Fumarole Steam Research Articles

Chemical and isotopic features of cold and thermal fluids discharged in the Southern Volcanic Zone between 32.5°S and 36°S: Insights into the physical and chemical processes controlling fluid geochemistry in geothermal systems of Central Chile

The Principal Cordillera of Central Chile is characterized by two belts of different ages and lithologies: (i) an eastern Mesozoic belt, consisting of limestone- and gypsum-rich sedimentary rocks at the border between Central Chile and Argentina, where the active volcanic arc occurs; and (ii) a western belt of Cenozoic age containing basaltic to andesitic volcanic and volcanoclastic sequences. This distinctive geological setting controls water chemistry of cold and thermal springs in the region, which are fed by meteoric water that circulates through deep regional structures. In the western sector of Principal Cordillera, water–rock interaction processes produce low TDS, slightly alkaline HCO3− dominated waters, although dissolution of underlying Mesozoic evaporitic rocks occasionally causes SO42− and Cl− enrichments. In this area, few Na+–HCO3− and Na+–SO42− waters occurred, being likely produced by a Ca2+–Na+ exchange during water–rock interactions. Differently, the chemical features of Ca2+–Cl− waters was likely related to an albitization–chloritization process affecting basaltic to andesitic rocks outcropping in this area. Addition of Na+–Cl− brines uprising from the eastern sector through the west-verging thrust faults cannot be excluded, as suggested by the occurrence of mantle He (~19%) in dissolved gases. In contrast, in the eastern sector of the study region, mainly characterized by the occurrence of evaporitic sequences and relatively high heat flow, mature Na+–Cl− waters were recognized, the latter being likely related to promising geothermal reservoirs, as supported by the chemical composition of the associated bubbling and fumarolic gases. Their relatively low 3He/4He ratios (up to 3.9 Ra) measured in the fumaroles on this area evidenced a significant crustal contamination by radiogenic 4He. The latter was likely due to (i) degassing from 4He-rich magma batches residing in the crust, and/or (ii) addition of fluids interacting with sedimentary rocks. This interpretation is consistent with the measured δ13C-CO2 values (from −13.2 to −5.72‰ vs. V-PDB) and the CO2/3He ratios (up to 14.6×1010), which suggest that CO2 mostly originates from the limestone-rich basement and recycling of subducted sediments, with an important addition of sedimentary (organic-derived) carbon, whereas mantle degassing contributes at a minor extent. According to geothermometric estimations based on the Na+, K+, Mg2+ and Ca2+ contents, the mature Na+–Cl− rich waters approached a chemical equilibrium with calcite, dolomite, anhydrite, fluorite, albite, K-feldspar and Ca- and Mg-saponites at a broad range of temperatures (up to ~300°C) In the associated gas phase, equilibria of chemical reactions characterized by slow kinetics (e.g. sabatier reaction) suggested significant contributions from hot and oxidizing magmatic gases. This hypothesis is consistent with the δ13C-CO2, Rc/Ra, CO2/3He values of the fumarolic gases. Accordingly, the isotopic signatures of the fumarolic steam is similar to that of fluids discharged from the summit craters of the two active volcanoes in the study area (Tupungatito and Planchón–Peteroa). These results encourage the development of further geochemical and geophysical surveys aimed to provide an exhaustive evaluation of the geothermal potential of these volcanic–hydrothermal systems.

Diffuse volcanic degassing and thermal energy release from Hengill volcanic system, Iceland

We report the first detailed study of spatial variations on the diffuse emission of carbon dioxide (CO2) and hydrogen sulfide (H2S) from Hengill volcanic system, Iceland. Soil CO2 and H2S efflux measurements were performed at 752 sampling sites and ranged from nondetectable to 17,666 and 722 g m−2 day−1, respectively. The soil temperature was measured at each sampling site and used to evaluate the heat flow. The chemical composition of soil gases sampled at selected sampling sites during this study shows they result from a mixing process between deep volcanic/hydrothermal component and air. Most of the diffuse CO2 degassing is observed close to areas where active thermal manifestations occur, northeast flank of the Hengill central volcano close to the Nesjavellir power plant, suggesting a diffuse degassing structure with a SSW–NNE trend, overlapping main fissure zone and indicating a structural control of the degassing process. On the other hand, H2S efflux values are in general very low or negligible along the study area, except those observed at the northeast flank of the Hengill central volcano, where anomalously high CO2 efflux and soil temperatures were also measured. The total diffuse CO2 emission estimated for this volcanic system was about 1,526 ± 160 t day−1 of which 453 t day−1 (29.7 %) are of volcanic/hydrothermal origin. To calculate the steam discharge associated with the volcanic/hydrothermal CO2 output, we used the average H2O/CO2 mass ratio from 12 fumarole samples equal to 88.6 (range, 9.4–240.2) as a representative value of the H2O/CO2 mass ratios for Hengill fumarole steam. The resulting estimate of the steam flow associated with the gas flux is equal to 40,154 t day−1. The condensation of this steam results in thermal energy release for Helgill volcanic system of 1.07 × 1014 J day−1 or to a total heat flow of 1,237 MWt.

Microbial diversity in nonsulfur, sulfur and iron geothermal steam vents

Fumaroles, commonly called steam vents, are ubiquitous features of geothermal habitats. Recent studies have discovered microorganisms in condensed fumarole steam, but fumarole deposits have proven refractory to DNA isolation. In this study, we report the development of novel DNA isolation approaches for fumarole deposit microbial community analysis. Deposit samples were collected from steam vents and caves in Hawaii Volcanoes National Park, Yellowstone National Park and Lassen Volcanic National Park. Samples were analyzed by X-ray microanalysis and classified as nonsulfur, sulfur or iron-dominated steam deposits. We experienced considerable difficulty in obtaining high-yield, high-quality DNA for cloning: only half of all the samples ultimately yielded sequences. Analysis of archaeal 16S rRNA gene sequences showed that sulfur steam deposits were dominated by Sulfolobus and Acidianus, while nonsulfur deposits contained mainly unknown Crenarchaeota. Several of these novel Crenarchaeota lineages were related to chemoautotrophic ammonia oxidizers, indicating that fumaroles represent a putative habitat for ammonia-oxidizing Archaea. We also generated archaeal and bacterial enrichment cultures from the majority of the deposits and isolated members of the Sulfolobales. Our results provide the first evidence of Archaea in geothermal steam deposits and show that fumaroles harbor diverse and novel microbial lineages.

Halophilic Archaea determined from geothermal steam vent aerosols

Hydrothermal vents, known as 'fumaroles', are ubiquitous features of geothermal areas. Although their geology has been extensively characterized, little is known about the subsurface microbial ecology of fumaroles largely because of the difficulty in collecting sufficient numbers of cells from boiling steam water for DNA extraction and culture isolation. Here we describe the first collection, molecular analysis and isolation of microbes from fumarole steam waters in Russia (Kamchatka) and the USA (Hawaii, New Mexico, California and Wyoming). Surprisingly, the steam vent waters from all the fumaroles contained halophilic Archaea closely related to the Haloarcula spp. found in non-geothermal salt mats, saline soils, brine pools and salt lakes around the world. Microscopic cell counting estimated the cell dispersal rate at approximately 1.6 x 10(9) cells year(-1) from a single fumarole. We also managed to enrich microbes in high-salt media from every vent sample, and to isolate Haloarcula from a Yellowstone vent in a 20% salt medium after a month-long incubation, demonstrating both salt tolerance and viability of cells collected from high-temperature steam. Laboratory tests determined that microbes enriched in salt media survived temperatures greater than 75 degrees C for between 5 and 30 min during the collection process. Hawaiian fumaroles proved to contain the greatest diversity of halophilic Archaea with four new lineages that may belong to uncultured haloarchaeal genera. This high diversity may have resulted from the leaching of salts and minerals through the highly porous volcanic rock, creating a chemically complex saline subsurface.

Sampling and analysis of geothermal fluids

AbstractSampling of geothermal fluids presents some problems not encountered when sampling surface and nonthermal ground waters. Specific collection techniques are required to obtain representative samples because of the elevated temperature and boiling of these fluids, the effect of exposing them to the atmosphere and cooling of the samples. Sample treatment during collection depends on the analytical method to be used. When sampling wet‐steam wells, both the liquid and the vapour fractions should be collected at the same fluid separation pressure. When sampling fumarole steam, maximum information is obtained if the total discharge is collected into a single container without separating the gas and the steam condensate fractions. Silica polymerization affects the solution pH. The only way to obtain reliable pH measurement of a water sample supersaturated with respect to amorphous silica is to measure it on site, before the onset of polymerization. This paper provides an outline of the geothermal sampling techniques and analytical methods currently in use in Iceland. Sampling of hot‐water and wet‐steam wells is described, as is sampling of hot springs, fumaroles and gas bubbling through hot‐spring waters. Detailed procedures are given for the analysis of total carbonate carbon and total sulphide sulphur in geothermal water and steam condensate samples.

Diffuse emission of CO 2 from Miyakejima volcano, Japan

Two soil gas surveys were carried out in May and September 1998 at Miyakejima volcano, in the Izu Mariana arc, Japan. CO 2 flux values for May and September surveys ranged from 0.1 to 18,150 g m −2 day −1 and from 0.1 to 9685 g m −2 day −1, respectively. Statistical graphical analysis showed three overlapping populations. The spatial distribution of these emissions correlated quite closely with the geothermal and geological characteristics of the studied area. The structure releasing higher CO 2 is the summit cone Oyama and surrounding areas, where the most obvious geothermal features occur. A total output about 100–150 t day −1 is estimated from this area. A good correlation was observed between soil CO 2 flux and soil temperature at the summit caldera indicating extensive condensation of fumarolic steam within the upper part of Miyakejima. Carbon isotopic analysis of selected samples inside the summit caldera ( δ 13C–CO 2=−0.90‰ to −5.70‰) suggests a mixing of carbon derived from marine limestone and magmatic CO 2 while a clear biogenic origin ( δ 13C–CO 2=−14.76‰ to −25.52‰) is observed for the diffuse degassing of CO 2 outside summit caldera.

Chemical and isotopic compositions of fluids at Cumbal Volcano, Colombia: evidence for magmatic contribution

We present chemical and isotopic data for fumarolic vapor and thermal spring discharges from Cumbal Volcano, SW Colombia. In 1988 Cumbal showed signs of apparent reactivation. Gases and steam condensates were sampled from summit fumaroles (83–375°C) of Cumbal in 1988–1996 and discharges from thermal springs (15–37°C) on its flanks in 1995–1996. Based on relative CO2, total S (H2S+SO2), and HCl contents, fumarolic discharges are principally magmatic in composition. Fumarolic steam condensates (1993–1996) have δ18O values of –11.4 to +2.5‰ and δD values of –91 to –43‰. δ18O and δD compositions indicate mixing between local meteoric and magmatic waters. 3He/4He ratios in 1993–1996 samples (5.3–7.9 Rcor) are consistent with addition of mantle-derived helium. δ13CCO2 values for 1996 samples (–6.7 to –5.0‰) likely indicate contribution of marine-carbonate, organic sediment, and mantle-derived CO2. δ34SStotal compositions (–4.6 to +5.6‰) of 1988–1996 fumarolic discharges have magmatic signatures and may reflect cycles of deposition and remobilization of native sulfur. Thermal waters are acid-sulfate or bicarbonate in composition. Relative concentrations of chemical constituents of thermal waters imply that the composition of waters is controlled by absorption of magmatic volatiles into shallow ground- and surface waters, dilution with meteoric waters along flow paths, and dissolution of host rocks. δ18O and δD compositions are consistent with a meteoric origin of waters. δ34SStotal values for thermal spring gas discharges (9.6–10.5‰) suggest deposition of δ34S-depleted sulfur minerals along flow paths. Chemical and isotopic compositions of 1988–1995 fumarolic discharges provide evidence for input of magmatic volatiles into the Cumbal hydrothermal system. From 1995 to 1996, geochemical data show increasing hydrothermal signatures, suggesting a decline in magmatic volatile input.

Contrasting hydrothermal activity at Sierra Negra and Alcedo volcanoes, Galapagos Archipelago, Ecuador

and approximately 85% SO2 of the total sulfur gas. Relative amounts of He, Ar, and N2 show a distinct hot-spot signature ( ). The δ13C–CO2 is approximately −3.6‰ and δ34ST is approximately +3.3‰. The δD/δ18O of fumarole H2O indicates steam separation from local meteoric waters whose estimated minimum mean residence time from 3H analyses is ≤40 years. Fumarolic activity at Alcedo is controlled by a caldera-margin fault containing at least seven hydrothermal explosion craters, and by an intracaldera rhyolite vent. Two explosion craters which formed in 1993–1994 produce approximately 15 m3/s of steam, yet discharge temperatures are ≤97°C. Water content of the total gas is 95–97 mol.%, noncondensible gas is 92–98 mol.% CO2, and sulfur gas is dominated by H2S. Relative amounts of He, Ar, and N2 show extensive mixing between hot spot and air or air-saturated meteoric water components but the average . The δ13C–CO2 is approximately −3.5‰ and δ34ST is approximately −0.8‰. The δD/δ18O of fumarole steam indicates separation from a homogeneous reservoir that is enriched 3–5‰ in 18O compared with local meteoric water. 3H indicates that this reservoir water has a maximum mean residence time of approximately 400 years and empirical gas geothermometry indicates a reservoir temperature of 260–320°C. The intracaldera hydrothermal reservoir in Alcedo is probably capable of producing up to 150 MW; however, environmental concerns as well as lack of infrastructure and power users will limit the development of this resource.

Formation environment of montmorillonite in Sumikawa landslide area on the basis of oxygen and hydrogen isotopic ratios and mineral assemblage

Oxygen and hydrogen stable isotopic ratios are measured about swelling montmorillonite occurring in Su-mikawa and Toroko landslide areas in Hatimantai geothermal area, Akita Prefecture and also Jizukiyama land-slide area, Nagano City. Formation temperatures of montmorillonite are estimated to be 50-100°, 27t and 50° in Sumikawa, Toroko and Jizukiyama areas, respectively. In Sumikawa area, isotopic ratios of water that yields the montmorillonite have feature of mixture of the present local meteoric water and fumarolic steam of Yake volcano where the Sumikawa landslide area is located on the hillside. On the other hand, in Toroko and Jizuki-yama area, the isotopic characters are far different from the expectation for the present local meteoric water, and shows intermediate one between expectation for meteoric water and sea water.From these results, formation environment of montmorillonite in Sumikawa landslide area is geothermal one, which may continue for recent a few thousand years. Toroko and Jizukiyama landslide areas might be in diage-netic condition, and the higher isotopic ratios may be attributed to admixing with sea water, meteoric water at warmer climate or evaporation effect from water pool.

Geochemical exploration of the Chipilapa geothermal field, El Salvador

Results of the geochemical exploration of the Ahuachapán-Chipilapa area are presented. The procedure for interpreting the chemical composition of very dilute thermal waters is emphasized. Three groups of thermal waters are described, two with a geothermal brine component and one resulting from steam condensation. The model for one of the groups (Type 2) leads to predictions of temperature and chloride concentration that are reasonably close to those of the geothermal liquid feeding Chipilapa well CH-7B. In particular, it was predicted correctly that the salinities in the Chipilapa area would be considerably lower than those in the Ahuachapán field.It is shown that the simultaneous modeling of the carbon dioxide concentration and isotopic composition of fumarole steam allows discrimination between primary and secondary steam. The composition of all fumarole samples is described as steam originating from a single reservoir fluid at 250°C, and composition δ 18O = −4.1, δD = −46, CO 2 = 5 × 10 −5 molar fraction.The total discharge composition of CH-7B confirms the trend observed in the Ahuachapán field of decreasing reservoir salinities towards the east. Postulating the existence of a “deep” reservoir brine in the eastern (Chipilapa) section of the system, with lower salinity but otherwise similar temperature and isotopic composition to the Ahuachapán brine, allows for the generation of relatively simple models that explain the formation of the CH-7B brine, and the three groups of thermal waters. Type 1 waters are noteworthy in the sense that they result from a ternary mixture of meteoric water, geothermal brine and high-temperature steam condensate.The possibility that the east-west trend in salinity results from a process of dilution of brine with condensate from steam separated at very high temperatures is discussed. The distribution of the different types of hydrothermal manifestations delineates a lateral discharge system, with the steam upflow zone to the south of the study area, on the northern slope of the volcanic range, and thermal water discharges several kilometers to the north. It is deemed reasonable that exploratory drilling should be directed towards the southern edge of the geothermal system, as far as topography and the indicators of probable secondary permeability permit.

Diffuse emission of CO 2 from the Fossa crater, Vulcano Island (Italy)

Numerous measurements of CO2 degassing from the soil, carried out with the accumulation chamber method, indicate that in the period April–July 1995 the upper part of the Fossa cone released a total output of 200 t d–1 of CO2, which corresponds to approximately 1000 t d–1 of steam. These large amounts of fluids are of the same order of magnitude as those released by the high temperature fumarolic field located inside the crater. The spatial distribution of soil gas fluxes shows that the main structures releasing CO2 are the inner slopes of the crater and a NW–SE line, located NE of the crater rim, which correspond to the main direction of Vulcano Island active faults. The comparison of the φCO2 maps with the soil temperature distribution, derived from both direct measurements and airborne infrared images, indicates the occurrence of extensive condensation of fumarolic steam within the upper part of the Fossa cone, whose total amount is comparable to the rainfall budget. Part of the condensate which originates from this process contributes to the recharge of the phreatic aquifer of Porto Plain, modifying the chemical and isotopic composition of the groundwater.

Gas geothermometry in selected icelandic geothermal fields with comparative examples from Kenya

Using data from selected geothermal fields in Iceland, several gas geothermometers are applied to calculate reservoir temperatures. Results for well fluids are compared to estimated inflow temperatures. The effects of gases from other sources, e.g. magmatic fluid entering reservoirs, condensation/boiling, loss of components and mixing of different fluids during upflow, are discussed. Those geothermometers that give results reasonably close to the inflow temperatures, and are not constrained by the need to know thermodynamic parameters, are used to estimate subsurface temperatures from fumarole steam composition. It is suggested that geothermometers based on H 2 and H 2S concentrations are relatively effective. The CO 2, H 2S and H 2 geothermometers are also evaluated by thermodynamic simulation using plausible mineral buffers. The CO 2 geothermometer generally gives slightly low temperatures for values in the range 100–200°C. The calibration of the existing H 2S and H 2 geothermometers needs improvement when reservoir temperatures are higher than 220°C. The mineral buffer controlling H 2S in reservoirs seems to be pyrite + magnetite + epidote + prehnite changing into pyrite + pyrrhotite + epidote + prehnite with increasing temperature.

Isotopic variation of the hydrothermal system on Vulcano Island, Italy

The isotopic and chemical composition of thermal waters and the isotopic composition of high-temperature fumarolic steam at Vulcano Island indicate an evolution of the hydrothermal system from its magmatic origin through interaction with rocks and mixing with meteoric water. Seawater is not a component in this system. The isotopic values of the fumaroles indicate a local magmatic composition of +7 to +8%. δ 18 O and −5 to +15%. δD. The high δD values of this vapor may reflect contamination of the magma by connate water. The isotopic and chemical composition of water from well W2 indicates the existence of geothermal fluid formed by mixing of HCl and SO 2-bearing condensates of volcanic origin with groundwater, with subsequent evolution via water-rock interaction. The estimated isotopic composition of the geothermal water is δ 18 O = +1%. and δD = −5%. Local shallow seismic events result in a temporary increase in fracture permeability, enabling the mixing of magmatic vapor with groundwater or geothermal water. Tectonic activity associated with deep structures appears to increase the contribution of magmatic vapor to the fumaroles.

The volcanic and magmatic evolution of Volcán Ollagüe, a high-K, late quaternary stratovolcano in the Andean Central Volcanic Zone

Volcán Ollagüe is a high-K, calc-alkaline composite volcano constructed upon extremely thick crust in the Andean Central Volcanic Zone. Volcanic activity commenced with the construction of an andesitic to dacitic composite cone composed of numerous lava flows and pyroclastic deposits of the Vinta Loma series and an overlying coalescing dome and coulée sequence of the Chasca Orkho series. Following cone construction, the upper western flank of Ollagüe collapsed toward the west leaving a collapse-amphitheater about 3.5 km in diameter and a debris avalanche deposit on the lower western flank of the volcano. The deposit is similar to the debris avalanche deposit produced during the May 18, 1980 eruption of Mount St. Helens, U.S.A., and was probably formed in a similar manner. It presently covers an area of 100 km 2 and extends 16 km from the summit. Subsequent to the collapse event, the upper western flank was reformed via eruption of several small andesitic lava flows from vents located near the western summit and growth of an andesitic dome within the collapse-amphitheater. Additional post-collapse activity included construction of a dacitic dome and coulée of the La Celosa series on the northwest flank. Field relations indicate that vents for the Vinta Loma and post-collapse series were located at or near the summit of the cone. The Vinta Loma series is characterized by an anhydrous, two-pyroxene assemblage. Vents for the La Celosa and Chasca Orkho series are located on the flanks and strike N55 W, radial to the volcano. The pattern of flank eruptions coincides with the distribution in the abundance of amphibole and biotite as the main mafic phenocryst phases in the rocks. A possible explanation for this coincidence is that an unexposed fracture or fault beneath the volcano served as a conduit for both magma ascent and groundwater circulation. In addition to the lava flows at Ollagüe, magmas are also present as blobs of vesiculated basaltic andesite and mafic andesite that occur as inclusions in nearly all of the lavas. All eruptive activity at Ollagüe predates the last glacial episode (∼ 11.000 a B.P.), because post-collapse lava flows are overlain by moraine and are incised by glacial valleys. Present activity is restricted to emission of a persistent, 100-m-high fumarolic steam plume from a vent located within the summit andesite dome. Sr and Nd isotope ratios for the basaltic andesite and mafic andesite inclusions and lavas suggest that they have assimilated large amounts of crust during crystal fractionation. In contrast, narrow ranges in 143Nd/ 144Nd and 87Sr/ 86Sr in the andesitic and dacitic lavas are enigmatic with respect to crustal contamination.

Isotopic composition of natural thermal discharges on Vulcano island, southern Italy

This paper reviews the isotopic 18 O 16 O and D/H ratios reported in the literature for phreatic waters in the plain of Vulcano island off Sicily and condensates from high-temperature fumaroles at the top of the summit crater. The Vulcano phreatic waters have a rather high average annual temperature (greater than 30|dgC), but do not seem to be heated directly by mixture with condensates of fumarolic steam. They seem to originate from rainwater and to undergo high evaporation, particularly in summer, owing to high conductive heat flow in the island. By comparing the 18 O 16 O and D/H ratios with Cl − (as HCl) content, it is estimated that 75% of the steam from the high-temperature fumaroles comes from seawater and 25% from low-salinity waters. The latter, however, do not seem to come directly from the shallow aquifer of Vulcano because the steam from the fumaroles does not contain radiogenic tritium. This mixing model and the lack of steam condensation in the Vulcano aquifer implies, indirectly, that there are no steam losses during the rise of deep fluids, and that some observed variations in the outlet temperatures without accompanying variations in the composition of the associated gas phase may be due to increased (or decreased) extent of fracturation in the volcanic edifice (as a result, for instance, of tectonic movements), which cause more (or less) vaporization of low-salinity fresh water into the volcanic ducts as well as faster (or slower) outflow of deep gases.

Volcanic steam output directly measured in fumaroles: the observed variations at Vulcano Island, Italy, between 1983 and 1987

The mass of steam emitted by the fumaroles of the crater of Vulcano (Aeolian Islands, Italy) was evaluated by using a direct measuring method. Measurements were effected with this method between 1983 and 1987 in nine field campaigns, the results of which are given in this paper. The steam flux measurements were performed by an instrument expressly designed for steam condensation. The estimates of the thermal energy transported daily by the steam gave values of up to 10E12 J. The sharp variations in the total mass outputs recorded in 1984 and 1985 are here interpreted as being a consequence of the probable accumulation of fluid pressure at depth, during the evolution of volcanic activity. The different results obtained from the various areas of the fumarolic field during the evolutionary phases have led to the hypothesis that different subsystems exist that feed the fumaroles, and that their activity is triggered by volcano-tectonics. This method can be applied in the evaluation of both fumarolic steam and thermal energy outputs, and therefore it is particularly useful in geothermal exploration, in the evaluation of the environmental impact of volcanic degassing, and in the forecasting of volcanic eruptions. The evaluation of the steam output is also useful for quantifying the release of other gaseous species and giving univocal interpretations regarding their variations in concentrations. Furthermore this direct method provides a new way of calibrating the flux estimates effected by other telemetric techniques (Lidar, Cospec, etc.).

Geothermal assessment of the island of ischia (southern Italy) from isotopic and chemical composition of the delivered fluids

The Ischia geothermal system is hosted by silicic rocks of the Quaternary Potassic Roman Province, in southern Italy. Exploration drilling down to 1156 m depth in the mid-1950s provided information on boiling profiles (up to 250°C) and on the depth and permeability of the potential reservoirs. Discharge fluid samples were collected and analyzed to define the inflow of surrounding seawater (C1 ranges from 2.5 to 20 g/kg) into the system. Analyses of samples from surface manifestations and shallow wells collected during 1983 and 1988 point to the existence of three distinct mixing regimes, involving three water components. A dishomogeneous body of diluted water (Cl less than 2.5 g/kg), that occurs at depths > 700 m and reequilibrates at 240°C at least, is overlain by an aquifer of groundwater variably mixed with variably seawater (Cl from 4 to 10 g/kg), which tends to reequilibrate at 160°C. Steam-heated waters locally develop and act as dilutants of the rising geothermal fluids. Dilution, mixing, and evaporation of the ascending chloride fluids are supported by oxygen and hydrogen isotopic data the thermal waters being enriched in 18O and D with respect to local meteoric water by up to 7 and 30‰, respectively. The relative composition of the major cations in thermal solutions was used to discriminate the two main groups of thermal waters, the reservoir temperatures of which are estimated from the Na/K-gethermometer. K-Mg geothermometer indicates reequilibration in near-surface conditions. The isotopic composition of the fumarolic steam varies from −7 to −12‰ in ∂ 8O and from − 35 to − 70‰ in ∂D, in agreement with a deep mixed fluid that boils adiabatically from 240 to 80°C. The deuterium content of the H 2O-H 2 pair gives enrichment factor of about 830‰, corresponding to equilibrium temperature conditions slightly higher than the surface boiling temperatures. The ∂ 13C of CO 2is almost constant at −4.5‰ (1 δ=0.4), suggesting an important magmatic contribution, and the ∂ 18O values of CO 2appears to in equilibrium with accompanying steam at the measured temperatures. The CO 2/Ar and H 2/Ar chemical ratios have been used to derive aquifer temperatures, the values obtained being consistent with those of solute geothermometers.

Isotope composition of rain water, well water and fumarole steam on the island of Vulcano, and their implications for volcanic surveillance

Abstract Water samples from eight wells in the Vulcano Porto area and fumarole steam from the crater of Vulcano (Southern Italy) were collected at monthly intervals over a two-year period from February 1987 to January 1989. Samples of bulk precipitation were also collected in three points at Vulcano Porto and Vulcano Piano (island of Vulcano) and Castroreale (Tyrrhenian coast of Sicily), again at monthly intervals. The weighted mean isotopic composition of rain water on Vulcano is δ D =−32‰ , and δ D 18 O =−6.4‰ . Results for well waters tend to support the existence of at least two water bodies: one of purely meteoric water and the other of brackish thermal water. An intermediate water body, probably the result of mixing of the other two, was also recognized. The isotope composition of fumarolic steam from the crater suggests the mixed contribution of both sea and rain waters, although mixing is not the only process occurring. The existence of a deep marine-brine body is inferred, characterized by δD and δ18O values more positive than those of Mediterranean sea water. A close relationship, indicating common source effects, is shown by temporal isotopic variations at some wells and the fumarolic steam, wich may be used for volcano survey at distance from the active crater.

Fluid-volcano interaction in an active stratovolcano: the crater lake system of Poás volcano, Costa Rica

Seismic and geochemical data collected at Poás volcano, Costa Rica, since 1978 suggest that temperature and chemical variations recorded in subaerial fumaroles and the crater lake are related to episodic release of heat and volatiles associated with hydrofracturing of the upper margin of the shallow magma body. Power outputs associated with these events approach 600 MW and are superimposed on a baseline energy flux of approximately 200 MW. The baseline heat flux suggests a magma solidification rate of 0.012 km3/yr and background volatile release rates of 1000 t/d H2O, 66 t/d, S, 13 t/d Cl, and 0.5 t/d F. These fluxes are comparable to fluxes of F, Cl and S exiting the magmatic/hydrothermal system through acidic flank springs and are about a factor of five less than estimated volatile fluxes through summit fumaroles during the high-temperature event of 1981–1983. Mass balance considerations suggest that heat released at the lake surface is primarily supplied by the ascent of heated brines supplemented by condensation of fumarolic steam. Calculated average seepage rates out of the lake (≈ 450 kg/s) indicate rapid convection of acidic lake brine through underlying lake sediments and pyroclastic deposits. Circulation of the extremely corrosive lake brine through the volcanoelastic material beneath the lake may enhance subsurface permeabilities. Fluxes of rock-forming elements observed in the Rio Agrio basin on the northwest flank of Poás suggest that dissolution and removal of volcano-elastic material occurs at a rate of approximately 1650 m3/yr. However, estimated sulfur fluxes from the cooling magma body suggest that porosity created by this dissolution could quickly be filled by the deposition of native sulfur. Finally,analysis of heat and water budgets for the crater lake over the period 1978–1989 indicates that the recent decline and disappearance of the lake was caused by variations in summit rainfall combined with an increase in subsurface heat flow to the lake in 1987–1988. The increase in heat flow is related to continued degassing of high-temperature volatiles following a hydrofracturing/magma ascent episode that occurred in 1986.

Indirect detection of subsurface outflow from a rift valley lake

Naivasha, highest of the Kenya (Gregory) Rift Valley lakes, has no surface outlet. However, unlike other Rift lakes it has not become saline despite high potential evaporation rates, which indicates that there must be some subsurface drainage. The fate of this outflow has been the subject of speculation for many years, especially during the general decline in lake water level during the 1980's. Particularly to the south of the lake, there are few opportunities to obtain information from direct groundwater sampling. However, the stable isotopic composition of fumarole steam from late Quaternary volcanic centres in the area has been used to infer groundwater composition. Using a simple mixing model between Rift-flank groundwater and highly-evaporated lakewater, this has enabled subsurface water flow to be contoured by its lakewater content. By this method, outflow can still be detected some 30 km to the south of the lake. Stable isotope data also confirm that much of the steam used by the local Olkaria geothermal power station is derived from lakewater, though simple balance considerations show that steam use cannot alone be responsible for the fall in lake level observed during the 1980's.