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

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

P. Delmellea,b,* , A. Bernarda, M. Kusakabeb, T.P. Fischerc, B. Takanod

aLaboratoire de Ge´ochimie, Universite´ Libre de Bruxelles, 160/02, Av. F. Roosevelt, 50, B-1050 Brussels, BelgiumbInstitute for Study of the Earth’s Interior, Okayama University, Misasa, 682-01 Tottori-ken, Japan

cDepartment of Geology, Arizona State University, Tempe, AZ 85287-1404, USAdDepartment of Chemistry, College of Arts and Sciences, The University of Tokyo, Tokyo 153, Japan

Abstract

Samples from Kawah Ijen crater lake, spring and fumarole discharges were collected between 1990 and 1996 for chemicaland isotopic analysis. An extremely low pH (,0.3) lake contains SO4–Cl waters produced during absorption of magmaticvolatiles into shallow ground water. The acidic waters dissolve the rock isochemically to produce “immature” solutions. Thestrong D and18O enrichment of the lake is mainly due to enhanced evaporation at elevated temperature, but involvement of amagmatic component with heavy isotopic ratios also modifies the lake D and18O content. The largeDSO4–S0 (23.8–26.4‰)measured in the lake suggest that dissolved SO4 forms during disproportionation of magmatic SO2 in the hydrothermal conduitat temperatures of 250,2808C. The laked 18OSO4

andd18OH2O values may reflect equilibration during subsurface circulation ofthe water at temperatures near 1508C. Significant variations in the lake’s 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 increasedSO2-rich gas input to the lake system.

Thermal springs at Kawah Ijen consist of acidic SO4–Cl waters on the lakeshore and neutral pH HCO3–SO4–Cl–Na watersin Blawan village, 17 km from the crater. The cation contents of these discharges are diluted compared to the crater lake but stilldo not represent equilibrium with the rock. The SO4/Cl ratios and water and sulfur isotopic compositions support the idea thatthese springs are mixtures of summit acidic SO4–Cl water and ground water.

The lakeshore fumarole discharges�T � 170, 2458C� have both a magmatic and a hydrothermal component and aresupersaturated with respect to elemental sulfur. The apparent equilibrium temperature of the gas is,2608C. The proportionsof the oxidized, SO2-dominated magmatic vapor and of the reduced, H2S-dominated hydrothermal vapor in the fumarolesvaried between 1979 and 1996. This may be the result of interaction of SO2-bearing magmatic vapors with the summit acidichydrothermal reservoir. This idea is supported by the lower H2S/SO2 ratio deduced for the gas producing the SO4–Cl reservoirfeeding the lake compared with that observed in the subaerial gas discharges. The condensing gas may have equilibrated in aliquid–vapor zone at about 3508C.

Elemental sulfur occurs in the crater lake environment as banded sediments exposed on the lakeshore and as a subaqueousmolten body on the crater floor. The sediments were precipitated in the past during inorganic oxidation of H2S in the lake water.This process was not continuous, but was interrupted by periods of massive silica (poorly crystallized) precipitation, similar tothe present-day lake conditions. We suggest that the factor controlling the type of deposition is related to whether H2S- or silica-rich volcanic discharges enter the lake. This could depend on the efficiency with which the lake water circulates in thehydrothermal cell beneath the crater. Quenched liquid sulfur products showd 34S values similar to those found in the banded

Journal of Volcanology and Geothermal Research 97 (2000) 31–53

0377-0273/00/$ - see front matterq 2000 Elsevier Science B.V. All rights reserved.PII: S0377-0273(99)00158-4

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* Corresponding author. Present address. Unite´ des Sciences du Sol, Universite´ Catholique de Louvain, Place Croix-du-Sud 2/10, B-1348Louvain-la-Neuve, Belgium. Tel.:132-10473628; fax:132-10474525.

E-mail address:[email protected] (P. Delmelle).

deposits, suggesting that the subaqueous molten body simply consists of melted sediments previously accumulated at the lakebottom.q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Kawah Ijen volcano; geochemistry; magmatic–hydrothermal system; stable isotopes

1. Introduction

Acidic fluids produced during shallow interactionof magmatic volatiles with ground water candischarge and accumulate in summit volcanic cratersto form lakes whose physico-chemical status dependson both atmospheric conditions and subsurface activ-ity. Scientists have studied such crater lake systemsand their products—including the liquid sulfur bodythat they often host—from geochemical, geophysicaland hydrologic viewpoints using chemical and stableisotopic data, thermochemical models, underwateracoustic surveys and energy and mass balance calcu-lations. Their results have helped (1) to understandwater–rock and gas–water interactions in magma–hydrothermal systems (e.g. Giggenbach 1974; Casa-devall et al., 1984; Christenson and Wood, 1993;Delmelle and Bernard, 1994; Pasternack and Vare-kamp, 1994; Delmelle et al., 1998; Taran et al.,1998); (2) to identify geochemical and geophysicalprecursors of volcanic activity (Brown et al., 1989;Takano and Watanuki, 1990; Rowe et al., 1992a;Ohsawa et al., 1993; Takano et al., 1994a); (3) toestimate long-term magma heat and volatile fluxes(Rowe et al., 1992b; Brantley et al., 1993; Ohba etal., 1994); (4) to describe the volcanic–hydrothermalore-forming environments (Christenson and Wood,1993; Arribas, 1995); and (5) to predict the composi-tion and spectral properties of volcanogenic sulfur onIo, a moon of Jupiter (Oppenheimer, 1992; Kargel etal., 1999).

In this paper we present the results of a chemicaland isotopic study of gases, waters and elementalsulfur collected from a crater lake, fumaroles andsprings at Kawah Ijen volcano from September 1990to August 1996. We use these data to clarify the originof the fumarole gas and thermal water dischargesassociated with the volcano–hydrothermal systemby considering various geochemical and hydrolo-gical processes. The occurrence and formation ofelemental sulfur in the crater lake system are alsodetailed.

2. The hydrothermal system of Kawah Ijen

Kawah Ijen (2386 m) is an active stratovolcano ofbasalt–andesitic to andesitic composition (Whitfordet al. 1979; Delmelle and Bernard, 1994) locatedwithin the Ijen caldera on East Java (Fig. 1). Themost recent magmatic eruption was in 1817, butfrequent phreatic and geyser-like activity have report-edly occurred since that time (Newhall and Dzurisin,1988), and the lake has lately shown signs of instabil-ity coupled with increased seismicity (SmithsonianInstitution, 1993; 1994a,b; 1997a,b). The visiblesurface manifestations of the hydrothermal systemassociated with Kawah Ijen consist of a crater lake,arguably the largest natural reservoir of hot acidicwaters on Earth (volume< 32× 106 m3, T . 358C;pH ,0.3; Delmelle and Bernard, 1994), crater fumar-oles and few thermal discharges. One of the hotsprings is in the crater and the others are found inBlawan village, located about 17 km below thevolcano (see Delmelle and Bernard, 2000 – thisvolume; Fig. 1). There is also a cold spring dischargeon the east flank at Paltuding.

Delmelle and Bernard (1994) discussed the lakewater chemistry and concluded that condensationand oxidation of volcanic gases containing SO2, H2Sand HCl into ground water account for the high acid-ity and high anion concentrations, whereas isochemi-cal dissolution of the rock builds up the cationcontents (total dissolved solid concentration,TDS$ 100 g/kg) until silica, gypsum and bariteprecipitate. Evaporation further concentrates thelake water in dissolved elements.

A local company mines elemental sulfur in thecrater by channeling the fumarolic gas throughmetal pipes. Sulfur mats conveyed to the lake surfaceand pyroclastic sulfur ejecta produced during theJuly–August 1993 phreatic activity (Delmelle,1995; C. Oppenheimer, pers. commun. 1993) indi-cate the likely occurrence of molten sulfur at thelake bottom. Elemental sulfur also occurs in partiallyeroded banded lake sediments (height,40 m)

P. Delmelle et al. / Journal of Volcanology and Geothermal Research 97 (2000) 31–5332

exposed on the west lakeshore (Brouwer, 1925;Delmelle, 1995). Their presence attests to a relativelyrecent drop in the lake water level. The depositsconsist of alternating fine- and coarse-grained lamina-tions. The fine-grained laminations contain eitherelemental sulfur or poorly crystallized silica precipi-tates and vary in thickness from a few millimeters upto 3 m. The coarse laminations are made of rock frag-ments mixed with sediments. The deposits also recordpast explosions through the lake’s molten sulfur body,because pyroclastic sulfur similar to the 1993 ejectaoccurs in a few coarse layers. The transition betweenone lamination to the next is not regular and is gener-ally abrupt. In contrast to the sulfur-rich bandedsediments of Tateyama volcano (Kusakabe andHayashi, 1986), the Kawah Ijen deposits do not reveal

a seasonally controlled deposition pattern (Delmelle,1995).

3. Field work and laboratory analyses

We sampled the fumarole gases, the crater lakewaters, the thermal spring and surface stream watersfor chemical and D,18O and34S analysis according toprocedures described elsewhere (Giggenbach andGoguel, 1989; Delmelle and Bernard 1994; Delmelleet al., 1998). We also collected various elementalsulfur materials in the crater, including floating spher-ules and slicks, pyroclastic ejecta, fumarole subli-mates and cliff sediments for sulfur isotopicdetermination. Most sampling sites are shown in

P. Delmelle et al. / Journal of Volcanology and Geothermal Research 97 (2000) 31–53 33

Fig. 1. Map of the summit area of Kawah Ijen volcano showing the locations of fumarole, crater lake, crater spring and elemental sulfur deposits(modified from Delmelle and Bernard, 1994). Black circles indicate sampling points for lake water. Inset map shows Kawah Ijen on East Java.Symbols: CS� crater hot spring.

Fig. 1. 18O and D contents in the waters weredetermined according to Bigeleisen et al. (1952)and Epstein and Mayeda (1953). Elemental sulfurand sulfides were oxidized into SO4 by reactionwith Br2 and HNO3, and the sulfur isotope ratioin dissolved SO4 was obtained using the methodof Yanagisawa and Sakai (1983). The oxygenisotope ratio in SO4 was measured followingSakai and Krouse (1971). All SO4-containing solu-tions were purified with cation and anionexchange resins before conversion into BaSO4.Total sulfur in a powdered sample of a fresh basalticandesite crater rock was prepared for isotopic analysisaccording to Sasaki et al. (1979). Isotopic results areexpressed in thed notation relative to SMOWor CDT standards. The reproducibility ofd valuesis ^1.5, ^0.05 and ^0.15‰ for D, O and S,respectively.

4. Chemical composition of waters

4.1. Crater lake waters

Analyses of the crater lake waters and thermal springwaters are shown in Table 1. The dissolved cation andanion contents in the hot lake�T � 33:8–43:28C� areconstant with depth, indicating complete mixing. Thelake composition did not vary significantly from 1990through 1996 and is similar to that reported in 1941(van Bemmelen, 1949). The relative Na, K, Mg andCa contents in Kawah Ijen waters are shown in Fig.2. All the lake compositions closely correspond toisochemical dissolution of the crater rocks, consistentwith their high acidity. The lake molar S/Cl ratio rangesbetween 0.9 and 1.2 and is systematically lower thanthat of the fumarole gas (S/Cl� 3.1–10.6, Table 2). Ifthe latter is representative of the volcanic vapor being


Fig. 2. Relative abundances (mg/kg) of Na, K, Mg and Ca of Kawah Ijen thermal discharges. Also shown is the composition of Kawah Ijen rocks(Delmelle and Bernard, 1994). The full equilibrium line represents the composition of waters in equilibrium with thermodynamically stablemineral assemblages of the rocks (Giggenbach, 1987). Symbols: CS� crater hot spring; BL� Blawan hot springs; PA� Paltuding cold spring.

condensed in the lake’s hydrothermal system, thisdifference would account for the preferential partition-ing of HCl over SO2 and H2S into the aqueous phase(Simonson and Palmer, 1993). It may also indicate thatdeposition of elemental sulfur occurs at depth, asdiscussed below. The lake waters contain significantpolythionate (SxO6, x� 4; 5 or 6) concentrations�SSxO6 � 343–1095 mg=kg�; which attest to thedischarge of SO2- and H2S-bearing volcanic gasesinto the lake–hydrothermal system (Takano, 1987;Takano and Watanuki, 1990).

Some acidic crater lakes have shown conspicuousvariations in their Cl-normalized SO4, Mg and SxO6

concentrations accompanying changes in the magma–hydrothermal activity (e.g. Giggenbach and Glover,1975; Rowe et al., 1992a; Takano et al., 1994a). AtKawah Ijen, the Mg/Cl and SO4/Cl ratios did notvary significantly between 1990 and 1996 (Fig. 3).However, theSSxO6/Cl ratio increased from 0.017in 1990 to 0.023 in 1993 and 0.048 in 1996. Further-more, there was a change in 1996 from the orderS5O6 . S4O6 . S6O6 to the order S5O6 < S4O6 . S6O6.The ratio increase may indicate an enhanced flux

of volcanic sulfur to the lake system, whereas thedistribution change may reveal condensation of agas enriched in SO2 (Takano et al., 1994a).

4.2. Crater, Paltuding and Blawan spring waters

Although less concentrated (TDS� 56 g/kg), thestrongly acidic crater spring�T � 618C; pH� 0:6�is chemically similar to the lake, suggesting that itconsists of lake seepage diluted with ground water.The thermal discharges�T , 608C� at Blawan areHCO3–SO4–Cl–Na waters with low TDS concentra-tions (,2 g/kg) and near-neutral pH values (pH. 6)Paltuding�T � 138C� is a moderately acidic�pH�4:8� and slightly mineralized (TDS� 0.3 g/kg) springwith a high SiO2 concentration (231 mg/kg). None ofthese spring discharges have reached equilibrium withthe altered rock (Fig. 2), pointing to the “immature”nature of the Kawah Ijen acidic hydrothermal system.

In Fig. 4, all thermal discharges are plotted for theirSO4 and Cl contents. There is a clear linear trenddefined by the springs and the average compositionof the crater lake, which strongly suggests that the


Fig. 3. Temporal variations of the Cl-normalized SO4, Mg, S4O6, S5O6, S6O6 andSSxO6 concentrations (mg/kg) in Kawah Ijen crater lake watersas measured in 1990–1996. Arrows indicate dates of phreatic eruptions. Note the increase inSSxO6 content and the concurrent increase in S4O6

concentration relative to the other polythionate species in 1996.

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Table 1Chemical composition of thermal waters from Kawah Ijen volcano (in mg/kg) (n.a.: not analyzed; n.m.: not measured)

Label Locality Date T(8C)

pH TDS(g/kg)

Na K Mg Ca B Al SiO2 Fe SO4 F Cl HCO3 S4O6 S5O6 S6O6

Crater lakeIJ90-1a 0 m 18/09/90 36.6 0.20 94.4 585 1702 688 1106 46 5490 143 1862 59305 1325 22146 n.a. 112 157 74IJ90-5a 71 m 18/09/90 n.a. 0.18 100.9 566 1732 686 1092 43 5513 138 1855 66552 1128 21517 n.a. 113 186 78IJ92-1a 0 m 25/10/92 33.8 0.19 102.0 982 1284 714 1027 54 6078 173 1923 63580 1682 24460 n.a. n.a. n.a. n.a.VSI-1 0 m 15/05/93 36 n.m. 100.2 790 1364 688 n.a. n.a. 4300 n.a. 1953 69551 n.a. 21528 n.a. 127 208 76VSI-2 0 m 15/06/93 40 n.m. 92.3 828 1426 666 n.a. n.a. 4394 n.a. 2056 62588 n.a. 20300 n.a. 134 263 108VSI-3 0 m 07/07/93 40 n.m. 104.4 768 1326 677 n.a. n.a. 4269 n.a. 2040 72397 n.a. 22933 n.a. 139 264 124VSI-4 0 m 31/08/93 42 n.m. 109.3 782 1364 679 n.a. n.a. 4437 n.a. 2079 76173 n.a. 23811 n.a. 124 253 86VSI-5 0 m 10/09/93 44 n.m. 109.9 768 1368 690 n.a. n.a. 4367 n.a. 2096 76943 n.a. 23670 n.a. 147 266 90IJ93-1a 0 m 17/09/93 43.2 0.28 107.3 940 1180 569 911 54 5530 175 1888 74133 n.a. 21832 n.a. 215 290 133VSI-6 0 m 11/10/93 42 n.m. 94.5 774 1382 682 n.a. n.a. 4346 n.a. 2052 67944 n.a. 21218 n.a. 166 270 102IJ93-3 0 m 22/12/93 41.6 0.09 114.0 812 1400 751 1262 43 4865 193 2170 77044 1926 23485 n.a. n.a. n.a. n.a.IJ94-1 0 m 17/08/94 n.m. 0.30 103.7 862 1289 561 n.a. 53 5209 n.a. 2004 70542 1015 22138 n.a. n.a. n.a. n.a.IJ95-1 0 m 20/09/95 n.m. 0.25 104.2 880 1278 509 1329 53 5365 190 2105 70104 998 21381 n.a. n.a. n.a. n.a.IJ95-2 0 m 26/07/95 42.5 0.39 107.1 1260 1360 767 873 58 6233 150 2370 69899 1663 22385 n.a. n.a. n.a. n.a.IJ96-1 0 m 08/08/96 35.6 0.29 106.9 1160 1473 630 968 53 5413 161 2062 71309 1045 22630 n.a. 396b 444b 255b

IJ96-2 165 m 08/08/96 n.m. n.m. 108.0 1140 1469 639 856 51 5182 150 1929 70692 1113 24780 n.a. n.a. n.a. n.a.IJ96-3 80 m 08/08/96 n.m. n.m. 97.0 1160 1510 658 1278 54 5349 148 2093 64477 915 19338 n.a. n.a. n.a. n.a.IJ96-5 155 m 08/08/96 n.m. n.m. 101.8 1160 1490 681 1036 54 5416 150 2074 67352 963 21357 n.a. n.a. n.a. n.a.IJ96-6 75 m 09/08/96 n.m. n.m. 105.1 1180 1464 702 824 53 5408 116 2052 69923 1001 22336 n.a. n.a. n.a. n.a.IJ96-7 90 m 09/08/96 n.m. n.m. 100.5 1140 1462 657 1000 52 5470 120 2074 66795 986 20720 n.a. n.a. n.a. n.a.

SpringsBL2-90 Blawan 20/09/90 51.2 6.58 1.9 97 67 110 109 1.6 ,0.5 139 ,1.0 302 n.a. 87 979 n.a. n.a. n.a.BL1-92 Blawan 26/10/92 47.1 6.41 0.8 200 46 74 87 1.6 0.1 131 ,0.1 228 n.a. 82 n.a. n.a. n.a. n.a.BL1-93 Blawan 24/12/93 48.2 6.20 1.8 126 45 99 150 1.6 0.5 165 1.1 242 n.a. 80 842 n.a. n.a. n.a.BL2-93 Blawan 24/12/93 50.2 6.32 1.9 130 48 115 128 1.4 0.3 169 1.0 291 n.a. 89 939 n.a. n.a. n.a.BL1-96 Blawan 13/08/96 31.3 6.43 1.4 90 32 89 250 n.a. 1.54 118 3.6 600 2 202 n.a. n.a. n.a. n.a.BL2-96 Blawan 13/08/96 46.7 6.45 1.2 176 50 77 102 n.a. 0.3 171 3.1 505 1.5 85 n.a. n.a. n.a. n.a.CS-95 Crater 26/07/95 61.0 0.58 56.4 663 856 510 887 40 3940 195 1110 36268 746 11161 n.a. n.a. n.a. n.a.PA-92 Paltuding 26/10/92 13.1 4.75 0.3 11 5.5 3.8 12,0.1 3.0 231 ,0.1 58 n.a. 15 n.a. n.a. n.a. n.a.

a From Delmelle and Bernard (1994).b Averaged value of eight water samples collected at a depth of 0, 25, 75, 100, 125, 150 and 155 m, the SxO6 concentrations did not vary significantly with depth (B. Takano,

unpublished data).

springs are related to a common SO4–Cl reservoir.We conclude that the spring discharges representvarious degrees of dilution with ground water of theSO4–Cl waters formed in the summit acidic hydro-thermal system. The high HCO3 content found in theBlawan springs probably resulted from the incorpora-tion of surface ground waters that contained a CO2-rich steam fraction boiled off the hydrothermal system(Ellis and Mahon, 1977).

5. Fumarolic gas chemistry

The chemical compositions of fumarole dischargesare reported in Table 2. The temperatures varybetween 169 and 2448C. Water is the most abundantgas species, followed by CO2 and the sulfur speciesH2S and SO2. In terms of the inert, minor species N2,He and Ar, Kawah Ijen gases lie in the field of typicalarc-type gases mixed with various amounts of air and


Table 2Chemical composition of fumarole discharges from Kawah Ijen volcano (mmol/mol) (n.a., not analyzed; n.r., not reported)

Label Date T (8C) H2O CO2 SO2 H2S HCl HF He H2 Ar O2 N2 CH4 CO

M40a ??/07/79 244 764000 225616 2289 9133 n.r. n.r. 1.20 22.20 n.r. n.r. n.r. 1.40 33.5M57a ??/07/79 244 843000 146795 2873 7348 n.r. n.r. 1.38 21.40 n.r. n.r. n.r. 0.80 20.3KIG-1b 17/09/93 187 864000 114000 8600 11500 1900 n.a. 0.18 2.2 1.52 10.4 386 0.1 n.a.KIG-2 26/07/95 169 880000 94100 2890 15670 6080 9 0.32 21.05 1.40 3.1 476 0.19,0.02KIG-3 09/08/96 217 967000 28000 1280 3110 560 3 0.02 1.39 0.03,1.7 24.3 ,0.57 0.05KIG-4 09/08/96 217 964000 30000 1470 3330 460 4 0.02 1.36 0.03,1.7 25.7 ,0.59 ,0.05

a From Allard (1986).b From J. Hirabayashi (pers. commun., 1993).

Fig. 4. Log SO4 vs. log Cl concentrations (mg/kg) in Kawah Ijen thermal discharges. Symbols are as in Fig. 2. The “average crater lake water”data point represents averaged analysis of all crater lake samples reported in Table 1.

air-saturated ground water, as observed at othersubduction zone volcanoes (Giggenbach, 1992a;Fischer et al., 1998) (Fig. 5). The H2–Ar geotherm-ometer of Giggenbach and Goguel (1989) yieldsequilibrium temperatures of 260–2908C.

Fig. 6 shows that all gas discharges are super-saturated with respect to liquid sulfur according toreaction:

SO2 1 2H2S, 3S0 1 2H2O �1�The metal pipes through which the gases flow arecoated on both sides with elemental sulfur, and liquidsulfur continuously “drains” from the outlet, consis-tent with its supersaturation at discharge tempera-tures. Removal of elemental sulfur from volcanicgases according to reaction (1) has been observedfrequently on active volcanoes (e.g. Mizutani andSugiura, 1966; Giggenbach, 1987). In SO2-dominatedvapors, the deposition of elemental sulfur willconsume 2 moles of H2S per mole of SO2 and leadto depletion of H2S in the gas discharge. By contrast,addition of a H2S-dominated hydrothermal vapor willalso result in the deposition of elemental sulfur andwill cause an increase in the H2S content of thedischarged gas.


Fig. 5. Classification of Kawah Ijen fumarole gases in terms of relative Ar–N2–He contents. Numbers correspond to sample labels in Table 2.The “arc-type” field corresponds to gases which are high in N2 and the “mantle-derived” field to gases enriched in He (Giggenbach, 1992a). Alsoshown are air and air-saturated water (asw).

Fig. 6. Saturation of Kawah Ijen fumarole gases with respect tonative sulfur. The theoretical lines are calculated according to reac-tion (1) (Mizutani and Sugiura, 1966; Giggenbach, 1987). Numberscorrespond to sample labels in Table 2.

To determine the controlling elemental sulfurdeposition process at Kawah Ijen, we have reportedthe relative abundance of CO2, SO2 and H2S and theirchanges throughout the sampling survey from 1979 to1996 in Fig. 7. The gas composition changed fromCO2-rich in 1979 to a more sulfur-rich gas in 1993,with the C/S ratio decreasing from,20 to ,5. Thiswas accompanied by a concurrent decrease in theH2S/SO2 ratio, indicating the addition of a relativelyoxidized, SO2 rich component. During that time, thevapors added to the gas discharges were dominated bySO2, which led to the depletion of H2S as elementalsulfur was removed. From 1993 to 1995, the H2S/SO2

ratio increased significantly and the deposition ofelemental sulfur most likely was the result ofaddition of a H2S-dominated, HCl-rich hydrothermalcomponent. This idea is supported by a drop in theCO2/HCl ratios from 60 in 1993 to 15 in 1995. Thesituation was again reversed in 1996 as more SO2-dominated magmatic vapors were supplied to thesystem. The crater lake may have also “chemically”

recorded this last episode, because the changeobserved in the distribution of its polythionatescorresponds to a lower H2S/SO2 in the subaqueousfumaroles, as noted above. The addition of a lowC/S, low CO2/H2S, reduced hydrothermal endmember component with a relatively high HClcontent to a more oxidized, high C/S magmaticcomponent has been recognized at Vulcano byChiodini et al. (1993). The chemical compositionsof the gas discharges at Kawah Ijen are similarlyaffected.

Fig. 8 represents the values ofRH � log�xH2=xH2O�

plotted vs. outlet temperatures. In this diagram, thelines of the redox “gas” (H2S–SO2) and “hydrother-mal” (FeO–FeO1.5 of the rock system) buffers corre-spond to the temperature dependence of theequilibrium constants (Giggenbach, 1987). The gasdischarges have H2/H2O ratios apparently reflectinginternal equilibration of the H2–H2O–H2S–SO2

system close to the surface. Accepting this, the H2S/SO2 equilibration temperatures can be evaluated


Fig. 7. Relative contents of SO2, CO2 and H2S for Kawah Ijen fumarole gases. Numbers correspond to sample labels in Table 2. The gascollected in 1979 has a more magmatic signature, whereas the samples of 1990s are more hydrothermal in nature.

according to:

ts � 10744=�Ls 1 3:66�2 273:2 �2�

where

Ls � log�xH2Sx3H2O=xSO2

x3H2�:

Therefore,ts ranges from 3308C in 1979 to 2258C in1996, consistent with the H2–Ar temperatures. Theresults suggest equilibration in more hydrothermalconditions in the 1990s and more magmaticconditions in 1979.

The redox diagrams for Kawah Ijen gases interms ofRH � log�xH2

=xH2O�; RCH4� �xCH4

=xCO2� and

RCO � �xCO=xCO2� are shown in Fig. 9. The dashed

lines correspond to equilibrium temperatures in asingle liquid or single vapor phase. The solid linesrepresent equilibrium with the “hydrothermal” buffer(Giggenbach, 1987) in the vapor, liquid or two-phasesystem. For the H2–H2O–CH4–CO2 system (Fig. 9A),the gas discharges lie within the area indicating equi-librium in a single vapor phase between 120 and2508C. In terms of CO, and using the detection limitfor CO of the gas chromatograph, the equilibrationoccurs in the liquid–vapor coexistence field attemperatures of no more than 1608C (Fig. 9B). The1979 samples have exceedingly high CO contents,which suggests equilibrium in a single liquid phase.Fig. 9A shows that CH4 is slow to equilibrate whereasCO equilibrates faster (Giggenbach, 1987) and

represents conditions in the shallower parts of thesystem.

6. Isotopic composition of the waters and gascondensates

6.1. Crater lake waters

Hydrogen and oxygen isotope ratios of the KawahIjen waters and gas condensates reported in Table 3are plotted in Fig. 10. The surface stream waters havedD and d 18O values plotting close to the meteoricwater line. We assume that these compositions char-acterize local meteoric water (LMW) recharging thesummit hydrothermal system, although seasonalvariability may slightly change the isotopic signatureof LMW.

The lake waters exhibit D- and18O-shifts of44,48‰ and 16–17‰, respectively. There is alwaysa haze above the surface of Kawah Ijen lake, whichsuggests that evaporation takes place continuously.This process variably concentrates the water in Dand 18O depending on the lake surface temperatureand on atmospheric conditions. IndD–d 18O space,cool water reservoirs generally fall on an evaporationline with a slope depending on the relative atmo-spheric humidity (Gonfiantini, 1986; Gat, 1996).However, hot lakes and geothermal pools showrelatively “flat” isotopic evolution lines, becauseenhanced kinetic isotope fractionation of18O relative


Fig. 8. H2/H2O molal ratio of the Kawah Ijen fumarole gases vs. measured outlet temperatures. Numbers correspond to sample labels in Table 2.The altered rock buffer (FeO–FeO1.5) and the gas buffer (H2S–SO2) are from Giggenbach (1987).

to D occurs at elevated water temperatures(Matsubaya and Sakai, 1978; Gonfiantini, 1986; Gat,1996; Varekamp and Kreulen, 2000 – this volume).

To investigate this process, we have represented thetheoretical evaporation line for Kawah Ijen lake ascalculated according to Gonfiantini (1986) and Vare-kamp and Kreulen (2000 – this volume) in Fig. 10.

Clearly, evaporation strongly influences thedD andd 18O of the lake water, but it cannot alone account forthe observed values, since the data points plot off andto the right of the theoretical evaporation line. Deines(1979) argued that thedD of vapor evaporated from a10 wt.% solution of HCl at 208C is 20‰ higher thanthat of vapor released from pure water. This effect


Fig. 9. Redox diagrams for Kawah Ijen fumarole gases in terms ofRH � log�xH2=xH2O�; RCH4

� log�xCO2=xCH4

� andRCO � log�xCO=xCO2� (x is

the mole fraction), according to Chiodini et al. (1993) and Taran et al. (1998). Numbers correspond to sample labels in Table 2. Hatchedsymbols show the detection limits of the gas chromatograph. Solid lines correspond to equilibria with the geothermal rock buffer in a vapor,liquid or two-phase system. Dashed lines correspond to equilibrium in a single vapor phase (upper isotherms) or liquid phase (lower isotherms).(A) in the RH–RCH4

system, data points show equilibrium in a single vapor phase. (B) CO was detectable only in the 1979 sample. For othersamples, the detection limit places the fumarole gases within the two-phase region.

could thus contribute to generate the lowdD–d 18Oslopes observed. However, we note that the KawahIjen water differs somewhat from Deines’ model byits composition (mixtures of H2SO4 and HCl), acidity(lower H1 activity) and temperature (.208C).

Strong isotopic enrichment has also been reportedat other hot acidic crater lakes. For example, the offsetfrom the theoretical evaporation line has been docu-mented at Yugama Lake and Keli Mutu (Ohba et al.,2000 – this volume; Varekamp and Kreulen, 2000 –this volume). Rowe (1994) estimated the isotopiceffect associated with water–rock interaction at hightemperatures for Laguna Caliente on Poa´s. This

process may increase thed 18O values of the thermalwaters feeding the lake but should not affect thehydrogen isotope ratios because the rocks typicallycontain little hydrogen compared to the amount ofwater involved (e.g. Taylor, 1974, 1986). Followingthe procedure described by Rowe (1994) and others(e.g. O’Neil and Taylor, 1967; Taylor, 1974), we esti-mated that the18O enrichment in LMW during inter-action of water with the basaltic–andesite rock�d18Orock � 17‰; Taylor, 1986) at high temperatures(.2508C) is at most,10‰. The water/rock ratio of28 (36 g rock/kg water) used here for the summithydrothermal system of Kawah Ijen is evaluated


Table 3Isotopic composition (‰) of thermal and meteoric waters and fumarole condensates from Kawah Ijen volcano (n.a.: not analyzed)

Label Locality Date dD d 18O d 34S (SO4) d 18O(SO4)

Crater lakeIJ90-1 0 m 18/09/90 n.a. n.a. 22.3 n.a.IJ92-1 0 m 25/10/92 n.a. n.a. 22.5 n.a.IJ93-1 0 m 17/09/93 n.a. n.a. 22.5 21.5IJ93-2 0 m 12/10/93 1.2 8.5 23.0 n.a.IJ94-1 0 m 17/08/94 21.1 9.9 22.2 22.3IJ95-1 0 m 20/09/95 21.9 7.8 22.3 20.8IJ95-2 0 m 21/09/95 22.8 9.0 22.5 21.8IJ96-1 0 m 08/08/96 0.8 8.7 22.3 22.1IJ96-2 165 m 08/08/96 20.8 8.7 22.3 22.1IJ96-3 80 m 08/08/96 0.6 8.7 22.4 22.7IJ96-4 0 m 08/08/96 20.7 8.7 22.5 21.9IJ96-5 155 m 08/08/96 0.9 8.8 22.2 22.0IJ96-6 75 m 09/08/96 23.1 8.3 22.1 21.8IJ96-7 90 m 09/08/96 20.5 8.8 22.3 22.1

SpringsBL1-93 Blawan 24/12/93 241.4 26.3 14.6 n.a.BL1-93 Blawan 24/12/93 240.6 28.0 n.a. n.a.BL2-93 Blawan 24/12/93 239.9 28.0 n.a. n.a.BL1-96 Blawan 13/08/96 248.3 28.1 n.a. 9.8BL2-96 Blawan 13/08/96 249.2 28.1 18.2 16.8BL3-96 Blawan 13/08/96 247.5 28.1 n.a. n.a.CS-95 Crater 26/07/95 29.4 8.3 n.a. n.a.PA-95 Paltuding 18/09/95 245.1 28.2 n.a. n.a.

MeteoricSodong Flank 18/09/95 239.6 27.2 n.a. n.a.Streamlet Summit 08/08/96 257.5 29.3 n.a. n.a.Streamlet Summit 08/08/96 249.3 28.5 n.a. n.a.Kalisat river Blawan 08/08/96 246.0 27.9 n.a. n.a.

FumarolesKIG-2 Lakeshore 26/07/95 223.9 3.2 – –KIG-2b Lakeshore 18/09/95 221.2 4.1 – –KIG-3 Lakeshore 08/08/96 214.0 4.7 – –

from the Na content in fresh rock (Delmelle andBernard, 1994) and in the lake water by assumingcomplete dissolution of Na (Gislason and Eugster,1987). This crude calculation suggests that the lake18O content may be influenced by water–rock inter-action in the subsurface hydrothermal system.

Aside from water–rock interaction, the contribu-tion of an isotopically heavy volcanic component tothe lake–hydrothermal system may play an importantrole in determining the finaldD andd 18O values ofKawah Ijen waters, and would be similar to the situa-tion found at other hot acidic crater lakes. For exam-ple, using time-series data on the Cl content andisotopic composition for Yugama Lake, Kusatsu-Shirane, Ohba et al. (2000 – this volume) deducedthat such a component accounts for 25,36% ofthe measured D- and18O-enrichment in the lake.The volcanic endmember could be a mixture ofmeteoric water and magmatic vapor. Varekamp andKreulen (2000 – this volume) argued convincingly

that the difference in the magnitude of the offsetsfrom the calculated evaporation lines exhibited bythe two lakes at Keli Mutu points to different volcanicgas inputs, i.e. larger inputs correlate with largeroffsets. These results suggest that condensation of avolcanic component withdD andd 18O values typicalof a volcanic arc-gas (Kusakabe and Matsubaya,1986; Taran et al., 1989; Giggenbach, 1992b) is likelyto occur at Kawah Ijen. This idea is also supported byour preliminary calculations of the lake’s Cl budget,which indicate that approximately 2× 107 kg=day ofvolcanic vapor condense in the lake–hydrothermalsystem (Delmelle, 1995).

6.2. Crater, Paltuding and Blawan spring waters

In Fig. 10, the crater hot spring plots slightly belowthe lake water compositions. Recalling that thisdischarge is fed by the crater lake, the differencemay reflect the decreased fractionation of D with


Fig. 10. Oxygen and hydrogen isotopic composition of Kawah Ijen thermal discharges and fumarole gas condensates. Symbols are as in Fig. 2.The meteoric water line is from Craig (1961). Local meteoric water (LMW) represents the average of the surface water compositions reported inTable 3. “Volcanic arc gas” is from Kusakabe and Matsubaya (1986), Taran et al. (1989) and Giggenbach (1992b). The “theroreticalevaporation curve” of Kawah Ijen lake is calculated according to Gonfiantini (1986) and Varekamp and Kreulen (2000 – this volume) witha lake temperature of 408C, an air temperature of 208C, air relative humidity of 80% and fractionation factors from Majoube (1971). Also shownis the observed “lake evaporation curve”. The difference between the two curves points to addition of a volcanic gas component with a heavyisotopic composition. The “single-step boiling curve” is calculated according to Giggenbach and Stewart (1982), where temperatures indicatetemperature at which boiling occurs.

respect to18O due to the higher temperature of thespring compared to the lake (Matsubaya and Sakai,1978; Gonfiantini, 1986; Gat, 1996). ThedD andd 18O values of the Blawan and Paltuding dischargesare similar to LMW, supporting a meteoric-dominatedorigin.

6.3. Fumarolic gas condensates

The dD andd 18O values of the fumarole conden-sates lie between the values of LMW and the craterlake (Fig. 10). Boiling of downward-seeping lakewaters cannot yield these compositions, because thesteam obtained from single-step boiling of a craterlake sample with an average isotopic composition�dDlake� 20:7‰; d18Olake� 18:7‰� is notablyenriched �dDsteam� 26:4‰; d18Osteam� 17‰�compared to the heaviest fumarole condensate�dD � 214‰; d18O� 14:7‰� (Fig. 10). Wesuggest that the fumarole isotopic values mainlyreflect mixing of ground water with a volcanic arcvapor and summit acidic hydrothermal fluids, asituation also observed at the magmatic–hydro-thermal systems of White-Island, Esan and Poa´s(Giggenbach, 1987; Hedenquist and Aoki, 1991;Rowe, 1994).


Table 4Isotopic composition (‰) of elemental sulfur, fumarole sulfur androck sulfur from Kawah Ijen volcano

Sample description Date d 34S

Floating S0 spherule 20/09/90 23.8Floating S0 spherule 20/09/90 22.6Floating S0 spherule 01/09/95 21.4Floating S0 spherule 08/08/96 24.1Floating S0 slick 20/09/90 24.2Floating S0 slick 20/09/90 23.1Pyroclastic S0

(erupted in July 1993)12/12/93 23.6

Pyroclastic S0

(erupted in July 1993)23/12/93 23.4

Ancient pyroclastic S0

(in S deposits)18/09/95 23.4

S0 sediment layer 1 18/09/95 23.1S0 sediment layer 2 18/09/95 23.2S0 sediment layer 3 18/09/95 23.5S0 sediment layer 4 18/09/95 24.4S0 sediment layer 5 18/09/95 23.2S0 sediment layer 6 18/09/95 22.5S0 sediment layer 7 18/09/95 23.5S0 sediment layer 8 18/09/95 22.8S0 sediment layer 9 18/09/95 23.8S0 sediment layer 10 18/09/95 23.3Fumarole S0 20/09/90 25.3Fumarole SO2 17/09/93 7.5Fumarole H2S 17/09/93 25.6Crater rock

PS 20/09/90 6.7

Fig. 11. Sulfur isotopic ratio of various sulfur-bearing materials from Kawah Ijen volcano.

7. Isotopic composition of sulfur compounds

7.1. Crater lake sulfate

7.1.1. Sulfur isotopesSulfur isotopic compositions from Kawah Ijen are

reported in Tables 3 and 4, and Fig. 11. The34Scontent of dissolved SO4 in the lake did not varysignificantly (122.1–123.1‰) from September1990 to August 1996 (Table 3), suggesting a stablesource for SO4. It does not vary in the water columneither. The observedd 34SSO4

values are among themost enriched ever reported for acidic SO4–Cl ther-mal waters (Kiyosu and Kurahashi, 1983; Williams etal., 1990; Ohsawa et al., 1993; Sturchio et al., 1993;Rowe, 1994; Kusakabe et al., 2000 – this volume;Varekamp and Kreulen, 2000 – this volume). Thisindicates that inorganic oxidation of volcanic H2S(typically with a light isotopic signature) is not rele-vant in the lake SO4 formation process because of thelack of significant isotopic exchange associated withthis reaction (Ohmoto and Rye, 1979). Instead,disproportionation (hydrolysis) of magmatic SO2

during condensation of volcanic gases in groundwater[Eqs. (3) and (4), Kusakabe et al., 2000 – this volume]and hydrolysis of elemental sulfur at elevatedtemperatures (100–3508C) [Eq. (5), Ellis and Giggen-bach, 1971] are the potential mechanisms that canyield SO4 in the lake–hydrothermal system:

4SO2 1 4H2O) 3H2SO4 1 H2S �3�

3SO2 1 3H2O) 2H2SO4 1 H2O 1 S0 �4�

4S0 1 4H2O) H2SO4 1 3H2S �5�During SO2 hydrolysis, reaction (4) is favored underrelatively high redox potentials, low temperatures andhigh total sulfur concentrations (Kusakabe et al., 2000– this volume). The sulfur isotope exchange in theSO4–H2S and SO4–S0 systems that accompanies reac-tions (3) and (4) is almost similar, and its rateincreases with acidity and temperature (Robinson,1973; Ohmoto and Lasaga, 1982; Kusakabe et al.,2000 – this volume).

Molten sulfur lying on the crater floor is associatedwith relatively elevated temperatures compatible withthe elemental sulfur hydrolysis reaction. For example,the in situ temperatures recorded at several crater

lakes range from 120 to 1708C (Oppenheimer andStevenson, 1989; Christenson, 1994; Takano et al.,1994b). At Kawah Ijen, the presence of pyrite in thesulfur spherules and pyroclastic ejecta (Delmelle andBernard, 1994; Delmelle, 1995) suggests tempera-tures above 1508C, because marcasite instead of pyriteis the stable iron sulfide at lower temperatures(Murowchick and Barnes, 1986; Shoonen and Barnes,1991; Takano et al., 1994b).

The evaluation of the isotopic compositions of thesulfur species that would be produced during hydro-lysis of elemental sulfur is complicated by the fact thatthe hydrothermal water involved in this reaction mustalready contain sulfur dissolved in some form. Thus,the isotopic mass balance calculations developedbelow will offer no more than a simplified view ofthe system. According to Eq. (5), the equilibriumd 34S values of the sulfur-bearing products are givenby:

d34SS0 � 1=4d34SSO41 3=4d34SH2S �6�

The DSO4–H2S is 41.3 and 32.4‰ at liquid sulfurtemperatures of 120 and 1708C, respectively (Ohmotoand Lasaga 1982). Substituting these values into Eq.(6) and assuming an initialdS0 � 23:3‰ (average ofthe floating and pyroclastic sulfur isotopic composi-tions, Table 4), we obtaind 34SSO4

between127.7 and121.0‰ andd34SH2S between213.6 and211.4‰,respectively, for equilibrium in a closed system. Wecan reasonably assume that if H2S produced throughreactions (3) or (5) enters the lake, it is quicklyoxidized into elemental sulfur or SO4, because H2Scould not be detected in the water column in 1990nor in 1996. Obviously, if all H2S produced throughreaction (5) is converted into SO4 during inorganicoxidation, thed 34SSO4

values observed in the lakewould never be so high. Since isotopic fractionationin the H2S–S0 system at the lake temperatures wouldslightly deplete elemental sulfur in34S (Ohmoto andRye, 1979), thed34SH2S values are also too low toaccount for the34S-content of elemental sulfur. There-fore, hydrolysis of elemental sulfur cannot be themain source of SO4 in Kawah Ijen lake. Rowe(1994) reached the same conclusion in his study ofLaguna Caliente and Poa´s.

Alternatively, we believe that disproportionation ofSO2 conveyed by high temperature vapors to the root


of the crater lake generates SO4. Thermochemicalmodeling results have also demonstrated that conden-sation of SO2-, H2S-, HCl-containing volcanic gasesinto meteoric water can produce the range of SO4 andCl contents observed in acidic crater lakes (Rowe etal., 1992a; Christenson and Wood, 1993; Delmelleand Bernard, 1994). The presence of polythionatesin these systems further reveals input of volcanicSO2 and H2S (Takano, 1987). Finally, the SO2 dispro-portionation reaction is commonly evoked to explainthe isotopic and chemical characteristics of acid SO4–Cl thermal waters discharged at shallow magmatic–hydrothermal systems (e.g. Kiyosu and Kurahashi,1983; Williams et al., 1990; Christenson and Wood,1993; Ohsawa et al., 1993; Sturchio et al., 1993;Rowe, 1994; Kusakabe et al., 2000 – this volume).

When we apply the equation from Kusakabe et al.(2000 – this volume), the isotopic fractionationmeasured between SO4 and S0 �DSO4–S0 �23:8 , 26:4‰� corresponds to an equilibriumtemperature of 250,2808C. Large isotopic fractiona-tions up to 21‰ also could result from initially slug-gish reaction kinetics if disproportionation of SO2

occurs at temperatures below 1508C in the lake(Kusakabe et al., 2000 – this volume). Since tempera-tures above this value are likely to prevail in thebottom molten sulfur, we may safely conclude thatthe observedDSO4–S0 values reflect equilibration inthe high heat flow zone of the hydrothermal vent feed-ing the lake rather than a kinetic effect at the lakewater temperature. In addition, low temperatureswould diminish further isotopic exchange betweenSO4 and S0, thus “freezing” the initial kineticDSO4–S0 at values a few per mils lower than thosemeasured in the lake waters.

The molar H2S/SO2 ratio, rs, of the condensingvolcanic gas can be deduced from the sulfur isotoperatios of SO4 and S0 if these compounds form accord-ing to the disproportionation of SO2. For the iso-topic equilibrium, we can write (Taran et al., 1996;Kusakabe et al., 2000 – this volume):

d34SSO4� d34SSS 1 DSO4–H2S0�rs=�1 1 r��

1 1=3DSO4–S0�1=�1 1 rs�� 7�Assuming aDSO4–H2S value corresponding to the SO4–S0 sulfur isotopic equilibrium temperature deducedabove and using thed 34S of total sulfur in a crater

rock (Table 4,d34SSS � 16:7‰�; we obtain r s �1:1 for the volcanic gas. This gas ratio is notablylower than that inferred from the distribution of thelake SxO6 (rs . 14 for S5O6 $ S4O6 . S6O6, Takanoet al., 1994a). We may explain this difference if thelatter forms from a volcanic vapor which had alreadyreacted and lost some SO2 in the subsurface hydro-thermal zone. Thers values (1.3–5.4) for the subaerialfumaroles are comparatively higher too, suggestingthat the gas being injected into the lake–hydrothermalsystem corresponds to a more SO2-rich vapor. Thissupports the idea that magmatic gases interact withthe hydrothermal cell underlying the crater to producea reduced, H2S-rich gas and an oxidized, SO4-richwater which subsequently discharge in the fumarolesand in the crater lake, respectively. Using a given gasratio value, one can further infer the temperaturets[Eq. (2)] at which sulfur species equilibrated in thegas phase being condensed at depth. Forrs � 1:1; andfor a RH value of the vapors represented by themagmatic gases of 1979,ts is approximately 3508C.

7.1.2. Oxygen isotopesIn Fig. 12, thed 18OSO4

andd 18OH2O values for theKawah Ijen thermal waters (Table 3) are plottedtogether with isotherms showing the temperaturedependence ofDSO4–H2O (Lloyd 1968; Mizutani andRafter, 1969). The 1993–1996 lake samples showequilibration temperatures in the narrow range 120–1438C, well above the measured lake water tempera-tures. The rate of oxygen isotopic exchange in the pairSO4–H2O increases with temperature (Lloyd, 1968;Chiba and Sakai, 1985). Therefore, thed 18OSO4

valuescould represent quenched equilibrium values if thelake waters circulate in the subaqueous hydrothermalconduit (Kusakabe et al., 2000 – this volume). Thereis a growing body of evidence that acidic crater lakesabove magma–hydrothermal systems sustain subsur-face recycling of their waters (Rowe et al., 1992b;Christenson and Wood, 1993; Ohba et al., 2000 – thisvolume). At Kawah Ijen, the heat and mass budgetanalysis also suggests that the lake water seepingthrough the crater floor is partly re-injected into thelake (Delmelle, 1995). However, the observed lakeDSO4–H2O will reflect this process if we assume that the

d18OH2O in the lake and in the underlying hydrothermalcell are identical. This condition is unlikely considering


that the isotopic composition of the former isstrongly affected by evaporation and that itsvolume is likely to be smaller than that of the hydro-thermal reservoir. Therefore, theDSO4–H2O measuredin the lake may also represent non-equilibriumvalues.

7.2. Blawan hot spring sulfate

7.2.1. Sulfur isotopesThe d 34SSO4

values of the Blawan thermal watersare lower than those in the crater lake (Table 3, Fig.11) but are strongly enriched compared to SO4

produced by the inorganic oxidation of H2S. Consid-ering that these springs are derived from the summitSO4–Cl reservoir, the sulfur isotopic compositionsmay simply reflect mixing between34S-enrichedSO4 formed by disproportionation of SO2 and

34S-depleted SO4 produced during oxidation ofH2S distilled off the margin of the hydrothermalsystem.

7.2.2. Oxygen isotopesAs shown in Fig. 12, the near-neutral pH thermal

waters at Blawan haved 18OSO4values corresponding

to SO4–H2O isotopic equilibrium temperatures in therange 48–948C. Since significant oxygen isotopicexchange between SO4 and H2O occurs only at lowpHs and elevated temperatures (Lloyd, 1968; Chibaand Sakai, 1985), the results imply that the watershave resided at depth for exceedingly long periodsof time. However, it is more likely that the18O-content of the spring SO4 reflects non-equilibriumdilution of the isotopically heavy SO4 derived fromthe summit SO4–Cl hydrothermal reservoir.


Fig. 12. Oxygen isotopic ratio in water and dissolved sulfate in Kawah Ijen thermal discharges. Symbols are as in Fig. 2. The theoretical grid isobtained using the equations of Lloyd (1968) and Mizutani and Rafter (1969).

7.3. Origin of sulfur sediment and molten sulfur in thecrater lake

Thed 34S values of ten sulfur-rich layers represent-ing the sediments on the lakeshore vary little (22.8 to24.4‰, Table 4, Fig. 11), suggesting that the sulfursource did not change significantly through time. Theoccurrence of distinct elemental sulfur- and silica-richlaminations in the deposits implies that precipitationof elemental sulfur in the lake was not continuous.The sulfur grains in the sediment do not show theglobular shape distinctive of bacteria-mediated preci-pitation (e.g. Krupp and Seward, 1987). This isconsistent with the low pHs of the lake water whichprevent biological activity. Three types of inorganicreactions can yield elemental sulfur in the lake: (1)disproportionation of SO2 [Eq. (4)]; (2) oxidation ofH2S [Eqs. (8)–(12)], (e.g. Rowe, 1994); and (3) sulfi-tolysis of SxO6 [Eq. (13), Takano and Watanuki,1990]:

H2SO4 1 3H2S) 4S0 1 4H2O �8�

H2SO3 1 2H2S) 3S0 1 3H2O �9�

SO2 1 2H2S) 3S0 1 2H2O �10�

Fe31 1 H2S) S0 1 Fe21 1 2H1 �11�

O2 1 2H2S) 2S0 1 2H2O �12�

SxO226 1 �3x 2 7�HSO22

3 ) �2x 2 3�SO224

1 �x 2 1�H1 1 �2x 2 4�S0 1 �x 2 3�H2O �13�

Sulfitolysis of SxO6 (Eq. (13)) can potentially give riseto significant amounts of elemental sulfur, but thesulfur isotopic effect associated with this reaction isnot known. The destruction of the 1996 lake SxO6

content would produce,107 kg of elemental sulfurin the lake, only a tenth of the estimated sulfur masspresent in the lakeshore deposits (Brouwer, 1925).Reaction (4) cannot account for the formation of thesulfur sediments, because magmatic SO2 is believedto react at high temperatures in the subaqueous hydro-

thermal vent feeding the crater lake. Nevertheless,SO2 can disproportionate according to Eq. (3), releas-ing 34S-depleted H2S into the lake–hydrothermal ventsystem. This H2S may subsequently precipitate aselemental sulfur during oxidation. The finalDSO4–S0

will be close to theDSO4–H2S initially set by the SO2disproportionation reaction if the oxidant is a non-sulfur bearing compound [reactions (11 and 12)].Hydrogen sulfide also can be brought directly intothe lake system by the volcanic vapor being injectedfrom depth.

Interestingly, neither the suspended solids nor thesediments sampled directly from the lake at depths of20–40 m in 1990 and 1996 contained elemental sulfur(Delmelle and Bernard, 1994; B. Takano, unpublisheddata); instead, poorly crystallized silica was the domi-nant precipitate. These observations are evidence forthe lack of elemental sulfur precipitation in the lakeitself, which simply may link with the absence of H2Sin the water. We propose that the factor controllingthe type of deposition relates to the ability of eitherH2S- or silica-rich volcanic discharges to enter thelake. This could depend on the efficiency withwhich the lake water circulates in the hydrothermalcell beneath the crater. During subsurface seepagethrough this region, the lake waters may dissolvemore silica as they become progressively heated.When re-entering the relatively cool lake, these hotwaters would be supersaturated with respect to silicapolymorphs, thus leading to massive precipitation.Meanwhile, the circulation of the relatively oxidizedlake waters at shallow levels may prevent H2S-richdischarges from reaching the crater lake, becauseH2S will be converted into elemental sulfur or/andSO4 at depth. In contrast, enhanced elemental sulfurand depressed silica deposition could result from slowhydrothermal recycling of the lake water, leavingmore H2S-bearing gas and less silica-rich water tobe introduced into the lake. Hydrothermal circulationof the lake water could cease temporarily as a result ofpartial sealing of the crater floor due to mineral preci-pitation in the cracks and fractures and encrusting ofmolten sulfur.

The spherules, slicks and pyroclastic materialrepresenting quenched molten sulfur products haved 34S values similar (except one sample in 1995) tothose of the lakeshore sulfur sediments (Table 4,Fig. 11). This may simply indicate that previously


deposited sulfur is remobilized at high temperature onthe crater floor, likely in the hydrothermal vent.Consequently, the production of SO4 in the lake–hydrothermal system in 1990–1996 during dispropor-tionation of magmatic SO2 preferentially occurred viareaction (3) instead of reaction (4). Again, this wouldagree with the generation of a H2S-rich hydrothermalvapor added to the lakeshore fumaroles.

7.4. Sulfur dioxide, hydrogen sulfide and sulfursublimate in the fumarole discharges

TheDSO2–H2S measured in Kawah Ijen fumaroles is13.1‰ (Table 4, Fig. 11), somewhat higher than the10‰ fractionation determined experimentally byGrinenko and Thode (1970). These authors notedthat sluggish reaction rates occur at temperaturesbelow 3008C, possibly explaining our values and theresulting high apparent equilibrium temperature of,3098C (Thode et al., 1971). The measuredDH2S–S0

is slightly negative (20.3‰), not consistent with theexpected direction of the isotopic exchange. However,this value is small (# 1‰) for the system H2S–S0 atT , 3008C (Grinenko and Thode, 1970; Ueda et al.,

1979), suggesting conditions close to equilibrium inthe Kawah Ijen gas discharges.

The 34S content of total sulfur (d34SSS) in sampleKIG-1 corresponds to:

d 34SSS � d 34SH2SxH2S 1 d34SSO2xSO2

�14�wherexH2

S andxSO2are, respectively, the mole frac-

tions of H2S and SO2 relative to total sulfur in thevolcanic vapor. The calculated value of 0‰ doesnot match the isotopic composition reported formagmatic sulfur in the crater rock (16.7‰) nor thattypical of island-arc lavas (15‰, Taylor, 1986). Thisprobably reveals removal of34S-rich SO2 from, andaddition of 34S-poor H2S to, the rising high-tempera-ture volcanic vapor during interaction with thesummit hydrothermal system. Remobilization ofelemental sulfur (Eq. (1)) coating the pipes throughwhich the fumarolic gas flows also may affect thed 34SSS value in the fumaroles.

8. Summary and conclusions

A schematic cross-section of the “immature”magma–hydrothermal system that can account for


Fig. 13. Conceptual cross-section of Kawah Ijen showing a model of the lake–hydrothermal system. See text for discussion. Symbols are as inFig. 2.

the chemical and isotopic characteristics of the ther-mal water and fumarole gas discharges at Kawah Ijenvolcano is depicted in Fig. 13. Absorption of magma-derived high-temperature gases into shallow ground-water produces a two-phase vapor–liquid hydrother-mal reservoir beneath the summit crater. The gasesbeing condensed are relatively oxidized�H2S=SO2 �1:1� and may have equilibrated in the liquid–vaporhydrothermal region at,3508C. The liquid phase ofthe hydrothermal reservoir consists of SO4–Cl waterswhich enter a convective cell through which the lakewater circulates. The lake SO4 is highly enriched withrespect to34S, a characteristic which accounts for thedisproportionation of magmatic SO2 at temperaturesof 250–2808C. The summit SO4–Cl liquid flows later-ally and mixes with meteoric-dominated water toproduce the spring discharges at Blawan and Paltud-ing. The crater spring originates in the same manner orsimply corresponds to diluted lake water seepages.The lake exhibits strong D and18O shifts which resultto a large extent from akinetic isotopic effect asso-ciated with evaporation at elevated lake watertemperatures. Added to this is the likely contribu-tion of the isotopically heavy magmatic vaporbeing condensed at depth. Water–rock interactionsin the subsurface hydrothermal system also mayinfluence the finald 18O values of the lake waters.The oxygen isotopic distance for the pair SO4–H2Oindicates quenched equilibrium temperatures closeto 1508C that could reveal sublimnic recycling ofthe lake waters.

The vapor phase in the hydrothermal reservoirconsists of a reduced, H2S-rich component whichmixes with the rising high-temperature volcanicgases to be subsequently discharged at the lakeshorefumaroles. Hydrogen sulfide comes from dispropor-tionation of SO2 and from the condensing hightemperature volcanic vapor. The gas in the fumarolesmay have equilibrated at,2608C. Time-series datashow that there is an interplay between the oxidized,high-C/S magmatic gas and the hydrothermally-derived, reduced, low-C/S vapor through time whichmay depend on the activity of the volcano. The fumar-oles in 1979 represented more magmatic conditionsthan in the 1990s. In 1996, however, the fuma-roles exhibited a trend towards more SO2-richcompositions.

Kawah Ijen lake produces two types of sulfur

material, one consisting of a precipitate accumulatedin banded deposits exposed on the lakeshore and thesecond occurring as a molten body on the crater floor.The precipitate was formed in the lake during oxida-tion of H2S released from the subsurface hydrothermalreservoir. Deposition of elemental sulfur is not contin-uous, however, but may ultimately depend on the lakedynamics. Rapid subsurface circulation of the lakewaters in the hot hydrothermal cell may cause rela-tively oxidized, H2S-poor, silica-supersaturated solu-tions to enter the lake, thus allowing massiveprecipitation of poorly crystallized silica. At theother extreme, elemental sulfur precipitates maydominate only if H2S-rich, relatively silica-poordischarges reach the crater floor owing to a limitedrecycling of the lake waters. The similard 34S valuesin the sulfur lakeshore deposits and molten sulfurproducts indicate remobilization at high temperatureof sulfur-rich sediments previously accumulated onthe crater floor.

The combination of chemical and isotopic dataobtained at Kawah Ijen in 1990–1996 haveprovided constraints on the origin of the fumarolegases, thermal waters and elemental sulfur. Thelocation and timing of polythionate formation andelemental sulfur deposition in the lake-hydrother-mal system deserve further attention. Our resultsshow that the geochemical dynamics of themagma–hydrothermal system may reveal changesin the volcanic activity.

Acknowledgements

P.D. was supported by a doctoral fellowship fromthe Fonds pour la Recherche dans l’Industrie etl’Agriculture (FRIA) in Belgium and by a VisitingResearch Scholarship from Mombusho in Japan.The Communaute´ Francaise de Belgique, theAcademie Royale des Sciences and the Alice vanBurren Foundation generously provided grants forfieldwork (PD). This research was a part of a FondsNational pour la Recherche Scientifique en Belgi-que (FNRS) program (AB). TPF acknowledgesfinancial support from a NASA Earth SystemScience Fellowship. We thank S. de Brouwer forhis fantastic assistance in the field and the sulfurminers of Kawah Ijen for their support and


hospitality during all these years. The staff of theVolcanological Survey of Indonesia offered excel-lent logistic support during the 1993, 1995 and 1996sampling campaigns. We are grateful to J. Hira-bayashi (Kusatsu-Shirane Volcano Observatory)for providing us with the 1993 gas compositionand SO2 and H2S gas samples, to S. Bottrell(University of Leeds) for helping with the isotopicanalysis of the 1990 water and sulfur samples and toH. Maeda (Kyushu University) for the rock sulfurisotopic analysis. PD and AB analyzed the anions inwaters at the Laboratory of Glaciology, Universite´Libre de Bruxelles with the assistance of R. Lorrain.An earlier version of this paper benefited fromdiscussions with D. Stevenson and G. Rowe.Comments from D.T. Gregory and an anonymousreviewer are acknowledged. J. Varekamp madeseveral suggestions that greatly improved the inter-pretation of the lake isotopic compositions. Wethank J. Stix for editing the English.

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

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