The crater lake and hydrothermal system of Mount Pinatubo,Philippines: evolution in the decade after eruption

The June 1991 eruption of Mount Pinatubo, Philippines breached a significant, pre-eruptive magmatic-hydrothermal system consisting of a hot (>300 °C) core at two-phase conditions and surrounding, cooler (<260 °C) liquid outflows to the N and S. The eruption created a large, closed crater that accumulated hydrothermal upwellings, near-surface aquifer and meteoric inflows. A shallow lake formed by early September 1991, and showed a long-term increase in level of ~1 m/month until an artificial drainage was created in September 2001. Comparison of the temporal trends in lake chemistry to pre- and post-eruptive springs distinguishes processes important in lake evolution. The lake was initially near-neutral pH and dominated by meteoric influx and Cl–SO₄ and Cl–HCO₃ hydrothermal waters, with peaks in SO₄ and Ca concentrations resulting from leaching of anhydrite and aerosol-laden tephra. Magmatic discharge, acidity (pH~2) and rock dissolution peaked in late 1992, during and immediately after eruption of a lava dome on the crater floor. Since cessation of dome growth, trends in lake pH (increase from 3 to 5.5), temperature (decline from 40 to 26 °C), and chemical and isotopic composition indicate that magmatic degassing and rock dissolution have declined significantly relative to the input of meteoric water and immature hydrothermal brine. Higher concentrations of Cl, Na, K, Li and B, and lower concentrations of Mg, Ca, Fe, SO₄ and F up to 1999 highlight the importance of a dilute hydrothermal contribution, as do stable-isotope and tritium compositions of the various fluids. However, samples taken since that time indicate further dilution and steeper trends of increasing pH and declining temperature. Present gas and brine compositions from crater fumaroles and hot springs indicate boiling of an immature Cl–SO₄ geothermal fluid of near-neutral pH at approximately 200 °C, rather than direct discharge from magma. It appears that remnants of the pre-eruptive hydrothermal system invaded the magma conduit shortly after the end of dome emplacement, blocking the direct degassing path. This, along with the large catchment area (~5 km²) and the high precipitation rate of the area, led to a rapid transition from a small and hot acid lake to a large lake with near-ambient temperature and pH. This behavior contrasts with that of peak-activity lakes that have more sustained volcanic gas influx (e.g., Kawah Ijen, Indonesia; Poas and Rincón de la Vieja, Costa Rica).Editorial responsibility: H. Shinohara     

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⁻, Ca²⁺ and SO₄²⁻ contents, experienced a dramatic change in 1998–1999, turning from a Na⁺–Cl⁻- to a Ca²⁺–SO₄²⁻-rich composition. In June 2000, a relatively sharp increase in Na⁺ and Cl⁻ contents was observed. At the same time, SO₂/H₂S ratios and H₂ 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.     

Using Crater Lake chemistry to predict volcanic activity at Poás Volcano,Costa Rica

Monitoring of crater lake chemistry during the recent decline and disappearance of the crater lake of Poás Volcano revealed that large variations in SO₄/Cl, F/Cl, and Mg/Cl ratios were caused by the enhanced release of HCl vapor from the lake surface due to increasing lake temperature and solution acidity. Variation in the concentration of polythionic acids (H₂S_xO₆, x=4–6) was the most reliable predictor of renewed phreatic eruptive activity at the volcano, exhibiting sharp decreases three months prior to the initiation of phreatic eruptions in June 1987. Polythionic acids may offer a direct indicator of changing subsurface magmatic activity whereas chloride-based element ratios may be influenced by surface volatilization of HCl and subsequent recycling of acidic fluids in crater lake volcanoes.     

Sulfur isotopic effects in the disproportionation reaction of sulfur dioxide in hydrothermal fluids: implications for the δS variations of dissolved bisulfate and elemental sulfur from active crater lakes

Sulfur isotope effects during the SO₂ disproportionation reaction to form elemental sulfur (3SO₂+3H₂O→2HSO₄⁻+S+2H⁺) at 200–330°C and saturated water vapor pressures were experimentally determined. Initially, a large kinetic isotopic fractionation takes place between HSO₄⁻ and S, followed by a slow approach to equilibrium. The equilibrium fractionation factors, estimated from the longest run results, are expressed by 1000 ln α_HSO4⁻–S=6.21×10⁶/T²+3.62. The rates at which the initial kinetic fractionation factors approach the equilibrium ones were evaluated at the experimental conditions.δ³⁴S values of HSO₄⁻ and elemental sulfur were examined for active crater lakes including Noboribetsu and Niseko, (Hokkaido, Japan), Khloridnoe, Bannoe and Maly Semiachik (Kamchatka), Poás (Costa Rica), Ruapehu (New Zealand) and Kawah Ijen and Keli Mutu (Indonesia). Δ_HSO4⁻–S values are 28‰ for Keli Mutu, 26‰ for Kawah Ijen, 24‰ for Ruapehu, 23‰ for Poás, 22‰ for Maly Semiachik, 21‰ for Yugama, 13‰ for Bannoe, 9‰ for Niseko, 4‰ for Khloridonoe, and 0‰ for Noboribetsu, in the decreasing order. The SO₂ disproportionation reaction in the magmatic hydrothermal system below crater lakes where magmatic gases condense is responsible for high Δ_HSO4⁻–S values, whereas contribution of HSO₄⁻ produced through bacterial oxidation of reduced sulfur becomes progressively dominant for lakes with lower Δ_HSO4⁻–S values. Currently, Noboribetsu crater lake contains no HSO₄⁻ of magmatic origin. A 40-year period observation of δ³⁴S_HSO4⁻ and δ³⁴S_S values at Yugama indicated that the isotopic variations reflect changes in the supply rate of SO₂ to the magmatic hydrothermal system. This implies a possibility of volcano monitoring by continuous observation of δ³⁴S_HSO4⁻ values. The δ¹⁸O values of HSO₄⁻ and lake water from the studied lakes covary, indicating oxygen isotopic equilibration between them. The covariance gives strong evidence that lake water circulates through the sublimnic zone at temperatures of 140±30°C.     

Subaqueous geothermal activity revealed by lacustrine sediments of the acidic Nakadake crater lake, Aso Volcano, Japan

Lacustrine sediments were sampled from the inaccessible acidic (pH = 0.43) Nakadake crater lake of Aso Volcano, Japan by a simple method. The sediments contain an extremely high content (74 wt.%) of sulfur, which exits as elemental sulfur, gypsum and anhydrite. The abundant elemental sulfur is likely formed by the reaction of SO₂ and H₂S gases and by the SO₂ disproportionation reaction in magmatic hydrothermal system below the crater lake. Based on the sulfur content of sediments and measurements of elevation change of the crater bottom, the sulfur accumulation rate at the Nakadake crater lake was calculated as 250 tonne/day, which is comparable with the SO₂ emission rate (200–600 tonne/day) from the Nakadake crater. The sediments include a small amount (9%) of clear glass shards that are apparently not altered in spite of the high reactivity of hyperacid lake water. This finding suggests that the clear glass shards are fragments of recently emitted magmas from fumaroles on the bottom of the crater lake and the magma emissions continuously occur even in quiescent periods.     

Estimating thermal inflow to El Chichón crater lake using the energy-budget,chemical and isotope balance approaches

El Chichón crater lake appeared immediately after the 1982 catastrophic eruption in a newly formed, 1-km wide, explosive crater. During the first 2 years after the eruption the lake transformed from hot and ultra-acidic caused by dissolution of magmatic gases, to a warm and less acidic lake due to a rapid “magmatic-to-hydrothermal transition” — input of hydrothermal fluids and oxidation of H₂S to sulfate. Chemical composition of the lake water and other thermal fluids discharging in the crater, stable isotope composition (δD and δ¹⁸O) of lake water, gas condensates and thermal waters collected in 1995–2006 were used for the mass-balance calculations (Cl, SO₄ and isotopic composition) of the thermal flux from the crater floor. The calculated fluxes of thermal fluid by different mass-balance approaches become of the same order of magnitude as those derived from the energy-budget model if values of 1.9 and 2 mmol/mol are taken for the catchment coefficient and the average H₂S concentration in the hydrothermal vapors, respectively. The total heat power from the crater is estimated to be between 35 and 60 MW and the CO₂ flux is not higher than 150 t/day or ~ 200 gm^− 2 day^− 1.     

Geochemistry of the acid Kawah Putih lake, Patuha Volcano, West Java, Indonesia

Kawah Putih is a summit crater of Patuha volcano, West Java, Indonesia, which contains a shallow, 300 m-wide lake with strongly mineralized acid–sulfate–chloride water. The lake water has a temperature of 26–34°C, pH=<0.5–1.3, S^tot=2500–4600 ppm and Cl=5300–12 600 ppm, and floating sulfur globules with sulfide inclusions are common. Sulfur oxyanion concentrations are unusually high, with S₄O₆²⁻+S₅O₆²⁻+S₆O₆²⁻=2400 – 4200 ppm. Subaerial fumaroles (<93°C) on the lake shore have low molar SO₂/H₂S ratios (<2), which is a favorable condition to produce the observed distribution of sulfur oxyanion species. Sulfur isotope data of dissolved sulfate and native sulfur show a significant ³⁴S fractionation (ΔSO₄–S_e of 20‰), probably the result of SO₂ disproportionation in or below the lake. The lake waters show strong enrichments in ¹⁸O and D relative to local meteoric waters, a result of the combined effects of mixing between isotopically heavy fluids of deep origin and meteoric water, and evaporation-induced fractionation at the lake surface. The stable-isotope systematics combined with energy-balance considerations support very rapid fluid cycling through the lake system. Lake levels and element concentrations show strong seasonal fluctuations, indicative of a short water residence time in the lake as well.Thermodynamic modeling of the lake fluids indicates that the lake water is saturated with silica phases, barite, pyrite and various Pb, Sb, Cu, As, Bi-bearing sulfides when sulfur saturation is assumed. Precipitating phases predicted by the model calculations are consistent with the bulk chemistry of the sulfur-rich bottom sediments and their identified mineral phases. Much of the lake water chemistry can be explained by congruent rock dissolution in combination with preferential enrichments from entering fumarolic gases or brines and element removal by precipitating mineral phases, as indicated by a comparison of the fluids, volcanic rocks and lake bed sediment.Flank springs on the mountain at different elevations vary in composition, and are consistent with local rock dissolution as a dominant factor and pH-dependent element mobility. Discharges of warm sulfate- and chloride-rich water at the highest elevation and a near-neutral spring at lower level may contain a small contribution of crater-lake water. The acid fluid-induced processes at Patuha have led to the accumulation of elements that are commonly associated with volcano-hosted epithermal ore deposits. The dispersal of heavy metals and other potentially toxic elements from the volcano via the local drainage system is a matter of serious environmental concern.     

Temporal geochemical variations in volatile emissions from Mount St. Helens, USA, 1980–1994

Fumarole discharges (95–560°C) collected from the dacite dome inside Mount St. Helens crater show temporal changes in their isotopic and chemical compositions. A δD vs. δ¹⁸O plot shows that condensed waters from the gases are mixtures of meteoric and magmatic components, but that the apparent magmatic end-member in 1994 was depleted by about 7‰ in δD relative to the apparent end-member in 1980. Based on δD modeling, approximately 63% of shallow, post-1980 magma has yet to degas. Surprisingly, Cl and F contents in the 1994 samples were only 0.47 and 3.8%, respectively, of the concentrations determined for end-member magmatic fluid in 1980. The data indicate that Cl (and F and B) is degassed from magma relatively quickly compared to water and/or that most of the Cl degassed in later years is dissolved into the shallow Mount St. Helens hydrothermal system. Because metals are often transported in magmatic and hydrothermal fluids as Cl complexes, rapid changes in surface volatile compositions may have implications for the timing and location of metals transport and deposition in some volcanoes.     

Guagua pichincha volcano, Ecuador: fluid geochemistry in volcanic surveillance

The densely populated metropolitan area of Quito is located on the slopes of the active Guagua Pichincha volcano at only 10 km from the crater. Recently, the Italian Ministry of Foreign Affairs sponsored a project for the mitigation of volcanic hazard in this area. The geochemical study carried out as part of this project was aimed at constructing a geochemical model of the zone for use in volcanic surveillance.According to this geochemical model, a hydrothermal aquifer (T = 200–240°C), fed both by meteoric waters and by fluids released by a magma body, lies at shallow levels beneath Guagua Pichincha crater. The crater fumaroles are essentially fed by steam boiled off from the hydrothermal aquifer. The high flow rate fumaroles located in the dome area show significant SO₂ contents, which suggest a relatively high contribution of magmatic fluids in the zone of the aquifer feeding them. The absence of SO₂ in the fumarolic discharges near the southern crater wall indicates instead that the magmatic fluids dissolve entirely into the aquifer here. The hot springs located at the western end of the crater represent the lateral discharge of the hydrothermal aquifer.On the basis of this model, it is likely that an increment in the flux of both the magmatic fluids and the heat from a magma body produces an increase, albeit small, of the pressure-temperature conditions of the hydrothermal system and consequent changes in flow rate and fluid chemistry of the fumarolic vents. In particular, total sulphur and possibly hydrochloric acid may increase in all the vents and sulphur dioxide may appear in other fumarolic discharges. The varying thermodynamic conditions in the hydrothermal aquifer can be evaluated on the basis of the equilibria among carbon species and hydrogen. Only minor delayed changes are expected in the physical-chemical characteristics of the springs located at the western end of the crater.     

The chemical and hydrologic structure of Poa´s Volcano, Costa Rica

Comparison of the chemical characteristics of spring and river water draining the flanks of Poa´s Volcano, Costa Rica indicates that acid chloride sulfate springs of the northwestern flank of the volcano are derived by leakage and mixing of acid brines formed in the summit hydrothermal system with dilute flank groundwater. Acid chloride sulfate waters of the Rio Agrio drainage basin on the northwestern flank are the only waters on Poa´s that are affected by leakage of acid brines from the summit hydrothermal system. Acid sulfate waters found on the northwestern flank are produced by the interaction of surface and shallow groundwater with dry and wet acid deposition of SO₂ and H₂SO₄ aerosols, respectively. The acid deposition is caused by a plume of acid gases that is released by a shallow magma body located beneath the active crater of Poa´s.No evidence for a deep reservoir of neutral pH sodium chloride brine is found at Poa´s. The lack of discharge of sodium chloride waters at Poa´s is attributed to two factors: (1) the presence of a relatively volatile-rich magma body degassing at shallow depths (< 1 km) into a high level summit groundwater system; and (2) the hydrologic structure of the volcano in which high rates of recharge combine with rapid lateral flow of shallow groundwater to prevent deep-seated sodium chloride fluids from ascending to the surface. The shallow depth of the volatile-rich magma results in the degassing of large quantities of SO₂ and HCl. These gases are readily hydrolyzed and quickly mix with meteoric water to form a reservoir of acid chloride-sulfate brine in the summit hydrothermal system. High recharge rates and steep hydraulic gradients associated with elevated topographic features of the summit region promote lateral flow of acid brines generated in the summit hydrothermal system. However, the same high recharge rates and steep hydraulic gradients prevent lateral flow of deep-seated fluids, thereby masking the presence of any sodium chloride brines that may exist in deeper parts of the volcanic edifice.Structural, stratigraphic, and topographic features of Poa´s Volcano are critical in restricting flow of acid brines to the northwestern flank of the volcano. A permeable lava-lahar sequence that outcrops in the Rio Agrio drainage basin forms a hydraulic conduit between the crater lake and acid chloride sulfate springs. Spring water residence times are estimated from tritium data and indicate that flow of acid brines from the active crater to the Rio Agrio source springs is relatively rapid (3 to 17 years). Hydraulic conductivity values of the lava-lahar sequence calculated from residence time estimates range from 10⁻⁵ to 10⁻⁷ m/s. These values are consistent with hydraulic conductivity values determined by aquifer tests of fractured and porous lava/pyroclastic sequences at the base of the northwestern flank of the volcano.Fluxes of dissolved rock-forming elements in Rio Agrio indicate that approximately 4300 and 1650 m³ of rock are removed annually from the northwest flank aquifer and the active crater hydrothermal system, respectively. Over the lifetime of the hydrothermal system (100's to 1000's of years), significant increases in aquifer porosity and permeability should occur, in marked contrast to the reduction in permeability that often accompanies hydrothermal alteration in less acidic systems. Average fluxes of fluoride, chloride and sulfur calculated from discharge and compositional data collected in the Rio Agrio drainage basin over the period 1988–1990 are approximately 2, 38 and 30 metric tons/day. These fluxes should be representative of minimum volatile release rates at Poa´s in the last 10 to 20 years.     

Downstream composition changes of acidic volcanic waters discharged into the Banyupahit stream, Ijen caldera, Indonesia

The crater lake of Kawah Ijen volcano contains extremely low pH (<0.4) waters with high SO₄ (70000 mg/kg), Cl (21000 mg/kg), F (1500 mg/kg), Al (5000 mg/kg), Fe (2000 mg/kg) and trace metal (Cu 0.5, Zn 4, Pb 3 mg/kg) contents. These brines seep outward through the western crater rim and reappear on the other side as streamlets, which form the headwaters of the Banyupahit stream. The Banyupahit first mixes with fresh rivers and thermal springs in the Ijen caldera and then irrigates a coastal agricultural plain which is 30 km from the summit crater.We discuss the downstream composition changes affecting the Banyupahit waters by using stable isotope, chemical and mineralogical data collected from sites along the stream length. The saturation of the stream waters with respect to minerals was evaluated with SOLVEQ and WATEQ4F and compared with the geochemical observations. An aluminous mineralogy (alunogen, pickeringite, tamarugite and kalinite) develops in the upper part of the Banyupahit due to concentration of the headwaters by evaporation. Downstream attenuation of dissolved element concentrations results principally from dilution and from mineral precipitation. The stream pH changes from 0 at the source to >4 close to the mouth. The δD and δ¹⁸O values and the relative SO₄–Cl–F contents of the Banyupahit waters indicate that the tributaries are mostly meteoric. Dissolved SO₄ in the acidic stream come only from the crater lake seepages and are not involved later in microbially mediated reactions, as shown by their δ³⁴S and δ¹⁸O values. Re-equilibration of the stream SO₄ oxygen-isotope composition with H₂O from tributaries does not occur.Calcium, SiO₂, Al, Fe, K and SO₄ behave non-conservatively in the stream waters. Gypsum, silica (amorphous or poorly ordered), a basic aluminum hydroxysulfate (basaluminite?), K-jarosite and amorphous ferric hydroxide may exert a solubility control on these elements along the entire stream length, or in certain stream sections, consistent with the thermochemical model results. Downstream concentration trends and mineral saturation levels suggest that precipitation of Sr-, Pb-rich barite and celestite consume Ba, Sr and Pb, whereas dissolved Cu, Pb and Zn may adsorb onto solid particles, especially after the junctions of the acidic stream with non-acidic rivers. We calculated that significant fluxes of SO₄, F, Cl, Al, SiO₂, Ti, Mn and Cu may reach the irrigation system, possibly causing serious environmental impacts such as soil acidification and induration.     

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 fO₂ 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 H₂S and SO₂. 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.     

Fumaroles in ice caves on the summit of Mount Rainier—preliminary stable isotope, gas, and geochemical studies

The edifice of Mount Rainier, an active stratovolcano, has episodically collapsed leading to major debris flows. The largest debris flows are related to argillically altered rock which leave areas of the edifice prone to failure. The argillic alteration results from the neutralization of acidic magmatic gases that condense in a meteoric water hydrothermal system fed by the melting of a thick mantle of glacial ice. Two craters atop a 2000-year-old cone on the summit of the volcano contain the world's largest volcanic ice-cave system. In the spring of 1997 two active fumaroles (T=62°C) in the caves were sampled for stable isotopic, gas, and geochemical studies.Stable isotope data on fumarole condensates show significant excess deuterium with calculated δD and δ¹⁸O values (−234 and −33.2‰, respectively) for the vapor that are consistent with an origin as secondary steam from a shallow water table which has been heated by underlying magmatic–hydrothermal steam. Between 1982 and 1997, δD of the fumarole vapor may have decreased by 30‰.The compositions of fumarole gases vary in time and space but typically consist of air components slightly modified by their solubilities in water and additions of CO₂ and CH₄. The elevated CO₂ contents (δ¹³C_CO2=−11.8±0.7‰), with spikes of over 10,000 ppm, require the episodic addition of magmatic components into the underlying hydrothermal system. Although only traces of H₂S were detected in the fumaroles, most notably in a sample which had an air δ¹³C_CO2 signature (−8.8‰), incrustations around a dormant vent containing small amounts of acid sulfate minerals (natroalunite, minamiite, and woodhouseite) indicate higher H₂S (or possibly SO₂) concentrations in past fumarolic gases.Condensate samples from fumaroles are very dilute, slightly acidic, and enriched in elements observed in the much higher temperature fumaroles at Mount St. Helens (K and Na up to the ppm level; metals such as Al, Pb, Zn Fe and Mn up to the ppb level and volatiles such as Cl, S, and F up to the ppb level).The data indicate that the hydrothermal system in the edifice at Mount Rainier consists of meteoric water reservoirs, which receive gas and steam from an underlying magmatic system. At present the magmatic system is largely flooded by the meteoric water system. However, magmatic components have episodically vented at the surface as witnessed by the mineralogy of incrustations around inactive vents and gas compositions in the active fumaroles. The composition of fumarole gases during magmatic degassing is distinct and, if sustained, could be lethal. The extent to which hydrothermal alteration is currently occurring at depth, and its possible influence on future edifice collapse, may be determined with the aid of on site analyses of fumarole gases and seismic monitoring in the ice caves.     

Chemical characteristics of the crater lakes of Popocatetetl, El Chichon, and Nevado de Toluca volcanoes, Mexico

Three crater lakes from Mexican volcanoes were sampled and analyzed at various dates to determine their chemical characteristics. Strong differences were observed in the chemistry among the three lakes: Nevado de Toluca, considered as dormant, El Chichón at a post-eruptive stage, and Popocatépetl at a pre-eruptive stage. Not surprisingly, no influence of volcanic activity was found at the Nevado de Toluca volcano, while the other volcanoes showed a correlation between the changing level of activity and the evolution of chemical trends. Low pHs (<3.0) were measured in the water from the active volcanoes, while a pH of 5.6 was measured at the Nevado de Toluca Sun lake. Changes with time were observed at Popocatépetl and El Chichón. Concentrations of volcanic-gas derived species like Cl⁻, SO₄²⁻ and F⁻ decreased irregularly at El Chichón from 1983 until 1997. Major cations concentrations also diminished at El Chichón. A 100% increase in the SO₄²⁻ content was measured at Popocatépetl between 1985 and 1994. An increase in the Mg/Cl ratio between 1992 (Mg/Cl=0.085) and 1994 (Mg/Cl=0.177) was observed at Popocatépetl, before the disappearance of the crater lake in 1994. It is concluded that chemical analysis of crater lakes may provide a useful additional tool for active-volcano monitoring.     

Water chemistry of Lake Quilotoa (Ecuador) and assessment of natural hazards

A geochemical survey carried out in November 1993 revealed that Lake Quilotoa was composed by a thin (14 m) oxic epilimnion overlying a 200 m-thick anoxic hypolimnion. Dissolved CO₂ concentrations reached 1000 mg/kg in the lower stratum. Loss of CO₂ from epilimnetic waters, followed by calcite precipitation and a consequent lowering in density, was the apparent cause of the stratification.The Cl, SO₄ and HCO₃ contents of Lake Quilotoa are intermediate between those of acid–SO₄–Cl Crater lakes and those of neutral-HCO₃ Crater lakes, indicating that Lake Quilotoa has a ‘memory’ of the inflow and absorption of HC1- and S-bearing volcanic (magmatic) gases. The Mg/Ca ratios of the lake waters are governed by dissolution of local volcanic rocks or magmas, but K/Na ratios were likely modified by precipitation of alunite, a typical mineral in acid–SO₄–Cl Crater lakes.The constant concentrations of several conservative chemical species from lake surface to lake bottom suggest that physical, chemical and biological processes did not have enough time, after the last overturn, to cause significant changes in the contents of these chemical species. This lapse of time might be relatively large, but it cannot be established on the basis of available data. Besides, the lake may not be close to steady state. Mixing of Lake Quilotoa waters could presently be triggered by either cooling epilimnetic waters by 4°C or providing heat to hypolimnetic waters or by seismic activity.Although Quilotoa lake contains a huge amount of dissolved CO₂ (3×10¹¹ g), at present the risk of a dangerous limnic eruption seems to be nil even though some gas exsolution might occur if deep lake waters were brought to the surface. Carbon dioxide could build up to higher levels in deep waters than at present without any volcanic re-awakening, due to either a large inflow of relatively cool CO₂-rich gases, or possibly a long interval between overturns. Periodical geochemical surveys of Lake Quilotoa are, therefore, recommended.     

Geochemistry of fluids associated with the 1995–1996 eruption of Mt. Ruapehu, New Zealand: signatures and processes in the magmatic-hydrothermal system

The 1995–1996 eruption of Mt. Ruapehu has provided a number of insights into the geochemical processes operating within the magmatic-hydrothermal system of this volcano. Both pre-eruption degassing of the rising magma and its eventual intrusion into the convective zone of the hydrothermal system beneath the lake were clearly reflected in lake water compositions. The eruptions of September–October 1995 expelled the lake, and provided the first-ever opportunity to characterise gas discharges from this volcano. The fumarolic discharges revealed compositions typical of andesite volcanoes and strong interaction with the enclosing meteoric and hydrothermal system fluids. Some 1.1 MT of SO₂ gas was released from the volcano between September 1995 and December 1996, whereas ca. twice this amount (2.2 MT equivalent SO₂) was erupted as soluble (i.e. leachable) oxyanions of sulphur. Significantly more sulphur was released from the volcano over this period than can be accounted for from the magma volume actually erupted. The evidence suggests that a sizable component of the evolved sulphur was remobilised from the long-lived hydrothermal system within the volcano during the 1995–1996 activity.     

Bathymetric and geochemical investigation of Kawah Ijen Crater Lake, East Java, Indonesia

A bathymetric survey of Kawah Ijen crater lake was conducted by acoustic sounding in 1996 to compare the lake morphology with those measured in 1922, 1925 and 1938, and to calculate the present lake volume. Even though the lake experienced several hydrothermal eruptions, the maximum depth became shallower (182 m) than before (200 m), resulting in a reduced lake volume (3.0×10⁷ m³).Fifty-two major and minor constituents including rare earth elements and polythionates (PT) of the lake waters at various depths were determined by ICP-AES, ICP-MS and HPLC, respectively. These ions except for several volatile elements are taken up by lake fringe through congruent dissolution of pyroclastics of Kawah Ijen volcano. Most ions are homogeneously distributed throughout the lake, although PT showed a considerable vertical variation. Rare earth elements (REE) in the Kawah Ijen water as well as those from other hyper-acidic crater lakes show distribution patterns likely due to the three rock dissolution (preferential, congruent and residual) types, and their logarithmic concentrations linearly depend upon the pH values of the lake waters.Using the PT degradation kinetics data, production rates of PT, injection rates of SO₂ and H₂S into the lake were estimated to be 114, 86 and 30 tons/day, respectively. Also travel time of the spring water at the Banyupahit Riverhead from Kawah Ijen was estimated to be 600–1000 days through the consideration of decreasing rates of PT. Molten sulfur stocks containing Sn, Cu, Bi sulfides and Pb-barite exposed on the inner crater slope were presumed to be extinct molten sulfur pools at the former lake bottom. This was strongly supported by the barite precipitation temperature estimated through the consideration of the temperature dependence of Pb-chlorocomplex formation.     

Geochemical and isotopic evidence for seawater contamination of the hydrothermal system of Taal Volcano, Luzon, the Philippines

 The hydrologic structure of Taal Volcano has favored development of an extensive hydrothermal system whose prominent feature is the acidic Main Crater Lake (pH<3) lying in the center of an active vent complex, which is surrounded by a slightly alkaline caldera lake (Lake Taal). This peculiar situation makes Taal prone to frequent, and sometimes catastrophic, hydrovolcanic eruptions. Fumaroles, hot springs, and lake waters were sampled in 1991, 1992, and 1995 in order to develop a geochemical model for the hydrothermal system. The low-temperature fumarole compositions indicate strong interaction of magmatic vapors with the hydrothermal system under relatively oxidizing conditions. The thermal waters consist of highly, moderately, and weakly mineralized solutions, but none of them corresponds to either water–rock equilibrium or rock dissolution. The concentrated discharges have high Na contents (>3500 mg/kg) and low SO₄/Cl ratios (<0.3). The Br/Cl ratio of most samples suggests incorporation of seawater into the hydrothermal system. Water and dissolved sulfate isotopic compositions reveal that the Main Crater Lake and spring discharges are derived from a deep parent fluid (T≈300  °C), which is a mixture of seawater, volcanic water, and Lake Taal water. The volcanic end member is probably produced in the magmatic-hydrothermal environment during absorption of high-temperature gases into groundwater. Boiling and mixing of the parent water give rise to the range of chemical and isotopic characteristics observed in the thermal discharges. Incursion of seawater from the coastal region to the central part of the volcano is supported by the low water levels of the lakes and by the fact that Lake Taal was directly connected to the China sea until the sixteenth century. The depth to the seawater-meteoric water interface is calculated to be 80 and 160 m for the Main Crater Lake and Lake Taal, respectively. Additional data are required to infer the hydrologic structure of Taal. Geochemical surveillance of the Main Crater Lake using the SO₄/Cl, Na/K, or Mg/Cl ratio cannot be applied straightforwardly due to the presence of seawater in the hydrothermal system. Received: 12 February 1997 / Accepted: 26 January 1998     

Evolution of fumarolic gases — boundary conditions set by measured parameters: case study at Vulcano,Italy

Physical, chemical and isotopic parameters were measured in fumaroles at the Vulcano crater and in drowned fumaroles near the beach. The data were used to define boundary conditions for possible conceptual models of the system.Crater fumaroles: time variations of CO₂ and SO₂ concentrations indicate mixing of saline gas-rich water with local fresh water. Cl/Br ratios of 300– 400 favour sea-water as a major source for Cl, Brand part of the water in the fumaroles. Cl concentrations and D values revealed, independently, amixing of 0.75 sea-water with 0.25 local freshwaterin furmarole F-5 during September 1982.Patterns of parameter correlation and mass balances reveal that CO₂, S, NH₃ and B originate from sources other than sea water. The CO₂ value of ¹³C = – 2%_o favours, at least partial, origin from decomposition of sedimentary rocks rather than mantle-derived material. Radiogenic⁴He(1.3 × lO^–3 ccSTP/g water) and radiogenic⁴⁰Ar(10.6 × 10^–4 ccSTP/g water) are observed, (⁴He/⁴⁰Ar)_radiogenic = 1.2, well in the range of values observed in geothermal systems.Drowned fumaroles: strongly bubbling gas at a pond and at the beachappears to have the same origin and initial compositionas the crater fumaroles (2 km away). The fumarolic gas is modified by depletion of the reactive gases, caused by dissolution in shallow-water. Atmospheric Ne, Ar, Kr and Xe are addeden route, some radiogenic He and Ar are maintained. The Vulcano system seems to be strongly influenced by the contribution of sea-water and decomposition of sedimentary rocks. Evidence of magmatic contributions is mainly derived from heat.     

Evaluation of volcanic gas analysis from surtsey volcano, iceland 1964–1967

Methods used previously to remove compositional modifications from volcanic gas analyses for Mount Etna and Erta'Ale lava lake have bean employed to estimate the gas phase composition at Nyiragongo lava lake, based on samples obtained in 1959. H₂O data were not reported in 11 of the 13 original analyses. The restoration methods have been used to estimate the H₂O contents of the samples and to correct the analyses for atmospheric contamination, loss of sulfur and for pre- and pest-collection oxidation of H₂S, S₂, and H₂. The estimated gas compositions are relatively CO₂-rich, low in total sulfur and reduced. They contain approximately 35–50% CO₂ 45–55% H₂O, 1–2% SO₂, 1–2% H₂., 2–3% CO, 1.5–2.5% H₂S, 0.5% S₂ and 0.1% COS over,he collection temperature range 102° to 960° C. The oxygen fugacities of the gases are consistently about half an order of magnitude below quartz-magnetite-fayalite. The low total sulfur content and resulting low atomic S/C of the Nyiragongo gases appear to be related to the relatively low f_O2 of the crystallizing lava. At temperatures above 800°C and pressures of 1–1.5 k bar, the Nyiragongo gas compositions resemble those observed in primary fluid inclusions believed to have formed at similar temperatures and pressures in nephelines of intrusive alkaline rocks. Cooling to 300°C, with f_O2 buffered by the rock, results in gas compositions very rich in CH₄ (50–70%) and resembling secondary fluid inclusions formed at 200–500°C in alkaline rocks. Below 600°C the gases become supersaturated in carbon as graphite. These inferences are corroborated by several reports of hydrocarbons in plutonic alkaline rocks, and by the presence of CH₄-rich waters in Lake Kivu — a lake on the flanks of Nyiragongo volcano.