Bokuichiro Takano

Geochemistry of the magmatic–hydrothermal system of Kawah Ijen volcano, East Java, Indonesia

Samples from Kawah Ijen crater lake, spring and fumarole discharges were collected between 1990 and 1996 for chemical and isotopic analysis. An extremely low pH (<0.3) lake contains SO 4 -Cl waters produced during absorption of magmatic volatiles into shallow ground water. The acidic waters dissolve the rock isochemically to produce immature solutions. The strong D and 18 O enrichment of the lake is mainly due to enhanced evaporation at elevated temperature, but involvement of a magmatic component with heavy isotopic ratios also modifies the lake D and 18 O content. The large Δ SO4-S D (23.8-26.4‰) measured in the lake suggest that dissolved SO 4 forms during disproportionation of magmatic SO 2 in the hydrothermal conduit at temperatures of 250∼280°C. The lake δ 18 O SO4 and δ 18 O H2O values may reflect equilibration during subsurface circulation of the water at temperatures near 150°C. Significant variations in the lakes bulk composition from 1990 to 1996 were not detected. However, we interpret a change in the distribution and concentration of polythionate species in 1996 as a result of increased SO 2 -rich gas input to the lake system. Thermal springs at Kawah Ijen consist of acidic SO 4 -Cl waters on the lakeshore and neutral pH HCO 3 -SO 4 -Cl-Na waters in Blawan village, 17 km from the crater. The cation contents of these discharges are diluted compared to the crater lake but still do not represent equilibrium with the rock. The SO 4 /Cl ratios and water and sulfur isotopic compositions support the idea that these springs are mixtures of summit acidic SO 4 -Cl water and ground water. The lakeshore fumarole discharges (T = 170∼ 245°C) have both a magmatic and a hydrothermal component and are supersaturated with respect to elemental sulfur. The apparent equilibrium temperature of the gas is ∼260°C. The proportions of the oxidized, SO 2 -dominated magmatic vapor anti of the reduced, H 2 S-dominated hydrothermal vapor in the fumaroles varied between 1979 and 1996. This may be the result of interaction of SO 2 -bearing magmatic vapors with the summit acidic hydrothermal reservoir. This idea is supported by the lower H 2 S/SO 2 ratio deduced for the gas producing the SO 4 -Cl reservoir feeding the lake compared with that observed in the subaerial gas discharges. The condensing gas may have equilibrated in a liquid-vapor zone at about 350°C. Elemental sulfur occurs in the crater lake environment as banded sediments exposed on the lakeshore and as a subaqueous molten body on the crater floor. The sediments were precipitated in the past during inorganic oxidation of H 2 S in the lake water. This process was not continuous, but was interrupted by periods of massive silica (poorly crystallized) precipitation, similar to the present-day lake conditions. We suggest that the factor controlling the type of deposition is related to whether H 2 S- or silica-rich volcanic discharges enter the lake. This could depend on the efficiency with which the lake water circulates in the hydrothermal cell beneath the crater. Quenched liquid sulfur products show δ 34 S values similar to those found in the banded deposits, suggesting that the subaqueous molten body simply consists of melted sediments previously accumulated at the lake bottom.

Sulfur isotopic effects in the disproportionation reaction of sulfur dioxide in hydrothermal fluids: implications for the δ34S variations of dissolved bisulfate and elemental sulfur from active crater lakes

Sulfur isotope effects during the SO 2 disproportionation reaction to form elemental sulfur (3SO 2 + 3H 2 O → 2HSO 4 - + S + 2H + ) at 200-330°C and saturated water vapor pressures were experimentally determined. Initially, a large kinetic isotopic fractionation takes place between HSO 4 - and S, followed by a slow approach to equilibrium. The equilibrium fractionation factors, estimated from the longest run results, are expressed by 1000 In α HSO 4 - -S = 6.21 × 10 6 /T 2 + 3.62. The rates at which the initial kinetic fractionation factors approach the equilibrium ones were evaluated at the experimental conditions. δ 34 S values of HSO 4 - and elemental sulfur were examined for active crater lakes including Noboribetsu and Niseko, (Hokkaido, Japan), Khloridnoe, Bannoe and Maly Semiachik (Kamchatka), Poas (Costa Rica), Ruapehu (New Zealand) and Kawah Ijen and Keli Mutu (Indonesia). Δ HSO4- -S values are 28‰ for Keli Mutu, 26%o for Kawah Ijen, 24%c for Ruapehu, 23‰ for Poas, 22‰ for Maly Semiachik, 21‰ for Yugama, 13‰ for Bannoe, 9‰ for Niseko, 4%o for Khloridonoe, and 0‰ for Noboribetsu, in the decreasing order. The SO 2 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 HSO 4 - produced through bacterial oxidation of reduced sulfur becomes progressively dominant for lakes with lower Δ HSO4- -S values. Currently, Noboribetsu crater lake contains no HSO 4 - of magmatic origin. A 40-year period observation of δ 34 S HSO4- and δ 34 S s values at Yugama indicated that the isotopic variations reflect changes in the supply rate of SO 2 to the magmatic hydrothermal system. This implies a possibility of volcano monitoring by continuous observation of δ 34 S HSO4- values. The δ 18 O values of HSO 4 - 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.

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 (H2SxO6, 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.

Monitoring of volcanic eruptions at Yugama crater lake by aqueous sulfur oxyanions

Abstract Variations of polythionates (sulfane disulfonates) and sulfate in the Yugama crater lake, Japan, have been monitored for more than 25 years. Just before the 1982 eruption at the crater lake, polythionate ions decreased to zero from the normal level of about 2000 ppm and sulfate ions increased from 2500 to 5000 ppm. During the 1982 eruption polythionate and sulfate ions varied inversely in concentration and the variations exactly coincided with the frequency of volcanic earthquakes and subsequent explosions. These observations are interpreted in terms of aqueous reactions of fumarolic SO2-H2S gases, resulting in precipitation of alunite. The behavior of polythionate and sulfate ions strongly suggests that they are useful indicator for prediction of impending volcanic hazards from active crater lakes.

Correlation of Volcanic Activity with Sulfur Oxyanion Speciation in a Crater Lake

The Yugama crater lake at Kusatsu-Shirane volcano, Japan, contains nearly 2200 tons (2800 parts per million) of polythionate ions (Sn O62-, where n = 4 to 9). Analytical data on lake water sampled before and during eruptions in 1982 showed that the concentrations of polythionates decreased and sulfate increased in response to the preeruption activities of the subaqueous fumaroles. These changes were observed 2 months before the first phreatic explosion on 26 October 1982. The monitoring of polythionates and sulfate in crater lake water is a promising means of anticipating potential volcanic eruption hazards.

High SO2 flux, sulfur accumulation, and gas fractionation at an erupting submarine volcano

Strombolian-style volcanic activity has persisted for six years at the NW Rota-1 submarine volcano in the southern Mariana Arc, allowing direct observation and sampling of gas-rich fluids produced by actively degassing lavas, and permitting study of the magma-hydrothermal transition zone. Fluids sampled centimeters above erupting lava and percolating through volcaniclastic sediments around an active vent have dissolved sulfite >100 mmol/kg, total dissolved sulfide 1 mmol/kg. If NW Rota is representative of submarine arc eruptions, then volcanic vent fluids from seawater-lava interaction on submarine arcs have a significant impact on the global hydrothermal flux of sulfur and Al to the oceans, but a minimal impact on Mg removal. Gas ratios (SO 2 , CO 2 , H 2 , and He) are variable on small spatial and temporal scales, indicative of solubility fractionation and gas scrubbing. Elemental sulfur (S e ) is abundant in solid and molten form, produced primarily by disproportionation of magmatic SO 2 injected into seawater. S e accumulates within the porous rock surrounding the lava conduit connecting the magma source to the seafloor. Accumulated S e can be heated, melted, and pushed upward by rising magma to produce molten S e flows and lavas saturated with S e . Molten S e near the top of the lava conduit may be ejected up into the water column by escaping gases or boiling water. This mechanism of S e accumulation and refluxing may underlie the relatively widespread occurrence of S e deposits of many sizes found on submarine arc volcanoes.

Experimental alteration of molybdenite: evaluation of the Re–Os system, infrared spectroscopic profile and polytype

Experiments have been carried out to clarify the effect of alteration on Re–Os system, near infrared (NIR)–infrared (IR) spectroscopic characteristics and polytype of a natural molybdenite mineral (MoS2). The molybdenite sample was placed in H2O and various media of 0.1 mol/L NaCl, NaHCO3, CaCl2, and AlCl3 solutions, and heated in a sealed quartz tube at a temperature of 180°C for 20 d. The unaltered and altered samples were subsequently used for analysis of Re and Os, NIR microscopic observation, and NIR–IR spectroscopy, and microfocus X-ray diffraction (XRD). Molybdenites subjected to NaCl and NaHCO3 solutions give younger Re–Os ages than that of the original unaltered molybdenite. No significant changes in d spacing and width of micro-XRD patterns can be found in these altered molybdenite, indicating the possibility of Re–Os fractionation without significant structural conversion of molybdenite mineral. These results strongly suggest that the Re–Os system in molybdenite would be frequently disturbed if it has experienced alteration, because alteration by the low salinity (<1%), low temperature (∼180°C) hydrothermal solution containing NaCl and/or CO2 is commonly found in the natural environment. We maintain, therefore, that one set of analyses of Re and Os in a sample is not enough to determine whether the obtained Re–Os age has been affected by postdepositional alteration, but systematic replicate analyses are indispensable. Additionally, pulverizing all the collected molybdenite in a sample might give misleading results because portions, which have been altered and experienced Re–Os fractionation, may possibly mix into the undisturbed sample and be homogenized. The molybdenite used for the experiment was originally opaque under NIR light. Infrared microscopic and spectroscopic profiles show that some parts of the molybdenite subjected to CaCl2 and AlCl3 solutions become transparent to NIR. Increased NIR transmittance is possibly attributed to the removal of the impurity band in molybdenite. It was also found in this study, however, that change in IR profile does not correlate with the Re–Os fractionation and, therefore, IR measurement solely is not useful to detect disturbance of Re–Os systematics of molybdenite in alteration.

Surveillance of Ruapehu Crater Lake, New Zealand, by aqueous polythionates

Abstract Aqueous sulfur speciation of samples of crater lake water from Crater Lake collected at Mt. Ruapehu, New Zealand since 1968 has been determined using high-performance liquid chromatography. Polythionates (S x O 6 2− ) are produced in the lake through reactions between sulfur dioxide and hydrogen sulfide gases from the subaqueous fumaroles. Their concentrations vary from 0 to a few hundred mg/l, corresponding to the activity of the volcano. The concentration of the total polythionates ( Σ S x O 6 2− = S 4 O 6 2− + S 5 O 6 2− + S 6 O 6 2− ) is found to be a good indicator of changes in the subaqueous fumarolic activity at Crater Lake. The molar S x O 6 2− /Cl − ratios of the lake water can be used to divide the volcanic activity of Crater Lake into four stages: 1. Stage I: Period of weak volcanic activity with no hydrothermal explosions. This stage is characterized by low total S x O 6 2− with a distribution order of S 5 O 6 2− > S 4 O 6 2− > S 6 O 6 2− . H 2 S is predominant among dissolved gases in the lake water. 2. Stage IIa: Quiescent period with minor hydrothermal explosions. The lake water contains high ΣS x O 6 2− with a distribution order of S 5 O 6 2− > S 4 O 6 2− > S 6 O 6 2− . Dissolved H 2 S and SO 2 gases are very low. 3. Stage IIb: Increased fumarolic activity with frequent hydrothermal explosions. Low ΣS x O 6 2− with a distribution order of S 4 O 6 2− > S 5 O 6 2− > S 6 O 6 2− . Sulfur dioxide but no H 2 S is detected in the water. 4. Stage III: Period of phreatomagmatic eruptions with lahars and intensive seismic activity. No S x O 6 2− exists but dissolved SO 2 is high in the lake water. The moderate-sized explosion of December 8, 1988, which was not preceded by any changes in lake water polythionate chemistry or seismic activity, is hypothesized to be a vent-cleaning eruption caused by blockage of ascending magmatic gases that was caused by solidification of a molten sulfur pool at the bottom of Crater Lake.

Quenching and liquid chromatographic determination of polythionates in natural water.

Polythionates in highly acidic, crater-lake water have been determined by ion-chromatography and high-performance microbore liquid chromatography. The first technique allows the determination of tri-, tetra- and pentathionate in excess of 10 ppm, and the second allows analysis for tetra-, penta- and hexathionate in excess of 0.2 ppm. The methods for preserving polythionates in natural solutions are also discussed. The recommended procedures for storage are to add hydroxylamine hydrochloride to sample solutions or to exclude atmospheric oxygen by using Winkler oxygen-determination bottles, followed by storage in a refrigerator at 5 degrees .

Characterization of sulfate ion in travertine

The mode of incorporation of sulfate ion in travertine was discussed on the basis of chemical compositions, i.r. and laser Raman spectra. These data strongly suggest that most of the sulfate ions in the calcitic travertine replace carbonate ions. This conclusion is in good harmony with the facts that calcite incorporates more sulfate ions than aragonite does and that the sulfate content of manganoan calcite decreases with increasing manganese content (Takano et al. 1977). Based on this conclusion, retarding effect of sulfate ion on the precipitation of calcite from solution was discussed.