Geochemical constraints for the origin of thermal waters from western Turkey

Applied Geochemistry 17 (2002) 163–183 www.elsevier.com/locate/apgeochem

Geochemical constraints for the origin of thermal waters from western Turkey . Avner Vengosha,*, Cahit Helvacıb, Ismail H. Karamanderesic a

Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, PO Box 653, Beer Sheva 84105, Israel . b Dokuz Eylu¨l U¨niversitesi, Mu¨hendislik Faku¨ltesi, Jeoloji Mu¨hendisligˇ Bo . ¨lu¨mu¨, 35100 Bornova-Izmir, Turkey c MTA Ege Bo¨lge Mu¨du¨rlu¨gˇ, 35042 Bornova-Izmir, Turkey Received 14 February 2000; accepted 14 February 2001 Editorial handling by R.L. Bassett

Abstract The combined chemical composition, B and Sr isotopes, and the basic geologic setting of geothermal systems from the Menderes Massif in western Turkey have been investigated to evaluate the origin of the dissolved constituents and mechanisms of water–rock interaction. Four types of thermal water are present: (1) a Na–Cl of marine origin; (2) a Na–HCO3 type with high CO2 content that is associated with metamorphic rocks of the Menderes Massif; (3) a Na– SO4 type that is also associated with metamorphic rocks of the Menderes Massif with H2S addition; and (4) a Ca–Mg– HCO3–SO4 type that results from interactions with carbonate rocks at shallow depths. The Na–Cl waters are further subdivided based on Br/Cl ratios. Water from the Cumalı Seferihisar and Bodrum Karaada systems are deep circulated seawater (Br/Cl=sea water) whereas water from C¸anakkale–Tuzla (Br/Cl < sea water) are from dissolution of Messinian evaporites. Good correlations between different dissolved salts and temperature indicate that the chemical composition of the thermal waters from non-marine geothermal systems is controlled by: (1) temperature dependent water– rock interactions; (2) intensification of reactions due to high dissolved CO2 and possibly HCl gasses; and (3) mixing with overlying cold groundwater. All of the thermal water is enriched in B. The B isotopic composition (d11 B=2.3% to 18.7%; n=6) can indicate either leaching of B from the rocks, or B(OH)3 degassing flux from deep sources. The large ranges in B concentrations in different rock types as well as in thermal waters from different systems suggest the waterrock mechanism. 87Sr/86Sr ratios of the thermal water are used to differentiate between solutes that have interacted with metamorphic rocks (87Sr/86Sr ratio as high as 0.719479) and carbonate rocks (low 87Sr/86Sr ratio of 0.707864). # 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction The chemistry of thermal waters has attracted the attention of numerous studies, in particular investigations of the influence of water–rock interactions and the large diversity of the ionic composition of fluids that are found in geothermal systems (e.g. Mahon, 1970; Tonani, 1970; White, 1970; Fournier and Truesdell 1973; Ellis and Mahon, 1977; Fournier, 1979; Giggenbach et al., 1983; Giggenbach, 1988). The chemical and environmental isotope compositions were used to determine the * Corresponding author. E-mail address: [email protected] (A. Vengosh).

origin of geothermal waters, in particular to distinguish between meteoric and sea water (e.g. Davisson et al., 1994). The geothermal fields of western Turkey provide a unique setting of extremely high enthalpy combined with a large variation in chemical composition. The distribution of the thermal systems follows the tectonic patterns of Turkey. The presence of active structural systems that characterizes western Anatolia is associated with young acidic volcanic activity, block faulting (grabens), hydrothermal alteration, fumaroles, and more than 600 hot springs with temperatures up to 100
C (C¸agˇlar, 1961; Ercan et al., 1985; 1997). The major high-enthalpy geothermal fields of Turkey are Kızıldere (200–240
C),

0883-2927/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(01)00062-2

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O¨merbeyli–Germencik (232
C), C.¸ anakkale–Tuzla
(174 C), Simav–Ku¨tahya (165
C), and Izmir–Seferihisar (232
C) (S¸ims¸ ek and Gu¨lec¸, 1994; Go¨kgo¨z, 1998; Fig. 1). The geothermal energy potential of western Turkey is used for electricity production. During 1998, Turkey produced enough geothermal heat for 50,000 houses and greenhouses of 200,000 m2 with 350 Mwt, as well as 190 hot springs with 285 Mwt (Go¨kgo¨z, 1998). However, the high concentrations of dissolved constituents, in particular high dissolved B in geothermal effluent, presents a serious environmental problem. For example, effluents from the power plant in Denizli–Kızıldere that have B concentrations of more than 20 mg/l are released into adjacent creeks and endanger natural biota that are sensitive to B. In addition, natural underground discharge of geothermal waters into overlying aquifers results in B contamination in the associated aquifers in western Anatolia (Filiz and Tarcan, 1995). Previous studies have investigated different aspects of the chemical and isotopic composition of geothermal waters in western Turkey (Filiz, 1984; Gu¨lec¸, 1988; Tarcan and Filiz, 1990; Ercan, 1993; Conrad et al., 1995; Mu¨tzenberg, 1997; Balderer, 1997; Go¨kgo¨z, 1998; O¨zgu¨r et al., 1998). This study presents the chemical and B (11B/10B) and Sr (87Sr/86Sr) isotopic compositions of major geothermal fluids from western Turkey. The aim is to provide an overall assessment on the origin of the thermal fluids, in particular the origin of the elevated B dissolved in the geothermal waters.

2. General geology of western Turkey and Menderes Massif Turkey is located within the Alpine–Himalayan orogenic belt. The distribution of seismicity and active regimes are concentrated along high strain zones, many of which are major strike-slip faults, such as the North Anatolian fault (Ketin, 1956, 1968), East Anatolian transform fault (Dewey and S¸engo¨r, 1979) and graben zones (e.g. Bu¨yu¨k Menderes graben, Ku¨c¸u¨k Menderes graben, Gediz graben, Simav, Manyas, Kızılcahamam) (Angelier et al., 1981; S¸engo¨r, et al., 1985). The broad tectonic framework of the Aegean region and the eastern Mediterranean region is dominated by the rapid westward motion of the Anatolian plate relative to the Black Sea (Eurasia) plate, and west to south-westward motion relative to the African plate (McKenzie, 1972, Dewey and S¸engo¨r, 1979). The Anatolian plate is considered a ‘‘floating’’ continental plate being pushed westward from the intercontinental Bitlis suture zone (the southern edge of the Arabian — Eurasian convergent strain zone), where its motion relative to Africa, is characterized by subduction at the Hellenic Trench (Dewey and S¸engo¨r, 1979). The Anatolian region consists of a mosaic of fragments of

continental crust originally scattered over Tethys. These fragments have been assembled as intervening oceanic crust has been eliminated by a series of subduction episodes during the past 200 ma (Crampin and Evans, 1986). The differential plate motions are responsible for the young, east and west Anatolian volcanic activities. Block faulting and North Anatolian transform movements apparently began in the mid-Miocene. The movement on the North Anatolian fault is right lateral strike-slip on an E–W fault, or normal to the movement between the major plates. The explanation of this remarkable observation is that the North Anatolian fault does not form a plate boundary between Eurasia and Africa, but the northern boundary of a small plate. The small plate is situated on central and western Turkey, and is rapidly moving westward, at about 40 mm/a (McKenzie and Yılmaz, 1991; Yılmaz, 1997). The motion in western Turkey yields a velocity of 70 mm/a in the front of the arc and an uplift of 2.4 cm/a between the Aegean and the Eurasian plates. The western Anatolian region is undergoing extension at some of the highest rates ever documented. Eyidogˇan (1988) reported extension rates of 13.5 mm/a over the last 40 years. The Menderes Massif (Fig. 1) is one of the largest metamorphic massifs in Turkey, with a lengths of about 200 km N–S between the Simav and Go¨kova grabens, and about 150 km E–W between Denizli and Turgutlu in western Anatolia (Ketin, 1983). Philippson (1910) described the Menderes Massif as a dome-like structure, broken by faulting during the Alpine orogeny whereas Dixon and Pereira (1974) regard the Menderes Massif as one of a number of ‘‘zwischengebirge’’, essentially microcontinental blocks, made up of pre-Mesozoic basement rocks having some of the characteristics of the cratons but displaying evidence of Alpine tectonic and magmatic . involvement (Blumental, 1951; Bas¸ arir, 1970; Izdar, 1971; Du¨rr et al., 1978; O¨ztu¨rk and Koc¸yigit, 1983). The crystalline Menderes Massif is divided into two major units: the core and the cover series. The core series consists of Precambrian to Cambrian high-grade schist, metavolcanic–gneisses, augen gneiss, metagranites, migmatites and metagabbros. The cover series is composed of Ordovician to Paleocene micaschists, phyllites, metaquartzites, meta leucogranites, chloritoid–kyanite schists, metacarbonates and a metaolistostrom. In many places, metabauxites, probably upper Jurassic to Cretaceous in age occur in the upper levels of the metacarbonate sequence (Dora et al., 1987, 1995; Candan et al. (1992) observed that high-grade metamorphic rocks are located along tectonic contacts within the schist, phyllite and marble series, which is enveloping the core. This is supported by the field data and drilling data from the Germencik–O¨merbeyli geothermal system (S¸imsek et al., 1983; Karamanderesi and O¨zgu¨ler, 1988; Karamanderesi et al., 1988). On a large scale, the post-metamorphic compressional phase conjugated with the paleotectonic evolution of

A. Vengosh et al. / Applied Geochemistry 17 (2002) 163–183

Fig. 1. General map of western Turkey and location of investigated geothermal systems.

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A. Vengosh et al. / Applied Geochemistry 17 (2002) 163–183

western Anatolia is in a N–S direction; and as a result it is pushed in a northward direction. This compressional force has given rise an extreme cataclastic structure. The post metamorphic granite plutons in Early Miocene have been strongly subjected to this compressional tectonics, and the allochtonous units are cut across by the graben systems of the neo-tectonic phase started in the Tortonian. It seems that the effective compressional tectonism in the Menderes Massif was during the Early– Middle Miocene period. Neogene sediments overlie the allochthonous and autochthonous groups of rocks with angular unconformity in the south of the study area. The neotectonic period of the Menderes Massif and surrounding areas has been the subject of regional research for many years (Ketin, 1966; McKenzie, 1972; Dumont et al., 1979; Angelier et al., 1981; Satir and Friedrichsen, 1986).

3. Background on the investigated geothermal fields Extensive tectonic activity and formation of E–W grabens have formed the shape of western Anatolia (Fig. 1). Of these, the Bu¨yu¨k Menderes and Gediz grabens host the main and most important geothermal fields of Turkey. The distribution of geothermal fields in Turkey closely follows the tectonic patterns. All of the hot springs with temperatures above 50–100
C in eastern and western Anatolia are clearly related to young volcanic activity and block faulting. The post-collosional volcanic activities, lasting from the upper Miocene to modern time have been responsible for heating up the geothermal fields (Demirel and S¸entu¨rk, 1996). The high thermal activities is reflected in the forms of widespread acidic volcanic activity with much hydrothermal alteration, fumaroles, and more than 600 hot springs with temperatures up to 100
C (C¸agˇlar, 1961). Table 1 summarizes the basic geological, temperature, water types, total dissolved salts, and lithological data of the investigated geothermal systems. Below the geological background of the investigated geothermal fields are described (Fig. 1 and Table 1). The Seferihisar geothermal field (samples HVK-1, HVK-2) is located on the Aegean coast of Turkey, 40 . km SW of Izmir close to the Aegean Sea within the C¸ubukludagˇ graben. The stratigraphic series of the Seferihisar area consist of Paleozoic metamorphic rocks . of the Menderes Massif, Upper Cretaceous ‘‘Izmir flysch’’, which are all metamorphosed to greenschist facies and include schists, phyllites, spilites, and metasandstone, and Neogene units of alternations of conglomerate, sandstone, and claystone. Six research wells were drilled to a maximum depth of 1417 m and indicated that the fluid-bearing formation, composed of sandstone and conglomerate, has a thickness of 200–400 m (Demirel and S¸entu¨rk, 1996). Sample HVK-1 was collected from well CM-1 that was drilled down to a

depth of 1417 m with temperature up to 146.5
C. Sample HVK-2 was collected from Dog˘anbey hot springs which have high temperature (71–77
C) and moderate salinity. The springs are located on the contact of the . Izmir flysch within the overlying Yeniko¨y formation, along the southern boundary of the Karakoc¸–Dog˘anbey horst in the SW of the Seferihisar geothermal area (Es¸ der and S¸ims¸ ek, 1975). The Germencik–O¨merbeyli geothermal field (HVK-3, HVK-4), one of the geothermal areas with high enthalpy, is located in the western part of Menderes graben (Fig. 1). The geological strata are composed of Paleozoic metamorphic rocks of sedimentary origin and Miocene to Quaternary detrital and alluvial deposits. The metamorphism has produced marble, calcschist, graphitic schist and some quartzite. The Miocene sediments also include lignite or coal-bearing horizons, interbedded mainly with conglomerate, sandstone, silts and claystone. The thermal water is derived from two major sources: a sedimentary shallow and a deep basement reservoir (Karamanderesi et al. 1985; Gu¨ner et al. 1986). Samples HVK-3 and HVK-4 were sampled from deep wells (O¨B-9 and O¨B-3, respectively) from depth intervals of 896.9–1465. and 657–1196 m, respectively. The Aydın Ilıcabas¸ ı Imamko¨y field (HVK-5, HVK-6) is composed of Paleozoic mica-schist, gneiss blocks, locally quartzite and marble, and Pliocene sediments. The later consist of cobblestone, sandstone, siltstone and claystone, and alluvial sediments on top of these units. Samples HVK-5 and HVK-6 were collected from wells AY-1 and AY-2, respectively at depth intervals of 220–471 and 250–350 m. It should be noted that the water samples were collected from Pliocene sediments. The Aydın–Salavatlı geothermal field (HVK-7) is located in the middle part of the Bu¨yu¨k Menderes graben, and is characterized by a normal fault structure. The stratigraphic sequence is composed of metamorphic rocks of the Menderes Massif and sedimentary rocks deposited during the Menderes Miocene rifting period. Field data suggests that there is a connection between tectonic development and periods of hydrothermal alteration. Several deep wells were drilled (AS-1, 1510 m and AS-2, 962m) revealing low resistivity zones (Karamanderesi, 1997). Kızıldere geothermal field (HVK-8, HVK-9) is located on the eastern part of Bu¨yu¨k Menderes graben, which extends for about 150 km in length with an E–W trend. The field was the first to produce electrical energy in Turkey. Metamorphic basement rocks which compose the stratigraphy, cover 4 sedimentary formations. The basement rocks are composed of Paleozoic Menderes metamorphic units that are characterized by alterations of . marble, calcschist, quartzite, schist, and gneiss (the Igˇdecik formation; S¸ims¸ ek, 1985). Pliocene sediments overlie the basement and are divided into 4 lithological units (S¸ims¸ ek, 1985). (1) Lower Pliocene

Table 1 General data on the investigated geothermal systems from western Turkey Location name

Production zone rocks

HVK-1

Seferihisar-Cumalı field

HVK-2

Seferihisar–Dogˇanbey

HVK-3

Germencik–O¨merbeyli

HVK-4

Germencik–O¨merbeyli

HVK-5

Aydın–Illıcabas¸ i

Tertiary sediments

HVK-6

Aydın–Illıcabas¸ ı

Tertiary sediments

HVK-7

Aydın–Salavatlı

Menderes massif metamorphics, marble

HVK-8

Denzili–Kızıldere

HVK-9

Denizli– Kızıldere

HVK-10

Manisa–Urganlı

Menderes massif metamorphics Menderes massif metamorphics Mesozoic serpantinite and limestone

Well KD-13 760 m Well KD-16 666.50 m Spring

HVK-11

Manisa–Salihli–Sart.

HVK-12

Manisa–Salihli– Kurs¸ unlu

HVK-13

Manisa–Salihli– Kurs¸ unlu–mineral water

HVK-14

Manisa–Salihli–MTA well

Menderes massif metamorphics Menderes massif metamorphics and Tertiary sediments Menderes massif metamorphics and Tertiary sediments Tertiary sediments

. Izmir flysch and Tertiary sediments. Rhyolite 12.5 ma . Izmir flysch and Tertiary sediments. Rhyolite 12.5 ma Menderes massif metamorphics, marble Dacite, 13.1 ma Menderes massif metamorphics, marble

Thermal source (springs or well)

Well deep temp. or springs temp.

TDS

Lithology

References

Well 1417.45 m

140
C

19800

Schists, phyllites, spillites and metasandstones

Well 350.00 m spring

78
C 62.5
C

22200

Schists, phyllites, spillites and metasandstones

Well number 9 1466 m

224
C

5200

Well number 3 1196.75m

232
C

3700

Well Ayter-1 471.25 m Well Ayter-2 335 m Well AS-1 1510 m

84.5
C

7000

101.5
C

4600

167
C

4600

201
C

4200

212
C

4600

82
C

2100

Menderes massif metamorphics and Tertiary sediments Menderes massif metamorphics and Tertiary sediments Tertiary and Quaternary sediments Tertiary and Quaternary sediments Menderes massif metamorphics and Tertiary sediments Menderes massif metamorphcs, marble Menderes massif metamorphics, marble Mesozoic serpantinite, limestone

MTA, 1996 Es¸ der and S¸ims¸ ek, 1975 Williamson, 1982 MTA, 1996 Es¸ der and S¸ims¸ ek, 1975 Williamson, 1982 Demange et al., 1989 MTA, 1996 Williamson, 1982a Demange et al., 1989 MTA, 1996

Spring

50
C

1200

Spring

94
C

1650

Spring

39.5
C

1200

Menderes massif metamorphics, marble

Yenal et al., 1976 MTA, 1996

Well MTA-1

94
C

–

Menderes massif metamorphics, marble

Erentu¨z and Ternek, 1968 MTA, 1996

Menderes massif metamorphics, marble Menderes massif metamorphics, marble

Karamanderesi et al., 1990 MTA, 1996 Karamanderesi et al., 1990 MTA, 1996 Karamanderesi et al., 1988 MTA, 1996 S¸ims¸ ek, 1985 MTA, 1996 S¸ims¸ ek, 1985 MTA, 1996 Erentu¨z and Ternek, 1968 Karamanderesi, 1972 MTA, 1996 MTA, 1996 Erentu¨z and Ternek, 1968 MTA, 1996

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A. Vengosh et al. / Applied Geochemistry 17 (2002) 163–183

Sample number

168

Table 1 (continued) Sample number

Location name

Production zone rocks

Thermal source (springs or well)

Well deep temp. or springs temp

TDS

Lithology

References

HVK-15

Manisa–Alas¸ ehir Fish farm Manisa–Alas¸ ehir Fish farm Manisa–Alas¸ ehir hot spring

Tertiary sediments

Well 33 m

17
C

–

Tertiary sediments

Karamanderesi, 1998

Tertiary sediments

Well 92 m

24
C

1800

Tertiary sediments

Karamenderesi, 1998

Menderes massif metamorphics, Tertiary sediments Menderes massif metamorphics Mesozoic limestone Mesozoic limestone Magmatic and volcanic rocks. Granodiorite 280.8. Ignimbrite 17.1a Magmatic and volcanic rocks. Granodiorite 280.8a

Spring

31
C

700

Menderes massif metamorphics

Karamenderesi, 1998

Spring

18
C

2300

Spring Spring Spring

55
C 34.5
C 102
C

1500 1300 59000

Mesozoic limestones Mesozoic limestone Trachyandesite, trachyte. Ignimbrite

Well T-1 814 m

174
C

65000

Trachyandesite, trachyte, Ignimbrite

Karakaya formation, Tertiary sediments. Granodiorite 23.5 ma Karakaya formation, granite, Tertiary sediments. Granodiorite 23.5 ma

Well Gu¨re-1 197 m

55
C

1000

Tertiary sediments

Well 33 m

70
C

800

Tertiary sediments

MTA, 1996

Well 29 m

100
C

1000

Quaternary alluvium

Bu¨rku¨t, 1966a MTA, 1996

Well BD-1 564 m Spring

140
C

1400

. Izmir flysch

62
C

850

Spring

33
C

35000

HVK-16 HVK-17

HVK-18

HVK-22

C¸anakkale–Tuzla

HVK-23

Edremit–Gu¨re

HVK-24

Edremit–Havran

HVK-25

Dikili kaynarca

Volcanic rocks and Tertiary sediments. Yunt dag˘ volcanics, 14.1 ma

HVK-26

. Izmir–Balc¸ova

HVK-27

. Izmir–Balc¸ova

HVK-28

Bodrum–Karaada

. Izmir flysch and Tertiary sediments. Dacite, 19.2 ma . Izmir flysch and Tertiary sediments. Dacite, 19.2 ma Limestone. Monzodiorite, 11.2 ma

a

19.2 ma and Borsi et al. a (production zone rocks and related magmatic and volcanic rocks and age datermined by).

Erentu¨z and Ternek, 1968

. Izmir flysch and Tertiary sediments Laminated cherty limestone

MTA, 1996 MTA, 1996 Karamanderesi, 1986 Borsi et al. 1972 Fytikas et al., 1976a Karamanderesi, 1986 Mu¨tzenberg, 1997 Borsi et al., 1972a Fytikas et al., 1976a MTA, 1996 Bu¨rku¨t, 1996a

JICA, 1987a MTA, 1996 Borsi et al., 1972a MTA, 1996 Borsi et al., 1972a Bas¸ kan and Canik, 1983 Pis¸ kin et al., 1983a

A. Vengosh et al. / Applied Geochemistry 17 (2002) 163–183

HVK-19 HVK-20 HVK-21

Manisa– Alas¸ ehir Sarıkız mineral water Denizli–Karahayit Denizli–Pamukkale C¸anakkale–Tuzla

A. Vengosh et al. / Applied Geochemistry 17 (2002) 163–183

Kızılburun formation-alternating red and brown conglomerates, sandstone, claystone, and lignite seams, up to 200 m; (2) Lower Pliocene Sazak formation-intercalated grey limestone, marls and siltstone, 100 to 250 m; (3) Lower Pliocene Kolonkaya formation- alternating layers of sandstone, claystone and clayey limestone, 500 m; and (4) The Plio-Quaternary Tosunlar formation- poorly consolidated conglomerates, sandstone, and mudstone with fossiliferous claystone, up to 500 m. The thermal water in the Kızıldere field is derived from two major sources: a shallow Pliocene sedimentary (Sazak Formation) reservoir with a temperature of 198
C and a deep Menderes metamorphic reservoir (Ig˘decik formation) with a temperature of 212
C. The Tekkehamam–Pamukkale–Karahayıt geothermal fields (HVK-19, HVK-20) are located in the topographic lows east of Kızıldere in the Tekkehamam area. Several fumerols are found on the mountain slopes of the area and hot springs with temperatures ranging between 30 and 100
C. The hot springs issue at the point where faults cut the valley. These springs deposit travertine and alteration minerals along the fault lines and in the vicinity of the springs. The hot springs of Pamukkale are located at the intersection of the Bu¨yu¨k Menderes and Gediz grabens. In this area the thickness of travertine reaches 85 m. The Pamukkale springs deposit snow-white travertine, whereas the Kızılleg˘en springs deposit red travertine due to high Fe concentrations in the fluid. Pamukkale and Karahayıt are tourist attractions, visited by 1.5 million tourists every year. The Manisa–Urganlı geothermal field (HVK-10) is located in the western part of the Gediz graben, and is characterized by normal fault structures. The stratigraphic sequence of the Urganlı geothermal field are composed of Paleozoic schist and marble that form the basement of the to Menderes Massif. Mesozoic limestone serpentinite and ophiolitic meˆlange overlie the basement units. The sequence continues with Pliocene conglomerate, sandstone, siltstone and limestone. Travertine and alluvium are the youngest sediments in the area. The general fault trends are W–E, NE–SW and also NW–SE. Also, a thrust zone is observed between the Mesozoic ophiolitic meˆlange and limestone in the NW of the area. The potential reservoir rocks are Paleozoic marbles, occasionally schist and Mesozoic limestone cut by fault zones in the region (MTA, 1996). The Manisa Salihli geothermal field (HVK-11, HVK12, HVK-13, and HVK-14) is located along the southern boundary fault of the Gediz graben. Salihli is known for its Hg mineralization of hydrothermal origin. The field is currently under reconsideration as a prospect for epithermal Au–Sb mineralization (Larson and Erler, 1993). The stratigraphic succession in the field includes the Paleozoic metamorphic of the Menderes Massif, Miocene and Pliocene conglomerate, sandstone, siltstone, limestone, clay, tuff and lignite layers, and Quaternary

169

travertine and alluvium unconformably overlay the metamorphic units. The major faults in the field trend dominantly E–W and NW–SW while N–S and NE–SW trending faults also exist on a smaller scale. In the Salihli geothermal field hot springs are concentrated in the Kurs¸ unlu and Sart areas. A total of 6 wells were drilled in the field. The highest temperature (150
C) was measured in the deep drill well, SC-1. The flow rate of this well reaches 2 l/s (Karamanderesi et al., 1995; MTA, 1996). The Alas¸ ehir and Kavaklıdere geothermal field (HVK-15, HVK-16, HVK-17, HVK-18) is located in the Gediz graben. Drilling to a depth of 750 m revealed temperatures of up to 116
C and production of natural gas with 15% CH4 and 85% CO2 and thermal water (Karamanderesi et al. 1998). Alas¸ ehir fish farm is a local shallow well with a depth of 92 m and temperature of 24oC into alluvium deposits. Alas¸ ehir mineral water has a temperature of 31oC (Erentu¨z and Ternek, 1968). The C¸anakkale–Tuzla geothermal field (HVK-21, HVK-22) is located 80 km SW of C¸anakkale, 5 km from the Aegean coast. The Tuzla field is a volcanic area. The stratigraphy of the field is composed of Permian metamorphic basement rocks, granodiorite intrusive rocks, Miocene volcanic rocks, including rhyodacitic, ignimbrite, trachyte and trachyandesite lavas, monzonite, and Quaternary and recent alluvium sediments (Karamanderesi, 1986; S¸ims¸ ek, 1997). Thermal water is derived from a shallow volcanic reservoir at a depth of 330–350 m and a deep granite reservoir at a depth of 1020 m. The thermal water of Tuzla is unique due to the extremely high dissolved salt content, up to 63 g/l. Samples were collected from hot spring and well T-1 at a depth of 814 m (HVK21 and HVK-22, respectively). The Edremit–Gu¨re and Havran geothermal fields (HVK-23, HVK-24) are located at the Edremit bay in the southern part of the Kazda˘g massif. The geological sequence includes the Paleozoic Kazdagˇ formation (composed of gneiss, amphibolite, and marble and crystallized limestone), Triassic conglomerate, arkose, siltstone, Permian and Carboniferous limestone and marble blocks, and Upper Miocene Bayramic¸ formation that consists of conglomerate, sandstone, claystone, shale and marl. Dikili-Bergama Kaynarca geothermal field (HVK-25) . is located in Western Anatolia, 90 km north of Izmir and includes more than 20 hot springs. Compressional fields that formed during late Miocene to early Pliocene control the geological structure. As a result, the area became a site of N–S oriented tensional stress fields. In an area between Dikili and Bergama, there are many hot springs whose distribution is controlled by fracture patterns. The geology of the Dikili–Bergama area comprises various rocks such as sedimentary and metamorphic rocks (Paleozoic to recent), Kozak granodiorite (Eocene to Oligocene), Yuntdagˇ Volcanics (Late Miocene to Pliocene), and Dededag˘ Basalt (Pleistocene). A deep well that was drilled by MTA (K-1) yielded temperatures of up to

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130
C. Since the volcanic activity and tectonic movements in the Dikili–Bergama area are very intense, it is assumed that the heat source of geothermal activity derives from both tectonism and volcanism (MTA and JICA, 1987) . The Izmir–Balc¸ova geothermal field (HVK-26, HVK. 27) is located approximately 10 km SW of .Izmir. The geological section includes Upper Cretaceous Izmir flysch composed of metasandstone, phyllite, limestone, serpentinite and diabase, the Miocene Yeniko¨y formation composed of conglomerates, sandstone and siltstone, and Pliocene Cumaovası volcanics, which includes andesite, agglomerate, tuff, and rhyolites. The geothermal systems are fed by the main NE–SW, NW–SE and E–W trending fault (Yılmazer et al., 1989). The Bodrum. geothermal field (HVK-28) is located south west of Izmir on the Agean coast. Hydrothermal mineral deposits and some mineral water exposures suggest a large geothermal potential of that field (Karamanderesi, 1998). The prospect covers the contact zone of the Menderes Massif and Taurus Belt (Ercan et al., 1984; Robert, 1995; O¨zc¸ic¸ek and O¨zc¸ic¸ek, 1977; Pis¸ kin, 1980).

4. Analytical procedures During the fall of 1995, 26 representative hydrothermal samples were collected from major thermal systems in western Turkey. Analyses of major ions were performed at the Analytical Laboratory of the Hydrological Service in Jerusalem. Lithium concentrations were measured by ICP–MS (Element, Finnigan) at the University of California Santa Cruz. Lithium intensities were normalized to the internal standard of Be. Spike-free samples were scanned before the analyses and no detectable levels of Be were found in the original samples. Bromine was determined by flow injection ion analyzer (QuickChem 8000) at the Hydrological Service laboratory in Jerusalem (Vengosh and Pankratov, 1998). Boron isotopes were measured by a negative thermal ionization mass spectrometry technique (NITIMS; Vengosh et al., 1989, 1994). Samples were analyzed by a direct loading procedure, in which B-free sea water and natural solutions were loaded directly onto Re filaments and measured in a reverse polarity NBS-style 12 solidsource mass spectrometer at the University of California Santa Cruz. A standard deviation of less than 1.5 % was determined by repeat analysis of NIST SRM-951 standard (11B/10B=4.013 0.005). Isotope ratios are reported as  p11B values, where h
i 
1 d11 B ¼ 11 B=10 B sample = 11 B=10 B NBS 951  1000 Strontium was separated by cation-exchange chromatography using standard techniques at the Department of Geology, Hebrew University of Jerusalem.

Isotope ratios were determined using third generation Faraday detectors in static mode on a VG-54WARP mass spectrometer at the University of California Santa Cruz. Zone refined Re filaments were used. All measured 87Sr/86Sr results were corrected to a 87Sr/86Sr ratio of 0.1194 using an exponential correction law. Correction for 87Rb was negligible for all samples. Using this procedure, NBS-987 87Sr/86Sr yielded a ratio of 0.71025 (  0.00001; n=5) during the period in which the unknowns were run.

5. Results and discussion The locations (Fig. 1), geological structure, source, temperature, salinity, water types, and lithology of the investigated thermal systems are presented in Table 1. Chemical and isotopic results are presented in Tables 2 and 3, respectively. The chemical composition (Fig. 2) suggests several water types with different distribution of the major ion composition. The different proportions of Cl, HCO3 and SO4 ions (i.e. their ratios to total dissolved constituents in meq/l, or the Cl–SO4–HCO3 diagram; Giggenbach, 1991) are used to determine 4 basic water types (Table 1; Fig. 2): (1) Na–Cl– (in thermal waters of Cumalı Seferihisar, Bodrum Karaada island and Tuzla–C¸anakkale); (2) Na–HCO3 (Aydın Ilıcabas¸ ı, Salavatlı, Denizli-Kızıldere, Urganlı, Salihli); (3) Na– SO4 (Dikili–Kaynarca–Bergama, Edremit–Gu¨re, Edremit–Havran); and (4) Ca–Mg–HCO3–SO4 (Karahayıt, Pamukkale). Some systems have mixed compositions . like Na–Cl–HCO3 (Germencik O¨merbeyli, Izmir–Balc¸ova). The variation of dissolved ions as normalized to Cl and evaporation-dilution of modern sea water are illustrated in Fig. 3. Most ions show enrichment relative to sea water with similar salinity. The temperature–ion concentration relationships are presented in Fig. 4. The 11 B values and 87Sr/86Sr ratios of the thermal water vary from 2.3 to 18.7% (n=6) and 0.707864 to 0.719479 (n=5), respectively. 5.1. Marine vs. non-marine sources In the following discussion the authors distinguish between soluble ions (e.g. Cl, Br, B) and rock-forming elements (e.g. Na, Ca, HCO3) in order to evaluate the origin of the geothermal water. Fig. 3 shows two distinctive correlation lines between Cl and other ions, particularly for the Cl–Na coordination. It is argued that high salinity, Na–Cl water composition, and the low (Na/Cl < 1) of thermal fluids from Cumalı Seferihisar and Tuzla–C¸anakkale suggest that most dissolved salts, in particular Cl and Na, are derived from a marine origin. On the other hand, all other thermal waters with significantly lower Cl concentrations and typically Na/ Cl>1 are non-marine, and thus most of the dissolved

Table 2 Chemical data of geothermal waters from western Turkey Water type Source

Date

1 2 3 4 5 6 7 8 9 10 11 12 13 16 17 18 19 20 21 22 23 24 25 26 27 28

Well CM-1 Dogˇanbey Kaplıcası hot spring Well OB-9 Well OB-3 Well AY-1 Well AY-2 Well AS-1 Power plant well number 13 Power plant well number 16 Hot spring Sart hot spring Kurs¸ unlu hot spring Kurs¸ unlu mineral water Well Well Alas¸ ehir hot spring Karahayıt hot spring Pamukkale hot spring Tuzla hot spring Tuzla well T-3 Well near Gu¨re Well Havran Kaplıcaları Well–1 Balc¸ova deep well Balc¸ova hot spring Black island hot spring

5/10/95 560 40 6300 1025 13.5 8.7 10930 5/10/95 100 60 1840 166 3180 5/10/95 20 15 1550 160 11.0 8.5 1570 5/10/95 20 1 1420 160 2.0 1470 5/10/95 6 16 1840 180 1.7 240 5/10/95 10 40 1130 160 1.5 4.2 220 5/10/95 6 1 1260 105 1.0 250 5/10/95 26 – 1360 170 0.4 100 5/10/95 1480 190 0.4 4.2 120 6/10/95 15 14 520 50 70 6/10/95 25 17 190 30 40 6/10/95 10 12 350 60 0.6 70 6/10/95 90 50 210 6 20 6/10/95 51 150 380 20 180 6/10/95 26 30 110 10 30 6/10/95 8 150 360 20 0.2 80 6/10/95 143 110 120 30 30 6/10/95 186 80 50 6 4.5 13 7/10/95 2840 70 18700 1970 154 35320 7/10/95 3154 110 20600 2060 166 18.3 39500 7/10/95 25 – 270 7 65 7/10/95 41 – 260 6 60 7/10/95 17 6 450 30 60 7/10/95 25 7 350 30 0.5 160 8/10/95 210 20 125 9/10/95 12600 520 21100

Cumalı Seferihisar Cumalı Seferihisar Germencik O¨merbeyli Germencik O¨merbeyli Aydın Ilıcabas¸ ı Aydın Ilıcabas¸ ı Aydın Salavantlı Denizli–Kızıldere Denizli–Kızıldere Urganli Salihli Salihi Salihli Alas¸ ehir fish farm Alas¸ ehir fish farm Alas¸ ehir hot spring Karahayıt hot spring Parmukkale C¸anakkale–Tuzla C¸anakkale–Tuzla Edremit–Gu¨re Edremit Havran Dikili–Kaynarca–Bergama . I. zmir–Balc¸ova Izmir–Balc¸ova Bodrum Karaada island

Ca

Mg Na

K

Sr

Li

Cl

Br

B

300 640 1600 1600 4700 3010 2900 2510 2730 1440 930 1110 820 990 510 1700 1040 940 70 130 60 30 480 540 480 510

37.4 7.5 6.6 6.3 1.3 1.7 2.2 0.3 0.3 2.5 0.1

16.1 2.7 19200 0.9 9.8 5.0 6000 0.9 54.2 5.6 5000 1.5 50.7 5.7 4720 1.5 49.6 3.9 7040 11.8 43.7 4.4 4610 8.1 51.1 15.5 4590 7.8 20.7 17.9 4200 20.5 25.9 23.1 4570 19.7 9.1 4.5 2120 11.5 16.4 1.0 1250 8.1 33.0 2.2 1650 8.4 1.3 1200 15.5 2.2 0.2 1770 3.3 1.2 0.3 720 6.5 29.4 0.5 2340 7.2 0.9 2.3 1470 6.5 1.1 1.0 1280 5.6 27.6 3.6 59060 0.8 29.0 3.9 65650 0.8 3.0 6.1 430 6.3 1.9 4.7 410 6.2 3.0 4.8 1050 12.0 10.9 6.0 1130 3.3 6.6 3.5 850 2.6 5.1 1.3 34800 0.9

160 200 80 20 70 100 150 630 670 8 30 90 180 640 80 140 830 600 150 190 430 460 530 140 260 2890

51.6 66.3 0.4

75.3

F

TDS

Na/Cl Br/Cl (103) B/Cl

HCO3 SO4

1.5 1.0 1.9 1.9 2.4 3.4 3.9 – 1.3 1.6 – – –

6.5 7.5

1.6

0.0048 0.0101 0.1131 0.1135 0.6777 0.6665 0.6756 0.6656 0.7325 0.4302 1.4763 1.6661 0.1952 0.0411 0.1421 1.2501 0.1054 0.2699 0.0026 0.0024 0.1529 0.0974 0.1691 0.2170 0.1719 0.0008

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ID Name

171

172

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Table 3 Isotopic data of geothermal waters from western Turkey ID

Name

Source

Date

1 3 5 9 20 22

Cumalı–Seferihisar Germencik–O¨merbeyli Aydın–Ilıcabas¸ i Denizli–Kızıldere Pamukkale C¸anakkale–Tuzla

Well CM-1 Well O¨B-9 Well AY-1 Power plant well number 16 Pamukkale hot spring Tuzla well T-3

5/10/95 5/10/95 5/10/95 5/10/95 6/10/95 7/10/95

constituents are derived from water-rock interactions. The Br/Cl ratios of the thermal waters can also be used to distinguish marine from non marine sources, as all of the Cl-enriched marine water has typically Br/Cl ratio4 sea water (Fig. 3). The different potential marine sources are deep circulation of modern seawater, fossil seawater, and dissolution of marine evaporites. Assuming that deep circulation of seawater was the source of the dissolved salts, one would expect to have seawater composition, particularly for conservative elements such as Cl and Br that are less affected by water-rock interactions. Karamanderesi and Helvacı (1994) measured the Br concentrations in different rock types in the Menderes massif and found negligible Br levels ( < 1 ppm). Thus, modern Mediterranean seawater would have a chloride content of <22,000 mg/l (i.e. lower concentrations can be derived from dilution with meteoric water) and a Br/ Cl ratio of 1.5103. The only water sources that have similar chemical characteristics are the saline water from Cumalı Seferihisar (with Cl content of 10,926 mg/l) and Bodrum Karaada island (21,097 mg/l) with marine Na/ Cl and Br/Cl ratios. The other geochemical features (i.e. the B/Cl, Li/Cl, F/Cl, Ca/Cl, Mg/Cl, and SO4/Cl ratios, d11 B value of 2.3%), however, are different from those of seawater and suggest that the original seawater was modified by intensive water-rock interactions. The depletion of Mg and enrichment of Ca, B, Li, and F, as well as the depletion of 11 B are typical of oceanic hydrothermal water (Spivack et al., 1987; You et al., 1994). This conclusion is consistent with the chemical and 18 O data reported by Conrad et al. (1995) who showed that Seferihisar thermal water originated from a mixture of sea water and local ground water. In contrast, the thermal water of the Tuzla system has a Cl concentration of 39,500 mg/l and a Br/Cl ratio of 0.7103, which are higher and lower than those of seawater, respectively. In addition, the Tuzla brines are characterized by a d11 B value of 18.7%, 87Sr/86Sr ratio of 0.709633 (Table 3), and d34S of 12% (Mu¨tzenberg, 1997). Balderer (1997) and Mu¨tzenberg (1997) suggested that the Tuzla brines were derived from lateral migration of fossil Miocene brines that were trapped in the

87

Sr/86Sr

0.710867 0.714490 0.719479 0.707864 0.709633

d11 B 2.31 0.17 2.34 1.79 2.51 18.70

Miocene sediments. The fossil brines could have originated from relics of evaporated sea water trapped in the sediments (e.g. Vengosh and Starinsky, 1993; Vengosh et al., 1994, 1998) or, alternatively, from dissolution of Messinian evaporites. Several lines of evidence suggest that the Tuzla thermal water could not be derived from evaporated sea water. First, relics of evaporated sea water or diagenetically modified sea water (e.g. Dead Sea) would have high d11 B values (d11 B >39%) as demonstrated recently by the composition of pore water from the Mediterranean with d11 B values of up to 66% (Vengosh et al., 2000). In contrast, salts derived from evaporite dissolution would have lower d11 B values (< 39% Vengosh et al., 1992; 1998). In high-temperature environments, however, a large fraction of the dissolved B is also derived from leaching of the rocks. Thus, the original isotopic composition could be modified. This is clearly demonstrated in the case of thermal water from Cumalı Seferihisar where marine Na/Cl and Br/Cl ratios are associated with non-marine low 11 B values (2.3%) and high B/Cl ratios. The d11 B values of the hypersaline Tuzla thermal water is 18.7% which is significantly higher than the values expected for leached B from local igneous rocks (granodiorite, trachyandesite, trachyte, rhyodacite, ignimbrite) with d11 B  0%. The relatively high d11 B can be interpreted as a reflection of modified high d11 B evaporated sea water that was modified towards lower d11B values due to water–rock interaction. Alternatively, the relative lower d11 B value may indicate dissolution of late-stage evaporites with d11 B< 39%. Second, the Na/Cl and Br/Cl ratios of the Tuzla water are not consistent with the ratios expected for evaporation of sea water. During >10-fold evaporation beyond the halite saturation stage, the residual evaporated sea water has Na/Cl <0.86 and Br/Cl>1.5103 (McCaffrey et al., 1987). Fig. 5 illustrates the evolution of evaporated seawater compared to the composition of thermal water from the Tuzla and Seferihisar thermal waters. The data points are not consistent with the evaporation line (i.e. low Br/Cl ratios below the seawater ratio) and thus rule out the relic sea water model.

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Fig. 2. Pie diagrams of the chemical composition (in meq l1) of selected geothermal fields from western Turkey.

173

174

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Fig. 3. Log chloride vs. log dissolved salts concentrations in geothermal waters from western Turkey.

Third, the 87Sr/86Sr ratio of 0.709633 and d34 S value of 12% are respectively higher and lower relative to the expected Miocene fossil evaporated sea water (87Sr/86Sr 0.7081; d34 S  20%). Consequently, it is suggested that the hypersaline thermal water of Tuzla is derived from dissolution of salt deposits. The high Na (20,000 mg/l), Ca (3000 mg/ l), K (2000 mg/l), and B (29 mg/l) concentrations reflect the mineralogical composition of these deposits with a possible mineral assemblage of gypsum and Ca- and Na-borates. This mineral composition is typical for many Neogene salt-deposits in western Turkey (Helvacı, 1994, 1995; Palmer and Helvacı, 1977).

5.2. The impact of water–rock interactions and origin of boron Following the Ellis and Mahon (1977) classification, HCO 3 waters are considered to occur in volcanic geothermal areas where steam containing CO2 condenses into the liquid phase. Bicarbonate water can also reflect interaction of CO2 charged fluids at lower temperatures and migration path as well as mixing with local ground water (Giggenbach, 1991). Sodium–HCO3 waters are common in geothermal systems associated with metamorphic rocks which is consistent with the general lithology of the Menderes Massif (Table 1) and the high

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175

Fig. 4. Source temperatures (as measured in the investigated thermal systems) vs. different dissolved ions (mg/l) in geothermal waters from western Turkey.

CO2 content that is typical of the thermal water of western Turkey (Filiz, 1984; Ercan et al., 1994). The origin of the high dissolved CO2 according to d13 C and He isotopic data is magmatic (Filiz, 1984; Ercan et al.,

1994). The Na–HCO3 chemical composition is therefore a combination of high CO2 flux and extensive waterrock interactions with metamorphic rocks. Similarly, the Na–SO4 water type can be derived from H2S condensing

176

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Fig. 5. Na/Cl vs. Br/Cl ratios of evaporated sea water as compared to those from thermal water from Tuzla and Seferihisar.

into the liquid phase as well as from interaction with sulfate minerals (e.g. Ellis and Mahon, 1977). The dependence of the ion composition on temperature is demonstrated by the correlations between different ion concentrations and temperatures measured in the thermal sources (Fig. 4), particularly for Na, K, Mg (reverse correlation), B and F. These positive correlations clearly indicate that much of the dissolved salts of the non-marine thermal waters are derived from water– rock interactions. The chloride–temperature relationship may also reflect an absorption of HCl gas into the liquid phase as well as extraction of Cl from the rocks. It seems that only in the Germencik O¨merbeyli system is Cl derived from such sources with significantly high Cl/ TDI ratio (0.3). It should be emphasized that mixing with local ground water also controls the chemical composition of the hydrothermal water. Thus, the linear correlations of most dissolved ions with chloride (Fig. 3) reflect both the original source of the thermal systems and mixing (i.e. dilution) with local cold ground water. Go¨kgo¨z (1998) showed that geothermometry temperatures calculated by applications of Na–K and Na–K–Ca geothermometers of geothermal water from Kızıldere area vary between 188 and 245
C which is consistent with actual temperatures measured at the bottom of research wells in that area. While the HCO3 ion can be derived also from mixing with cold shallow groundwater, it seems that the HCO3/ TDI ratio, which would be less affected by dilution, can be a useful tracer for delineating the sources of the salts. Positive correlation between the HCO3/TDI ratio and Na/Cl, K/Cl, and B/Cl ratios (Fig. 6) probably reflects the role of CO2 in water–rock interactions. Similarly, the correlation between Cl and other dissolved salts (Fig. 3) may also derive from the influence of HCl gas.

The CO2 and HCl gases can thus be considered as the triggers for the intensified water–rock interactions and enhance leaching of dissolved ions in the thermal water. The Br/Cl ratio of most of the non-marine thermal water is higher than that of sea water (Fig. 3). The relative enrichment of Br can be explained by extraction of Br from organic matter in the Tertiary sediments, or, from preferential degassing of Br gases from deep sources. The high linear correlation between Cl and Br that characterizes the thermal water favors the second possibility. Thermal waters from western Turkey have typically high B content, which also causes environmental and operational problems. The association of high B and high CO2 levels led Tarcan (1995) to suggest that B is also derived from a deep mantle source. Demirel and S¸entu¨rk (1996) also suggested that high B, NH4, and CO2 concentrations in thermal water from the Kızıldere geothermal field reflect ascent of magmatic emanations from depth although there is no evidence of recent volcanic activity in the area. Based on 3He/4He ratios, Gu¨lec¸ (1988) argued that the involvement of mantlederived He, in the Kızıldere geothermal field does not exceed 30%. Two models should therefore be considered for the origin of B in the thermal water: (1) dissolved Cl, HCO3, and B are derived from deep mantle flux of HCl, CO2 and B(OH)3 gasses; or (2), water-rock interactions leach B to the liquid phase. Next, these two conflicting models will be evaluated. Karamanderesi and Helvacı (1994) and Karamanderesi (1997) measured REE and other elements extracted from well cuttings and core rock samples from different geothermal fields (Fig. 7) and surface rock samples in the Menderes Massif. Their data showed that: 1. Different rock units from the Salavatlı geothermal field have high concentrations of B (range of 800 to 1600 ppm) relative to those (independent of lithology) of the O¨merbeyli field (O¨B-7, a range of 50–230 ppm). The difference in the B content of the rocks is also reflected in relatively higher B/Cl ratio in the associated thermal waters from these two systems (0.7 relative to 0.1), whereas the absolute B concentrations are similar. 2. Boron is unevenly distributed among different rock types. Boron is particularly enriched in (decreasing order) quartz vein, tourmaline gneiss, illite–chlorite–feldspar zone, and quartz–chlorite schist zone. Boron is relatively depleted in marble and gabbro. 3. The vertical distribution of B (and Li) with depth is not uniform and is heavily dependent on the lithology. Boron is depleted in the marble zone in the O¨merbeyli field (  50 ppm B at depth of 1400 m) relative to the albite-amphibolite schist zone (  200 ppm,  1400 m).

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177

Fig. 6. HCO3/TDI ratio vs. Na/Cl, K/Cl, and B/Cl ratios of non-marine water from western Turkey.

If indeed all of the B was derived from deep mantle flux as argued by Tarcan (1995) one would expect to have uniform B composition with similar B/Cl ratios in all of the thermal systems (i.e. the B/Cl ratio is used to eliminate the dilution factor). Moreover, one would not expect to have any relationships between B contents in local rocks and thermal waters, and yet the Salavatlı geothermal field is significantly enriched in B relatively to the O¨merbeyli field. Boron is easily leached from rocks and remains in its volatile form even at lower temperatures relative to the HCl that is converted to less volatile NaCl. The B/Cl ratio can thus be used to assess the maturity of the thermal system. Fluids from older systems are expected to be depleted in B relative to young systems (Go¨kgo¨z, 1998). The large difference between the Salavatlı and O¨merbeyli fields may be related to this factor. Consequently, it seems that B is mainly derived from local water-rock interactions and the source rocks strongly control the B, concentration in the water (e.g. quartz

vein or tourmaline gneiss versus marble). Nevertheless, the overall B budget of a geothermal system can also be controlled by the original B concentrations in the rock or original parent magma fluids, as well as the degree of maturation in which water-rock interactions can contribute B to the thermal system. Since the lower mantle reservoir is enriched in primordial 3He with respect to shallow MORB and radiogenic 4He is generated by the decay of unstable isotopes of U and Th and radiogenic 3H in the crust, the 3He/4He ratio can be a sensitive tracer to detect the presence of mantle helium in thermal water (Gu¨lec¸, 1988; Hoke et al., 2000). The 3He/4He ratio is normalized to atmospheric He (R/Ra=1) and consequently deep manlederived He would have high R/Ra values (>30) whereas crustal He production has a low ratio (R/Ra values of a typical continental crust are 0.005 to 0.02). While springs from the vicinity of Germencik–O¨merbeyli yielded R/Ra values of 0.2 to 0.8 (n=3), which is typical of a crustal source, a spring from the Denizli

178

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Fig. 7. Variations of B concentrations in rocks with depths in OB-7 and AS-1 drill holes (data from Karamanderesi and Helvacı, 1994 and Karamanderesi, 1997).

area had a significantly higher value of 2.5 (Gu¨lec¸, 1988). Consequently, the available 3He/4He data in springs from western Turkey does not exlusively indicate on the origin of He, which in turn cannot support any evidence for the origin of B. Moreover, in the Germencik area thermal water has a low 3He/4He ratio that indicates a crustal source, while the B content is the highest among the investigated thermal waters. Previously, special attention has been given to the B isotope composition of hydrothermal fluids from a marine setting. The B isotope composition of hydrothermal fluids such as those venting from mid-ocean ridge crests (d11 B ¼26.7 to 36.8%) suggests a mixture between seawater B and MORB-derived B leached from the basalt without resolvable isotopic fractionation (Spivack and Edmond, 1987). Hydrothermal fluids from a sediment-starved back-arc spreading center (Mariana Trough; d11 B ¼22.5 to 29.8%; Palmer 1991) and from a classic sediment-hosted basin (Guaymas Basin and Escanaba Trough; d11 B=10.1 to 23.2%; Spivack et al., 1987; Palmer, 1991) are characterized by lower d11 B

values and higher B concentrations, reflecting interactions with the hosted rocks. Thermal fluids from continental geothermal fields are characterized by even lower d11 B values (Salton Sea, California, d11 B ¼ 2.6 to 1.1%; Yellowstone National Park, d11 B ¼ 9.3 to 4.4%; Palmer and Sturchio, 1990), reflecting the isotopic compositions of the source rocks. The influence of seawater B in geothermal systems has been traced in central Japan (d11 B ¼ 5.8 to 27.1%; Musashi et al., 1988) and Iceland (d11 B ¼ 6.7 to 30.7%; Aggarwal et al.,1992). The B isotopic composition cannot be used to distinguish between mantle flux and rock leaching processes due to the overlap in the isotopic composition of these two sources. The d11 B range of the Na-HCO3 waters is 2.3 to 1.8% (Table 3; n=3) and can thus reflect both leaching of igneous rocks and flux of mantle B (e.g. Spivack and Edmond, 1987). The B-isotope fractionation is controlled by the B species as B with tetrahedral coordination is isotopically depleted (low d11 B) relative to B with trigonal coordination. Selective formation and

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179

Fig. 8. Schematic illustration of different geothermal water types from western Turkey.

removal of B(OH)3 species may cause a relative depletion of d11 B in the residual fluid. Nevertheless, it seems that the magnitude and thus the effect of this isotopic fractionation is negligible in high-temperature environments. 5.3. Distribution of the water types The thermal systems of western Turkey exhibit a wide range of chemical composition that reflect the complex nature and different sources of thermal waters (Table 1).

As shown above, the authors distinguish between 4 major groups that reflect different origin and mechanism of water-rock interactions. The Na–Cl type originated from deep circulation and water–rock interactions of modern sea water in the case of Seferihisar and Bodrum systems and from deep fossil brines originated from dissolution of Miocene evaporites in the case of Tuzla geothermal waters. The Na–HCO3 type characterized thermal waters from the systems of Aydın Ilıcabas¸ ı, Salavatlı, Urganlı, Alasehir, Denizli–Kızıldere and Salihli. Thermal waters

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. from Germencik O¨merbeyli and Izmir–Balc¸ova have a mixed Na–Cl–HCO3 water composition. d18 O–dD data (Filiz, 1984; Ercan et al., 1994) suggest that the origin of these waters is meteoric whereas the temperature–ion concentrations relationships suggest that most of the dissolved constituents (Fig. 4) were derived from extensive water–rock interactions. As shown above, the high CO2 content that characterizes these waters, presumably derived from a mantle source (Filiz, 1984; Ercan et al., 1994), enhances water–rock interaction. In most cases the local bedrocks of the geothermal systems are the metamorphic of the Menderes Massif (Germencik O¨merbeyli, Aydın Ilıcabas¸ ı, Salavatlı, Urganlı and Salihli). Yet in other systems, where the local rocks are composed of other rock units (e.g. Manisa–Urganlı– serpantinite and limestone) the associated thermal waters also have a Na–HCO3 composition. The 87 Sr/86Sr ratio of Na–HCO3 type thermal water (0.710867 in Germencik O¨merbeyli and 0.71449 in Aydın Ilıcabası) reflect leaching of Sr from a highradiogenic source, which suggests that the source rock has a high Rb/Sr ratio). The system of Denizli-Kızıldere is one of the highest enthalpy geothermal field and most producing field in Turkey. The d18 OdD data indicates that the origin of the water is meteoric, modified by intensive water-rock interactions. In addition, Go¨kgo¨z (1998) showed that calculated temperatures based on chemical geothermometers are similar to measured temperatures of up to 245
C. The high 87Sr/86Sr ratio (0.719479) of the geothermal water from Kızıldere system suggests that the deep aquifer units (schist and quartzite) are the predominant rock sources of Sr while the shallow limestone unit has negligible effects on the dissolved Sr budget in the thermal waters. The Na–SO4 type characterizes thermal waters from Edremit–Gu¨re and Havran and Dikili–Kaynarca geothermal fields, which are located at the Edremit bay in the southern part of the Kazdag˘ massif. Sulfate can be derived from hydrogen sulfide condensing into the liquid phase as well as dissolution of sulfate minerals (e.g. Ellis and Mahon, 1977). Since the local geology (see above) is not different from other thermal systems in the Menderes Massif with a Na–HCO3 composition, it seems that the second explanation can be ruled out. The Ca–Mg–SO4–HCO3 type characterizes geothermal systems from Karahayıt and Pamukkale. It seems that this composition reflect shallow sources and interaction with shallow carbonate rocks. The Pamukkale hot spring has a 87Sr/86Sr ratio of 0.707864 that is distinctively low relative to the other non-marine thermal systems. This low 87Sr/86Sr signature reflects interaction with carbonate rocks of the Pliocene Sazak formation that consists of intercalated limestone, marls and siltstone, or the Pliocene Kolonkaya formation composed of alternating layers of sandstone, claystone and clayey limestone. The low 87Sr/86Sr ratio rules out interaction

with the underlying Paleozoic Menderes metamorphic, which is consistent with the chemical composition of this water type.

6. Conclusions The chemical data, combined with isotopic data for B and Sr of thermal waters from western Turkey reveal 4 types of water, which originate from marine and nonmarine sources. The marine source has a Na–Cl composition and Na/Cl ratio <1 whereas the non-marine waters typically have Na/Cl>1 (Fig. 8). The Br/Cl ratio is used to distinguish between direct penetration of sea water or recycled marine salts in the form of evaporite dissolution. The non-marine water shows 3 types of chemical compositions, reflecting different source rocks and depth of circulation. Na–HCO3 and Na–SO4 compositions reflect deep circulation and interactions with metamorphic rocks and gneiss while Ca–Mg–SO4– HCO3 composition is associated with shallow circulation in carbonate rocks and mixing with cold ground water. The 87Sr/86Sr ratio further constrains the nature of the source rocks (i.e. igneous and metamorphic versus carbonate rocks). Systematic changes in Na, K, Ca, and Mg with temperature (Fig. 4) show that concentrations of these dissolved constituents are largely dependent on the temperature and depth of circulation. Water–rock interaction results in high concentrations of dissolved constituents such as Na, K, and B. The data suggest that B is derived from water–rock interaction rather then deep mantle flux of B(OH)3 gas. The high B concentration in the thermal water is typical of many non-marine geothermal fields, worldwide, and thus can be used as a sensitive tracer to monitor advection and mixing of underlying geothermal fluids with shallow groundwater.

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