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36C1/Cl Ratios in Geothermal Systems:Preliminary Measurements from the Coso Field

GregoryJ. Nixnz,JosephN. Moore andPaul W. KasameyerLawrenceLiverrnoreNationalLaboratory

This paperwas preparedfor submittalto theAnnualMeetingof GeothermalResources Council

San Francisco, CaliforniaOctober 12-15,1997

July 1997

—

DISCLAMER

‘llds document was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor the University of California nor any of theiremployees, makes any warranty, express or implied, or assumes any l~al liability or respona~bility forthe accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed,or represents that its use would not infringe privately owned rights. Reference herein to any specificcommercial product, process, or service by trade name, trademark, manufacturer, or otherwise, doesnot necessarily constitute or imply its endorsement, recommendation, or favoring by the United StatesGovernment or the University of California. The views and opinions of authors expressed herein donot necessarily state or reflect those of the UNted States Government or the University of California,and shall not be used for advertising or product endorsement purposes.

Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore NationalLaboratory under Contract w-7405-ENG43.

%1/Cl RATIOS IN GEOTHERMALSYSTEMS:P~mY WSW~ FROM~ COSO~

Gregory J. Nh12Lawrence Livermore National Labomtory

Joseph N. MooreEnergy and Geoscience institute, University of Utah

Paul W. KasameyerLawrence LNermom National Laboratory

The *l/Cl isotopic composition of chlorine in geothermalsystems can be a useful diagnostic tool in charac-ghydrologic structure, in determining the origins and age ofwaters within the systems, and in differentiating the sources ofchlorine (and other solutes) in the thermal watera. The ~llClvalues for several geothemd W- samples and mselv03t hostrock samples from the Coso, Californi& geothcrd field havebeen measured for these purposes. llle results indicate thatmost of the chlorine is not derived from fhe dominant granitoidrocks that host the geothermal system. If the chlorine w%originally input into the Coso subsurface through rneteor3crecharge, that input occurred at least 1-1.25 million years ago.The results suggest that the thermal waters could be connatewaters derived from sedimentiuy formations, psumablyunderlying or adjacent to the gmnitic rocks,, wkch kverecently migrated into the host rocks. Alternatively, mqst ofthe cldmine, but not the water, may have been recently inputinto the system from magmatic sources. In etther case, theresults indicate that most of the chlorine in the thermal watershas existed within the granitoid host rocks for no more thanabout 100,000-200,00 years. This residence time for thechlorine is similar to residence times suggested by otherresearchers for chlorine in deep groundwaters of the MonoBasin north of the Coso field.

~1 IN NATURAL HYDROLOGIC SYSTEMS

The concentmitions of chlorine in geothermal fluids have beenused as indicators of hydrologic and chemical processes thatare dficult to detect or assess by other means. Chlorine hasthereby become. a basic tool in gythe~ pms~ting,resource evaluation, system chamckmm tiom and numericalmodeling (Ellis and Mahon, 1977; Hedenquist et al., 199XRichards et al., 1992; Kissling et al., 1996). YeL the origin ofchlorine in natural waters, geotherrnrd included, is often indkpute (e.g., S= Nords@m ~ ~.t 1989)” we ‘t @ ‘nknown for many years that thermal waters reacting with rockcan produce a fluid with a chemical composition similar togeothermal fluids (Ellis and Mahon, 1964; 1%7), the vephigh concentrations of chlorine (and other solutes) present mmany geothermal fluids would imply that unrealistically highvolumes of rock would have to be involved in the reaction(Nicholson, 1993). The older concepts that much of thechlorine is instead of magmatic origin, or is derived fromconnate formation waters, are therefore still considered viablepossibilities (Nicholson, 1993; cf. I?Mllips et al., 1995). ‘IMspaper presents the findings of an investigation into the uses ofthe isotopic ratio 36~cl @ w the 03’@hS of chlorine ‘itim

geothermal waters.

Chlorine has three naturally occurring isotopes, 3~Cl(composing 75.77% of all chlorine), 37C1 (composmg24.23%), and ‘Cl which is radioactive and about 12-15 ordersof magnitude less abundant than the other two isotopes. Thehalf-life of 36CI is 301,000 years, which makes it useful for

@ternary gecwhronology (Jannik et al., 1991; Nolte et al,1991; til et al., 1994 Bierman et al. 1995). Because older

‘tCl, they can be rudysystems will contain very little or nodistinguished frum younger systems. This feature can be usedto ident@ old groundwater within aquifer systems (geothermalor otherwise)

The ability to routinely measure ‘Cl in natural systems is arelatively new technique employing accelerator mass ,spectrometry (AMS). The chlorine atoms am accelerated asnegative ions to 8.3 MeV prior to mass faltering (separation)and analytical detection. The acceleration permits the ion beamto be stripped of interfering particles and lowers the detectionbackground. Even then, a detector system capable ofdisdn~uishing behveen the very slight mass difference in ‘Cland S is necessary in oder to remove 3’% interferences.Typical %3/Cl ratios found in nature are on the order of 10’3.

Wkhout a constant natural source of production of ~Cl, it allwould have decayed away long ago. In fac~ *1 is producedon ~ by severrd different mechanisms. Its primaryproduction is in the -phe~ where it is created from argon(’”*) through spal@op reactions caused by cosmic radiation.The rate of productmn Mabout 1.lE-03 atorna/cm2/sec (IA andPeters, 1967). The ‘Cl atoms are then incorporated intoprecigitionandentergroundwa~rsystemsthroughrecharge.The C1/Clratioincnxtsesacrossa continentwithdistancefromthe ocww sincethe ~ _ c~ofi~e *ms toptipitation andoceanic~1/Cl ratiosm exrmnelylow. IntheUnitedStates,%/Cl ratiosinmodemprecipitationrangefromabout50E-15atthecoaststo about650E-15 intheuppermidwest(Wyoming-D*o@; BentJeyetd.. 1986).Anothersourceof ‘Cl, which is much smallerbut stillsignifkan~isprovidedbythenati neu~n activ~onof 35C1withinrocksoftheEarth’scrust.Thesourceoftheneutronsisthespontaneousf~sionofnaturaluraniumandthoriumandtoa lesserdegreetheradioactivedecayofU andTh. Overtime-aboutfivehalf-livesof 36C1- m uilibriumwill developbetweentheproductionanddecayof‘% 1 suchthata constant~C1/Clvaluewillbemaintainedintherock. Rockswithhighconcentrationsof U and Th will have higherequilibriumY1/Cl valuesthanrockswithlow U andTh concentrations.For commoncrustalrocks,therangeof equilibrium3@Clvaluesis about5E-15 for typical limestones (U poor) to overIOOE-15for granitic rocks (U rich).

A third source of 36CIis from the atmospheric nuclear testingof the 1950’s to 1960’s. A signifkant amount of ‘Cl was putinto the atmosphere by the tests, prim@Y from neutronactivationof 3SC1in seawaternear the tests. This produced a“bomb-pulse” of *I in the precipitation which was about 1-3orders of magnitude higher than the typical natural precipitationvalues. l’bis bomb-pulse clearly fingerprints groundwaterrecharged during the period of atmospheric testing, and

. provides a mechaniim for identifying groundwater whargedduring this time from groundwater recharged prior to the1950’s. The abiihy to make thk dkinction has significant

I

1

UWIOndCknefi~ ~d h= bUII U~ h ~Y mundwa~rresource evaluations (e.g., Bentfey et al., 1982 Ptmdy et al.,1987; ScaldOn, 19$% Ctiw et ~.. 1992).

USE OF WI fri GEOTf=M SY8=MS

MI of tJWfactors ‘ust discusxd lead to a vad@Y of difl-t4,,tiwoirs,, of y I cm F.$JUI,which produces a Wiety Of

~~cl ~tiCISwithin crust waters. These VtiatiOm should k“sef”l itI ahg and cbaractaizing processes * tivolvecbimitw, pardcuIady gmthennd PIOIXSSCSwhere a ~de ~ge

of chlorine cOn=nmtiOns ~ -r ~~ a ~la@Y ,Sdlregion. Owing prifily m tie ~ty Of AMS f~la~ m *PWL tbe~ have been very few W smdies 2.syet conducti mgeothermal fields.

TIIe earliest puhlisbed smdy W?.Scondd hy filliP2 * .~.(1984), who used ~lfCl ratios itt mnjuncdm with chlorine

E Wvme.n the effectz of chlorinemnce.ntmtions to di~kddng from host inks. md -g Of wa~r ~1* wi~OIevdk gmtbmml sywem. Although only a few ~P!eswett ma$zed for tMs study, a wide rmge Of 3’C~Cl rau’?swere observed (-13E-15 m -7W~ 15), md @em @Wcwaters witin the vdles mdem wem *JY dlff~%~from gmthe~ wa~rs. l%e most valuable observation,b~~fxr, WaS tht ‘e ~tic” ~a~. ‘m~v~r

gmundwate.r W du~ng sballOw, c!~lahOngmtbemmliy bted zmes) muld ~ disunWh~ frOm d~pcirculating “production” waters.

fITmotber study, Hedenquist et al. (1990)&the v~~ ~WVCIvalues bewen ne~-s~~ flOudwa@r. dmp Ol$ergmundwater (where the cbltie figfi~ fmm % W?faK&s), and bomt-puls pwipi~ti~ tO def~e ~ c~u~b~pattern of thermal watm in the Mo~ gmtb~ sYsEm OfNew Zealand. The data suggested a circuladon ‘WY’extending several kilometem into tie crest to a depth teneatbthe thick suflcid silicic volmic MCkS md ~~ h~rnentgmywackes. upward tiula~g wa~m hen Pmmslve?ymid with fmt older gmundwa@ tb~ ~em ~~ncwater, as it neared the stim md MOV~ OUtW~ Over ~kilometm from the thermal mne.

Fehn et al. (1994) used ‘~UC1 ratiOs of _ hd% dthe calculated subsurface neutron fluxes m deteme maxtmummidence dmes for wa*~ ~~ln ~ s~mn Sea g~ti!dsystem. The relatively high values of W1/Cl measured m thesampies necessititi the conclusion tit ~ hOst r~~ m!sthave quite high U ml m Wn=nmtion(fOr b litbologl!spresent). Even the highest re~nable VAU~ fOr U md ~ mthese rocks would indicak tit the gm~e~ wa~rs we=pm~”t h * formations for at least 5MW3 Y~s. wtheory behind ties: conclusions, and h metbd used for thecalculations, are dwxssed below.

fn a similar study, Febn et al. #9!?2),us@ ~UCl ratios alongtith ‘mm ratios (the origin Of I MSIINIWm ~ Of 3SC1.~dit is also measured by AMS) to dktin~ish ~~mn ,*i,fferem ttodks of water - connate fOtmaaOn wa~rs, ~~~a~, md ~teotic waters - in the Clea Ne Wthd Wat .f -fk G.?Y%rs, WLfOmi& They ~~~~~maximum residen= times for these waters:MCUC1compositions, spring waters tigina~g at depth ~ OWFranciscan Formation wem diffemntia~ fmm sPnng wa~rsoriginating at depth in the G@ V~eY Wuenm. md ~tbcould be dktimguishd fmm m~~ ~~riC w~~m. .At ~Earth’s surface, most Of h springs wem ~~la~ ~ti MClear lake Volcanics and the depth of origin of the waters ortheir subsurface host rocks were indektinahle.

TO w knowledge, tbe~ 8X M OnlY s~di=WUCI d-m far conducted m gedbe~l sYs@~.

employing

WI fN THE COSO GEOTHEH FfELDTIE pi-csentstudy itIwti@s h n3ktd-P @@n hchlorine in geotbetmzl waters and the cblmne m OWWbhosting those waters through an examirwdon Of WC1 tiOsbt WaEr samples and rock cores from the COW g~~~field. COW is located itt & soutbemmost porhon of OwensVdIq h mtemctilfOmia aPPIUtiEIY z50 kM nO* Off-OSAngeles and 230 km west of k Vegm. ‘fbe 8@JW~wtivity is associated with the coso vol~c field @g. 1).Which is composed of basaktic to rbyObbC mk emp~

khwen 6 million and 40 thousand years ago (TMffield et al..19so). TIIe volcanic rocks rest on ba~rnent IWk fiat ~mOStIYgranitic in composition, although slgnlfiw~ aIW-TWItZOf~emmm-phosed COWVI’Yrocks (including tWM.$dlMen@) dsOoccur (Duftield and Bacon, 1981).

Figure L tbmlii geology ~ of the ,COSO~a. ~M gdwmal pmductmn area !s outlined., ~Mmmic grmtitic and lll&MWWkC tock2 (flgh~t

my) m dominmt subsurfaw Iitbology.

w pmbwtion me of the COSOgcotltermd field is ~~these basement reds, and afl of the water and rock samplesused in this study are fmm the basemmt IC@S. ,’ffw COSJgeothermal field is o- by the Caltfmma BnergyCompany, and currently has a production of 240Mw.

TIIe water samples analyzed in this study wet’e detivd fmrnPtiuctim wells withh the interior pOttiOns Of b ~gwtb~tifield. The W1/Cl WAWSfor tb- UPl~ wgiven in Table 1. ‘fhe values range from 9.lE-15 to 16. lE-15,a WY narrow range of V~U= (reladve m tie ~~. rmge fOrnatural watem) considering tit the =P1e lc@JO!S wepseveral kilome~ apti. me n~Ow fmge Of, ~ ~mplcWI”eS arc dso noteworthy in contrast to the wide mge Of

chlorine concmtiatiOns within the COsO gmthe~ wa~m(,fightly k than 2cCQppm m almost WOOPprn. Mm et al..1995).

2

NIIIWMooread K-IIWCX

expectedrange for Sierran granitoidrocks similar to the Coso

CL3988CL3w

sainples.

i 14.OE-15 4.8E-15II 12.4E-15 2.lE-15 ICL399.

a_.3g; 9. IE-15 1.lE-1511.4E-15 1.6E-15

CL3994 1O.6E-15 1.OE-15Sierra Nevada FrecIpltadoncoso Subsuflace ROCkS w“qGN197-07 (granite)GN197-11 (diorite) 81.7E-15 18.OE-15GN197-12 (diorite) 30.5E-15 1.7E15GN197- 13 @lot. schist) 12.9E-15 3.4E-15

59.8E-15 2.3E-151

Table 1. *l/Cl Data for Coso Samples

The measured %Y/Cl values are extremely low for naturalwaters, and ate appmxirnatdy an order of magnitude lowerthan modem Sierra Nevadan precipitation, measured recentlywithin Yosemite National Park (Table 1; Nirnz, unpublisheddata). The modem preeipitadon values for the Coso area mustbe very similar to those measm’d in Yosemi@ ~ ~Yare approximatelyequidktant from the Ptilc ocean (Bentleyet al., 1986).

The rock samplea analyzed in this study were solid drill cores(GN197-7, GN197-12, GN197-13) or drill-return eutdngs(GN197-11) from boreholes within the central portions ofthe Coso field. The ~1/Cl values for these samples are givenin Table 1. The exterior, drill-polished, portions of the eo~swere removed from the solid core samples to avoidcontaminadom and all samples were washed and then mildlyleaehed for several hours to several days in nitric acid toremove any surface ~1 (from tic or clrilling sources).The samples were then washed again with ultrapme water,dried and crushed prior to dissolution and extraction ofchlorine for the ‘Cl analysis. Because the rocks containseveral 10’s of micrograms of chlorine per gram of roeL andbetween 30 and 70 grams of rock were proeeased, it isstrongly believed that chlorine contaminadon (carrying Wl)would not & possible. Several n’d~- of ~~tchlorine would be required to signit%antly affect the %lC~l&fthe analyses. bboratory blank values ~period of the analytical work show an average *l./Clprocedural blank value of 7.OE-15.

The %XCl values of the Coso grsnitoid rocks (30-82E-15)were very much higher than those of the water samples (9 to16E-15). Ordy the value of the metamorphic sample (Iiotitesehisc 12.9E-15) is within the range observed for the waters.These rock values are within the range expected for granitoidrocks with elemental compositions like those of the SierranProvince. It is possible to dcuk+te the equilibrium ~C~Clvalue for any reek given its bulk chemical composltlon,including its U and Th concentrations (Andrews et al., 1986).Figure 2 shows the range of YUC1 values expected forgranitoid rocks of the Sierran province, based. on uraniumconcentrations and the range of expected ThlLJ ratms.

The Coso granitoid rocks are also plotted on Figure 2 based ontheii ~C1/Cl values, giving a predction for the uraniumconcentrations of the samples (U concentrations have not yet&en measured). For average Sierran ThAJ ratios, ~pleGN 197-11 would be prwhcted to have about 10 ppm,uramum,sample GN197-07 would have about 8 ppm uramum, andsample GN 197-12 would have about 4 ppm uranium. F@e2 indicates that measured al values as well as the inferreduranium concentrations are all reasonable dative to the

120

100

*~tlo

p

*4O

Co&! ➤Watere

o0 246 8 10 12

Figure 2. The calculated range of ‘C1/Cl values for typic-alizmnitoid reeks of the Sierran Province. Shaded fieldkdkatea the typical range of U concentrations, andtherefore expected ~1/Cl ratioa. Average value forthe Coso water samples is shown at the lefL Thelarge open &lea mpreaent the three Coso granitoidsamples, and are plotted along the TWU = 3.2 lineaccordiig to their measured %3/Cl values. The miceelements listed at the top are signifkant neutronabsorbers. The concentrations listed are typicalvalues for granitoid rocks, and these values am usedin the calculations. ‘Ihe granitoid rock compositiondata are taken from Dodge et al. (1982) and Parker(1967).

The wl/cl ratio of the Coso biotite schist (GN197-13) ismuch lower than the values for the granitoid rocks, and withinthe range of the Coso watera. F@re 3 shows the expected~1/Cl ranges for lypieal sedimentary rocks, baaed on thesame calculation methodology used for the Sierran granitoidreeks, Because uranium concentrations are lower in typicalsediintary reeks, and the ThAJ ratio-is generally lower forsandstones, the expected ‘CVCI ratI.m ~ lower. Theobserved*l/Cl ratio for the Coso btowe selust indicates thatthe U-Th concentrations are lower in this sample than for theCoso granitoids, and Figure 3 suggests that the pre-metamorphic protolith was a sedimentary rock (as would beappropriate for a biotite schist).

DISCUSSION

There are at last three significant observations that ean be maderegarding ~C1/Cl within the Coso system. FnL the %211C1ratios indkate that most or ti of the chlorine in the waters isvery old. If the chlorine is derived from country rock (forexample, like the blotite schist), the chlorine is obviously asold as that reek. If instead we assume that the chlorine wasderived from precipitation at sometime in the PSSLand that theprecipitation had ~C1/Cl ratios similar to modem-dayprecipitation in the Coso area, it would take approximately 1-1.25 million years for the %1 levels to decay to their presentvalues (Figure 4). Thus the chlorine is at least that old.

Seeond, most of the chlorine in the Coso waters apjmendywas not derived from the granitoid reeks that are known toampose the bulk of the Coso geothermal field. The YVC1ratios of the granitoid reek samples are two to five times higher

3

NU Moore andKasamcycc

than the ratio3 of the waters. This isatoddswithmanystudiesthatsuggest significant input of Cl from the host rock ingeothermrd systems (e.g., Nordstromet al., 198%Ellis andMahon, lM, 1967). However, it is consistent with theargument that it requires leaching of umealistically largevolumes of rock to derive the very high concentrations of Cland other solutes observed in many geothermal fields,includiifz fields such as Coso with several thousand mrn Cl(Nichol~on, 1993).

. .

L

-50

40

0.0 1.0 2.0 3.0 4.0 5.0 6.0

U (ppm)

Fimue 3. The calculated ramze of %3/Cl values for typicalliieatones and sands&es in addition to the gra&oidcompositions shown on Figure 2. The calculationmethod is the same as for Figure 2, using rockcomposition data from Parker (1967).

The third observation that can be made regarding %3/cl withinthe Coso system is that the chlorine must be derived from (or,if of meteoric origin, been existent for >1.25 million yearswithin) an environment with low uranium and thoriumconcentrations. High U-Th concentmtions would inevitablylead to higher %XCl ratios. As Figure 3 indicates,aedhnentary rocks-perhaps lii a GN197-13 protolith - wouldprovide the appropriate environment. It is important to notethat not only the Cl within a rock becomes activated by theambient neutron flux, but so does the 3SC1withii water hostedby those rocks. The free mean path of a neutron produced byU-Th spontaneous fission is tens of centimeters (Andrews etal., 1986). Only the 35C1in watrx in the center of rockfrachrres wider than a few tens of centimeters would escapethk neutron flux. Thus if the chlorine is originally of meteoricorigin, either as groundwater aged >1.25 million yearsago or as connate water originating coeval with the rockformation, the host rock must below in uranium and thorium.

These observations imply that the chlorine in the waters of theCoso geothermrd field has been transported into the host recksfrom which the water samples me now taken. Theobservations also imply that the chlorine in the water has onlyrecently entered into the host granitoid rocks. If the chlorinehad existed in these rocks for a sufficiently long period of time(about five half-lives of %21 ==1.5 million years), the ~C1/Clvalues present in the water would be the same as those in thehost rock. The host-rock neutrons would have activated the3@ in the water to produce the same equilibrium value ss therock. Since the chlorine in the water has a substantially Iower~C1/Cl value, a maximum raidence time for the within thewater of the COW granitoid rocks ean be calculated based onthe neutron flux (i.e., the U-Th concentrations) and the build-up (minus the decay) time of Yl in that water (Fehn et al.,

1992). That ix How long would it have taken the Coso watersamples to arrive at theii currently observed ~1/Cl values,having been continuously hosted by the Coso granitoid rocks?

zoo, 1 1 1 , t i

t \% ewowsbramnpk 11

‘“;~o %010s 1.010’ 1.310’ 2010’ 2s 10’ 3.010’

Time (yin)

Figure 4. A simple decay curve for the decay of ‘Cl withtime. The initial value for ~1/Cl chosen is similar tothat observed in Sieman precipitation (Table 1). Byplotting the measumd values for the Coso watersamples on the decay curve, the “age” of the chlorinecan be ted from the abscissa. If the chlorineoriginated in the Coso samples durin recharge, andthe only process lowering the 4 l/Cl ratio isradioactive decay, then the water samples would be-1 to 1.25 million years old.

While we have not yet determined the U-Th concentrations ofthe COW rock samples, we can assume average U-Th valuesbased both on the average for Sierran grsnitoids and on theimplied values for the three Coso samples (i.e., 4, 8, and 10ppm). This will permit us to make an ewimate of the maximumredence times for the Coso water samples. Figure 5 showsthe results of these calculations based on an average uraniumconcentration of 6 ppm and a Th/U rado of 3.2 (the Sierranaverage). A maximum ieskience time would be given by thecircumstance where the original W1/Cl ratio (before enteringinto the granitoid rocks) was zero. This would be about180,000 years (F@re 5). If insteacL the original equilibrium~1/Cl ratio was 6.OE-15 (an appropriate value for water fromaverage limestones and sandstones, see Figure 3), themaximum residence time would be about 110,000 years. Anysignificant amount of leaching of Cl from the granitoids wouldlower the calculated residence time even further.

The above suggests two possible models for the evolution ofthe Coso geothermal field. In the fmt model, the waterexisting today in the Coso geothermal system was not therebefore 100,000 to 200,000 years ago. Other water might havebeen presen~ or it may have been a hot-dry system. At sometime after 100,000-200,000 years ago water of connate originfrom adjacent (underlying?) sedimentary country rock enteredinto the granitoid rocks. A connate origin for the water wouldbe consistent with the 6D/6]*0values observed in Coso waters(Fournier and Thompson, 1980). The cause of the watermigration could be a change in regional tectonic stress, or anunusually strong rise in regional potentiometric heads. Suchbroad regional affects leading to local dramatic changes inhydrologic systems have never been documented. A variationon this rndel would one in whwh the water has just veryrecently moved into the granitoid rocks, as for example in alarge convection cell where water moves into and out of thegranitoids.

4

Ni~ Moore,andKasameyer

In the second model, the chlorine input to thes stem -200,000:years ago is of magmatic origin. The 8D/& O data indicate

that the water in the COsosystem is not magmatic, so in thismodelonly the $Morine (* other solutes) is input to the system.Magrnahc chlorine produced from a magma derived from uppermantle rocks (low U-Th), would have very low %_X/Clvalues.‘Ibis model is similar to one proposed by Phillips et al. (1995)for deep groundwater within the Mono Basin to the north ofCoso. Based on ctdorine mass-balance. arguments, they havesuggested that waters within the Mono Basin received asignifk.ant input of chlorine between 100,000 and 450,000years ago. They suggest that the chlorine was from “volcanic”sources. The upper bound of this age range coincides with a

, climatic shift to more arid condhions which could have closedMono Basin and led to an uccumdutw“ n of the volcaniccldorine. However, it is not clear how a climatic change couldlead to an accumulation of msgmadc chlorine within the Coso

. geothermal system. It is striking, however, that two studies atopposite ends of the Owens Valley have each seen strongevidence for an immt of chlorine to regional hydrologicsystems within the 100,000-400,000 year tinieframe. - -

300 1 I 1 1TMJ=32 -a, ” f

o 4 8 12 16 20%X/Cl ● E-15

Figure 5. Maximum nxidence times for water samples hostedin rocks of the indkated uranium con&ntrations.36C1“build-up” CUNCSshown for rocks with 4 ~mand 10 ppm (dashed curves), and 6 ppm uramum(solid curves). A concentration of 6 ppm is taken astypical (average) for the Sierran Province (see Figure2). Curves starting at the origin arc for waters thatbegin with no 36C1at all, which is improbable. Thecurve starting at ~1/Cl = 6B 15 (an averagesedimentary rock ratio, see Figure 3) is a morereasonable scenario. The vertical and horizontaldashed lines indicate the maximum residence timesfor water with ‘C1/Cl = 16.lE-15 (i.e., Coso sampleCL3988). For this sample, a residence time of about180,000 is derived for a 6 ppm uranium environment

. with an Mid YVCI ratio of zero, and a residencetime of about 110,000 is derived for a 6 ppmenvironment with an initial ratio of 6E-15. If theaverage uranium concentrations are lower than 6

. ppm, the derived residence times becomesignificantly higher, as can & seen by the curve forthe 4 ppm uranium environment.

CONCLUSION

This study has examined the ~1/Cl isotopic compositions ofgeothermal waters and their host rocks from the Coso

geothemud field. TIE purpose of tk study was to determinewhether the predominant granitoid rocks of the field wem thesource of the chlorine in the waters, and if no~ whether thewater *l/Cl values cdd indicate what other sources mightprovide the chlorine. In general, this information might ~provided either by distinctive ~1/Cl ratios, or bychronological information provided by either the decay ofradioactive ‘bCl, or the subsurface production of ‘Cl byneutron activation.

The following conclusions were reached concerning the Cosogeothermal system

1) The host granitoid rocks am not the source of most of thechlorine in the thermal water samples.

2) Most or all of the chlorine in the thermal waters has existedfor more than 1 miUion years in a low U-Th environmentsimilar to that found in typical sedihnentary rocks, or within theupper mantle. This suggests either that the source of thechlorine is comate formation waters (ii which case 8D/8180data is permissive of the conclusion that Coso thermal wateritself is formation water), or that the source of the chlorine ismagmatic (ii, which case 6D16180 data require that only thechlorine was input mto the Coso system, not magmatic water,i.e., the water could be meteoric).

3) Given typical U-Th concentrations for the average Cosogranitoid host rock the chlorine in the Coso water samplescould have resided withh the host rocks for no more thanabout 100,000-200,000 years. This is similar to residencetimes for chlorine in the Mono Basin north of Coso found byother researchers.

ACKNOWLEDGMENTS

We would like to thank the California Energy Company forproviding us with the Coso water and rock samplea. John F.Copp was instnunental in obtainiig the water samples during1995. Alex Schricner provided help in the review process forthe California Energy Company. This work was performedunder the auspices of the U.S. Dcptmcnt of Energy by theLawrence Livcrmore National Laboratory under Contract W-7405-Eng-48.

REFERENCES

Andrews, J.., .Edmunds, W.M., Smedley, P.L., Fontes, J.-Ch., FWield, L.K., and G.L. AlIan, 1994. “Chlorine-36 intzmundwatcr as a oalaeoclimatic indkatm the East MidlandsTriassic sandstoni aquifer (UK)”, ~ ,vol. 122, p. 159-171.

Andrews, J.N., Fontcs, J-Ch., Michelo~ J-L., and D.Elmore, 1986. “In-situ neutron flux, 36CIproduction andgroundwater evolution in crystalline rocks at StriptL Sweden”,

t. Sci. ~ , vol. 77, p. 49-58.

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Bentley, H.W., Phillips, F.M., Davis, S.N., Gifford, S.,Elmore, D., Tubbs, L.E., and H.E. Gove, 1982.“Therrnonuclcar %21pulse in natural waters”, -, Vol.300, p. 737-740.

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Fehn, U., Moran, J.E., Teng, R.T.D., and U. Rae, 1994.“Dating and tracing of fluids using *nI and ~k Results fromgeothermal fluids, oil field brines and formation waters”, N.u.GL~ VO1.B929 P. 380-384.

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Purdy, C.B., Mignerey, A.C., Helz, G.R., Drummond,D.D., Kubik P.W., Elmore, D., and T. Hemmic~ 1987.‘~k A tracer in groundwaters in the Aquia Fommtion ofSouthern MSIYlWId”9~. VO1.B29.F. 372-375.

Richards, H.G., Savage, D., and J.N. Andrews, 1992.“Granite-water reactions in an experimental Hot Dry Rockgeothermal reservoir, Rosemanowes test site, Cornwall,U.K.”, ~DO1. Geoc hem., Vol. 7, p. 193-222.

Scanlon, B. R., 1992. “Evaluation of liquid and vapor waterflow in desert soils based on chlorine 36 and tritium tracers andnonisotherrnai flow simulations,” Water ~, Vol.28, p. 285-297.

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