has a faint fumarolic activity at the summit. The fumarolic gas losses most of water vapor prior to the discharge due to a conductive cooling and condensation of the vapor up to the summit. A model calculation evaluates the original composition of the fumarolic gas from Ohachi volcano. The original composition resembles to those of the fumarolic gases from the summit of Shinmoe volcano.

Origin of fumarolic fluids from Tupungatito Volcano (Central Chile): interplay between magmatic, hydrothermal, and shallow meteoric sources

Tupungatito is a poorly known volcano located about 100 km eastward of Santiago (Chile) in the northernmost sector of the South Volcanic Zone. This 5,682 m high volcano shows intense fumarolic activity. It hosts three crater lakes within the northwestern portion of the summit area. Chemical compositions of fumarolic gases and isotopic signatures of noble gases (3He/4He and 40Ar/36Ar are up to 6.09 Ra and 461, respectively), and steam (δ18O and δD) suggest that they are produced by mixing of fluids from a magmatic source rich in acidic gas compounds (SO2, HCl, and HF), and meteoric water. The magmatic–hydrothermal fluids are affected by steam condensation that controls the outlet fumarolic temperatures ( 200 °C and redox conditions are consistent with those inferred by the presence of the SO2–H2S redox pair, typical of fluids that have attained equilibrium in magmatic environment. A comprehensive conceptual geochemical model describing the circulation pattern of the Tupungatito hydrothermal–magmatic fluids is proposed. It includes fluid source regions and re-equilibration processes affecting the different gas species due to changing chemical–physical conditions as the magmatic–hydrothermal fluids rise up toward the surface.

Constraints on magma processes, subsurface conditions, and total volatile flux at Bezymianny Volcano in 2007–2010 from direct and remote volcanic gas measurements

Direct and remote measurements of volcanic gas composition, SO2 flux, and eruptive SO2 mass from Bezymianny Volcano were acquired between July 2007 and July 2010. Chemical composition of fumarolic gases, plume SO2 flux from ground and air-based ultraviolet remote sensing (FLYSPEC), and eruptive SO2 mass from Ozone Monitoring Instrument (OMI) satellite observations were used along with eruption timing to elucidate magma processes and subsurface conditions, and to constrain total volatile flux. Bezymianny Volcano had five explosive magmatic eruptions between May 2007 and June 2010. The most complete volcanic gas datasets were acquired for the October 2007, December 2009, and May 2010 eruptions. Gas measurements collected prior to the October 2007 eruption have a relatively high ratio of H2O/CO2 (81.2), a moderate ratio of CO2/S (5.47), and a low ratio of S/HCl (0.338), along with moderate SO2 and CO2 fluxes of 280 and 980t/d, respectively, and high H2O and HCl fluxes of ~45,000 and ~440t/d, respectively. These results suggest degassing of shallow magma (consistent with observations of lava extrusion) along with potential minor degassing of a deeper magma source. Gas measurements collected prior to the December 2009 eruption are characterized by relatively low H2O/CO2 (4.13), moderate CO2/S (6.84), and high S/HCl (18.7) ratios, along with moderate SO2 and CO2 fluxes of ~220 and ~1000t/d, respectively, and low H2O and HCl fluxes of ~1700 and ~7t/d, respectively. These trends are consistent with degassing of a deeper magma source. Fumarole samples collected ~1.5months following the May 2010 eruption are characterized by high H2O/CO2 (63.0), low CO2/S (0.986), and moderate S/HCl (6.09) ratios. These data are consistent with degassing of a shallow, volatile-rich magma source, likely related to the May eruption. Passive and eruptive SO2 measurements are used to calculate a total annual SO2 mass of 109kt emitted in 2007, with passive emissions comprising ~87–95% of the total. Total annual volatile masses for the study period are estimated to range from 1.1×106 to 18×106t/year. Annual CO2 masses are ~8 to 40 times larger than can be explained by degassing of dissolved CO2 within eruptive magma, suggesting that the eruptive magma contained a significant quantity of exsolved volatiles sourced either from the eruptive melt or unerupted magma at depth. Variable total volatile fluxes ranging from ~3000t/d in 2009 to ~49,000t/d in 2007 are attributed to variations in the depth of gas exsolution and separation from the melt under open-system degassing conditions. We propose that exsolved volatiles are quickly transported to the surface from ascending magma via permeable flow through a bubble and/or fracture network within the conduit and thus retain their equilibrium composition at the time of segregation from melt. The composition of surface CO2 and H2O emissions from 2007 to 2009 are compared with modeled exsolved fluid compositions for a magma body ascending from entrapment depths to estimate depth of fluid exsolution and separation from the melt. We find that at the time of sample collection magma had already begun ascent from the mid-crustal storage region and was located at maximum depths of ~3.7km in August 2007, approximately 2months prior to the next magmatic eruption, and ~4.6km in July of 2009 approximately five months prior to the next magmatic eruption. These findings suggest that the exsolved gas composition at Bezymianny Volcano may be used to detect magma ascent prior to eruption.

Chemical and isotopic composition of fumarolic gases at Iwate volcano, Japan, during and after seismic activity in 1998: implications for the modification of ascending volcanic gases

In 1998, there were many volcanic earthquakes recorded at Iwate volcano, Japan. Although an eruption was anticipated, it never occurred. Fumarolic gases were sampled at the volcano on six occasions during 1998 and 1999, and were analyzed for their chemical compositions and isotope ratios. The fumarolic gases were again sampled in 2004 and 2006, after the period of seismic activity. The HCl concentrations and isotope ratios of the H2O in the gas samples collected from an active geothermal area at the volcano were high in 1998 and 1999, but decreased significantly after 2004, irrespective of the relatively stable concentrations of CO2 and sulfur-bearing gases. A notable feature of the fumarolic gases is the high isotope ratio of H2O, similar to that of pure magmatic gas, which is typical of andesitic volcanoes, although the temperature at the outlet was less than 146°C in the present case. These features can be explained if the enthalpy of the magmatic gas was equivalent to that of water vapor at 252°C to 370°C under a pressure of 1 bar. The correlations observed among CO2/H2O and HCl/H2O, and the δ18O values of the fumarolic gases suggest partial condensation of H2O vapor during the ascent of the volcanic gas to the surface. The estimated CO2/H2O molar ratio of the magmatic gas was 0.008, which is less than the 0.03 reported previously for magmatic gas sampled during an effusive eruption at Unzen volcano, Japan. These data suggest that the magma at Iwate volcano is depleted in volatiles, because CO2 is preferentially degassed from the silicate melt relative to H2O. Such depletion in CO2 might explain the failed eruption at Iwate volcano in 1998.

Signals from the Campi Flegrei hydrothermal system: Role of a “magmatic” source of fluids

This is a parametric study that was carried out to investigate the signals generated by a hydrothermal system fed by a pulsating source of magmatic fluids. This study focuses on the effects that selected properties of the source have on the evolution of hydrothermal activity at Campi Flegrei, Italy. Numerical simulations are carried out to describe a multiphase and multicomponent hydrothermal system. Each simulation describes a short unrest phase, followed by a prolonged quiet period. During the unrest, specific properties of the fluid source (flow rate, fluid composition, source size, and unrest duration) are modified with respect to selected baseline values. The evolution of the system is tracked by looking at two parameters that can be monitored in active volcanic areas: the composition of fumarolic gases and gravity changes. The results describe the temporal evolution of these two observables and allow comparisons of the effects of different source properties. All of the simulated unrest events cause measurable changes in gas composition and gravity. For the geometry and system properties considered, these changes always last beyond the end of the unrest period, and can often persist for decades. Fluid flow rate is the source property that mostly affects the observable evolution. Gravity is more sensitive to source properties than gas composition, and it undergoes the largest and quickest changes. The results also highlight the major role that rock properties and initial conditions have in the evolution of these observable signals.

Statistical evaluation of compositional changes in volcanic gas chemistry: a case study

The geochemical analysis of fumarolic gases collected at quiescent and active volcanic systems over time is one of the main tools to understand changes in the state of activity for surveillance and risk assessment. The continuous output of chemical species through fumarolic activity, which characterizes the inter-eruptive intervals, has also a major and general influence on the environment. The mobilization of chemical species due to weathering of volcanic rocks, or the input of gaseous components from fumarolic activity, results in some kind of modification of the environment affecting, in particular, water, soils, and the consequent growth of the plants present in these areas. In this paper, an investigation on the chemical composition of fumarolic gases collected at Vulcano island (Sicily, southern Italy) is performed, with the aim to discover how data changes during the monitored period of time, and to design a strategy for the environmental surveillance of volcanic systems taking into account the nature of the analyzed data. In order to summarize the contribution of all the components that can affect the chemical composition of volcanic gases, a multivariate statistical approach appears to be suitable. Since many of those methods assume independent observations, the possible presence of time-dependent structures should be carefully verified. In this framework, given the compositional nature of geochemical data, we have applied recent theoretical and practical developments in the field of compositional data analysis to work in the correct sample space and to isolate groups of parts responsible for significant changes in the gas chemistry. The proposed approach can be generalized to the investigation of complex environmental systems.

Chemical and isotopic equilibrium between CO 2 and CH 4 in fumarolic gas discharges: Generation of CH 4 in arc magmatic-hydrothermal systems

The chemical and isotopic composition of fumarolic gases emitted from Nisyros Volcano, Greece, and of a single gas sample from Vesuvio, Italy, was investigated in order to determine the origin of methane (CH 4) within two subduction-related magmatic-hydrothermal environments. Apparent temperatures derived from carbon isotope partitioning between CH 4 and CO 2 of around 340°C for Nisyros and 470°C for Vesuvio correlate well with aquifer temperatures as measured directly and/or inferred from compositional data using the H 2O-H 2-CO 2-CO-CH 4 geothermometer. Thermodynamic modeling reveals chemical equilibrium between CH 4, CO 2 and H 2O implying that carbon isotope partitioning between CO 2 and CH 4 in both systems is controlled by aquifer temperature. N 2/ 3He and CH 4/ 3He ratios of Nisyros fumarolic gases are unusually low for subduction zone gases and correspond to those of midoceanic ridge environments. Accordingly, CH 4 may have been primarily generated through the reduction of CO 2 by H 2 in the absence of any organic matter following a Fischer-Tropsch-type reaction. However, primary occurrence of minor amounts of thermogenic CH 4 and subsequent re-equilibration with co-existing CO 2 cannot be ruled out entirely. CO 2/ 3He ratios and δ 13C CO 2 values imply that the evolved CO 2 either derives from a metasomatized mantle or is a mixture between two components, one outgassing from an unaltered mantle and the other released by thermal breakdown of marine carbonates. The latter may contain traces of organic matter possibly decomposing to CH 4 during thermometamorphism.

Chemical composition of fumarolic gases and spring discharges from El Chichòn volcano, Mexico: causes and implications of the changes detected over the period 1998–2000

Since the March–April 1982 eruption of El Chichòn volcano, intense hydrothermal activity has characterised the 1-km-wide summit crater. This mainly consists of mud and boiling pools, fumaroles, which are mainly located in the northwestern bank of the crater lake. During the period 1998–2000, hot springs and fumaroles discharging inside the crater and from the southeastern outer flank (Agua Caliente) were collected for chemical analyses. The observed chemical fluctuations suggest that the physico-chemical boundary conditions regulating the thermodynamic equilibria of the deep rock/fluid interactions have changed with time. The chemical composition of the lake water, characterised in the period 1983–1997 by high Na +, Cl −, Ca 2+ and SO 4 2− contents, experienced a dramatic change in 1998–1999, turning from a Na +–Cl −- to a Ca 2+–SO 4 2−-rich composition. In June 2000, a relatively sharp increase in Na + and Cl − contents was observed. At the same time, SO 2/H 2S ratios and H 2 and CO contents in most gas discharges increased with respect to the previous two years of observations, suggesting either a new input of deep-seated fluids or local variations of the more surficial hydrothermal system. Migration of gas manifestations, enhanced number of emission spots and variations in both gas discharge flux and outlet temperatures of the main fluid manifestations were also recorded. The magmatic-hydrothermal system of El Chichòn is probably related to interaction processes between a deep magmatic source and a surficial cold aquifer; an important role may also be played by the interaction of the deep fluids with the volcanic rocks and the sedimentary (limestone and evaporites) basement. The chemical and physical changes recorded in 1998–2000 were possibly due to variations in the permeability of the conduit system feeding the fluid discharges at surface, as testified by the migration of gas and water emanations. Two different scenarios can be put forward for the volcanic evolution of El Chichòn: (1) build-up of an infra-crater dome that may imply a future eruption in terms of tens to hundreds of years; (2) minor phreatic–phreatomagmatic events whose prediction and timing is more difficult to constrain. This suggests that, unlike the diminished volcanic activity at El Chichòn after the 1982 paroxistic event, the volcano-hydrothermal fluid discharges need to be more constantly monitored with regular and more frequent geochemical sampling and, at the same time, a permanent network of seismic stations should be installed.

Chemical and isotopic composition of fumarolic gases and the SO 2 flux from Volcán de Colima, México, between the 1994 and 1998 eruptions

Volcán de Colima is the most active volcano in Mexico, including a significant effusive eruption in 1991 and a weak explosive event in 1994. Geochemical monitoring of the volcano started in January 1996 with collection of fumarolic gases approximately every 3 months from two sites in the summit crater (Z3, ∼800°C, and Z2, ∼400°C). Chemical compositions of volcanic gases from the Z2 fumarole showed a peak in S/Cl in the middle of 1997, 14–15 months before the start of the 1998 lava eruption. High-temperature volcanic vapors from Z3 changed progressively in water isotopic composition and showed a weak trend of increasing HCl/HF and a peak in S/Cl at the same time. Starting from the summer of 1997, the volcanic water became gradually enriched in deuterium, which probably indicated an increasing contribution from deep, less degassed magma. A negative correlation between δD and Cl content for the Z3 fumarolic gases is explained by shallow degassing of the Colima hot lava plug. This hypothesis is consistent with published data on Cl contents and solubilities in magmas and supported by the observed positive trends of S/Cl and HCl/HF with δD for the Z3 fumarole. Until September 1998, the SO 2 flux from the volcano was at a low level (<100 tons/day), and often below the detection limit of the instrument (∼30 tons/day). A sharp increase in the SO 2 flux was recorded 1 month before the eruption started. It jumped from 400±50 tons/day to 1600 tons/day 2 days before new lava emerged on 20 November 1998. The SO 2 flux correlated well with seismicity.

Variation in fumarolic H 2 gas and volcanic activity at Nasudake in Japan

The chemical and isotopic compositions of fumarolic gases collected from Nasudake in northeastern Japan were determined in order to investigate the correlations between the behaviour of H 2 and volcanic activity. On the basis of the hydrogen isotope ratio for volcanic H 2, the temperature of the thermal fluid reservoir producing H 2 was estimated at ~ 160 †C. Large variations in the δD and δ 18O values of fumarolic condensates around the summit were observed. Hydrogen content in the fumarolic gases has been gradually increasing since 1980, resulting from rock-water interaction under redox conditions governed by a mineral buffer such as pyrite and magnetite in the underlying shallow hydrothermal systems. Hydrogen concentration and isotopic composition of volcanic steam are strongly influenced by the proportion of local meteoric water circulating underground. The measured content and D H ratios of volcanic H 2 suggest that the contribution of magmatic gases to the fumarolic discharges has decreased with time.

The chemical and isotopic composition of fumarolic gases and spring discharges from Galeras Volcano, Colombia

Galeras fumarole discharges have been collected since its reactivation, in 1988, through December 1995. The gases are dominated by H 2O, CO 2, S (as SO 2 and H 2S) and HCl. The relative proportions of these gases classify them as ‘magmatic’. Thermodynamic equilibrium temperatures of the gases range from 260 to > 600 °C. The relative abundance of inert gases, N 2, Ar and He, can be used as ‘tracers’ to identify the source of the fumarole discharges. At Galeras the majority of the samples have a composition characteristic of gases originating from arc-related magmas, with relatively high N 2 contents and minor He and Ar. During 1993, the year of frequent eruptions, the gas composition changed to basaltic or ‘mantle-derived’ gases, with significantly higher He contents. This is interpreted to be the result of injection of volatiles from a basaltic magma body at depth prior to and during the increased eruptive activity of 1993. The δ 13C values for CO 2 in fumarole discharges are typical of andesitic volcanoes and may indicate addition of MORB-derived CO 2. The δ 15N values for N 2 may indicate significant contribution of N 2 from marine sediments and only minor contribution of MORB-derived N 2. The δ D and δ 18O values of the discharging steam lie on a mixing trend between the isotopic composition of ‘arc-related’ magmatic water and 18O-shifted meteoric water. The most magmatic discharges have δ D values of −30 to −35‰; while the most meteoric discharges have values of −70 to −75‰, similar to Galeras thermal spring waters. Galeras thermal water discharges consist of acid sulfate and bicarbonate waters.S/Cl ratios in the acid sulfate waters are similar to fumarole ratios, suggesting direct absorption of magmatic gases into shallow ground waters. This is supported by the essentially meteoric δD and δ 18O values of the discharges and by elevated 3He/ 4He ratios of thermal spring waters. The absorption of acid S- and Cl-rich gases yield acid waters which are capable of dissolving rocks. The thermal waters, however, are far from equilibrium with Galeras lavas and pyroclastic rocks, providing evidence of the immaturity of the Galeras hydrothermal system. The SO 4 and Cl content, as well as the O and H isotopic composition of Galeras thermal springs vary with the activity of the volcano. The 7-year sampling program at Galeras revealed intriguing results concerning the activity of Galeras, its magmatic-hydrothermal system and the origin of the volatiles. Despite decreasing outlet temperatures since 1992, deep temperatures remain high, implying continued unrest in the Galeras magmatic system.

Water and fire: Vulcano island from 1977 to 1991.

The fluctuating behaviour of the volcanic system of Vulcano, which caused changes in chemical-physical properties of surface manifestations, moderate ground inflation and seismicity, is interpreted to be the result of different contributions of the deep-seated magma (fire) and the near-surface aqueous environment (water). The phenomena observed during the last fifteen years are explained by variations in these two contributions. A factor analysis method was applied to the variation of temperature and chemical composition of fumarolic gases. The results support the model. The absence of major seismic events rules out eruptive activity caused by ascending magma. Significant thermal output from the deeper portions of the system into shallow aquifers, however, caused vaporization of water and, consequently, pressure increase and inflation of the volcano. Landslides in the weakest sectors of the crater slopes can be caused by inflation. The resulting pressure decrease beneath the volcano may have triggered the emission of gases.

Geochemistry of hot springs and fumarolic gases from the Banda Arc

Geothermometry carried out on hot springs occurring on the volcanic non-active Islands of Ambon and Haruku indicates minimum subsurface temperatures of 180°C; for Alor Island we obtained a temperature of 170°C. The hydrothermal systems of these islands are likely crack and fault dominated. Hot springs on the islands with active volcanism indicate subsurface temperatures of 180 to 250°C.The chemical and isotopic compositions of fumarolic gases from the active volcanoes Banda Api in the Banda Archipelago, Serawerna on Teon, Wurlali on Damar and Ili Lewotolo on Lomblen are reported here for the first time. The measured fumarolic gas temperatures ranged from 98°C to 170°C for Banda Api, Laworkawra, Legatala, Serawerna, Wurlali and Sirung volcanoes and reached 490°C for Ili Lewotolo volcano. The main gas components are H2O, CO2 and SO2; CO2 predominates over (SO22 + H2S). The calculated gas equilibrium temperature for Wurlali is 700°C and for Ili Lewotolo more than 1000°C.Stable carbon isotope data for Wurlali (δ13C = −3.3 % 0) and for Ili Lewotolo (δ13C = −2.9 % 0) volcanoes are similar to those for other Indonesian volcanoes. The bulk sulfur is relatively rich in the heavy isotope. The 3He/4He (R/RA) ratios vary between 1.0 and 3.6, which is low for subduction related volcanoes. A contribution from continental crust or sediment-derived radiogenic 4He is suggested.

Temperature increase and chemical change of fumarolic gases at Momotombo Volcano, Nicaragua, in 1982–1985: Are these indicators of a possible eruption?

Active volcanoes of Nicaragua, including Momotombo volcano, were monitored in 1982–1985 to determine volcanic hazard and to establish criteria for prediction of eruptions. In 1982 the chemical composition and isotopic ratios of fumarolic gases were found to depend on temperature. In the products of fumarolic activity the materials of crustal origin are of great significance. Sharp variations in the gas composition depending on exogenic factors were noted in 1983; the H2S/SO2 ratio was influenced the least. The temperature of fumarolic gases at Momotombo gradually increased from 740°C in 1974 to 895°C in 1984. During the same period the contents of H2S, SO2, CO, and H2 and the values of S/Cl, S/C, H2S/SO2, H2/H2O, and CO/CO2 ratios have risen, and the isotope ratios of hydrogen, sulfur, and especially carbon have changed. Future work in the crater of Momotombo will show whether the changes in the temperature and composition of the fumarolic gases are the precursors to a forthcoming eruption.

Chemical and isotopic compositions of fumarolic gases from Kuju-Iwoyama, Kyushu, Japan.

Variations in chemical and isotopic compositions of gases discharged from Kuju-Iwoyama, a fumarolic area developed in the 1738 crater of the Hosshozan volcano in Kyushu, Japan, were investigated from 1960 to 1984. The isotopic data suggest that the fumarolic condensate collected from KH-1 fumarole (the hottest fumarole of Kuju-Iwoyama) in 1960 is likely to be almost pure magmatic water, and that those collected from other low-temperature fumaroles in the early 1960s are mixtures of the magmatic water and local groundwaters. The relationship between the temperature and composition of the fumarolic gases shows that the mixing of high-temperature gases with liquid groundwater took place near the surface in the early 1960s. In 1984, the steam samples from KH-1 fumarole were found to be 1 : 1 mixtures of the steam, which had been derived from the groundwater by total evaporation, and magmatic steam, and those from other fumaroles to be mixtures of KH-1 steam and liquid groundwater. The mixing of the magmatic steam with the goundwater-derived steam suggests the penetration of groundwaters into deeper parts of the geothermal system along with the decline of fumarolic activity of the system. The isotopic and chemical geothermometer study on the gases suggests that the deep reservoir of volcanic gases was maintained at temperatures above 600°C during this investigation, and that the addition of some gaseous species (e.g., CH4, H2S and SO2) into volcanic gases took place during the passage of the gases through the geothermal system.

Geochemical surveillance of the Solfatara of Pozzuoli (Phlegraean Fields) during 1983

Geochemical surveillance of the Phlegraean Fields area has been intensified since 1983, in response to the increased uplift rate (brady-seismic activity). Fumarolic gases from Solfatara (Pozzuoli) were sampled and analyzed monthly. A Reducing Capacity (RC) monitoring unit was installed at Soffione, the most active fumarole in the Solfatara system. The preliminary analysis of the RC temporal variations suggest they are consistent with the rate of the seismic energy release. The composition of fumarolic gases indicates that the equilibrium temperature and pressure are higher than those of sampling. The observed variations in CH4 content are explained as an increase of pressure (from 1982 to the end of 1983) of about 70% at an extimated depth of 3–3.5 km. Finally, during 1983, there were no geochemical indications of both rising magma and significant accumulation of energy at shallow depth.