t077

Canadian MineralogistVol. 30, pp. 1077-1092 (1992)

CI.BEARING AMPHIBOLE IN THE SALTON SEA GEOTHERMAL SYSTEM.CALIFORNIA

MASAKI ENAMI-, JTIHN G. LIOU nTvn DENNIS K. BIRDDeparttnent of Geology, Stanford University, Stanford, Califumia 94305, U.S.A.

ABsrRAcr

Calcic amphiboles with up to 2.7 wt.fo Cl occur in metasandstones, metabasites and veins at depths berween 3, 100 to 3,180 mand temperatures in excess of 350'C in the State 2-14 well of the Salton Sea geothermal system (California). These amphiboleswere formed by reactions involving high-salinity geothermal fluids, with 15.4 to 19.7 wt.Vo total dissolved Cl. Coexisting phasesinclude quartz, plagioclase, K-feldspar, epidote, clinopyroxene, apafite, and titanite. The Cl-bearing amphiboles range incomposition from hastingsitic (Cl > 1 wt.Vo) to actinolitic (Cl < 0.5 wt.7o). Texturally complex intergrowths of actinolitic andhastingsitic amphiboles occur at depths greaterthan 3,140 m, suggesting a miscibility gap benveen the two amphiboles. Measuredcompositional variations suggest a crystal-chemical control on the Cl content in the calcic amphiboles: (1) the chlorine content ofamphiboles increases with increasing edenite substitution {tAl1Na6;t+lAltr-lSi-t}; (2) the maximum observed Cl content of anamphibole increases with increasing X(Fez*) value. Comparison of Cl content in amphiboles from the Salton Sea geothermalsystem, submarine metabasites, skams, high-grade metamorphic rocks and igneous rocks implies that a main factor in controllingCl-for-OH substitution in amphibole is different in low- and high-chlorinity environments. In low-chlorinity environments, the Clcontent of an amphibole increases with increasing chlorinity of the coexisting fluid, and is defined by partitioning of Cl betweenthe two phases, as well as the crystal-chemical constraints imposed by (Na+K), Fe, and Al substitution. On the other hand,amphiboles coexisting with the Salton Sea and more saline fluids are enriched in Cl; Cl content strongly depends on X(Fez+) andthe edenite content of the amphiboles. They may achieve a maximum Cl content, and the extent of Cl-for-OH substitution iscrystal-chemically controlled.

Keywords: Cl-bearing amphibole, crystal chemistry, salinity, geothermal system, Salton Sea Califomia.

SoruMarnE,

Nous trouvons des amphiboles caJciques ayant des teneum en Cl jusqu'h 2.7Vo (poids) dans des grbs mdtamorphis6s, desm6tabasites, et des veines d profondeul entre 3 I 00 et 3 I 80 m, et h des tempdratures au dessus de 350"C dans le puits State 2-l 4du systbme g6othermique de la mer de Salton (Califomie). Ces amphiboles ont 6td form6es par rdaction impliquant des saumuresgdothermiques, I salinit6 entre 15.4 et l9.7Vo en Cl dissout. Sont aussi prdsents quartz, plagioclase, feldspath potassique, 6pidote,clinopyroxbne, apatite et titanite. Les amphiboles chlorifdres ont une composition hastingsitique (>lVo en poids de Cl) dactinolitique (< 0.57o de Cl). Une intercroissance complexe des deux amphiboles, rencontr6es b une profondeur au deli de 3,140m, t6moignerait de I'imporrance d'une lacune de miscibilit6 entre les deux amphiboles. l.es variations mesur6es en teneur en Clseraient r6gies par des contraintes. cristallochimiques: ( I ) Ia teneur des amphiboles en chlore augmente avec I'impoftance d'unesubsti tut ionverslep6le6deniteI l^l(Na,K)t4lAltrtsi r ] ,et(2)lateneurmaximumobserv6eenCl augmenteavecX(Fez+).Unecomparaison de la teneur en Cl des amphiboles provenant du systdme g6othermique de la mer Salton, des m6tabasites, des skams,des roches m6tamorphis6es h un facies 6lev6, et des roches magmatiques indique qu'un facteur diffdrent rdgit la substitution duCl dans des milieux i chlorinitd faible et 6lev6e. Dans un milieu l faible chlorinitd, la teneur d'une amphibole en Cl augmenteavec la chorinit6 de la phase fluide coexistante, et ddpend de la r6partition du Cl entre les deux phases, et des contraintescristallochimiques dues d I'incorporation de (Na+K),Fe et al. D'autre part, les amphiboles qui coexistent avec les saumures de lamer Salton et des saumures encore plus salines sont enrichies en Cl; leur teneur en Cl ddpend de X(Fez+) et de la teneur en 6denite.Elles peuvent aueindre la saturation en Cl, et la port6e de la substirurion du Cl au OH esi ici rdgie par des contraintescristallochimiques.

(Traduit par la R6daction)

Mots-clds: amphibole chlorifore, chimie cristalline, salinitd, systdme g6othermique, mer de Salton, Califomie.

"Present address: Department of Earth and Planetary Sciences, School of Science, Nagoya Univenity, Nagoya 46zt-0 l, Japan.

1078 THE CANADIAN MINERALOCIST

INTRODUcnoN

High concentrations of Cl in calcic amphiboles havebeen reported from various rock-types: submarinemetabasite, skarn, amphibolite, granulite, granitic andgabbroic rocks, but the conditions necessary for theformation of Cl-rich amphiboles are notwell understood(see review by Suwa et al. 1987). Determination of Clpartitioning between calcic amphibole and a coexistingfluid is important for evaluating the geological cycling,amount and distribution Cl in the Earth's crust.

Relationships between Cl content and the major-element chemistry of amphiboles on one hand, and Clcontent of the coexisting fluid have been discussed bymany authors. Volfinger et al. (1985) and Kamineni( 1986) emphasized that Cl content of a calcic amphiboleincreases with increasing of Fe2*. Ito & Anderson(1983) studied calcic amphibole in metamorphosedgabbros from the Mid-Cayman Rise and consideredthat A1 substitution at tetrahedral sites allows increas-ing substitution of Clfor OH.These authors emphasizeda crystal-chemical control of Cl content in amphibole.On the other hand, Vanko (1986) concluded that theCl content of amphibole from the Mathematician Ridgevaries as a function of the Cl activity of coexistinghydrothermal fluid, and pointed out that Cl does notsimply replace OH wherever Fe is available in thefluid phase. Vanko (1986) also showed that some am-phiboles in greenschists from the Mathematician Ridgeare more Cl-rich than those in amphibolites. Theseobservations and occurrences of Cl-rich amphibole inigneous rocks (e.9., Kamineni 1986) and high-grademetamorphic rocks (e.g., Sharma 1981) suggest that Clsubstitution for OH in calcic amphiboles can occur overwide range ofpressure, temperature, and fluid composition.

The Salton Sea Scientific Drilling Project success-fully drilled Colorado River sediments within the SaltonSea geothermal system to a depth of 3,220 m, wheretemperatures exceed 350oC. Hydrothermal solutions ofthe Salton Sea geothermal system a.re NaCl-rich brineswith Cl contents of approximately 15 wt.7o (White1968). Fluid inclusions within anhydrite contain up to507o crystals of halite, sylvite and carbonates in aNa{a-K{l brine (e.9., White 1968, McKibbenet al.1987). During the course ofour study ofamphibolite-fa-cies mineral assemblages within core and chip samplesof the State 2-14 well, we identified and analyzed grainsof Cl-rich (up to 2.7 wt.Vo) calcic amphibole in hy-drothermally altered metasandstones, metabasites andveins at depths greater than 3,140 m. To our knowledge,this is the first description of Cl-rich amphibole from anactive geothermal system. In this paper, we report thechemical characteristics of the Cl-bearing amphibolefrom the Stats 2-14 well, and discuss the crystal-lographic constraints of Cl substitution in calcic amphi-bole in geothermal, metamorphic, and igneous environ-mems.

GEoLocICAL SEI'ING

AND PETROGRAPIilC DESCRTP,NON

The Salton Sea geothermal system lies near thesoutheastern end of the Salton Sea" within the SaltonTrough of southern Californi4 which is the landwardextension of the Gulf of California Rift system (Whiteet al. 1963, Helgeson 1968). The high+emperature(>350'C) and high-salinity brine (> 15 wt.Vo totaldissolved solids) is responsible for crystallization ofsilicate, sulfide, and oxide minerals throughout thegeothermal system. Muffler & White (1969) andMcDowell & Elders (1980) described greenschist-faciesmetamolphism occurring at temperature above 300oC at2-3 km depth. On the basis of mineral parageneses inmetasandstones from the Elmore I well (cl Fig. 1), threemineral zones were identified with increasing tempera-ture, and are referred to as the chlorite, biotite, and garnetzones (McDowell & Elders 1980). These studies haveshown dramatic mineralogical modifications of Colo-rado Riverdelta sediments within and nearthe numerousthermal anomalies at the Salton Trough. In particular,systematic changes in mineralogy of authigenic layersilicates (McDowell & Elders 1980) and alkali feldspar(McDowell 1986) with increasing temperature suggestan approach to chemical equilibration between theauthigenic minerals and the coexisting fluid phase in thegeothermal system.

The State 2-14 well is located near the northeasternflank of the Salton Sea geothermal system Gtg. 1).Grsenschist- and amphibolite-facies metamorphism ac-companying hydrothermal metasomatism observed inmetasandstone and metashale core samples have beenreported by Cho et al. (1988) and Shearer et al. (1988).Parageneses of secondary minerals along veins andfractures have been extensively investigated (e.9.,Caruso et al. 1988, McKibben & Elders 1985, McKib-ben & Eldridge 1989).

On the basis ofdegree ofhydrothermal alteration ofmetasandstone" three zones have been described withincreasing depth and metamorphic temperature: chlo-rire--calcite (6lG-2,480 m), biotire (2,480-3,000 m) andclinopyroxene (3,00V3,220 m) zones. Characteristicassemblages of minerals of metasandstone, in additionto epidote, quartz, albite, apatite and titanite, are: chlorite+ K-feldspar + phengitic mica + calcite for the chlorite-calcite zone, biotite + chlorite + K-feldsparfor the biotitezone, and diopside + actinolite (or actinolitic horn-blende) + K-feldspar + oligoclase for the clinopyroxenezone(Cho et a/. 1988, Enami et al.,inprep.). tow-gradeamphibolite-facies assemblages of minerals, includinghastingsitic amphibole and andesine or more calcicplagioclase, occur in metasandstone, metabasite andveins at depths greater than 3,140 m in the clinopyroxenezone. A chain silicate with composition intermediatebetween actinolite and diopside (Ca-bearing pyribole)was reported in a sample from the lowest-grade part ofthe clinopyroxene zone (Cho et al. 1988). Agarnet-bear-

l fullEt lsland 11f35'f,

Salton Sea

Geothermal wel I

Quaternaryrhyol ite dome

o o

'o'GlCIor;-il

oo--J o

ing assemblage reported from the Elmore I well(McDowell & Elders 1980) was not found in the samplesstudied.

SAMPLE DESCRIPTONAND ANALYT'ICAL PROCEDLRES

The workreportedherewas done on ten sampJes fromdepths of 3,100 to 3,180 m in the Stile 2-14 well (Fig.2), where the measured temperature exceeds 350"C(Sass er a/. 1988). Excluding core sample 9907b, all thesamples were drilling chips less than 0.5 cmin maximumdimension. The chips include mixtures of metashale andmetasandstone with subordinate metabasite and aggre-gates of vein minerals. These samples were pulverizedto 0.1-0.35 mm with a disk crusher. Amphibole-bearingfractions were concentrated with an isodynamic separa-tor, impregnated with epoxy resin and polished forpetrographic observation and electron-probe microana-lysis.

Chlorine-bearing amphibole was identified using aKEVEX energy-dispersion spectrometer. Quantitativechemical analyses were done on automated JEOLelectron-probe microanalyzers JCXA-733 at StanfordUniversity and Nagoya University. Accelerating volt-age, specimen current and beam diameter were 15 kV,12 nA and 3 pm, respectively. Well-characterizedminerals and synthetic phases, including sodalite andCl-rich hastingsite (for Cl), were used as standards.

tw9

Precision (1o level) of Cl microanalysis is 0.1 wt.%io incount statistics on Cl-rich amphibole. Fluorine contentfor all analyzed grains is below the detection limit of0. Iwt.Vo. Fe3+ contents were estimated using the methodsof Papike et al. (1974).

AMPHTBOLES AND COEXISTING MTNSRALS

Mineral assemblages associated with Cl-bearing am-phibole are given in Table l. Owing to the small size ofthe chips, it was not always possible to determinewhether the protolith is sandstone, basaltic material, orvein material. In such cases, quartz-bearing chips areconsidered to be metasandstone. Among undifferenti-ated quartz-free chips, monomineralic or K-feldspar-bearing chips are considered to be vein material, andwhere amphibole and plagioclase are dominant phases,the chip is considered to be metabasite.

The amphibole-bearing metasandstone chips ana-lyzed ue composed mainly of quartz, plagioclase,amphibole, epidote and K-feldspar; clinopyroxene, apa-tite, biotite, titanite, pyrite and zircon also occur as minorphases. The metabasite chips consist of amphibole,plagioclase and epidote, with minor amounts of clinopy-roxene, quartz, apatite, and pyrite. The major phases ofthe veins are amphibole, epidote, and plagioclase;clinopyroxene, K-feldspar, quafiz, apatite, titanite, andpyrite occur in some vein chips. Most grains of plagio-clase in the metasandstone have An contents less than

CI-BEARING AMPHIBOLE IN THE SALTON SEA GEOTI{ERMAL SYSTEM

Ftc. l. Location map of the State 2-14 well, together with sites of previous drilling in the

Salton Sea seothermal field.

TTIE CANADIAN MINERALOGISTr080

F,{l?F_.?t! I[:]rri,N::l II IG -

-II

@

r-n4trsil

ilnlL-,

| | lletasandstone

! uetauasiteffi vein

5 t 0 1 5Alz0r (wt%)

2(wt%)

0 tc l

3,tw/

$Tr*

_ ) /f L f

l+0510

Alz0r (rt%)

Major-element chemistry of amphiboles

More than 200 chemical analyses of calcic amphi-boles were carried out; representative compositions aregiven in Table 2. Al-poor actinolitic amphibole occursin all the samples studied (Fig. 2). Only five samples(10300, 10330, 10360" 10390, and 10430) from depthsbetween 3,140 and 3,180 m contain coexisting Al-richhastingsitic and Al-poor actinolitic amphiboles (Fig. 3a).There is a dramatic increase in the Cl and Al contents ofamphibole between 3,130 m (sample 10260) and 3,140m (sample 10300). This also corresponds to an increasein the abundance of metabasite sills and dykes in the

t]FE- '

l1tll l -

0 1Cl (rt%)

Frc. 2. Variations ofthe Cl and Al2O3 contents (wr.7o) ofcalcic amphiboles in metasandstone, metabasite and vein chips from theSahon Sea State 2-14 well, with depths of chip samples. Some of the data on Al2O3 for samples 10160 and 10230 are fromCho etal. (1988).

50 mol%o; some plagioclase in metabasite and veinchips are as calcic as Ane6. The albite content of theK-feldspar coexisting with plagioclase is about7-8 molVo, suggesting equilibrium temperatures of350-400'C using the two-feldspar geothermometerproposed by Green & Usdansky (1986) and Fuhrman &Lindsley (1988). The Al2O., CaO contents, and X(Fe2*)[= Fe2*(Fez* + Mg)] value of clinopyroxene are < 0.5wt.%o, 23.5-24 wt.%o, and 0.3-0.5, respectively. Phaserelations, parageneses and chemical compositions ofother minerals in the clinopyroxene zone of the State2-14 well will be presented in a subsequent communi-cation.

CI-BEARING AMPHIBOLE IN THE SALTON SEA CEOTI{ERMAL SYSTEM

TABLE 1. MINERALASSEMBTAGES OF ANALYZED CHIPS

l08 l

Cam Pt Ep Qa Kfs Cpx Samples

6 , 71 , 6J

4 , 82 , 3 , 71 , 525 , 6< 1

2

Mstasandstone tAp,fitsTit

r Ap, Ti! ZrnrTit, Pyt Ap, Titt Ap, fit Zrn,Py,Bt

+ + + + ++ + . f . t+ + ++ + + + ++ + + . t+ + ++ + ++ + + ++ + ++ +

4 , 83 , 4 , 6 , 744 , 654 ,s4

+ r P y!AP

+

Metabasite f + +f + f+ ++ t+ + ++ ++

c P y! P y

t

4 , 7

)4 , 6

2 , 4 , 5 , 6 ,7 ,8

+ + ++ + ++ ++ + ++ f ++ . t ++ +

tAp,Tit

s P yr P y

Note: Abbrwiations are: Cam, calcic amphibole; Pl plagioclase; Ep, epidote; Qtz, quare;Kfs, K-feldspac Cpx, clinopyroxeneaAp, apatitei Tit, titanite; Zm' zircon; Py, pynte; Bt' biotile;t, 101 60; 2,- 1D50i 3, lO?KOi 4, 103CXi, 5; 10330; 6, 10360; 7, 10390; 8, I 0430.

chips recovered. The maximum Cl and Al contents ofamphibole at3,140 m are essentially the same as thoseobserved at the bottom of the drill hole (3, I 80 m).

Most of the compositions corespond to actinoliticand hastingsitic amphiboles (Fig. a). Some of theintermediate compositions may refer to mixtures of twoor more amphiboles that could not be resolved with theanalytical techniques used. Hastingsitic amphibole isdark bluish green and occurs as prismatic subhedralcrystals 0.02-0.1 mm in size. Chemical variations fromferro-homblende, through ferro-pargasitic hornblendeand hastingsitic hornblende, to hastingsite (Fig. )reflect both chemical variations among different drillchips and zonation within single grains. Some grainsshow complex zoning represented by patchy inter-gowths of hastingsite with either hastingsitic horn-blende or ferro-pargasitic hornblende (Fig. 3b). Thehastingsitic amphibole in metabasites and veins has avalue ofX(Fe2+) between 0.5 and 0.85; in metasandstonechips, values are between 0.45 and 0.65. The atalyzedhastingsitic amphibole contains 034.7 vtt.Vo TiO2,similar to Cl-rich amphibole from submarinemetabasites and skarns (e.g.,0.3V1.14 wt.7o, Jacobson1975). The total alkali content (Na+K) is between 0.5and 0.8 pfu (per formula unit for O=23), and K contentincreases from 0.3 to 0.6 pfu with increasing total alkalicontent. Sodium is fairly constant at 0.27 t 0.03 pfu. Theactinolitic amphibole is colorless or pale green andoccurs mostly as acicular subhedral crystals (less than

0.1 mm long). Some grains of actinolitic amphibole areintergrown with hastingsitic amphibole (Fig. 3a).

Two lines of evidence suggest that both hastingsiticand actinolitic amphiboles are formed in the present-daygeothermal system: (l) they show a prismatic andsubhedral habit, and occur as pore fillings in somemetasandstones, and (2) Al, Na and K contents ofamphibole increase with increasing depth and tempera-ture (Fig. 2).

The coexisting actinolitic and hastingsitic amphibolessuggest a miscibility gap in the calcic amphibole series(Cooper & Lovering 1970, Tagiri l9ll,Marayama etal. 1983, Ishizuka 1985). In coexisting amphiboles,average contents of tetrahedrally coordinated aluminum,t4lAl, andA-site alkali contents, h1154+K), are 0.224.54and 0.02-0.13 pfu, respectively, for actinolitic amphi-bole, and 1.42-1.79 and 0.474.73 ptu for hastingsiticamphibole (Fig. 5). The compositional range of themiscibility gap in terms of Tschermak substitution {=f4lAl - fAl(Na+K)] is from 0.22 to 1.06 for a Mg-poorpair (Mg is 2.3 pfu in actinolitic amphibole and 0.9 ptuin hastingsitic amphibole), and from 0.41 to 0.98 ptu fora Mg-rich pair (3.1 ptu in actinolitic amphibole and 2.0ptu in hastingsitic amphibole). The miscibility gapbecomes narrower with increasing Mg content.

Cl content of amphibole

Relationships between Cl content of calcic amphi-

1082 TTIE CANADIAN MINERALOGIST

TABI,E Z REPRESENTATIVE ANALYSES OF CI-BEARJNG CArcIC AMPHIBOLES

Sample

Chip

103m 1(D60 10250 10160

01b 01b 01b 05v 05v 05v 14s 01s 04s 09v 06b 08s Lzv 11b 16v 06s 10s 01s

53.6 52.6 52.90.07 0.15 0.113.80 3.20 2.89n.d. n.d. n.d.

16.0 18.0 13.0o.23 0.37 0.L7

L2.2 LL.8 15.6t2.5 L2.3 L2.60.37 0.31 0.230.07 0.16 0.180.04 0.13 0.M

42.8 52.90.51 0.039.89 3.380.00 0.10

18.2 10.6034 0.229.62 16.9

L2.1 12.6L.O1 0.281.52 0.081.(B 0.05

39.O 42.t 5r.8 52.2o.32 0.33 0.16 0.03

10.9 9.86 4.08 1.69n.d. n.d. n.d. n.d.

26.4 23.5 t3.7 25.7o.28 0.29 0.32 0.69s.37 7.44 L4.4 6.88

Lt.I tz.O L2.4 rl.go.75 0.87 0.31 0.182.39 L.4L 0.14 0.04LA O.93 0.08

CI-BEARING AMPHIBOLE IN THE SALTON SEA GEOTHERMAL SYSTEM

6.5

1083

6.0

8.0

tA l (Na + K) 0.5 , t6 lA l>Fe3+

Fr-EdcHbl

Fr-PrgcHblo%' Fr-Prg

Fn PrgcHbl Fn Prgo

Edc Hbl

Prgc Hbl Prg

6.5 6.0Si (pfu. )

tAl(Na + K) 80.5, t6 lAl< Fe3+

Fr-EdcHbl o (

o o

Hsc Hbl

t;dHs

[|gn HsTsF'

o

HblI'lgnHsc

Edc HblHgs-Hsc

Hbl Hgs-Hs

6.5Si (p fu . )

00

Frc. 4. Chemical characteristics of calcic amphiboles from the Salton Sea S@te2-14 well as a function of X@e2+) [= Fe2*/(Fe2*+ Mg)l , numbers of atoms of Si and (Na+K) in theA-site {lAl(Na+K)} performula unit (pfu). The nomenclature of amphibolesfollows l.eake (1978). Abbreviations are: Act actinolite, Actc actonolitic, Edc edenitic, Frferro, Fn ferroan, Hs hastingsite,Hsc hastingsitic, Hbl hornblende, Mgn magnesian, Mgs magnesio, Prg pargasite, hgc pargasitic, Tr tremolite, Trc tremolitic,Tsc tschermakitic.

Tsc Hbl(wt%)

o ct

1084 THE CANADIAN MINERALOGIST

0 . 6

tAr1tr16 + 1) 1ptu.)

j 1 . 5

e.=r 1 . 0

Frc. 5. Variations in proportion of Cl (wt.%o\ as a function of the number of atoms of Al inthe tetrahedral site 1f+iat, -6 tallNa+Kl in amphiboles from the Salton Sea State2-14well. Five pairs of coexisting amphiboles are shown by tie lines thal represent averagecompositions of coexisting actinolitic and hastingsitic amphiboles.

bole,lithology anddepth in thedrill hole are summarizedin Figure 2. Actinolite at depths less than 3,125 m(samples 10160, 10250, 10260) contains usually lessthan 0.2 wt.Vo Cl. The coexisting amphiboles at depthsgreater than 3,140 m (samples 10300, 10330, 10360,10390, and 10430) have Cl contents in the range0.0-2.7wt.7o. Amphiboles in metabasites and veins have similarranges in Cl content, but at any given depth, amphibolegrains in metasandstones have lower Cl content (usuallyless than I wt.Vo). The Cl-rich amphibole in themetabasites and veins also has higher X(Fe2+) andta(gatK) contents than the amphibole in metasand-stones at similar depths. This relationship suggests thatthe variations in extent of Cl-for-OH substitution in theamphibole structure are largely controlled by crystal-chemical constraints rather than differences in tempera-ture or Cl-content of the coexisting fluid phase.

On the basis of a variety of drill hole and fluid-inclu-sion experiments, measured temperatures, pressures andfluid compositions, Helgeson (1968) and McKibben etal. (1987, 1988) have identified large gradients insalinity within the Salton Sea geothermal system. Ahigh-salinity geothermal brine is overlain by a lower-sa-linity fluid; steep gradients in salinity and perhapsdensity occur at depths between 1,000 and 1,500 m atthe center of the geothermal system. Near the centralgeothermal anomaly (at depths > 1,900 m), the measuredCl content of the geothermal brine is fairly constant at15.4-19.7 wt.7o (Michels 1986, Thompson & Fournier1988). In addition, apatite in the Cl-rich and Cl-pooramphibole-bearing samples have similar Cl contents(0.37 t 0.16 wt.Vo Cl in samples 10360 and 10390, 0.34+ 0.08 wt.7o in samples 10250 and 10260). The near-

constant chlorinities predicted for the deep geothermalfluid and for apatite suggest that both the Cl-rich andCl-poor amphiboles in the samples studied were formedunder similar chlorinities of the fluid. We conclude thatvariation in Cl content of amphiboles (Fig. 2) is not aconsequence of amphibole formation in fluids of widelyvarying chlorinity, but rather of crystal-chemical con-straints among the calcic amphiboles, as discussedbelow.

Crystal-chemical control of Cl content in amphibole

Several authors have noted that the Cl content ofcalcic amphibole is crystal-chemically controlled. Ito &Anderson ( I 983) noted that Al substitution at tetrahedralsites coupled with Fe3* substitution at octahedral sites or(Na+K) substitution at the A site allow increasingreplacement of OH by Cl. Vielzeuf (1982) showed apositive correlation between Cl and K contents ofamphiboles in chamockite from Sakeix, French Py-ren6es. Volfinger et al. (1985) and Kamineni (1986)emphasized that substitution of Cl for OH is accompa-nied by Fe2* substitution at octahedral sites. All thesesubstitutions increase the unit-cell volume and may alsoenlarge size of the cavity normally occupied by OH, sothat Cl, with an ionic radius of 1.81 A, can be accommo-dated. The unit-cell volume of Cl-rich hastingsite is 3-47o larger than that of Cl-poor or Cl-free hastingsite(Suwa et al. 1987).

In the Salton Sea geothermal system, Cl is preferen-tially incorporated in hastingsitic amphibole relative tocoexisting actinolitic amphibole (Table 2). Grains of thehastingsitic amphibole are compositionally heterogene-

)- o/oDn/

ous, and the Cl-rich domains are rich in Fe2+, t4lAl andh]1|r{a+K) (Fig. 3b, Table 2). Variations of Cl content inamphibole (Fig.6) as functions of tAl(Na+K) andX(Fe2*)suggest a crystal-chemical control on Cl content. Twocharacteristic features are apparcnt: (l) Cl content ofamphibole increases with increasing tAl(Na+K), and (2)the maximum Cl content of amphibole increases withincreasing X(Fe2*). Amphibole compositions with Cl inexcess of 0.5 tt'rt.%o plot along an edenite{t4115416;ta14ltr-rsi-r} substitution vector (Fig. 5).This fact indicates that the extent of tschermakitesubstitution {t4lAlt6lAlsi-r(Mg, Fe2*)-r } in cl-rich am-phibole is relatively constant {0.9 < I41Al - tallNa+K) <1.0). The chemical characteristics shown in Figure 6thus indicate that (1) Cl variation of the Salton Seaamphibole is controlled primarily by variations in theedenite component; (2) X(Fe2\ value of the amphiboleshows little correlation with Cl and edenite contents, butis correlated with the maximum Cl content of amohibole.Increasing trlg166;tal41n-,Si-, and FeMg-, iubsritu-tions favor incorporation of Cl in the amphibole struc-rure. Both hl15n+19lal41n ,Si_, (Ito & Anderson 1983)and FeMg-1 (e.9., Volfinger et al. 1985) substitutionsseem essential but not sufficient for Cl enrichment.Where coupled, the two substitutions seem to increasethe content of Cl incorporation in amphibole. Thisscheme is consistent with the fact that examples ofCl-rich calcic amphibole (> 3 wt.Vo Cl) reported in

1085

literature are mostly ferro-hastingsite with X(Fe2*) >0.75, (Na+K) > 0.9 pfu, and I4lAl between 1.8 and 2.3pfu (total Fe as FeO and O = 23; Suwa et al. 1987).

Figure 7 shows the relationships among Cl,tatgla+K), X(Fe4), and t4lAl contents of the Salton Seaamphiboles. The data indicate that (l) on Figure 7a"

Dn",/dnlAllNa+K) - 0.6 for t4lAl < 1.6 anddn./dntAl(Na+K) = 1.q for t4lAl >1.6,

and (2) on Figure 7b,dn"y'EX(Fe2*) = 0.3 for talQr{a+K) < 0.55 andEn6/dX(Fe2*) = 0.8 for tel(\arK) 2 0.55,

where n is the number of atoms of the subscriptedelements.

Yolfinger etal. (1985) demonstrated the fundamentalrole of the local structure of the anion site in exchangeof anions in silicate minerals. In the case of amphibole,the closer the symmetry of the ring of six tetrahedra inthe double chain to ideal hexagonal symmetry, the largerthe sizes of anionic and alkali sites. The adaptation ofthe tetrahedral chains to the octahedral strips is control-led particularly by their iron content. This schemeexplains the positive correlation between Cl andtatlya+K) and that between Cl and X(Fe2+) in calcicamphiboles. Figure 7, however, shows that (1) the extentof Cl substitution for OH as a function of edenitesubstitution at tAl(Na+K) > 0.55 is twice that for anamphibole with ntlNa+K) < 0.55, and (2) the Cl content

CI-BEARING AMPHIBOLE IN TI{E SALTON SEA GEOTHERMAL SYSTEM

Salton Sea

:s3

5

Detect i on (26 ls'ral)0.01-

0 0 . 5 1 . 0r^t( i la + K) (pfu.)

FIc. 6. Variations in proportion of Cl (wt.7o) as a function o1 tAllNa+K) and X(Fe2+) inamohiboles from the Salton Sea State 2-14 well.

1086 THE CANADIAN MINERALOGIST

0 . 8

^ 0 . 6;

50.4

U , Z

- ' - ' t l r

a - tN1p6 + K )0.55

o o . z 0 . 4 - .

0 . 6 0 . 8 1 . 0

XFe'-

FIc. 7. Variations in the,proponion of C\ (wt.Va) as a function of (a) tAl(Na+K) and t4lAl,

and (b) X(Fe'*) and tor(Na+K), in amphiboles from the Salton Sea State 2-14 well. Solidand broken lines indicate the regression lines for amphiboles with t4lAl < 1.6 and > 1.6in (a), and those *it5 FllNa+K) < 0.55 and > 0.55 in (b), respectively.

(a)

E_,,ril;ll l

-*-tanr>r.o I E /

a

o

a o la

E t r

E nE'

. c D l ".e q56'btrt - t ' o o

" EpF- .{tl

/ t r 8

6 t t r

o

E

t r 6ru:tr

_ t r & 6u _ . ' e" - d " F wd.E' ^q,

r " En E r - -

. t - '

d-

^ 0 . 6

0 . 4

ata

o1 ta1(Na+K)-rich amphibole is higher than that oftAl(Na+K)-poor amphibole for a fixed X(Fe2+). Totalexpansion of unit-cell volume with increasing edenitecomponent may also make the increased substitution ofCl-for-OH possible.

Cl panitioning between amphibole andfluid

Figure 8 summarizes the relationships among Cl,X(Fe}), -4 tal(Na+K) in calcic amphibole composi-

tions reported in the literature. The data pertain toamphibole formed under various geological environ-ments: submarine metabasites @g. 8a), skarns (Fig. 8b)"high-grade metamorphic rocks (Fig. 8c), and igneousrocks @g. 8d). Available data on fluid composition andP-T conditions of the representative examples aresummarized in Table 3. Although the Cl-bearing amphi-boles shown in Figure 8 were mostly formed underhigher P-T conditions (0.4-10 kbar, 450-850'C) thanthose in the Salton Sea samples (0.3 kbar, 350'C), most

(a) Submarine netabasites [96

CI-BEARING AMPHIBOLE IN THE SALTON SEA GEOTT{ERMAL SYSTEM 1087

(d) lgnoous rocks 11361

0.0130.5 1 .0

r^11,1, * K) (ptr.)

wt.7o) amphibole can form at lower temperatures thancoexisting Cl-poor (less than 1.0 wt.Vo) amphibole.

Some amphiboles in granodioritic charnockite(Karineni et a/. 1982) and anorthosite-gabbro complex(Kamineni 1986) have Cl-rich compositions exceedingthe maximum Cl-for-OH substitution of the Salton Seaamphiboles @gs. 8c, d). This material has a highercalculated Fe2O3 content (5.+-7.9 wt.Vo) than the othersamples (Fe2O, = 3.8 + 2.2 wt.7o). Increasing Fe3*Al-,

r';i,

o

*E

-o -

&to o

o o o o

o

, @

oo

O xr."5.aO 0.4sxFo&

1088 TTIE CANADIAN MINERALOGIST

TABIE 3. MODB OF OCCURRENCE OF SOME CI-BEARING CAI,CXC AMPHIBOLES

Cl-content Cl-content EouiliMumofanphibole offluid cbndition

References

Submarine t 6.03-6.5L wt.Vom€tabasite .0.024.6wtVa

t O.OL-4.O2'ttt.Vo' 0.01-3.fi) wt7a

amphibolite Jacobson (1975)550-75fC Io and Anderson (1983)amphibolitef, Vanko(1986)greenschistf. Vanko(1986)

> sw(?)> sw(?). sw(?)> $*,(?)

Skam t 7.24,ttt.fot 0.L3-2.68,tttTo

n.d. pyroxeneh(?) Krutov(l93Qn.d. pyroxenehf(?) Gutyaevaaal.(l98Q

High-glade t 4.LSrxtVamamorphic I O.O4.65,*t.Vo

rock

grmulile lGmineni et at. (1982)8m-85(fc Matsubara and

&10 kbar Motoyoshi (1985)

n.d.nd.

Igneousrock' O.O7-2.44' t t tVo. 0.@-2.08 q't.7a

' 0.024.78,ttt,Vo

n.d. ? Czamanskeetal,(L9l)nd. 4O0-60fC lGmineni (1986)

1-2 kbar46wt.Vo ir 500-75fC Bird et al. (1986)naximum(?) 0.4-0.7kbar

Geothermal t 0.0-2.7 wt.Vometamorphic

rock

L5-19.7'xtVo 350-40trC Thisstudv0.3 kbar

No!e: Abbreviations arc: n.d., not determined; sw, seawater; amphibolite f., amphibolitc facies;greenschist f., greenschist facies; pyroxene hf, pyroxene homfels facies.

substitution expands the entire chain ofthe amphibole(Ito & Anderson 1983) and thus may facilitare substiru-tion of Cl for OH.

Variable Cl contents in amphibole have been corre-lated with water-rock interaction and Cl fixation duringmetamorphism and hydrothermal alteration of oceaniccrust. In submarine metabasite and skarn. chlorinecontents of some amphiboles attain7.2 wt.7o (Figs. 8a,b), which is distinctly higher than that of the Salton Seaamphiboles (up to 2.7 wt.Va). Many authors haveconsidered that Cl-rich amphibole grew from extremelychlorine-rich fluid (e.g., Ito & Anderson 1983, Vanko1986). Such fluids may exist in oceanic hydrothermalsystems and during skarn formation (e.g.,Tan & Kwak1979, Vanko 1988). However, cases of Cl-rich amphi-bole with more than 3 wt.Vo Cl have greater tAI(Na+K)than the Sallon Sea amphiboles. Therefore, the high Clcontent of amphiboles is considered also to be due toincreasing volume-expansion. The similarity in themaximum extent of Cl-for-OH substitution with fixedtAl(NarK) among the Salton Se4 submarine metabasite,and skarn amphiboles implies that the extent of Cl-for-OH substitution in these amphiboles is not sensitive tovariations in fluid chlorinity.

Amphiboles in high-grade metamorphic rocks andigneous rocks contain less Cl than the maximum extentof Cl-for-OH substitution found in the Salton Seasamples. These amphiboles were formed underconditions of lower chlorinity than those in the SaltonSea geothermal system. Nabelek ( 1989) showed that theCl content of calcic amphibole in mafic hornfelsesaround the Laramie anorthosite complex, Wyoming,

increases systematically from 0.03 to 0.37 wt.Vo wilhdecreasing proportion (vo1.7o) of amphibole; heinterpreted this finding in terms of progressive lossof water and preferential concentration of Cl in amphi-bole during contact metamorphism. These observa-tions imply that the Cl content of amphiboles coexistingwith a low-chlorinity fluid is sensitive to variationsin fluid chlorinity and is controlled by Cl partition-ing between the two phases as well as the crystal-chemi-cal constraints imposed by (Na+K), Fe and Al substitu-tions.

Figure 9 illustrates Cl partitioning between coexistingamphibole and fluid, inferred from the chemical data ofthe Salton Sea geothermal system (this study) andMathematician Ridge (Stakes & Vanko 1986, Vanko1986, 1988, Suwa & Enami, unpubl. data). In low-chlorinity environments, the Cl content of amphibole isdefined by partitioning of Cl between amphibole andfluid, and increases with increasing Cl content in fluid,tAl(Na+K), and X(Fe2+) of the amphibole. The estimatedratio of 3(X6)a'p to E(Xcr)nuid is approximately 3 at 0.4< Ietgla+K) < 0.6 and 0.4 < X(Fez+) < 0.7, andapproximately 4 or 5 at 9.6 < Ial(Na+K) < 0.8 and 0.4 <X(Fe2) < 0.7. An amphibole *i6 tal(Na+K) > 0.4 andX(Fe}) > 0.4 seems to preferentially concentrate Clrelative to the coexisting fluid in low-chlorinity environ-ments. On the other hand, amphiboles coexisting withthe Salton Sea brine or with more saline fluids havenearly constant Cl content for fixed X(Fe2+) andtAl(Na+K) contents, and seem independent of variationof chlorinity in the coexisting fluid.

CI.BEARINC AMPHIBOLE IN THE SALTON SEA GEOTHERMAL SYSTEM

XFe2*l lilfl

* o.ssc^loa 0.6stl t

r090 T}IE CANADIAN MINERALOGIST

of this research is gratefully acknowledged. Criticalreviews by Philip A. Candela David A. Vanko, Craig E.Manning, Moonsup Cho, Robert F. Martin, NoriyukiNakasuka and two anonymous referees improved thismanuscript.

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Received August 7, 1991, revised mnnuscipt acceptedJanuary 30, 1992.