GeochemicalJourn al,Vol.28,pp.263to287,1994

O x y g en, h y d ro ge n, a n d s ulfu r isoto p e syste m atics o f th e crater la k e syste m o f P o ~s V olc a n o, C osta R ic a

G ARY L. R OW E, Jr.*

Departm entofG eosciences, The Pennsylvania State University, University Park, PA 16802,U.S.A.

(Received M ay 6,1993,･Accepted April 24, 1994)

O xygen, hydrogen, and sulfurisotope data for fluids and m inerals associated with the craterlake of po~s Volcano, Costa Rica, are interp reted in the context ofthe chem ical and hydrologic stru cture ofthe volcano.Oxygen and hydrogenisotope dataw ere obtained forrain,spring,and riverw ater,Iow-tem perature fum arole condensates, and acid brines collected from the hotcraterlake before its disappearance in A pril 1989. Flank river and spring w aters whose solute com positions have been m odified by volcanic and hydrotherm al activity have, with one exception,isotopic com positions similar to local m eteoric water. Acid chloride-sulfate brines ofthe sum m it craterlake are extrem ely enriched in 180 with respectto local m eteoric water;in the m ost enriched brines 180 shifts are greaterthan 200100.The 180 shiftisrelatedto a kinetic isotope effectassociated with theintense evaporation atthe surface ofthelake. These sam e brines exhibit only m inim alshiftsin theirD/H ratios. The apparentlack ofdeuterium fractionation in the brines is attributed to an increase in the flux ofisotopically light steam into the craterlake and/or a decrease in the deute rium fractionation factor for evaporation that occurs atthe surface ofthe lake. The decrease in deuterium fractionation is correlated with large increases in lake-brine acidity and dissolved solids con- centration thatpreceded the disappearance ofthe lake. Sulfurisotope data are presented for H2S and S02 gas collected from low tem perature fum aroles;dis- solved sulfatein spring,river, and craterlake w aters;and native sulfur and gypsum found in the acid lake and active crater area. As02-H2s forlow tem perature gasesis approxim ately 240100indicating an equilibra- tiontem peratureof165'C. As04-H2S forlow temperatu re H2S andlake brine sulfateisapproxim ately 230100, allindicating subsurface equilibration occured at 265'C. The H2S and native sulfur are both highly de- pleted in 34S (834S = -8to -1Ioloo). 634S valuesof34S-depleted H2S and 34S-enriched sulfate in lake brine are produced by disproportionation ofS02 released by the shallow m agm a body.N ative sulfuris form ed bythe oxidation of34S-depleted H2S by non-sulfu r-bearing oxidantssuch as atm ospheric oxygen and ferric iron. M ass-balance calculations indicate that sulfitolysis of polythionic acids could also result in the deposition of significant quantities ofnative sulfur.Im plications ofthe isotopic com position of present- day fluids observ ed at Po~s V olcano with respectto the isotope system atics of acid-sulfate ore deposits are considered.

the isotope system atics of crater lakes and their INTRODUCTION associated hydrotherm al system s is driven by Crater lakes hosted by active volcanos repre- econom ic, scientific, and hazard m itigation con- sent unique geochem icalenvironm ents. Such lakes cern s. For instance, stable isotope data are com - are the s urface expression ofhigh-level geotherm al m only used to evaluate the econom ic potential of system s and act as condensers for fluids and gases geotherm al system s associated with active volca- re leased by sh allow m agm a bodies (Giggenbach, nos (e.g., Traineau et al., 1989; Giggenbach and

1974; Bran tley et al., 1987, 1993). R esearch into C or ales, 1992). G eochem ical processes that oc-

*Presentaddress' U.S. GeologicalSurvey. Water Resources Division,975 West ThirdAve.,Columbus, OH 43212, U.S.A.

263

264 G. L. Rowe, Jr. cur in shallow, volcano-hosted hydrotherm al sys- 1). po~s lavas are calc-alkaline basalts and tem s are considered to be analogousto those that andesites; deposits exposed in the sum m it area and occur during the form ation of epitherm al acid active crater are com posed of approxim ately equal sulfate precio us m etal deposits(H ayba et al.,1985; volum es oflava and w eathered pyroclastic m ate-

Stoffgren, 1987; Rye etal., 199 2; H edenquist and rial (Prosser and C ar, 1987). Eruptive activity in

A oki, 1991; Rye, 1993; H edenqu ist et al., 1993). historic tim es has been confined to the active crater

The role of acid fluids generated by crater-lak e and consists of continuous low -level degassing, hydrotherm al system s in the form ation of acid frequent geyser-like phreatic eruptions in the cra- sulfate ore deposits has also received attention ter lake, and rare phreato-m agm atic eruptions

(R ow e, 1991; C hristenson and W ood, 1993). Fi- (Casert ano etal., 1983). nally, tem poral variations in the sul fur isotope A sm all but vigorous m agm atic-hydrotherm al com p osition of w ater collected from Y ugam a system at the sum m it of Po~s V olcano is charac- crater lake have been used to assess recent v olca- terized by (1) a hot, acid crater lake (300 m eters nic activity at K usatsu-Shirane V olcano, Japan in diam eter) w hose level, tem perature, and w ater

(O hsaw a et al., 1993). chem istry vary sharp ly in response to changes in

Th e purp ose ofthis paper is to sum m arize and rainfall,therm alpower output, and sum mitseismic interp ret oxygen, hydrogen, and sulfurisotope data activity (Brow n et al., 1989; R ow e et al., 1992a, rece ntly obt ained a t Po~s V olcano in the context b), and (2)the rem nants ofa 30-m eter-high pyro- ofa chemical and hydrologic m odel developed for clastic cone form ed during the last phreato-m ag- the sum mitcraterlake and associated hydr otherm al m atic eru ption of Po~s in 1953-54 (C asertano et system (R ow e et al., 1992a, 1994). These data al., 1983). This cone, on the southern edge ofthe p rovide insight into the geochem ical and hydro- pit crater occupied by the lake,is the site ofsub- logic processes associated w ith the form ation and aerial fum arolic activity of variable tem perature circulation of acidic chloride-sulfate brines ofthe and intensity (Casert ano et al., 1983, 1987; R ow e crater lake. The isotope data also provide infor- et al., 1992a). The crater-lake brines are am ong m ation regarding the origin of the sulfuric acid the m ost acidic natural w aters ever sam pled; pH responsible for the extrem e acidity of the lake ofthe acid-chloride-sulfate brines is usually near brin e,the origin of native sulfurin t he craterlake zero (T able 1). and its sedim ents, and the origin of the diverse Previous studies of the crater lake system solute com positions ofriversthat drain the flanks (C asertano et al., 1987; R ow e et al., 1992a) indi- of Po~s V olcano. Finally, im plications ofthe re- cate that hydrotherm alcirculation is driven by the sults of this study w ith resp ect to the isotopic release of heat and volatiles from a pipe-like system atics of epit herm al ore deposits are briefly m agm a body w hose upper m argin lies approxi- discussed. m ately 500 m beneath the floor ofthe active cra-

ter(R ym er and Brow n, 1987, 1989). H eattransfer

Geochemica/ and hydrogeologic structure of Pods is by a heat-pipe m echanism in w hich condensed

Volcano hydr otherm al steam and seeping lake brines are 'po~s V olcano,along with the adjacent volcanic revaporized near the chilled upper part of the centers of Platanar-Porvenir, B arva, and lrazti- m agm a body to supply heatto the overlyingliquid- Turrialba, is part of the Q uaternary C ordillera dom inated convection cell (H urst et al., 1991; C entral ofcentral Costa Rica(Fig.1). The sum m it R ow e et al., 1992a). R ecycling of lake brine and area ofPo~s consists ofthree nested calderas and condensed steam is a key feature ofthe craterlake an active crater bordered by tw o com posite cones: hydrotherm al system : volatile fluxes calculated V on Frantzius to the north (E = 2639 m) and from pow er outputestim atesindicatethatlessthan

B otos, the site of a 400 m eter diam eter cold 1Oolo ofthe heat su pplied to th e crater lake is de- freshw ater lake,to the south (E = 2708 m ) (Fig. rived by condensation ofm agm atic steam degassed

O,H and S isotope system atics of Poas V olcano 265

84'20' 84'16' 84'12' ~ ~¥ 1¥~00;// 1o'16' ,., ｢r

l ~ ~'d20'~ ¥ ' ¥o /i¥ ' b/: ~ ¥~( ~~ CERR r~ ¥b~i~) ' (/e '4 ~;' l: ( / ~ ~i ~Cl ~ l E'V / l ' {, ~ ~ 3~t, f~le I ¥ ~ :~f ' '~ '~ ¥ /./6;~] ¥,¥ 10'12' ~~ )!~.~;~2i¥ 4~i)~_~:~~ "~ "'A!/~:7n::~~ TZ~.~,1/;i1e~/f:~~{2 " / /~ ¥ ' / l ｢:l i /~ ' V ))~ <~ ~ / ( ( ' ( ~ ' 'IA~Inv ¥ / 5~ ~' .~1 ¥ - 9 T~ l l ' f ' ~ ~" VOpLCA (B~O ]NOES'j;~ ~: ( / j ~ IU~ / <:~~~~' ~.¥ . ~ t Ei /1 //l/ l ¥ prl 10'08' '/ // ' 7 2000 / OO l iI '>' *' *. /' .BAR~ *~ )// ' ~~-~:1"- + c! 11' ' / . , ~ p;oasIo ｢ ' ' / ｢' ' ' l'2Qb ~~ // COSTA' / l l'/ / /' RICAf (C'J . '. . l l ' . pacific ocea" , ! ~ 10004' 85' 84" 83'

o 2 4, KILO METERS

EXPLANATIO N I l l RIO AGRIO W ATERSHED TYPE OF WATER SITES--nvmberedsites have com positionaldata availabie O Acidchloride-sulfate l Neutralsulfate El Acidsulfate C) Neutralbicarbo nate - 2000 - ELEVATION--in m etersabo vesealevel. Contourintervai400 meters Fig. 1. M ap showing m ain topographic features of Pods Volcano and the distribution of fl ank water types. Num bered dots correspondto site num bersgiven in Tables 2 and 3.Inset.'M ap showing location of Pods Volcano (largertriangle) relativeto other volcanos ofthe Cordillera Central and northwestern Costa Rica (M othfi edfrom Rowe et al., 1994). directly from the m agm a (R ow e etal., 1992a). w as several hundred kilogram s per second (R ow e H eat and w ater m ass-balance calculations in- et al., 1992a). C hem ical and geologic data (R ow e dicate that dow nw ard-directed seepage of lake et al., 1994)indicate that leakage of acidic brine brines is an im portant com ponent of the hydro- is focused tow ards the Rio A grio w atershed on 10gic budget ofthe craterlake. Evaluation of data the northw est flank of the volcano. Fluxes of collected at Po~s during 1978-89 indicates that fluoride, chloride, and sulfur exitingthe Rio A grio the average rate of brine seepage from the lake w atershed require brine leakage rates of only 20-

266 G. L. Rowe,Jr.

Table 1. Comparison of key geochemicalproperties of Pods watersl Parameter Crater Agrioacid Fumarole AcidrainW Acidrain Crater NW flank Neutral Neutral lake Cl-S04 condensate craterrim W flank2 acid-S04 acid S04 sulfate HCp3-

pH 0.0 2.34 l.06 1.10 2.90 2.55 3.94 6.85 7.20 [FJ l270 25.3 114 39.4 3.6 33.7 l.21 0.37 0.lO [Cl] 221OO 460 5760 8240 22.0 366 28.6 7.1 2,9 [S04]T 59500 1180 3180 251 36.0 2760 131 41 10,l F/Cl 0.057 0.055 O.020 0.005 0.16 0.092 0.042 0.049 0,035 S04/Cl 2.69 2.56 O.55 0.03 l.63 7.54 4,56 5.86 3,39 Cl/B l247 l394 ND ND ND 1355 660 311 98.0 Ca/S04 0.018 0.067 O.009 0.06 ND O.21 0.21 0.35 0,88 IDatafrom Rowe etal.(1994). 2Valuesgiven are means of 17acid rainsamples collected betweenM ay 1985 and July 1986 atasite O.5kilometersouthwest ofwestcraterrim site (Rosario-Alfaroetal.,1986). ND = Notdetermined.

25 kgls (R ow e et al., 1994). This rate ofleakage tem (flank acid chloride-sulfate w aters); (5) neu- is only a few percent ofthe overall seepage flux tralization of acid sulfate w aters by m ixing and estim ated by R ow e et al. (1992a), an indication w ater-soil-rock interactions (neutral sulfate w a- that m ost brine is stored in the sum m it hydrother- ters);and (6) chem icalw eathering of volcanic soil m al system . B ecause seeping lake brines are and rock by w atersatu rated with C 02 derived from largely recycled, the hydrologic stability of the non-volcanic sources (neutral bicarbonate w aters). lake depends on the balance betw een w ater gained by rainfall-derived recharge (annualrainfall ~4 m / M ETH ODS yr) and w ater lost by evaporation from the hot lake surface. Sam pling locations are show n in Fig. I with Several w ater types have been found at Po~s nam es ofspecific sitesrefer ed to subsequently in V olcano including acid chloride-sulfate, acid sul- the textgiven in Tables 2 and 3,respectively. Rain, fate, neutral sulfate, and neutral bicarbonate w a- crater-lake, fum arole-condensate,river and spring- ters;the latteristhe m ost abundanttype (Table I, w ater sam ples w ere collected in 30 m illiliter Fig. I). C om parison of key chem ical properties polypropylene bottles. O xygen and hydrogen iso- (pH, dissolved solids concentration, conservative tope ratios of w ater and brine sam ples w ere ana- anion ratios)Ied R ow e et al.(1994)to propose that lyzed by use ofstandard techniques (Epstein and the different w ater types originate from a variety M ayeda, 1953; Colem an et al., 1982). Distillation of processes including (1) condensation of m ag- ofthe m ostconcentrated brines w as notattem pted; m atic volatiles beneath the crater lake com bined how ever, duplicate analyses ofa brine sam ple split with evaporative concentration atthe lake surface agreed within laboratory precision lim its expected (acid chloride-sulfate brines);(2) oxidation ofH 2S for random duplicate analyses (ｱ20100 for deuterium and native sulfur in the active crater (crater acid and ｱ0.10Voo for oxyge n) (R. H. Reesm an, written sulfate w aters);(3) acidification ofvapor,rain,and com m ., 1987). The 6180 values reported in Table river w ater by oxidation of S02 and H2S in gases 2 are uncorected for salinity effects because cor- released by the lake surface and subaerial fum a- rection factors estim ated from the m ethod ofSofer roles (acid rain and flank acid sulfate w aters);(4) and G at(1972) w ere within analytical uncertainty. verticaland lateralseepage ofacid chloride-sulfate R esults of hydrogen and oxygen stable isotope brines generated in the sum m it hydrotherm al sys- analyses given in T able 2 are reported relative to

O,H and S isotope system atics of Po~s V olcano 267

Standard M ean O cean W ater (SM O W ) (Craig, values for all sulfur sam ples w ere determ ined on 1961). a V G-Isogas Prism m ass spectrom eter and are re- 634S values w ere determ ined for several sul- ported relative to the C D T (Canyon D iablo Troi- fur-bearing phases atPo~s V olcano,including low- lite)reference standard.A nalyi icalprecision ofthe tem perature fum arolic gases (S02 and H 2S), dis- 634S valuesis ｱ0.10/00. s olved sul fate in cr ate r-lake brine s, acid chlo ride- sulfate spring and river w aters, acid sulfate river RESULTS w aters, a nd n ative sulfur and gypsum (Table 3).

S 02 gas w as absorb ed i nto a 4 N N aO H solution, Hydrogen and oxygen isotopes oxidized w ith H20 2, and then precipitated as Seasonal variations in the deuterium and oxy-

B a S04 by use of 0.1 M BaC12. B aS04 w as con- gen isotopic com position ofrainw ater at Po~s are v erte d to H2S by use of Kibareagent (Sasakietal., considerable. A vailable data indicate that rain

1 97 9 ), pre cipitated as Z nS, and then converted to falling during the dry season (D ecem bel~ A pril)is

A g2 S u sing a 0.1 N A gN 03 solution. T he A g2S only slightly to m oderately depleted relative to w a s then com busted (T > 9 00'C) under vacu um SM O W , w hereas rain falling during the rainy in the prese nce of C uO to produce S02 for m ass season (M ayLN ovem ber) is highly depleted (Fig. spe ctrom etric analysis. Dissolved sulfate in w ater 2, Table 2). The average isotopic com position of sam ples and total sulfur in a pow dered sam ple of the rainw at er collected at Po~s (6 D = -54'/oo and basaltic andesite lava eru pted in 1954 w ere simi- 6180 = -7.40/0') is sim ilar to that of flank spring larly prepared forisotopic analysis.Fum arolic H2S and river w aters (6D = -470100 and 5180 = 7.00Voo) w a s pr ec ipitated as As 2S3 afte r absorp tion into a (Fig.2, Table 2). The averageisotopic com position l N perchloric acid solution contai ning 0.1 N of surface w ater collected during periods of base

A s20 3 (Gig gen bach, 1975). A s2S3 w as then con- fl ow is likely to represent the annual, seasonally- ve rtedto A g2S by use of a 0.1 N A gN 03 solution. w eighted isotopic com position of m eteoric w ater

N ative sulfur sam ples w ere slow ly heated to recharging flank aquifers. This value is used to l15 'C to r ecrysta llize the sulfur and w ere then characterize the b ulk isotopic com position of dissolved in H PLC grade acetone. To produce cu- ground w ater in sum m it and fl ank aquifers unaf- prous sulfide for isotopic analysis, the acetone f ected by hydrotherm al activity and w ill be re-

fered to subsequently as "average m eteoric w ater " solution w as m ixed with a solution of cuprous (A M W ). Rain,spring, and river w aters plot on or am m onium hydroxide and hydroxylamine hydro- very close to t he m eteoric w ater line (M W L) of chloride: C raig (1961). A n exception is w ater from the

2Cu2+ + S. + N H40 H + 2N H20 H･H CI + 2e~ therm al spring ofthe A ffluente de A grio,in w hich

180 is shifted approxim ately 30100 to the right of ~ Cu2S(s) + N H3 + N2 + 3 H20 + 2Cl- + 4H+ (1) the M W L (Fig. 2). Isotopi c data for fum arole

co ndensates is limited to four low -tem perature

sam ples( l08' to 330'C) collected in January and (M ack and H am ilton, 1942). Cu2S yields ranged June 198 8 . 6 D values of the condensates are from 77 to 127010. D eficient yields m ay be related s imilar to those of A M W ; however, the conden- to difficulty in recovering all ofthe colloidal Cu2S s ates are enr iched s eve ralperrnilin 180 relative to precipitate from polycarbonate filter m em branes A M W . used to filter the Cu2S-bearing acetone solutions. Crater-lake brines sam pled betw een January Excess yields m ay be related to incom plete drying 1987 an d M arch 1990 are highly enriched in 180; ofthe Cu2S precipitate. The Cu2S w as then burn ed 1 80 -shifts ra nge from 14 to 20.5~.. relative to w ith C uO to produce S02 for isotopic analysis. A M W (T able 2, Fig . 2). M axim um D enrichm ent D uplicate sam ples ofnative sulfur prepared by this m ethod yielded 634S values w ithin ｱ0.5Voo. 834S is 36%o, although several brines collected after

268 G. L. Rowe, Jr.

40

MW L 20

o -*- J ~ l l UGJ: ･20 ,L e l Z A A A A A l O -40 AAJL I A l c,O AD I

40

~o o e

･100 -14 ･12 ･10 4 S ･4 8 IQ 12 14 16

6180, IN PER MIL

EXPLANATIO N

dL Fum arole Oondensate W ater D Riverand Spring W ater

A Brine Po olW 8ter I Crater-Lake Brine

O Rainwater _*_ SM OW Fig. 2. 6D-6180 plot of various water types collected at Pods Volcano. M WL = M eteoric water line of Craig (1961). SM O W = Standard M ean Ocean Water.

June 1988 show little deuterium fractionation with lakeshore. Pool w aters are enrichedin 180 by 4 to respect to A M W . A trend tow ards less enriched 19.50/00 relative to A M W but are only slightly en- com positions for deuterium and 180 isindicated by riched in D (8 to 270yoo, Table 2). the datain Table 2 for sam ples collected after June 1988. Also show n are the isotopic com positions Sulfur isotopes of several sm all brine pools that form ed in lake The range of 634S values determ ined for sul- sedim ents exposed by the decline in lake level. fur-bearing fluids and solids of Po~s V olcano ex- These pools ranged from sm all depressions a few ceeds 25%o (Fig. 3). Sulfurisotope data for Po~s centim eters across to pools several m eters in di- gases are restricted to low -tem perature S0 2 and am eter. Tem peratures of the pools ranged from H 2S collected from three low -tem perature (94- 56' to 98'C. Tem perature ofthe pools w as posi- 95'C) fum aroles exposed by the decline in lake tively cor elated with the intensity of subaqueous level sam pled in Febru ary 1989. 634S values of fum arolic activity atthe base ofthe pool. M ost of H2S in these sam ples range from -8.0 to -10.20100 the brine pools sam pled in this study w ere at the and average -9.4%o. S02 from the sam e sam ples base of the eroded cinder cone at the south has a 834S value of14.50~oo. The 634S value ofS02

O, H and S isotope system atics of Po~s V olcano 269

Table 2 . Deuterium and oxygen isotope datafor Pods waters Locationl Date Sam ple description (sample nurnber#), Water 5D (o/on) 6180 (o/*o) temperature,and elevation type2 AC3 1/24/85 Crater Lake Brine, T= 45'C,E = 2310 m ACS Nodata 5.6 AC 1/9/87 CraterLake Brine, T= 61'C,E = 2300 m ACS -14 12.3 AC 11127187 CraterLake Brine, T= 61'C,E = 2300 m ACS -26 9.8 AC 1/26/88 CraterLake Brine, T =64'C,E = 2300 m ACS -13 13.5 AC 6/24/88 CraterLake Brine, T= 65'C,E =2300 m ACS -38 12.l AC 2/7/89 CraterLake Brine, T= 77'C,E = 2300 m ACS -39 7.3 AC 8/8/89 CraterLake Brine, T =87'C,E = 2300 m ACS -33 7.3 AC 3/2/90 CraterLake Brine, T =94'C,E = 2300 m ACS 47 10,6 AC3 l/24/85 Fumarole Cond, T= 584'C,E =2320 m ACS No data 3.0 AC 1/10/88 Furnarole Cond, T= 300'C,E =2320 m ACS -49 4,6 AC 1/12/88 Fumarole Cond, T= 330'C,E =2320 m ACS ~37 0.0 AC 1/19/88 Fumarole Cond, T= 108'C,E = 2320 m ACS -4O -2,0 AC 6/24/88 Fumarole Cond, T= 286'C,E = 2320 m ACS -4O -2,6 AC 6/24/88 Lake Brine Pool, T = 98'C,E = 2300 m ACS -20 12.5 AC 1/27/89 Lake Brine Pool, T = 91'C,E = 2300 m ACS -37 -3.0 AC 1/27/89 Lake Brine Pool, T = 86'C,E = 2300 m ACS ~33 -2.4 AC 1/27/89 Lake Brine Pool, T = 61'C,E = 2300 m ACS -34 -1.3 AC 1/27/89 Lake Brine Pool, T = 64'C,E = 2300 m ACS -39 8.9 AC 1/27/89 Lake Brine Pool, T = 56'C,E = 2300 m ACS ~34 -O.6 AC 1/27/89 Lake Brine Pool, T = 68'C,E = 2300 m ACS -35 2.1

#7 3/7/90 Affluente Spring, T = 56'C,E = 2000 m ACS -46 -4.O #4 l1127187 Rio Agrio, T= 19'C,E = 1280 m ACS -43 -6,6 #3 11127187 Rio Desagtie, T= 17'C,E = 1320 m AS ~39 -6,l #2 11127187 Rio Anonos, T = 18'C,E = 1420 m NS -48 -7,6 #9 1/7/88 Desagiie Spring, T= 13'C,E = 2390 m AS -5O -7,4 #8 1/8/88 Agrio S pring #1, T = 20'C,E = 1800 m ACS -46 -7,O #8 1/8/88 Agrio S pring #2, T = 22'C,E = 1780 m ACS -51 -7.l #6 1/9/88 Agrio Spring #4,T = 20'C,E = 1520 m ACS -47 -7.0 #6 1/9/88 Affluente de Agrio,T = 19'C,E = 1510 m ACS -47 -6.5 #1 1/13/88 Rio Gorrion, T = 18'C,E = 1440 m NC -49 -7.0 #4 8/7/89 Rio Agrio, T = 19'C,E = 1280 m ACS -50 -6.8 #6 3/12/90 Agrio Spring #4,T = 20'C,E = 1520 m ACS -48 -7.4 AC 1/19/88 Rainwater-1953 54 Cone, E = 2320m AS 2 -1.0 AC 6/26/89 Rainwater-Sum mitEast Rim, E = 2450 m AS ~77 -lO.8 #3 6/26/89 Rain water-Rio Desagtie,E = 1320 m AS -86 -11.1 #4 6/26/89 Rainwater-Rio Agrio,E = 1280 m AS -92 -12.l AC 8/8/89 Rainwater-Summit W est Rim, E =2440 m AS -47 -6.5 AC 3/17/90 Rainwater-Summit W est Rim, E =2440 m AS -24 -3.1 ILocationsgiven on Fig.1. 2ACS = Acid chloride-sulfate water.AS = Acidsulfate water.NS = Neutralsulfate water.NC = Neutralbicarbonate water. 3D atafrom J.R.Reyolds (written comm., 1987). pum ped from a high-tem perature (T = 584'C) fu- from the exposed core ofthe 1953-54 cinder cone m arole condensate collected in 1985 is 7.10Voo (J. (Table 2). R. R eynolds, written com m ., 1987). This value is 834Ss04 values of dissolved sulfate in crater sim ilar to the 834SｷS value of 8.30/00 for an unal- lake brine colle cted betw een M arch 1985 and tered sam ple of basaltic andesite lava obtained M arch 1990 range from 12.3 to 15.5%o and aver-

270 G.L. Rowe,Jr.

Table 3. Sulfurisotope dataforPods Volcano

Sitel Date Sample descripti on/watertype 534s AC 3/4/85 CraterLak eBrine S04;A CS2 15.5 AC 1/9/87 CraterLak eBrine S04;ACS 14.4 AC 1/26/88 CraterLak eBrine S04;ACS 13.4 AC 6/24/88 CraterLak eBrine S04;ACS 13.5 AC 2/7/89 CraterLak eBrine S04;ACS 12.9 AC 8/8/89 CraterLak eBrine S04;ACS 12.3 AC 3/2/90 CraterLak eBrine S04;ACS 13.1 AC 1/27/89 Lak eBrinePoolS04;ACS 7.1 AC 1/27/89 Lake BrinePoolS04;ACS 13.2 AC 1/27/89 Lake BrinePoolS04;ACS ll.3 AC 1/27/89 Lake BrinePoolS04;ACS 13.2 AC 1/27/89 Lake BrinePoolS04;ACS ll.6 AC 1/27/89 Lake BrinePoolS04;ACS 13.3 #7 3/7/90 Affluente SpringS04;ACS 8.8 #4 2/14/87 Rio Agrio S04;ACS 11.6 #4 11/27/87 Rio Agrio S04;ACS 10.1 #4 1/9/88 Rio Agrio S04;ACS 11.9 #2 1l/27/87 Rio Desagtie S04;AS 4,1 #8 1/8/88 Agrio Spring #1 S04;ACS 10,l #8 1/8/88 Agrio Spring #2 S04;ACS 10.0 #6 1/9/88 Agrio Spring #4 S04;ACS 11.2 #5 1/9/88 QuebradaPilasS04;AS 11.2 AC 6/24/88 CraterSpringS04;AS 7.5 AC 1/20/85 S02pumpe dfrom lakebrine2 15.1 AC 1/20/85 S02pumpe d from gascondensate2 7.0 AC 2/2/89 S02from gascondensate T= 94'C 14.5

AC 2/2/89 H2S from gassample T= 94'C -8.O AC 2/2/89 H2S from gassample T= 94'C -9.9 AC 2/2/89 H2S from gassample T= 94'C -10.2 AC 1/21/88 S1-Pyroclastic sulfur ejecta(1/88 eru p) -10.6 AC 4/15/88 S2-Pyroclasticsulfurejecta(4/88 erup) -11.1 AC 8/8/89 S3-Pyroclasticsulfurfrom SE cones -11.4 AC 2/2/89 S4-Pyroclastic sulfurfrom 1979-1980 -11.0 AC 1/5/87 S5-Lak eshoresulfurtube -10.4 AC 3/2/90 S6-M oltensulfurfrom NE cone -11.O AC 10/20/89 S7-Sulfurflow from N cones -9.4 AC 2/2/89 S8-Green Sulfurflow atbase53-54 cone -10.5 l/1/88 Gypsum,DesagtieCanyon E - 2300m 7.1 AC 1/1/88 Gyps um,Low-Tfumaroleon 53-54cone 3.0 AC 2/7/89 Gypsum,Crater Lak e Brine Pool 14.6 Isitelocationsgiven on Fig.1. 2Datafrom J.R.Reynolds (written comm.. 1987). age 13.5%o (Table 3). 634Ss0+ values oflak e brine and 1989 have lighter 634Ss0+ values, that aver- age 11.7%o. Sulfate from the hottest pool (T = declined fr om a peak of 15.5'/oo in early 1985 to 12.3%o in mid-1989. Brine pools fonned on the 91'C), i s notably lighter (634Sso* = 7.10Vo o) than lake botom by the decline in lake level in 1988 sulfate collected from the other brine pools (Table

O, H and S isotope system atics of Po~s V olcano 271

634s

-15 -1O -5 O +5 +1O +15 +20 Lake Brine S04 I I Brine PoolS04 I I l l Fiank Acid Cl-S04 Acid S04 I l I Fum arote S02 Fumarole H2S I Native Sulfur I l l I Gypsum I 1953-54 Andesite ｷS

Fig. 3. Range of 634S valuesfor sulfur-bearingfl uids and mineralsfound atPods Volcano.

3). This pool w as the sm allest pool sam pled (vol- DISCUSSIO N um e <1 Iiter) and w as heated by an H 2S-rich fu- Hydrogen and oxygen isotopes m arole. 634Ss04 values of acid chloride-sulfate Flank acid chloride-sulfate and acid sulfate waters w aters draining the Rio A grio w atershed range O xygen and hydrogen stableisotope data for flank from 8.8 to 11.90100 and average 10.70/00. 834Ss04 acid sulfate and acid chloride-sulfate w aters of values of dissolved sulfate in the tw o acid-sulfate po~s V olcano plot on or near the m eteoric w ater rivers draining the northw est fl ank are quite dif- line of Craig (1961) indicating a m eteoric origin ferent,ranging from 4.1 to 11.2%o(Table 3). forthese w aters (Fig. 2).These results are consis- 834S values w ere determ ined on eight native tent w ith the hypotheses of R ow e et al. (1994), sulfur sam ples and three gypsum sam ples (Table w hich suggestthatacid-sulfate w aters are produced 3). Several types of native sulfur w ere sam pled by the interaction of fl ank surface w ater and including (1) native sulfur ejected by phreatic shallow ground w ater withthe S02-rich gas plum e eru ptionsin the craterlake (S1, S2, S4);(2) pyro- em itted by the active crater and acid chloride-sul- clastic ejecta and spatter associated with the for- fate w aters are produced by m ixing of m eteoric m ation of sm all sulfur cones on the lake bottom w ater with seeping crater lake brines. C om pari- (S3, S5-S7) (see O ppenheim er, 1992); and (3) son ofthe average concentrations of the conser- hollow sulfur tubes form ed subaqueously in the vative dissolved species F, Cl, S0 4 and B in av- sedim ents lining the lake bottom (S5) (Table 3). erage lake brine w ith their respective concentra- N ative sulfur is uniformly depleted in 34S with tions in acid chloride-sulfate spring w aters yields respect to C D T: 634S values range from -9.4 to dilution factorsthatrange from 48 to 55 (R ow e et -11.40Voo and average -10.70Voo (n = 8). These val- al., 1994) (Table l). D espite the extrem e isotopic ues are similarto those reported by O ppenheim er enrichm entshow n by thelake brines (180 shifts of (1992)(average 634S = -10.70/00,range 834S = -9.4 14 to 200/00, D shifts as great as 360Voo),the degree to -12.3%o). 634S values of gypsur n are positive of dilution indicates that recognition of an en- and range from 3.0010 for a sam ple of gypsum lin- riched-brine com ponent in acid chloride-sulfate ing the vent of a low -tem perature fum arole (T = w aters draining the northw est flank of Po~s will 290'C) to 14.60/00 for gypsum precipitating in a be difficult (D or 180 shifts of 0.2-0.5%o only). pool of evaporating lake brine. A sam ple of gyp- A clearly discern able 180-shift of about 30/00is sum coating the w alls of a canyon in the upper only seen in the concentrated acid chloride-sul- drainage of Rio D esague (at 2300 m ) has a 634S fate w ater collected from the A ffluente de A grio value of 7.IoVoo. hot spring (Fig. 2). Although 180 shifts in geo-

272 G. L. Rowe,Jr.

therm al w aters have often been attributed to w a- D value is consistent with the isotopic com posi-

ter-rock exchange (e.g., Craig, 1963), the shiftin tion defined by Giggenbach (1992) for m agm atic

the isotopic com position ofthe A ffluente de A grio w aters degassed from andesi tic m agm as a t con-

therm al spring can also be explained by sim ple vergent m argins. Tem poral trends in F, Cl, and S

m ixing of a D and 180-enriched brine with m ete- concentratio ns of fum arole condensate sam ples

oric w ater. The average dilution factor calculated collected betw een 1981 and 1988 w ereinterp reted

from the F, C1, S04, and B concentration data re- by R ow e etal.(1992a) as representing progressive

ported by R ow e et al. (1994) forthe A ffluente hot contam ination of cinder cone fum aroles by

spring and average lake bri ne is about 5.5. This volatiles derived from acid brines that invaded

dilution factor indicates that the A ffluente spring fum arole conduits as m agm atic gas fluxes de-

w ater is mixed w ater com posed of 820/0 m eteoric clined.

w ater and 18010 enriched b rine. A sim ple isotope Sim ilar to fum aroles of W hite Island, N ew

m ass-balance calculation m ade by assum ing th at Zealand (Giggenbach, 1987), the fum arolic dis-

A ffluente spring w ater (8D = -46%o, 6180 = -4.0) charges of Po~s are pr obably com posed ofa m ix-

is produced by nlixing with A M W (8D = -47%o, ture of high-tem perature m agm atic w ater, acid

6180 = 7.00Voo ) according to the m ixing percent- brine generated in cooler parts ofthe sum m it hy-

ages defined above requires an enriched-brine drotherm al system, and m eteoric w ater. The rela-

com ponent w hose isotopic com position is 8D = tive proportio ns ofthese com ponents will vary as

-41%o and 8180 = +9.5. These values are sim ilar a fun ction oftem perature,m agm atic gas flux,and

to the 6D and 6180 values of P0~s lake brines the rate that infiltrating rainw ateris vaporize d in

show n in Table I. Conserv ative anion ratios (S04/ the hottest parts of cinder cone. T he intensity of

Cl, F/Cl, Cl/B) in the A ffluente spring w ate r are fum arolic activity on the 1953 54 cone had de-

sim ilar to thos e reported for average crater lake clined dram atically by 1988, and the condensates

brine (R ow e et al., 1994). Taken together, these probably containe d a signi ficant com ponent of

observations support the hypothesis that the 180 m eteoric w ater. This hypothesis is sup ported by

shift in the A ffluente de A grio spring w ater is the 6D values ofthe 1988 condensates, w hich are

caused by m ixing and dilution of acid brine by similar to those of flank spring and river w aters m eteoric w ater. (Fig. 2); how ever, the 6180 values of the 1988

F um arole condensates Several cyclical trends in condens ates are enriched in 180 by 2.4 to 7.00Voo tem perature and chem istry w ere noted atsubaerial relative to A M W (Table 1). A hig h-tem perature fum aroles at Po~s betw een 1978 and 1990. Sharp condensate(T = 584'C) colle cted in Febru a ry 1985 increases in fum arole tem peratures w ere cor elated had a 6180 value of +3.0%o (J. R. R eynold s, writ- withincreasesin sum mitseism ic activity th atw ere ten com m., 1987). interp reted by R ow e et al. (1992a) to represent H ypothesesto explain the observed enrichm ent hydrofracturing of the chilled m argin of the in con densate 180 values include (1) boiling of m agm a body. A fter a m ajor period of s eismic ac- 180-enriched acid brine at depth, (2)isotopic ex- tivity in July 1980, cinder-cone fum arole tem - change with high-tem perature w allrock, or (3) peratu resincreased from lessthan 1000C to greater m ixing of m eteoric w ater with 180 -enriched than 10200C (C asertano et al., 198 3). High- tem - fluid(s). Single-step boiling of the least lgO-en- perature gases sam pled in 1981 w er e near-equi- riched lake brine (6 D = -330Voo, 6180 = 7 .3%o, librium m ixtu res with respect to hydrogen, c ar- ITinitial = 3500C T = 100'C) produces steam , boihng bon, and sulfur redox couples and had undergone with an isotopic com position of 6D = 54%o, 6180 only m inor chem ical m odification during as cent = 2.1%o. Th is com position is enriched in 180 or sam pling (D elorm e, 1983; Row e, 1991). T he relative to the heaviest condensate w ater and de-

8D value of a 940'C condensate w ater collected pleted in D relative to A M W (Fig. 4). Although in early 1981 w as -260Voo (D elorm e, 1983). This boiling of dow nw ard-seeping la ke brine is an in-

O, H and S isotopesystem atics of Po~s V olcano 273 tegral part of the heat and w ater-m ass balance w hereas the bulk isotopic com position ofunaltered m odel ofthe craterlake proposed by R ow e et al. po~slavas and pyroclastic m aterialin the sum m it (1992a), it cannot be the dom inant process con- edifice is equivalent to 6D = -700Voo, and 8180 = trolling the isotopic com position of low-tem pera- 7.00Voo (Field and Fifarik, 1985). The exchange ture steam released by the subaerial fum aroles of curv esindicate that m axim um enrichnre nts of 170100 the cinder cone. in D and 100100 in 180 are pos sible at high tem - The effects of w ater-rock exchange at high peratures (350'C) and low w ater-rock ratios (W / tem perature and varying w ater-rock ratios w ere R = 0.01) for static pore fluids. For the m ore re- evaluated by use ofthe equations and fractionation alistic case ofm eteoric w aterundergoing exchange factors given by O'Niel and Taylor(1967) and H. during heating from 100 to 350'C sim ilar enrich-

P.T aylor (1974). The isotopic com position ofthe m entsin D and 180 are observ ed although greater initial w ater w as assum ed to be equivalentto av- enrichm ents in D (up to 30%.) are observed dur- er age m eteoric w ater (8D = -470Voo, 6180 = -70%oo) ing the initial exchange period (T 's < 150'C) at

20

M W L _*_

o Andesitic Sin9le-Step W aters Boilin9 curve -20 I SJ o 200 2500 eoOQ //r J'-1 C 3500 IJ'~ UJ A ~ 40 / / Z // I Q ll/ Primary ec Water-Roc kExchangeCurves ,r 40 Magmatic Waters

~Do

-100-10 dB ~ 4 .2 6 8 IQ 12 14 6180,IN PER MIL

EXPLA NATIO N I Crater-Lake Brine A Fum arole Condensate W ater

-*- S MO W e Average Meteoric W ater(A MW ) Fig. 4. 8D-8180 plotshowing trends in isotopic composition caused by isotopic exchange between AM W (6D = -47%o, 8180 = -7.0%o) and sum mitcountry rock (8D = -70%o, 6180 = 7.0%o). The exchange trends atconstant temperature were calculated by varying the w ater-rock ratio (W/R)from O.O1 to 30. Also shown is the trend in isotopic composition of steam produced by single-step boiling oflake brine (Tinitial = 350'C, Tboting rangedfrom 300 to 100'C). D and 180 fractionation factors for single-step boiling arefrom Truesdell et al. (1977). Boxes outlining the isotopic composition of prim ary magm atic water and andesitic water are from Sheppard et al. (1969), and Giggenbach (1992). respectively.

274 G. L. Rowe,Jr.

low w ater-rock ratios. These shifts are com patible cannot be solely attributed to the effects ofisoto- w ith the range of 8D and 6180 values reported for pic exchange w ith sum m it volcanic rocks. 180

low -tem perature Po~s condensates (Fig. 4). shifts of 14 to 200100 w ould require equilibration

The 6180 and 8 D values of the lo w-tem pera- w ith rocks w hose 6180 values are b etw een 12 and ture condensate w aters can also be produced by 180100 (at r s > 250'C). Such 8180 v alues are out- m ixing of m eteoric w ater with an 180 -enriched side th e range of 6180 values norm ally recor ded fluid. This fluid could be high-tem perature m ag- for calc-alkaline basalts and andesites (T aylor,H . m atic w ater, Iake brine, or steam derived from P., 1986) and are significantly higherthan the 6180 single or m ultistage boili ng oflake brine at depth value of 6.5%o reported by M ontigny et al.(1969)

(Fig. 4). M ixing of fluids of m eteoric, m agm atic, for andesites outcropping in the ac tive crater of and hydrotherm al origin is com patible with the po~s. Furtherm ore, th e a verage sodium concen- cyclic variations in fum arole tem perature and tration of the lake brine indi cates an integrated chemistry reported by R ow e et al.(1 992a); there- w ater-rock ratio near 30 (R ow e et al., 1994). fore, even though the 180-shifts observed in the M easurable enrichm ent with respectto deuterium condensate w aters could be produced by w ater- will not occur under such high w ater-rock ratios, rock exchange at elevated tem peratures (Fig. 4), although 180 shifts approaching 90Voo could occur the dom inant process controlling the isotopic at tem peratures approa ching 35 0'C (Fig. 4). com position of the low-tem perature condensate The refore, the preferre d hypothesis for ex- w atersis probably mixing ofinfiltrating m eteoric plaining the D and 180 enrichm ents in the lake w ater with vary ing am ou nts of rem obilized hy- b rines is a kinetic isotope effectthat accom panies drotherm al brin e or m agm atic w ater. evaporation of w ater from the hot lake surface.

Crater-lake brines Crater-lake brines are the m ost Evaporation from heated pools and lakes at el- isotopically evolved w aters on Po~s V olcano and evated tem peratures (60'-80'C) can resultin iso- display m axim um D and 180 enrichm ents of 36 topic enrichm ents as great as 50%o in D/H and and 20.50Voo relative to A M W (Table 1). Som e of 17.6%o in 180/160 (M atsubaya an d Sakai, 19 78; the brine sam ples plot nearthe range ofcom posi- Giggenbach and Stew art, 1982). 8 D-6180 slopes tions considered t o be representati ve of prim ary calculated for Po~s lake brine are less than 2.9, m agm atic w aters (Sheppa rd et al., 1969). Other the slope predicted by the net fractionation factors sam ples plotcloseto or within the field ofisotope reported by M atsubaya and Sakai (1978) for ratios representative of w aters equilibrated with steady-state evaporation at60' or 80'C.T he slope andesitic m agm as produced at convergent plate of the line connecting the m ost D -enriched lake boundaries (G iggenbach, 1992) (Fig. 4). Three brine (6 D = -110/00, 8180 = 12.30 Voo) to A M W is hypotheses can be advanced to explain the D and 1.9, w hereas the slope connecting A M W to the

18 0 enrichm ents in the lake brin e: (1) fraction- m ost180-enriched sa m ples (5D = -130Voo, 6180 = ation associated with evaporation from the hotlake 13.50Voo)is 1.7.These slopes a re sim ilarto the slope surface, (2) isotopic exchange betw een w ater and of 1.6 given by the line connecting the isotopic volcanic rock at high tem per atures, and (3) direct com pos ition of steam -heated, acid sulfate pools degassing of high- tem perature m agm atic gases. to local m eteoric w ater atthe ElT atio geotherm al

Th e third hypot hesis can be discounted on the system in C hile (Giggenbach, 1978) . basis ofheata nd m ass-balance calculations w hich A ccording to Gigg enbach (1978) ,the slope of indicate that direct degassing of high-tem perature a line (a), co nnectin g the isotopic com position of m agm atic volatiles accounts for only a sm all per- st eam -heated pools affected by evaporative frac - centage ofthe lake's heat and w ater budget(R ow e tionation is etal., 1992a).

The exchange curves show n in Fig. 4 indicate 6 Dsi ~ 6 D . + 8' thatthe extrem e 180 enr ichm ents ofth elake bri ne s (T = wr D (2) 6180si ~ 6180 wo + 8'180

O, H and S isotope system atics of Po~s V olcano 275 w here 5w and 6* refer to the D/H or 180/160 ra- total acidity ofthe brine. Total acidity is approxi- tios ofthe w ater or steam phase,respectively, and m ated asthe m olalsum ofthe strong-acid anions, the subscripts i and o refer to nuids entering or chloride and sulfa te (m a + m s04)' A plot of the leaving the pool. 8'isthe net fractionation factor data show s that 8 Db*i** - 8 D AMW rs negatively for D or 180 for evaporation from the surface of cor elated with m a + ms04 (Fig. 5). H ow ever,the the pool at elevated tem perature. Equation (2) corelation is only m ode rate (r = 0.80 by le ast- im plies that reduced 6D -8180 slopes can be pro- squares regression). The absence of a strong cor- duced by decreasing the values of 6*i (i.e., m ak- relation suggests that the increase in lake-brine ing the ascending fluid-vapor m ixture that heats acidity w as o nly partially responsible for the low the lake isotopica lly lighter). This decrease w ould 8Dbrin e ~ 5D AMW values observ ed after m id-1988. be possible if hydrotherm al brines boiled at shal- Thu s , it is li kely that a com bination of factors low er depths and,therefore atlow ertem peratures. (increased acidity, declining lake level,increased

The shallow er slopes defined by the data show n boiling atshallow e r depths) contributed to the low on Fig. 4 (a's near 1.0) cor espond to lake brines 8D-6180 slopes observ ed at Po~s before thelake's collected after m id-1988, w hen the sharp est drops disappearance. in lake level w ere observ ed (R ow e et al., 1992a). I n sm all pools of brine that form ed as lake

A Iow erlakelevel(i.e.,Iow er hydr ostatic pressure) level d eclined, 8D values are sim ilar to Po~s is consistent with shallow er and cooler boiling A M W , but 180 shifts range from 4 to 19.50100 beneath the lake. (Table 2). Field observation s indicate that w ater

The apparentlack ofD fr actionation in the lake i n the se po ol s is der ived from (1) brine derived brine afte r June 1988 could al so be related to in- from th e receding lake, (2) fum arolic steam con- creasesin lake-brine acidity that accom panied the densing at the bottom ofthe pools, and (3) rain. decline in lake level in 1988 and 1989 (R ow e et A plot o f l og dissolved solids as a function of al., 1992b). Theoretical m odeling of the isotopic 8180 (Fig. 6) reveals a linear trend thatis indica- fractionation that accom panies the evaporation of tive o f m ixing betw een a concentrated, isotopi- concentrated acid solutions, indicates that vapor call y heavy brine and dilute, isotopically light from highly acidic solutions will be enriched in w ater. The 8 D and 8180 val ues of the l ightest deuterium relative to vapor derived from pure brine-pool w aters are sim i lar to those of low- w ater (D eines, 1979). D uring evaporation, vapor te m peratu re fu m arole condensates (Fig. 3). This released from an acidic solution w ill be enriched sim i larity suggests that w ater in these pools is in D relative to vapor released fr om pure w ater. prim arily com p osed oflow-tem peratu re condensate

Fractionation factors for the various acids as a w aters sim ilar to those degass ed from subaerial function oftem perature and acidity have not been fum aroles ofthe 1953-54 cinder cone. determ ined. A cc ording to Deines (1979) how ever, evaporation of a 10 w tolo H CI solution at 20'C Sulfur isotopes w ou ld yield w ater vapor that is approxim ately H ydrogen sulfi de and sulfur dioxide in subaerial

200Voo heavierthan w ate r vapor released from pure fumaroles Low -tem perature gases collected from water atthe sam e tem peratu re. lake-bottom fum aroles in Febru ary 1989 have a

The calculations of D eines(1979)indicate that As04-H2S Value of 23.90Voo (T able 3, Fig. 2). A c- the low 6 D -6180 slopes of the lake brine are cording to the fractionation factors given by partly related to large increases in lake brine O hm oto and R ye (1979)the observ ed fractionation acidit y that accom panied the decline and disap- betw een S02 and H2S indicates an isotopic equi- pea r ance of the lake (R ow e et al., 1992b). This librium tem perature of approxim ately 165'C. Al- h ypothesis is supported by com pari ng the degree though physically reasonable, the validity of this ofdeuterium enric hm entin the lake br ines relative tem perature is questionable because these gases to A M W (6 Db*i** - 6 D AMW) as a function ofthe are notin chemical equilibrium . Results of equi-

276 G. L. Rowe, Jr.

40

D D :J~ 30 a: UJ ,L Z 20 D ~: = Q,, l ~ 10 l l QaD ~ o

-10 1 2 6 7 [CI+ SO d,IN M OLES PER KILO QRAM

EXPLA NATIO N

D Lake Brine(Sam ptes Collected before June,1988)

l Lake Brine(Sam ples Collected afterJune1988)

Fig. 5, D euterium enrichment of craterlake brines as afunction of m a + mS04'

librium m odeling of high-tem perature (800'- and S0 2 with respecttotal sulfur in the gas. U se 10000C) gases collected at Po~s during 1981-83 of the average S0 2/H 2S ratio cited above (0.18) indicate that Po~s gases are fairly oxidized with and the average 634Ss02 and 834SH2S Values of f02 Values near the N i-N iO oxygen buffer 10w-tem perature gases sam pled in Febru ary 1989

(D elorm e, 1983; Rowe, 1991). Sim ulated cooling (14.5 and -9.40Voo respectively) yields a 634SｷS ofhigh-tem perature Po~s gasesto IOO'C indicates value of -5 .80/00. This value is signi fica ntly low er that the S02/H2S ratios ofthese gases w ill never than the 634S value of7.Ioloo for S02 pum ped from drop below ten ifequilibrium is m aintained during a high-tem perature fum arole condensate (T = cooling;yet,the average S02/H 2S ratio ofthelow- 5840C) and is also low er than the 634SｷS Value tem perature gases is 0.18 (R ow e, 1991). It is determ ined fortotalsulfurin basaltic andesite lava therefore likely thatthese gases have been m odi- eru pted during the phreatom agm atic eruptions of fied by secondary reactions during their ascentto 1953-54 at Po~s (8.30Voo, Table 3). B oth ofthese the surface. values are consistent w ith 834SｷS Values reported 634SｷS can be estim ated from the datain Table for calc-alkaline basalts and andesites (Taylor, B. 3 by use ofthe isotope m ass balance equation: E., 1986). Therefore, as noted by Giggenbach (1987) for W hite Island,the depleted 634SｷS Value 534SｷS = 834Ss02X S02 + 634SH2SX H2S, (3) oflow tem perature gas at Po~s probably represents rem obilization ofpreviously deposited native sul- w here XH2S and Xs02 are the m ole fraction of H2S fur.

O, H and S isotope system atics of Po~s V olcano 277

6.0 5.8

=., 5.6 I *ac (o o o o 5.4 I l I A l i :~ O ~- U) a: s.2 A l o tlJ uJ t >J (O 5.o l O = A A (O ": (o a: 4.8 A A 5 0_ O J 4.6 A OJ ~ ~ 4.4 4.2

4.o.5 ~, .2 o 10 12 14 15 ,6180,IN PER MIL

EXPLA NATIO N A Brine-PootW aters l La ke Brines Fig. 6. Dissolved solids (mg/kg) as afunction of 6]80 forlake brine and brine-poolwaters.

Crater-lake and brine-poolsulfate The observ ed 350'C and low pressure (i.e., fluid pressures near fractionation factor betw een low-tem perature fu- the liquid-vapor curve) the disproportionation re- m arolic H2S (-9.4%o) and the m ostenriched lake- action shifts to the right w ith decreasing tem pera- brine sulfate sam ple (13.50/00) is 22.9%o. A ccord- ture (O hm oto and Rye, 1979). R eaction (5) de- ingto the H 2S-S0 4 fractionation equation given by scribes the hydrolysis of sulfur at elevated tem - O hm oto and Lasaga (1982) isotopic equilibrium peratures (100'-350 'C) (Ellis and Giggenbach, probably occured at a tem perature near 265'C. 1971). Kiyosu and K urahashi (1983, 1 98 4) c on- The form ation of 34S-enriched sulfate in m ag- cluded th at di sprop ortionation o f m agm atic S02 m atic-hydrotherm al system s can be attributed to w as responsible for 634S variations observed in one or both of the follow ing reactions sulfate and hydrogen sulfide in acid sulfate-chlo- (Giggenbach, 1987): ride therm al w aters of volcanic areas in north- eastern Japan. Similar conclusions w ere reached 4H O + 4SO ~ 4H SO by Sturchio et al. (1988) and W illiam s et al. 2 (g/1) 2(g) 2 3(*q) (1990) regarding the acid sulfate-chloride springs ~ 3H 2S04(*q) + H 2S(g), (4) ofN evado del Ruiz volcano in C olum bia. At Po~s how ever, the abundance of native sulfur in lake- + 4 H O ~>3 H S + H SO (5) bottom sedim ents suggests that significant quan- 4Se(1/s) 2 (g/1) 2 (g) 2 4(aq)' tities of sulfuric acid and seconda ry H 2S could be generat ed by h ydr olysis o f native sul fur. Reaction (4) represents the disproportionation The im portance ofthe hydrolysis of S02 rela- (hydrolysis) of S02. A t tem peratures less than tive to the hydrolysis of nati ve sulfur at Po~s can

278 G. L. Rowe,Jr. be evaluated by assum ing thatsulfatein lake brine River- and spring-w ater sulfate Chem ical and represents sulfate in isotopic equilibrium with H2S hydrogeologic evidence indic ates that acid chlo- gas at depth and by considering the isotopic frac- ride-sulfate springs of the Rio A grio w atershed

(Fig. 1) are derived by m ixing and dilution of tionation that w ould accom pany the dispropor- tionation of high-tem perature S0 2 w ith 634S = h ydrotherm al acid brine with flank m eteoric w aters

7.00/00 orthe hydrolysis of native sulfur with aver- (Row e et al., 1994). 634Ss0+ values of the acid age 634S = 11.0 (Table 3). For either process, chloride-sulfate w aters ofthe Rio A grio w atershed the 834S of product H2S04 and H2S will be con- are depleted about 30Voo on average relative to the strained by the equilibrium fractionation factor 634Ss0+ value of average lake brine (Table 3). As04-H2S and the m ass-balance equation relating The su lfate in the the acid chloride-sulfate spring the isotopic com position of the reaction products w aters ofthe Rio A grio w atershed m ay there fore to the source sulfur (e.g., 634SｷS02' 834SｷS.): reflectthe disproportionation of S02 under higher

tem perature conditions in deeper, hotter parts of

(6) the s um mit hydrotherm al system . Alternatively,it 634SｷSo = O 756 + 0 258H S, 2 ' s04 ' 2 could represe nt the contribution of isotopic ally

light sulf ate derived from the oxidation of H 2S or

634SｷS, = O.256s04 + O.756H2S' (7) native sulfur.

O nly three 634Sso* values are available for

If reactions (5) and (6) are assum ed to occur at a acid-sulfate w aters of P o~s V olcano. The positive tem perature of 265'C (AHS04 H2S = 21.80/00),then 634 Ss0+ values of all three sam ples clearly indi- hydrolysis of S0 2 yields 63 4Ss04 = 12.5%o and cate thatthe sulfateis notsolely derive d from the

63 4SH2s = ~9･30Voo w hereas the hydrolysis of na- inorganic oxidation of H2S or native sulfur be- tive sulfur yields 634Ss04 = 6.50/00 and 634SH2S = cause little fractionation occurs during the inor-

- 15.3. The results indicate that m ost sulfate in ganic oxidation ofreduced sulfur speciesto sulfate crater-1ake and brine-pool w aters is derived from (O hm oto and Rye, 1979; O hm oto, 1986). The the disproportionation of S02. Thus, the extrem e 63 4Sso* value of Q uebrada Pilas sulfate (11.20 /o) acidity ofthe lake brines is caused,to alarge ex- is sim ilar to the 63 4Ss0+ values ofsulfate in acid tent, by the hydrolysis of S02, and resultant pro- chloride-sulfate w aters ofthe adjacent Rio A grio duction of sulfuric acid. w atershed; how ever, com positio nal data indicate

A sim ilar hypothesis is advanced to explain that sulfatein the acid sulfate w aters ofQ uebrada the enriched 63 4S values of dissolved sulfate Pilas (as w ell as Rio D esagtie) is derived from s04 in brine pools that form ed in the exposed lake sulfur dioxide in the plum e of acid gases em itted bottom during the lake' s disappearance. The by the active crater (R ow e et al., 1994). D ata in

834Ss04 Value of one sam ple w as notably lighter Table 3 indicatethatthe 534 S values ofS02in the than all other crater-lake and brine-pool sulfates plum e of gasesreleased by the active crater range

(7.Io~oo, Table 3). This sam ple w as from the hottest from 7.0 to 15.0%o.

(91'C) and sm allest pool(volum e ｲ 2 L) sam pled; The 634Ss0+ value of sulfate in Rio Desagu e

D and 180 data indicate thatthe pool w aters are is 4.1"/o', depleted in 34S relative to gypsum pre- largely a m ixture of m eteoric w ater and low-tem - cipitating on the w alls ofthe Rio D esagtie can yon perature fum arolic steam. The pool w as fed by a (83 4Ss0+ = 7.1%.); how ever, the m olar Ca/S04 vigorously discharging 95'C fum arole rich in H 2S ratio of Rio D esagtie w aters is m uch less than l

(63 4SH2S = -l0.0, Table 3); hence, the 634Ss04 indicating that m o st sulfate in the riveris derived value ofthis particular pool probably reflects the from other sources such as dry deposition of S02, ox i dation o f32S -enric hed H2S. acid rain, or native sulfur. A ltho ugh sulfate de-

O, H and S isotope system atics of Po~s V olcano 279 rived from plum e S02 could be depleted in 34S (lO) H 2SO 3(*q)+ 2H 2S(g)~ 3 S.(1/') + 3H 20(g/1) relative to the gypsum found in D esagtie can yon x (e.g., see "gypsum form ed on cinder cone" in Table 3), the m ore likely source of 34S-depleted or sulfur w ould be the oxidation of native sulfur. Such sulfur w as ejected from the active crater 3 during the m ore vigorous phreatic eru ptions of SO 2(g)+ 2H 2S(g)=>2 H 2O (g)+ -x S*(1/s) (1l) 1988 and 1989. In addition, coatings of fine par- ticles ofnative sulfur, evidently precipitated from (O ana and Ishikaw a, 1966; Giggenbach, 1987). plum e gases, w ere occassionally found on plant Subaerial venting of S0 2 and H 2S willresultin leaves and rocks dow nwind of the active crater precipitation of native sulfur by reaction (11) or during field w ork in 1989 and 1990. O xidation of by direct oxidation of H 2S w ith atm ospheric 02: such sulfurcould contribute34S-depleted sulfate to the Rio Desagi ie watershed. 2 C alcium -rich, acid sulfate w aters ofthe active 2H 2S(g)+ 0 2(g)~ 2 H 2O (g)+ ~ S*(*). (12) crater have a interm ediate 834Ss04 value of7.50/00. These w aters are saturated with respectto gypsum R eactions (11) and (12) are expected to occur in (R ow e et al., 1994) and contain several hundred subaerial fum aroles of the 1953-54 cinder cone m glkg of dissolved calcium so that a significant as w ell as subaqueous fum aroles exposed to the proportion of the sulfate (~il5-250/0) is probably atm osphere by the decline and disappearance of derived from the dissolution of gypsum . The bulk the lake. At their outlet tem perature, Iow -tem - of the sulfate in this w ater how ever, m ust have peratu re (94-95'C) fum arole ga ses collected from been derived from other sources such as S02, H2S, lake-botto m fum aroles are extrem ely supersatu- native sulfur, or natroalunite [N aA13(S04)2(O H)6], rated with respectto the m etastable depos ition of w hich is found in old lake sedim ents exposed on sulfu r by reaction (11) or(12) (R ow e, 1991). Thus, the w alls of the active crater (Prosser and Car, precipit ation of native sulfur by either reaction

1987; Row e, 1991). shoul d occur upon m ixing with cooler air. Sulfur

N ative sulfur N ative sulfur can form through a precipitation w ere clearly visible at Poas in 1989 variety of reactions. U nder the relatively low - and 1990; vigorous steam plum esissuing from the tem peratu re (<300'C),extrem ely acidic, w ater-rich lake floor often had a pronounced yellow tinge conditions that are thought to exist in the liquid- due to precipitation of native sulfur, w hereas ar- dom inated zone beneath the crater lake, S0 2 eas around venting fum aroles w ere com m only

should disproportionate quantitatively, producing coated with a layer of bright yellow, m illim eter- native sulfur directly by the reaction: sized sulfur cry stals.

N ative sulfur can also be produced by the

1 breakdow n of polythionic acids. Sulfitolysis reac- 3SO + 2H O => 2H SO + - Sx(S/1)' (8) 2(g) 2 (g/1) 2 4(aq) x tions responsible forthe breakdow n ofpo lythionic

acids yield sulfate and native sulfur as their final

or indirectly by oxidation of H2S produced by reaction products by the reaction:

disproportionation of S02: H2S406 + 5H 2S03 4 ~ 5H SO~ + 5H+ + H20 + 4S* (13) H2S04(*q) + 3H 2S(g) ~ ~ S.(1/') + 4H 2O (g/1) (9)

(Takano and W atannk i, 1990). A cc ord ing to reac- or tion (13),sulfi tolysis oft etra thioni c a cid w ill yields

280 G. L. Rowe, Jr.

0.5 m oles of native sulfur (as S8) per m ole of lated w ith an average 634Se Value of -11.OoVoo and tetrathionic acid. From m id-1986 to m id-1987, an average 834Ss04 value of 13.50 Voo is 24.5%o. total polythionic acid concentrations at Po~s de- A ccording to the data of K usakabe and K om oda clined from a m axinu Jm concentration of18 m m ol/ (1992), this could represent isotopic equilibrium kg to lessthan 0.1 m m ol/kg (R ow e etal.,1992b). at a tem perature of approxim ately 275 0C. Alter- This decrease in polythionic acid conce ntration natively, it could represent sluggish reaction ki- corespondsto precipitation ofabout 9 m illim oles netics if disproportionation occur ed at tem pera- of nati ve sulfur per kilogram of lake brine, or tures below 150'C. about 1600 m etric tons (=1600 M g) of sulfur in A n additional hypothesis is that the isotopic term s of the entire lake (volum e about 7 x I05 com position of native sulfur reflects the direct m 3, Row e et al., 1992a).Although this quantity of oxidation of H 2S produced by the disproportion- native sulfur is less than I percent ofthe am ount ation of S0 2. C om parison of the average 634S of native sulfur estim ated to be stored in Po~s value of H2S (-9.40/00) with the average 634S crater lake sedim ents (about 5 x 105 M g; R ow e, value ofnative sulfur (-10.70/00) indicates that na- 1991) the sulfur deposition rate for this period tive sulfur is depleted by approxim ately I.30100. (about 1600 M g/yr) is com parable to the long- This fractionation cor esponds to So_H 2S equili- term sulfur deposi ti on rate for the crater lake of bration occuring at approxim ately 80'C according 1700 M g/yr recently estim ated by Brantley et al. to the fractionation factors given by O hm oto and (1993).T h ese calculations indicat e that over short R ye (1979). This w ould be consistent with depo- t im e p eriods, sulfitolysis of polythionic acids can sition of native sulfur in the cooler, Iiquid-satu- result in the deposition ofsignificant quantities of rated sedim entsim m ediately below the craterlake, native sulfur. O xidation of H 2S by other sulfur species as de- A ssessing the relative im portance ofthe vari- scribed by reactions(9)to (1l)is unlikely to pro- ous reaction sequences described above and their duce native sulfur w hose isotopic com position is influence on the isotopic com position of native as depleted as that observ ed at Po~s, prim arily sulfur found at Po~sis d ifficult. O ana and Ishikaw a because these reactions incorp orate 34S in reac- (1966) studied the sulfurisotope fractionation that tant S02, H2S03, and H 2S04 m to product native accom panies the form ation of sulfur by reaction sulfur. (8) and found that native sulfur produced by the To preserve the isotopic signature of 34S-de- initial disproportionation of S02 w ould react w ith pleted H2S produced via reaction (7) requires a w ater to produce H 2S and sulfuric acid via reac- non-sulfur containing oxidant. U nder subaerial tion (5). This H 2S w ould react with additional conditions, atm ospheric oxygen is the probable sulfurous acid to form native sulfur (Eq. (10)). oxidant(e.g., Eq.(12)). U nder subaqueou s condi-

Aso*-s. values determ ined for the overall reac- tions how ever, dissolved oxygen is unlikely to be tion (Eq. (11)) range from 16 to 210Voo over the present at m ore than trace levels in lake brine tem perature range of 150' to 300'C. R ecent re- because it readily com bines with sulfurous acid s e arch by K us akabe a nd K om oda (1992) h ow ev er, to form sulfuric acid (Takano and W atanuki, indicatesthatthe Aso*-s. valuesreported by O ana 1990).The only other non-sulfur oxidant found at and Ishikaw a (196 6) w ere not representative of significant concentrationsin the lake brine is dis- equilibrium fractionation, and instea d,represented solved feric iron (Fe3+ = 0.01-0.1 m ol/kg). Fer- an initial, kinetically con trolled frac tionation fa c- ric iron (Fe3+) is readily reduced by H 2S under tor. K usakabe and K om oda (1992) found that acidic conditions resulting in the precipitation of sluggish reaction kinetics at low er tem per atures native sulfur: (150 -250') resulted in observed As04-s . values that w ere 5 to 100Voo sm aller than equilibrium 16Fe3+ + 8H2S => 16Fe2+ + S8 + 16H+. (14) As04-s. values. At Po~s the As04-s* value calcu-

O,H and S isotope system atics ofPo~s V olcano 281

R eaction (14) indicates that periods of enhanced S0 2, follow ed by acid attack of vent w allrock, sulfur deposition will occur w hen the Fe3ﾖ/Fe2+ results in the release of calcium and the precipi- ratios ofthe lake brine are low.A tPo~s,Iow Fe3+/ tation of gypsum.A sim ilar hypothesisis advanced

Fe2+ ratios are cor elated with periods o flow lake to explain the 634Ss0+ value of7.10100 determ ined tem perature and no phreatic activity (O hsaw a, for gypsum on the w alls of Rio D esagtie canyon 1992). D uring these periods, the flux of h igh- dow nwind ofthe active crater. tem pe rature ga sesto the craterlake is presum ably low. A ccording to reaction (14), 16 m oles of fer - ric iron are required to produce I m ole of native IM PLICATIONS FOR E pITHERM AL sulfur (as S8) . A ssum ing reacton 14 goes to O RE D EPOSITS com pletion,a feric iron concentration of 0.05 m ol/ W ith respectto sulfur isotope system atics,the kgin lake brine (volum e = 7 x I05 m 3) could yield fractionation factors betw een reduce d and oxidized

2.5 x 106 m oles of S8 (640 M g) w hich is about sulfur phases observ ed at Po~s are sim ilarto those

40010 ofthe am ount of native sulfur produced by predicted for m agm atic-hydrotherm al system s the sulfitolysis of polythionic acids. w here hydrolysis of S02 is the dom inant m echa-

A lthough the role of dissolved oxygen in the nism controlling theisotopic com position ofsulfur subaqueous environm ent at Po~s has been dis- (Rye et al., 199 2; R ye, 1993). O w ing to the shal- counted,itis probable thatlarge quantities ofH 2S low depth of em placem ent of m agm a at Po~s, S02 are oxidized to native sulfur by atm ospheric oxy- is the dom inant sulfur species in high-tem pera- gen at the surface of the lake. Sulfur slicks and ture gases. C ondensation and disproportion ation spheru les are com m only observed at Po~s (and of m agm atic S02 resultsin the form ation ofacidic other acid crater lakes- G iggenbach, 1974; oxidiz ed condensatesin the tw o-phase convection

C asadevalletal.,1984) and native sulfu r spheru les zone that underlies the crater lake (Fig. 7). Ther- are com m only found in exposed lake sedim ents. m odynam ic m odeling of the condensate w aters

The author has observed precipitation of native indicates that precipitation of m etal sulfid es w ill sulfur on the surface of brine pools exposed by not occur except at very low -tem peratures (R ow e, the declinein lake levelin 1988 and 1989. Sulfur 1991). The aci d conde nsates m ix w i th m ete oric precipitated at the lake surface eventually settles w ater, are furt her concentrated by evaporation at out and is incorp orated in the lake sedim ents. the lake surface, and are neutralized and reduced

Gypsum 83 4S values were determ ined on three by dissolution o f volcanic rock. N eutralization gypsum sam ples collected at or near the active reactions how ever, are slow relative to the flux of crater. G ypsum crystals precipitating from a pool acid volatiles into the sum m it hydrotherm al sys- of evaporating lake brine (T = 19' C) had a tem , hence the brines rem ain highly acidic during

63 4Ss04 value of 14.6%o w hich is I.60100 heavier circulation throughout the upper parts ofthe vol -

canic edifice. Such brines are highly im m ature and than the 634Ss04 value ofdissolved sulfatein lake cur ently,are only capable ofprodu cingthe vuggy brine at that tim e. The observed fractionation is silica alteration assem blage found in the m ost in- consistent w ith the I.50100 enrichm ent observ ed for tensely altered parts of acid sulfate ore deposits. gypsum precipitating from seaw ater at 25'C M odeling results indicate that alunite and ka- (Thode and M onster, 1965). The 634Sso*value of olinite are severely undersaturated in the present- gypsum collected from the vent ofa 290'C fum a- day lake brines a t all tem peratures below the role located on the top ofthe 1953-54 cone w as criticalpoint and thatthese m inerals will not pre-

3.Ooloo. This vent lacked native sulfur crystals be - cipitate until 50 to IOO gram s of andesitic rock cause ofelevated tem perature (T = 31O' C) and the ha ve reacted w ith a kilogram ofacid brine (R ow e, low 634Ss04 can be attributed to oxidation of fu- 1991). Except for pyrite, w hich precipitates early m arolic S0 2 by oxygen. O xidation of fum arolic in the reaction sequence, m ost m etal sulfides re-

282 G. L. Rowe,Jr.

CUl)J

o.~ O N C¥i .~ ･~ :t~_ ~) ~ I Il ~: ~ ･~ ~) * O .~ Q1 ~~: a' ~ / ~ ~~ ;~ ~;~ -- - > ~C:)~.O Ce ~) ~) -c ~r~eo e) ~r~ c~Y~ae! ec'n Ce A:~ ~:S ~~*: ~, 'C(CO ,~ o 'v .c - - c:1 ~:)+ ~) ~~ ~~* ;~) N !x O !: ~3 'o ~ ~i~ ~*) ~~ C~ E .OQ ~_:.c ~~ 02 Lt) - ~ ~S C,~ ~~ ~) ~:) ,*O ~ IS~ q, (:, l' a) ,qJ, CQ ,~) ~) -~ ~~ ~~ U) c ~ CD CE~I O u -~2~5 d ~o) c~ ~ ~: ~ ~) > .-o & o ~S ~0 lrco / O'E *.g2 lO¥.~- .2 ~) ;1:~ ~ ~:~.;~_~ U) 7S'!>S cD- (1) .~2c.~-!g~.c-Q~ QL) ~) ;:~ ~S C~ ~ !'O c,a, (¥t e, ~Si ~ae ~S ~ t._ o_ ce_ > *C~:) O ~ ~: ~S 1aJ5 '9 o I 1~ r¥ ~e .N_ 10 c' oOc_E: ~! ~:)~ ~) ~~>･~~- go:'=c IS 'co: ~'r L-o 215 O~'O:SC~ tl) ~: ~, ･- ~) o-~;s co: !: ~} d, :Gs"~ nv*~ al~) ~ ?~~~;!:~; i~~: ~~: ~ Q~' O .t - ZJ~!c/)v.:~: H1o0 ~ ~ .~: e).~ c~5' c~:'iC:Dc iIo~ ~ ~ u 7 ~ < A : c e:! 6 '~S n ~ Oc'D.Cl()1~ae 'CD SE' ~:3 ~~ C~) ~~ ~: o* ~ ~ ~ < Q ,, N':-:: G'- CQ ~) i~ ~~ ~ OL <･ -- ･o c,.oo ,:a'~1C;'?,E: :~ o~ .~~* ~*~ ~) F~:~~･~: c>o a'~;:o2.cS:~O: ~ COL ~~･~J~ ~: - Q) ~'5 ~1~ ~ cv ~ O 1~ ~) ~ -- co ~ 0 > c~ "~ ~) eJ~(:･;:X ~ O F~ ~ Cb) ~C)~ ~(~:)~ C~) _+~ u)o =* ･--tL~:~'~ ~, ~ ~) C~D5 *; .~S E-･~ ~~ ~:) ~:~ - , (¥, ~ _~ ~,) ~X)* ~) ~) ¥ a, L ~:i:~i;j~:(:.!~'; ¥ .../'/_+' L/t _t Ct¥/ ~) t~: ~; Is>,t,~2 _ _,> ~* ' /_ _' _~ *- ~) ~ ;~ ~C~S) e~':) ~: ,, L ~.,. ~' ¥'//_'' *' ¥ 1- ~~ ~ ~ ~ ~ ~ 1( O "-'"-::.･i: ~ - ~- :o': CO *~~ ~I~ Q~)'- ~:} ~ ;::: o c9 0 o ~ ~: - ~) ~) .g O?- oc Eoo g ~ C) ~) _ ~ ~~ ~ CO N O_'O~C,~1, ~O eL~yO.~~~S F~:.1:~_ O ~::Ecv.~ ~ :ci ~ :~. ~ ~ ~b:~O XO~ ~ ~~ ~: ~) a' O'v _< =~,1¥8C/l)hu], i~~ ~ ~:_O ~ ~ ~?.~ ~ cl' co Oo Jl* ~ ~: C~ C> :LEL o o~~ O hre'c o_, ~ S~1~S O ~oo ~5~ , ~~ ~:3 ･~) N ~ ~ C5 J Jct!) :~ cl ~ :: a)~c,o (1'~ H F a: ~･:~*~ ~ ･S~;~C:~) ~ ~ n c: ~ edO O o O a) ~)e :~ ~~ e~, ~.~_ C(!)5d a) ~~co > ~) ~: ) ~) e' C5 :;g,~,O lb* ¥ ~; O e:i ~S ~(; ¥~~ E lb cl) Q5 < ~ ~e CJ :, 7~:.e?)c ecｽ ¥ ~) ~ ~e:~ ~~ ~) E: ~ .qJ ~ F,~ ~ ~S~ ~* *

O, H and S isotope systematics of Po~s V olcano 283 m ain unde rsaturated until solution pH is near- in the hydrotherm alsystem at Po~s. The only sili- neutral and (or) tem perature decreases below cate m ineral that is predicted to be stabl,e in

150'C. A ccording to R ye (1993),sulfide m inerals present-day lake brines is quartz. Although quartz that precipitate from ox idizing, a cidic brines with is not found in the silica-rich lake sedim e nts, pre- high S04/H 2S ratios will be d epleted in 34S rela- cipitation of secondary quartz is expected to oc- tive to sulfi des precipitated from brines with low cur as dow nw ard-seeping lake brine s are heated

S04/H 2S ratios i f 634SｷS rem ains relatively con- and then cooled. If such quartz equilibrates w ith stantthroughout the dur ation of hydrotherm al ac- circulating lake brines at hydrotherm al tem pera- tivity. The latter brines will have higher pH 's and tures (150-3000C)it w ould be enriched in 180 by will have low er S04/H 2S ratios because o freduc- approx im atley 7 to 150Voo according to the quartz- tion by fer ous iron in w allrock. They are likely w ater fractionation factors given by Clayton et al. to be found in deeper,cooler parts ofthe volcanic (1972). Given lake brines w ith 6180 val ues of 14 edifice w here brine circulation rates are slow (Fig. to 20%o, quartz precipitated from hydrotherm al

7). A s indicated by R ye (1993), the isotopic sig- brines circulating at Po~s could have 6180 values nature of oxidized and red uced f orm s ofsulfur i n approaching 20 or 300100.This reverse fractionation these fluids will ultim ately be dependent on the is in m arked contrast to the 180 depletions typi- redox state of the brines, tem perature, pH, and cally found in rocks hydrotherm ally altered by the 634S values of bulk sulfur released by the m eteoric w ater (Taylor, H. P., 1974 , 1986) and m agm a body. has only been noted before at alteration zones as-

The m od erating therm alinfluence of the lake sociated w ith low-tem perature subaerial fum a- causes hydrolysis of S02 to occur at low er tem - roles of Satsum a-Iw ojim a V olcano in Japan. A t peratures. This increases the fractionation factor Satsum a-Iw ojim a, altered sam ples of andesite betw een product H2S and sulfate and results in the yielded w hole -rock 8180 values ranging from +25 precipitation of som e of the m ost 34S-depleted to +290Voo (K usakabe et al., 1978). Secondary al- native sulfur report ed. The lake and the underly- unite or kaolinitethatprecip itate on the m argin of ing hydrotherm a l system also serv e as a condense r the vuggy silica zone as a r esult of neutralization for sulfur released by the m agm a body. This sul- of 180 -e nriched lake brines w ould also have en- fur is retained by the sum m it hydrotherm al sys- riched 6180 (and perhaps D) signatures. Lateral tem as gypsum (a nhydrite) and native sulfur (F ig. leakage of crater-lake b rines through perm eable

7). The q uantities pr eserv ed are notinconsequen - stratigraphic units or flank fractu res andlineam ents tial as ch emical sed im ents that line the lake floor (as o ccurs in the Rio A grio w atershed at Po~s- are estim ated to contain 450 M g of sulfur(Row e, Fig. 7) w ould also produce acid-sulfate alteration

1991). O f this approxim ately 7 5010 is present as zones characterized by secondary m inerals (alunite,

34S-e nriched gypsum w hereas the rem aining 25010 jarosite, kaolinite) with unusually enriched 180 is present as 34S-depleted native sulfu r. The S04/Se signatures. r atio of thelake sedim ents (~ｭ3/ 1)is consistentwith the S04-H 2S ratio predicted by the S02 hydrolysis C ONCLUSIONS reaction (Eq.(4)). O xidation ofpreviously dep os- ited 34S-depleted native sulfur could be an im por- O xygen and hydrogen isotopic data indicate t ant sourc e of3 4S-depleted sulfur as hydrotherm al- that fl ank acid sulfate and acid chloride-sulfate m agm atic activity w anes and supergene alteration w aters are predom inately of m eteoric origin. A v- of sulfur-rich lake sedim ents occurs. erage isotopic com position ofthese w atersis close

Circulation of crater-1ake brines m arkedly en- to the m ean seasonal isotopic com position of riched in D and 180 by evaporative fractionation rainw ater collected near the active crater. A n ex-

could affectthe isotopic com position of hydrogen ception is the therm al w ater of the A ffluente de

and oxygen-bearing alteration m inerals that fo rm A grio hot spring; it exhibits a m oderate 180 shift

284 G. L. Rowe,Jr. ofapproxim ately 30/00, w hich is consistent with the at approxim ately 265'C. The sulfur isotopic com - dilution ofisotopically heavy hydrotherm al brine position of crater-lake sulfate and coexising H 2S by m eteoric w ater and (or) w ate r-rock exchange. suggests that hydrolysis of S02 is the dom inant

C oncentrated acid chloride-sulfate brines reaction responsible forthe production ofsulfuric form ed in the craterlake are the m ostisotopically- acid and H2S at Po~s. e volved fluids on the volcano. Crater-1ak e brines Sulfate of the acid-chloride-sulfate w aters of are enriched in 180 from 14 to 200Voo relative to the Rio A grio w atershed has 634S values that are

A M W and are enrichedin D by as m uch as 360Voo. 2 to 30100 Iighter than sulfates of the crater-lake

Isotopic equilibration of m eteoric w ater w ith brine. M ost sulfate in these w aters also w as prob- sum m it volcanic rocks under variabletem perature ably derived by hydrolysis of S0 2 although m inor and fluid-rock ratios cannot produce the isotopic contributions o f34S-depleted H 2S or native sulfur enrichm ent observ ed in the crater-lake brine. In- are possible.In contrast,the 634S value ofsulfate stead, the extrem e enrichm ents are attributed to from Q uebrada Pilasisthoughtto be derived from kinetic isotope effectsthat accom pany the evapo- the oxidation of S0 2 in the gas plum e em itted by ration of w ater from the heated lake surface at the active crater w hereas the 634 S value ofsulfate high tem peratures (500 to 940C). in Rio Desagi ie river w ater reflects significant

8D-8180 slopes oflines connecting crater-lake contributions of sulfate derived by the oxidation brine to A M W range from I.O to I.9 and are sig- of34S-depleted native sulfur. nificantly less tha n the 6 D -8180 slope of ap- 634S values of native sulfur are close to the proxim ately 2.9 predicted by the 600 to 80' frac- 634S values of low -tem perature fum arolic H2S t ionation fa ctorsr eported by M atsubaya and Sakai w hich suggeststhat m ostnative sulfuris produced

(1978). 6D -8180 slopes connecting lake brine by the oxidation of H 2S produced by the hy- sam ples to A M W approached I.O as lake level drolysis of S02 or previously deposited native declined sharp ly after mid-1988. Tw o explanations sulfur. To preserv e the lightisotopic signature of for this phe nom enon are advanced (1) the shal- the source H 2S, oxidation by non-sulfur contain-

10w er slopes are caused by increasing b oiling and ing oxidants (oxygen,fericiron)isrequired.M ass concur ent production of isotopically light steam balance calculationsindicate thattypical fericiron

(as a result of boiling atlow er tem peratures) that concentrations in lake w ater are capable of de- occur ed in response to the decreasein hydrostatic positing significant quantities of n ative sulfur. pressure cause d by the decline in lake level, or Sim ilar m ass balance calculations indicate that

(2) the extrem e acidity of the crater lake brine sulfrtolysis of polythionic acids can also yield resulted in increased fractionation of deuterium significant quantities ofelem entalsulfur.H ow ever , into the vapor released by the lake resulting in it is likely that m ost sulfur in the lake sedim ents decreased D -enrichm ent in the lake brines. is produced by oxidation of H 2S by atm ospheric

634S values of S02 from high-tem perature fu- oxygen atthe lake surface. m arolic gases, as infer ed from high-tem perature The crater-lake system at Po~s V olcano repre- fum arole condensate sam ples and th e 534S values sents a rather extrem e geochem ic al environm en t. ofgypsum form ing at fum arolic vents, range from Although it currently produces a hydrotherm al

3 to 7%o. S02 collected from low-tem peratu re,Iake brine with a distinct stable isotope signature, the bottom fum a roles is m uch m ore enriched (634S = ephem eral nature ofthe crater lake suggests that

14.5%o), w hereas H2S is highly depleted (634S = its long-term influence on the isotopic com posi-

-9.4). The As02-H2S Value of 23.9%o indicates tion of any epitherm al ore deposit that form s at isotopic equilibr ation near 1650C. T he observ ed po~s will be m inim al. Indeed,the m ore im portant fractionation betw een craterlake sulfate and low - role of the crater lake w ill b e to function as a tem perature fum arolic H 2S (As04-H2s = 230100) condenser of m etals and sulfur released by the w ould reflect attainm ent of isoto pic equilibrium shallow m agm a body. This prevents the release

O, H and S isotope system atics of Po~s V olcano 285 of m etals and sulfurto atm osphere and results in at El C hichon volcano, M exico. J. Volcanol. the deposition of large-quantities of 34S-depleted Geotherm.Res.23,169-191. Casertano, L., Borgia, A. and Cigolini, C. (1983) El native sulfur w hose oxidation could influence su- V olc~n Po~s, C osta Rica; cronologia y pergene alteration during the w aning stages of characteristicasde actividad.G eofis.Int.22(3),215- hydrotherm al activity at the volcano. The distinct 236 (in Spanish). isotopic signature ofthe Po~slake brines suggests Casertano,L.,Borgia, A., Cigolin i, C., M orales, L. D., how ever, that a hot, acid crater lake, if present, M ontero,W ., Gom ez,M .and Fernandez,J.F.(1987) w ould be recognizable in the isotopic record of A nintegrated dynamic m odel forthe volcanic activ- ity atPo~s V olcano, Costa Rica. Bull. Volcanol.49, alteration m inerals affected by circulating crater- 588 598. lake brines. Christenson, B. W .and W ood, C.P.(1993) Evolution of a vent-hosted hydrotherm al system beneath Acknowledgm ents- The author wishes to acknowl- Ruapehu crater lake, N ew Zealand. Bull. Volcanol. edge Hiroshi Ohm oto who m ade his sam ple prepara- 55,547 565. Clayton,R.N., O'Neil,J.R.and M ayeda,T.K.(1972) tion and analytical facilities at Penn St ate U niversi ty availa ble to the author f or the sulfurisotope analyses . Oxygen isotope exchange betw een quartz and w ater. The as sistance of Sim o n Pou lso n and Ta kesh i J. Geophys. Res. 77, 3057-3067. K akega wain preparing and analyz ing the s ulfuri sotope Colem an, M . L., Shepard, T.J., Durham,J. J., Rouse, sam ple s collected d uringthis study was invaluabl e. The J. E. and M oore, G. R. (1982) Reduction of water a uthor th anks Jo se F. Fer nandez (Instit uto de with zinc forhydrogenisotope analysis.A nal. Chem.

C ostarricense de Electricidad) and Jorge B arquero 54, 993-995. (U nive rsid ad N acional, H ered ia , Costa Ri ca) forth eir Craig, H. (1961) Isotopic variations in m eteoric wa- a ssi stance inthe field . The authoralso wishestoth an k ters. Science 133, 1702-1703. Pe ter D ein es, Susan Brantley, R obert Rye, and Phi l Craig, H. (1963) The isotopic chemistry of w ater and

B ethke for discussions and criticism s regarding this carbon in geotherm al areas. N uclear G eology in G eotherm al Areas (Tongiorgi, E., ed.), 17-53, w ork. Thorough reviews ofthe initi al m anusc ript by A ntonio A rriba s Jr. and Robert Rye are gratefully ac- Consiglio N azionale delle Ricerche, Laboratorio de know le dged. E dito ria l reviews of the rev ise d ve rsion Geologia N ucleare,Pisa,Italy.

ofthis manuscript by Mike Eberle (USGS, Colurn bus, D eines, P. (1979) A note on hydrogen isotope frac- O hio) and M in o ru Kusakabe are als o acknowle dged. tionation involving acidic and basic solutions. This w o rk was support ed by gr ants from the N a tion al G eochim. Cosm ochim. Acta 43,1575-1577. G e ograp hic Soci ety (#3714 -87) an d the Da vid an d Delorm e,H.(1983)Com positio nchimique etisotopique Lu cile Packar d Fo undation,b ot hto Susan B rantley.T he de la phase gazeuse de volcans calcoalcalins;

author also acknowledges tw o years of Fellowship A m erica Centrale et Soufriere de la Guadeloupe ap- sup port fro m the P enn syl vania State U niversity Ea rth plication a la surveillance volcanlogique. Ph.D.

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