awalehetal2015-jge obock finalversion

8/20/2019 AwalehEtAl2015-JGE Obock FinalVersion

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The geothermal resources of the Republic of Djibouti — I:Hydrogeochemistry of the Obock coastal hot springs

Mohamed Osman Awaleh a,⁎, Farhan Bouraleh Hoch a, Ibrahim Houssein Kadieh b, Youssouf Djbril Soubaneh c,Nima Moussa Egueh a, Mohamed Jalludin a, Tiziano Boschetti d

a Centre d'Etudes et de Recherches de Djibouti (CERD), Route de l'aéroport, B.P. 486, Djibouti — ville, Djiboutib Laboratoire Régional, Newalta Châteauguay, 125 Rue Bélanger, Châteauguay, J6J 4Z2 Québec, Canadac Département de biologie, chimie et géographie, Université du Québec à Rimouski, 300, Allée des Ursulines, Rimouski, QC G5L 3A1, Canadad Department of Physics and Earth Sciences “ Macedonio Melloni” , University of Parma, Parco Area delle Scienze 157/a, 43124 Parma, Italy

a b s t r a c ta r t i c l e i n f o

Article history:

Received 8 May 2014

Accepted 1 February 2015

Available online 7 February 2015

Keywords:

Hot springs

Geothermometers

Chemical composition

Isotopic composition

Obock

Djibouti

This paper examines the hydrochemical features of the waters from the Obock coastal geothermal eld. As the

submarine waters at the ridge, their chemical and isotopecomposition shows af nities with sea water–basalt in-teractions at hydrothermal temperature. Moreover, good linear correlations were obtained between hot springsand of sea water sample points when plottingnormalized concentrations of elements to chloride (Mg/Cl, SO4/Cl,

K/Cl, Ca/Cl, SiO2/Cl)versus concentrationof Li/Cl. Thiswould indicate thattwo end members exist (seawater andreservoir uid). After extrapolation of the Mg and SO4 concentrations to zero, the obtained value of SiO2 corre-

sponding to the reservoir uid end member was used to estimate the reservoir temperature by quartzgeothermometers. The obtained temperature of 187 °C is in good agreement with that obtained from multiplemineral equilibrium approach (180–200 °C), cationic geothermometers (172–191 °C) and by the evaluation of

isotopic equilibrium between water and sulfate molecule (207 °C). Summarizing all the employed approaches,

a mean temperature of 197 ± 10 °C has been estimated.The isotopic δ34S(SO4) signature of the dissolved sulfates in Obock thermal waters conrms that these watersresult from the mixture of a hot seawater-derived uid (absence of sulfates) with cold seawater. However,

water isotope data did not exclude the presence of a small contribution from fresh groundwater.© 2015 Elsevier B.V. All rights reserved.

1. Introduction

The Republic of Djibouti is one of several African countries locatedon the East African Rift System where geology is also resulting fromtwo other ridges of Red Sea and Gulf of Aden. As in other rifting zones,

the activity of the East African Rift System corresponds to large seismic,tectonic and volcanic activities (Barberi et al., 1975; Mlynarski andZlotnicki, 2001).

In Djibouti, most of the widespread geothermal activity, manifested

in the form of numerous hot springs, fumaroles and hydrothermal alter-ation, is located mainly in thewestern part of thecountry and along theGulf of Tadjourah ridge (Fig. 1A).

The most geologically active area in Djibouti is the Lake Asal area,

and the Asal rift is one of two emergent oceanic ridges in the world,the other being Iceland (Mlynarski and Zlotnicki, 2001). Accordingly,numerous geological and geophysical studies were completed in Lake

Asal in order to understand the phenomena related to sea oor

spreading (Mlynarski and Zlotnicki, 2001; Pinzuti et al., 2010). More-

over, geothermal studies undertaken in the late 1960s and early 1970sallowed selecting theAsal prospect as the most favorable area forcarry-ing out deep drilling exploration (Demange et al., 1971; Lavigne andLopoukine, 1970). Therefore, six geothermal wells (Asal 1 to 6) with

various depths, between 1137 m and 2105 m, were drilled in theAsal prospect during the last decades (Aquater, 1989; BRGM, 1975).However, the high enthalpy Asal geothermal uids (about 350 °C)have had high salinity (116 g/kg) (D'Amore et al., 1998).

On the other hand, since the main national electricity productionsources depend on petroleum products, Djibouti is relativelydependenton diesel fuel and fuel oil imports to meet its energy needs. As a conse-quence, the electricity cost is exorbitant in the Republic of Djibouti,

where in average 1 kWh costs about 0.23 U.S. $ (GBAD, 2011). To miti-gate this energy burden, which put a brake on the rate of economicgrowth in this country, a national program for development of geother-

mal resources (NPDGR) was implemented in 2010. This program aimedto conduct multidisciplinary studies (geochemical, hydrogeological,geophysical, geological andreservoirengineering) on allareas withgeo-thermalactivities in the Republic of Djibouti (CERD, 2011, 2012; Awaleh

et al., submitted for publication).

Journal of Geochemical Exploration 152 (2015) 54–66

⁎ Corresponding author. Tel.: +39 253 77 84 68 55; fax: +39 253 21 35 45 68.

E-mail address: [email protected] (M.O. Awaleh).

http://dx.doi.org/10.1016/j.gexplo.2015.02.001

0375-6742/© 2015 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Journal of Geochemical Exploration

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On the basis of new pre-feasibility studies related to the NPDGR, theWorld Bankgranted, in2012,a loanabout31 M U.S. $ to the Republicof Djibouti to perform four new geothermal wells in the area of Lake Asal(World Bank, 2013). As part of this NPDGR, the hot springs of the

Obock beach were also studied in 2013 (CERD, 2013). Previously, 3samples of Obock hot springs in the intertidal zone were investigatedto develop a spa center (Aquater, 1982). Nowadays, the thermal watersof Obock beach are used mainly for hydrotherapy. In the early 1990s,

Houssein et al. (1993) studied the geochemistry of four Obock beach

thermal springs.

The main purpose of the present study is to characterize the hydro-thermal activity from known and newly investigated hydrothermalsprings and thus to provide a framework for future studies of theObock geothermal system, one of the northern active provinces of the

Republic of Djibouti. Towards this aim, detailed geochemical investiga-tions have been carried out on the cold groundwaters (well waters andboreholes) and the most representative thermal waters (7 hot springs)from Obock area in order to understand their geochemical evolution

and also estimate the reservoir temperature through chemical and

isotopic geothermometry as well as mineral equilibrium approach.

Fig. 1. A: Simplied geological map and hydrothermal activity in the Republic of Djibouti. B: Digital elevation model (DEM) of the Afar Rift system with the location of Obock ( Doubre,2004) (vertical color scale = altitude in meter; the black area = Obock region). C: Tectonic map of the Gulf of Tadjourah. It is noteworthy the extension of the fault lines, from the

coast to open ocean (after Manighetti, 1993). D: Magnetic anomaly map of the Gulf of Tadjourah (after Courtillot et al., 1980). In the key, numbers are milliTeslas (mT).

55M.O. Awaleh et al. / Journal of Geochemical Exploration 152 (2015) 54–66


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Furthermore, the geochemical and isotopic study of the thermal watersof Obock beach, combined with geological and tectonic information aswell as regional hydrogeology, have been used to propose a conceptualmodel for the Obock beach geothermal system.

2. Geological and hydrogeological settings

2.1. Geology of the area

The Republic of Djibouti is located in the Southeastern part of

the Afar Rift which is at the triple junction between two nascentmid-oceanic ridges (Red Sea and Gulf of Aden) and a continental rift(East African Rift) (Fig. 1B, Doubre, 2004). The main expression of crustal rifting process in Afar is, in addition to extensional faulting, a

bimodal magmatism which began as early as 30 Ma by the emplacementof trap volcanism in Yemen and Ethiopia (Barberi and Varet, 1977).

The Obock area located in the northern part of the country consistsof a Quaternary formation of coralline limestone (Faure et al., 1980;

Gasse et al., 1983; Manighetti, 1993).The Debergade paleo-valley is characterized by conglomerates with

large block which have a thickness up to 10 m probably coming fromthe neighboring reliefs (Mablas rhyolites andDahla basalts). The Mablas

rhyolitics correspond to a wide range of domes and lava ows, eruptedbetween 18and 11m.y (Manighetti, 1993) and observed in two sectorson both sides of Tadjourah Gulf (Manighetti et al., 2004). This series un-conformably overlies the Dahla basaltic series (8.6–3.8 m.y) whichform

the (i) outer rim of the Ali Sabieh antiform and (ii) the western ank of the Danakil depression (Barberi et al., 1975). The Dahla basaltic seriesform the outer belt of the Danakil depression, where their thicknessreaches 1000 m in the Randa area (south of the Danakil depression).

Along the coast, alluvial deposits are associated to coral reef materialsand shape small parallel hill which are reminiscent an ancient littoral(Gasse et al., 1983). The North West of the study area is characterizedby red silt plated to coral reef where the clay fraction mainly consists of

illite, smectite and traces of kaolinite and quartz, feldspar, goethite andchlorite minerals (Gasse et al., 1983). Rhyolite pebbles from the Mablasseries form a reg on the surface of the coral reef, in response to climaticchange. The Gulf basalts constitute the basis of these formations (Faure

et al., 1980).From a tectonic point of view, the coral reefs are raised by vertical

movements associated with the opening of Tadjourah Gulf (Faure,

1976). The Gulf basalts, detrital materials, and coral reefs are affected by

normal faults trending N 20 to N 50° (Faure et al., 1980; Gasse et al.,1983; Richard, 1979). These normal faults take gradually the E–W direc-tion of the Tadjourah ridge axis (Gasse et al., 1983). Two major tectonics

structures with N 110 orientation connecting the two bank of theDebergade paleo-valley with the Obock pit (1600 m of depth) character-ize the Obock area (Fig. 1C). These structures have also been observed inthe magnetic map produced by Courtillot et al. (1980) (Fig. 1D).

The Obock area is known to be the onshore prolongation of theTadjourah spreading ridge (Manighetti, 1993). These prolongationsallow explanation of the large elevation of the coral reefs (Gasse et al.,1983; Manighetti, 1993). The Gulf basalts have been dened as the

chronological markers of the dislocation of Tadjourah ridge (Faure,1976; Gasse et al., 1983; Manighetti, 1993). Geophysical data gatheredduring the IV Orgon cruise (1981) on the axial valley of the Tadjourahridge reveals (i) the presence of a huge volcano and (ii) two types of sediment clearly differentiated by their reector. The Obock pit was

drilled at 1070 m depth and the petrography based on cores consistsof clay, carbonate enriched on pelagic organisms, dated from LatePleistocene to Holocene (Moyes et al., 1981).

2.2. Hydrogeology of the area

The piezometric map of Obock region is based upon few points of

measurements (Fig. 2), it therefore remains indicative and the valuesof groundwater hydraulic heads and hydraulic gradients could bemodied according to new measurements. About ten points irregularly

Fig. 2. Piezometric map of the Obock region.

56 M.O. Awaleh et al. / Journal of Geochemical Exploration 152 (2015) 54–66


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spread over an area of 1000 km2 allows drawing up the general featureof the underground ows. The groundwater ow, oriented from theWest to East and the South-East across the Obock plain, seems to bemainly controlled by the mountains of Mabla rhyolites in the West

and the mountains in the North where the main recharge occurs fromthe wadi Sadai and its tributaries.

The piezometric levels evolve rapidly from Obock coastal zone to the

upstream area in the mountains. Close to the Obock town large diame-

ter wells taping infero

ux underground

ow indicates a piezometriclevel of 1 m above sea level. Within the wide sedimentary plain thewells of Oulma and Soublali give respectively 5 m and 20 m. In the

mountainous zone, the piezometric levels reach 142 and 198 m on thewells Assassan and Illisola respectively. More Westward the site of Waddi shows a piezometric level of 324 m. Consequently, the hydraulicgradients are generally high varying from 4.3‰ to 13‰.

3. Material and methods

A totalof seven geothermal springs, the seawater and nine wells andboreholes cold waters in the Obockarea were sampled in January 2013.Coastal thermal waters were collected at very low tide, in which periodthese waters upraised without any mixing with surface seawater.

Unstable hydrochemical parameters, including temperature (±0.1 °C),pH (±0.01 unit) and electrical conductivity (±1 μ S/cm), were measuredon site with, respectively, hand-held meters Hanna CheckTemp, EutechInstruments pH 610, and Eutech Instruments COND 610 that were

calibrated in the eld prior to sampling.Water samples were collected in polyethylene containers after

ltration through 0.45 μ m membrane lters. All samples used fordeter-mination of cations were acidied after collection through addition of

Suprapure® HNO3 (Merck) to bring the pH below 2.Analyses of anions and majorcationswere carried out by ionic chro-

matography with a Dionex ICS 3000 Ion Chromatograph using analyti-cal procedures and quality assurance for geothermal water chemistryreported by Pang and Armannsson (2006). AS4A SC-4 mm analytical

column (250 mm × 4 mm ID) coupled with AG4A SC 4 mm guard andCS12A analytical column (250 mm × 4 mm ID) coupled with CG12A

guard were used respectively for anions and cations analyses. The ionchromatograph was calibrated through repeated analysis of ve work-

ing anion and cation standards (with concentrations within the rangeof analyses). Peaks were identied using Chromeleon software (Dionex,Sunnyvale, California). The analytical precision was estimated at ±5%.

For the analysis of aqueous SiO2, the water samples were diluted

tenfold using deionized water to prevent the SiO2 precipitation. SiO2contents were determined by colorimetry and analyzed using a Jenway6300 spectrophotometer, while HCO3 was analyzed by titration with0.1 M HCl. The ionic balance was below 3%.

Additional samples of untreated waters were collected in 50 mL glass bottles (Quorpak) for stable isotopes analyses of the watermolecule, δ2H(H2O) and δ

18O(H2O), and 1000 mL plastic bottles fortritium (3H) analysis, all analyzed at the Bureau de Recherche

Géologique et Minéralogique (BRGM) in Orleans, France. The deuteriumand oxygen isotopic ratios were analyzed using a Finnigan MAT 252mass spectrometer and reported in per mil notation (‰) versus theVienna Standard Mean Ocean Water (V-SMOW) standard following

δ = [(Rsample / Rstandard) − 1] × 1000, where R is the 2H/1H and18O/16O absolute isotope amount ratios. The average precision, basedon multiple analyses of various samples and laboratory standards, was±0.1‰ for δ18O(H2O) and ±0.8‰ for δ

2H(H2O). Tritium activity was

measured by direct liquid scintillation counting. The detection limitwas 0.6 TU (Tritium Unit with 1 TU equal to 1 tritium atom in 10 18

hydrogen atoms).The samples for sulfur and oxygen isotope analysis of dissolved

sulfate were collected at the outlet of columns using 250 mL pre-acidwashed plastic perplexbottles. Cd-acetate was already added in the bot-

tles (5% v/v) prior to sample collection, tox sulfuras CdS, and then the T a b l e

1

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I B ( % )

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( m g

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T h e r m a l w a t e r s

O b o c k

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4 3 ° 1 7 ′ 2 1

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6 . 8

1

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1

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6

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2

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4

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M g

2 0 6

1 5 5

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2 7

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9

1 6 0 7

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1 8

2

0 . 0

8 7

0 . 7

7

4 . 7

4 2

. 1 4

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2

4 3 ° 1 7 ′ 2 1

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1 1 ° 5 8 ′ 2 4

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7 . 4

9

2 8 5 1

1 9 3 9

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C l – C a –

M g

1 4 0

8 1

. 8

3 2 0

1 6

8 1 . 9

3

8 5 5

8 7

. 7

1 7

1

0 . 0

0 0 5

0 . 3

6

2 . 7

2 4

. 2 4

K h o m a

A d o u

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1 1 ° 5 6 ′ 5 0

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2 7 . 8

7 . 7

2

1 0

, 5 3 0

7 4 0 7

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C l – M g

4 0 5

3 1 0

1 5 1 7

7 3

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8 2 . 0

5

3 7 4 0

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1

0 . 0

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5

1 2

1 5

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A l l a

- e l l a

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7 . 9

4

1 3 2 2

9 2 2

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C l – H C O 3

3 2

. 1

9 . 1

7

2 7 9

6 . 7

3 5 9

. 5

2 0 9

7 3

. 8 2

4 2

2

0 . 0

0 0 2

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6

0 . 8

4

6 1

. 3 9

T e r o

- e l a

4 3 ° 0 8 ′ 3 3

. 7 ″

1 2 ° 0 3 ′ 1 7

. 5 ″

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7 . 0

1

2 5 9 0

1 9 3 3

N a –

C l – H C O 3

1 1 4

7 3

. 3 7

3 7 8

8 . 8

4 0 6

. 8

6 6 5

1 5 7

4 1

- 2

0 . 2

1

0 . 7

2 . 6

n . a .

S e a w a t e r

4 2 ° 3 1 ′ 2 6

. 0 ″

1 1 ° 3 1 ′ 3 6

. 5 ″

2 7 . 6

8 . 0

6

5 2

, 1 9 0

4 1

, 1 0 0

N a –

C l

4 0 1

1 3 8 9

1 2

, 2 0 0

3 5 0

1 4 8

. 1

2 1

, 4 0 0

3 0 3 1

8 . 2

0

0 . 2

1 5

1 . 7

0

6 7

3 . 6

3

n

. a .

=

n o t a n a

l y z e

d ;

I B :

i o n

i c b a

l a n c e

.



5/13

aliquot was ltered through a 0.2 μ m nitrocellulose lter before chemi-cal determination of residual sulfate. Dissolved sulfate was precipitatedas BaSO4 at pH b 4 (in order to remove HCO3

− and CO32− species) by

adding a BaCl2 solution. The isotopic analyses on BaSO4 were carried

out using a Delta + XP mass spectrometer coupled in continuous-owmode to a Thermo Elemental Analyzer in BRGM laboratories. Theδ34S(SO4) and δ

18O(SO4) isotope compositions were reported in the

usual δ-scale in ‰ with reference to V-CDT (Canyon Diablo Troilite)

and V-SMOW (Vienna Standard Mean Ocean Water), respectively. Theanalytical precision was of ±0.3‰ versus V-CDT for δ34S(SO4) and±0.5‰ versus V-SMOW for δ18O(SO4).

4. Results

4.1. Hydrochemistry

Hydrothermal activity in the Obock beach is characterized by low-

owhot springs (1.8L s−1) (Aquater, 1982). The temperature, pH, elec-trical conductivity (EC), total dissolved solids (TDS), sampling locations,

hydrochemical types and major and minor elements of the sampledthermal and cold waters are listed in Table 1. Thermal waters fromObock beach are moderately acid to neutral (pH = 6.36–7.01) with atemperature range of 58.6 to 71.4 °C (Table 1). The chemical composi-

tions of thermal waters are similar to that of seawater, with EC valuesin the range of 41 to 47 mS/cm, while in the cold well and boreholewaters from Obock area ranged from 1.3 to 4.2 mS/cm (Table 1). The

relatively high EC (10 mS/cm) of Khoma Adou well, which is locatedinland 200 m from shoreline, likely indicates seawater intrusion.

Chemical compositions of the waters are plotted in the diagram of Piper (1944), as shown in Fig. 3. Classication of the water samples in

Table 1 was made according to principles of IAH (1979). Total equiva-lents of cations and anions separately were accepted as 100% and ionswith more than 20% (meq/L) were taken into consideration in thisclassication.

The chemical composition of the waters studied described interms of relative concentrations of main anion and cation allows us todistinguish the following two groups of waters (Table 1, Fig. 3):

(1) Thermal waters from the Obock coastal hot springs are of theNa–Cl type;

(2) Cold well and borehole waters from Obock area are mostly of the

Cl–HCO3–Na–Ca–Mg type.

According to Giggenbach (1991), the chemical composition of the

thermal water samples from Obock beach can be classied aschloride-rich mature waters (Fig. 4A).

The cold well and borehole waters also show dominant chloride,except the Alla Ella well water that plots at the chloride–bicarbonate

border (Fig. 4A). When comparing results of this study with previousdata (Aquater, 1982; Houssein et al., 1993), no signicant variations in

uid geochemistry were observed between 1980 and 2013 (Fig. 4B).

Such consistent chemical character suggests uniform reservoir for theObock beach geothermal system (OBGS).

4.2. Geothermometry

The Na/1000–K/100–Mg0.5 ternary plot of Giggenbach (1988) canbe usedto discriminate mature waters,which have attainedequilibriumwith relevant hydrothermal minerals, from immature waters andwaters affectedby mixingand/or re-equilibration alongtheir circulation

path (Fig. 5). This provides an indication of the suitability of thewaters for the application of solute geothermometers. Since allhot springs from OBGS were partially equilibrated (Fig. 5), chemical

geothermometers can be used to estimate geothermal reservoir tem-perature (Arnórsson et al., 1983; Fournier and Potter, 1979; Fournierand Truesdell, 1973; Giggenbach, 1988). Therefore, classical and

Fig. 3. A: Piper diagram of seawater and hot springs waters from Obock beach; B: cold borehole and well waters from Obock area.



6/13

isotopic geothermometers as well as geothermometrical modeling

approach were used to estimate the reservoir temperature of OBGS.

4.2.1. Chemical geothermometers

4.2.1.1. Cationic geothermometers. The Na–K geothermometer was ini-tiallydeveloped to locate the major upow in high enthalpy geothermalsystems, because a general decrease in Na/K ratios of geothermal uids

with increasing temperatures was observed (D'Amore and Arnórsson,2000). This geothermometer is related to the variation of sodium andpotassium in thermal waters due to ion exchange of these elementsbetween coexisting alkali feldspars (Nicholson, 1993). Na–K equations

are adapted for reservoir temperatures in the range 180–350 °C(Ellis, 1979), but are limited at lower temperatures, notably less than120 °C (Nicholson, 1993).

Several Na–K geothermometers have been proposed and applied to

calculate reservoir temperature in the past three decades. The Na–Kgeothermometer of Giggenbach (1988), based on thermodynamic

data of albite and K-feldspar, gives an equilibrium temperature of

deep parent uids of about 169–195 °C (Table 2). Furthermore, theNa–K geothermometer of Fournier (1979) and Arnórsson et al. (1983)estimated the temperature of the geothermal reservoir respectively atabout 156–185 °C and 153–179 °C (Table 2). Those temperatures are

in agreement with the values obtained with Na–K–Ca geothermometer,173–192 °C (Table 2). It is of interest to note that Houssein et al. (1993)obtained almost the same range of estimate temperature for OBGS

(Table 2).

The Na–

Li geothermometer was initially developed by Fouillac andMichard (1981) from a statistic study about groundwaters in graniticand volcanic domains. Later on, Na–Li equation was recalibrated by

Kharakaet al.(1982)and Verma andSantoyo (1997) obtaining differentequations. Moreover, the Na/Li geothermometer appears to be sensitiveto the total dissolved solids of the water at depth, locally controlled bycontributions of Na–Cl, and the rock type (Sonney and Vuataz, 2010).

Based on these results, these Na/Li geothermometers cannot be consid-ered reliable for the Obock beach geothermal system.

However, a Na/Li geothermometer adapted for high-temperature

uids derived from seawater interacting with basalt in emerged

rifts was recently proposed (Sanjuan, 2010). Although it might be

affected by mixing, adsorption and precipitation of secondary phases,the temperature of the OBGS was estimated at 210 °C (Sanjuan, 2010).In our calculation, temperatures obtained by the Na/Li and other

geothermometers are in agreement, except for the Obock-HS1(Table 2). This exception might be explained considering that theObock-HS1 water showed low silica, high SO4 and high Mg concentra-tions, which indicate a higher contribution of cold seawater.

4.2.1.2. Silica geothermometers. The silica geothermometers, basedon the content aqueous silica and the solubility of different silicaphases as a function of temperature, can be used to determine the

temperature of water–rock interaction processes at depth in a rising

uid before its discharge. However, among the several existing silicageothermometers, in this study only equations for quartz were used be-cause this is the main silica phase found in the local rift (D'Amore et al.,

1998; Zanet al., 1990).In Table 3, the results obtained from the applica-tion of the quartz equations proposedby Fournier (1977) are presented.

Estimated temperatures in the OBGS reservoir range between 125 and147 °C, in agreement with values (111–137 °C) obtained by using previ-ous data (Houssein et al., 1993). Quartz temperatures were generallylower than those obtained with cationic geothermometers (Table 2).Such differences might be due to disequilibrium between quartz and

the hot water, probably caused by mixing between waters of differenttemperature andsilicacontent(Fig.6). In fact, the cold terrestrial watersof meteoric origin, probably representing an end-member, appearequilibrated with silica phases (amorphous silica, opal, chalcedony)

more soluble than quartz (Fig. 6). Precipitation of silica phases duringmixing between the hot uid with cold seawater, as typically occurs inhydrothermal vents (e.g. Bethke, 2008), could also explain the lack of equilibrium with respect to quartz.

4.2.2. Geothermometrical modeling The state of equilibrium between water and mineral is a

function of temperature; therefore saturation indices can be used asgeothermometers (Pang and Reed, 1998; Reed and Spycher, 1984). Inother words, estimation of reservoir temperature can be achieved bysimultaneous consideration of the equilibrium state between specic

water and many hydrothermal minerals as a function of temperature.Geothermometrical modeling calculations have been performed

for the OBGS hot springs using the SOLVEQ computer program (Reed,1982).

Fig. 7 depicts the mineral saturation indices versus increasingtemperature for Obock coastal hot springs. For those hot springs, satura-tion indices with respect to quartz, chlorite, brucite, epidote, prehnite,microcline, forsterite and monticellite minerals tend to get closer to

the zero (SI = 0) around the temperature of 180–200 °C at which

Fig. 4. Cl–SO4–HCO3 classication diagram of Giggenbach (1991). A: Thermal and cold

waters fromthe OBGS(this study);B: hotspringswatersfrom Obock beach (1981: Aquater,

1982, 1989: Houssein et al., 1993; 2013: this study).



7/13

Fig. 5. Relative Na/1000, K/100 and Mg1/2 contents in mg/L basis of Obock beach hot spring and cold waters from the area (data from this study and Houssein et al., 1993). Dashed and

dotted lines depict the K/Mg and Na/K isotherms at 100, 200 and 300 °C, respectively. Arrow represents the isochemical dissolution of 100 g of basalt in 1 kg of chloride water

(Giggenbach, 1988).

Table 2

Cationic geothermometry results for the Obock hot spring water samples (all values displayed in °C).

Reference Hot spring code Na–K–Ca Na–K Na–K Na–K Na–K Na–Li

Fournier and Truesdell

(1973)aFournier

(1979)bArnorsson et al.

(1983)cGiggenback

(1988)dVerma and Santoyo

(1997)eSanjuan

(2010)f

This study Obock-HS1 173 156 153 169 155 127





This study Obock-HS6 183 172 168 183 170 176This study Obock-HS7 188 177 172 188 175 171

Mean (except HS 1) 187 178 173 189 176 172Standard deviation (except HS 1) 3 4 4 4 4 7Houssein et al. (1993) Obock-HS1 190 185 179 195 182 143

Houssein et al. (1993) Obock-HS2 186 175 170 186 173 115



Mean 189 181 175 191 178 –

Standard deviation 3 5 5 5 5 –

In the equation a and f , concentration of Ca, Na, K and Li is in mol/Kg, whereas Na and K are expressed as ppm from b to e.

–: data not used in the evaluation of the reservoir temperature.a T = 1647/[2.24 + Log(Na/K) + β ∗ Log(Ca0.5/Na)] − 273.15; β = 4/3 for T b 100 °C and b = 1/3 for T N 100 °C and Log(Ca0.5/Na) b 0.b T = 1217/(1.438 + Log(Na/K)) − 273.15.c T = 1319/(1.699 + Log(Na/K)) − 273.15.d T = 1390/(1.75 + Log(Na/K)) − 273.15.e T = 1390/(1.75 + Log(Na/K)) − 273.15.f

T = 1390/(1.75 + Log(Na/K)) − 273.15.



8/13

temperature these minerals are assumed to be in equilibrium with

water giving rise the estimated reservoir temperature (Fig. 7).

4.2.3. Water isotope, sulfate isotope and their equilibriumThe results concerning the isotope composition of the water mole-

cule and the dissolved sulfate from fourselected Obock hot springs sam-ples and the local seawater are in Table 4. In the δ2H(H2O) versusδ18O(H2O) plot of Fig. 8A, all the samples fall on or near the RedSea–Gulf of Aden line of Craig (1969): δ2H(H2O) = 6 × δ

18O(H2O). In

particular, the local seawater sample, with δ2H(H2O) = +5.2‰ andδ18O(H2O)= +1.1‰, is enriched in heavier water isotopes, accordinglyto a coastal seawater from an arid climate and in comparison with the

mean values of the Obock's springs, δ2H(H2O) = +2.6 ± 0.7‰ andδ18O(H2O) = + 0.55 ± 0.17‰, which are slightly depleted in compari-son to the typical Gulf of Aden values (Fig.8A). This isprobably due to thefact that: i) the main recharge of the hot springs didn't occur near their

outpouring sites but in a more open (and deep) ocean environment;ii) a possible mixing with freshwater of meteoric origin. According tothis latter hypothesis, the analyzed sample with lower salinity falls onthe local meteoric water line (LMWL in Fig. 8A; Fontes et al., 1980), prob-

ably due to the ow of local groundwater towards the coast (Awalehet al., submitted for publication). However, the δ34S(SO4) and δ

18O(SO4)values of the springs conrms a signature quite similar to the local(Fig. 8B) and mean ocean water, i.e. δ34S(SO4) = +21.1‰ (Coplen

Table 3

Silica geothermometry results for the Obock hot spring water samples (all values displayed in °C).

Reference Hot spring code Quartz Quartz Quartz Quartz

Fournier (1977)a Fournier (1977)b Fournier and Potter (1982)c Verma (2001)d

This study Obock-HS1 127 124 127 124







Houssein et al. (1993) Obock-HS1 127 132 128 124




a T = {1309/[5.19 − Log(S)]} − 273.15.b T = {1522/[5.75 − Log(S)]} − 273.15.c T = −42.2 + 0.28831 ∗ (S) − 3.6686 ∗ 10−4 ∗ (S)2 + 3.1665.10−7 ∗ (S)3 + 77.034 ∗ Log(S).d T = {−1175.7/[Log(S) − 4.75]} − 273.15; where (S) = silica concentration as SiO2 (ppm).

0

20

40

60

80

100

120

140

160

180

200

220

0 100 200 300 400 500 600 700 800

seawater

m i x i n

g

m i x i n g

Entalpy (kJ/kg)

S i O ( p p m )

2

a m o r p h

o u s s i l i c

a

( G u n n a r s s o

n & A r n ó r s s o

n ,

1 9 9 8 )

o p a l

( F o u r n i e

r , 1

9 9 1 )

q u a r t z

( V e r m

a , 2 0 0

1 )

q u a r t z n

o s

t e a m

l o s s

s t e a m

l o s s

c h a l c

e d o n y

( A r n o

r s s o n

e t a l

., 1 9 8

3 )

reservoir

this study

boreholes

wells

1993

hotsprings

coldwaters

Fig. 6. Dissolved silica-enthalpy diagram of the Obock hot springs from this study and literature ( Houssein et al., 1993), local cold waters and seawater. The calculated dissolved silica

content of the local geothermal reservoir is SiO2 = 212 mg/L. Quart steam and no-steam loss curve are from Anórsson (2000). Amorphous silica-water equilibrium (Gunnarsson and

Arnórsson, 2000) and opal (Fournier, 1991) curves are also represented for comparison.



9/13

et al., 2001)and δ18O(SO4)=+8.6‰ (Boschetti and Iacumin,2005). Fur-thermore, a comparison with the deep Red Sea waters and hydrothermalbrines of Atlantis II (Craig, 1969; Zierenberg and Shanks, 1986) reveals anisotopic af nity with the former, and at present a trend towards more

enriched δ34S(SO4) should be excluded (Fig. 8B).

The application of bisulfate–water or sulfate–water isotopicgeothermometer, which are based on the equilibrium exchange of oxygen isotopes betweenaqueousHSO4

− orSO42− and H2O, respectively,

depends by variousfactors, mainly i) the dominant dissolved sulfur spe-

cies in solution; ii) the kinetic of equilibration; and iii) the presence of conductive or mixing cooling (Boschetti, 2013; Boschetti et al., 2003).Considering the evident mixing between Obock thermal waters and sea-

water and to check if the mother thermal water is equilibrated withHSO4

− or SO42−, the δ18O(SO4) and δ

18O(H2O) values obtained in thisstudy was plotted on a 103lnα(SO4–H2O) versus 10

6/T2 diagram andusing the temperatures obtained by the chemical geothermometers

(Fig.8C).Similarly to the Atlantis II brine from the RedSea, Obock thermalwaters plot between the HSO4

−

–H2O and SO42−–H2O equilibrium lines:

103

lnαðHSO−

4 –H2OÞ ¼ 3:26106=T

2−5:81 ð1Þ

103

lnαðSO2−4 –H2OÞ ¼ 2:54510

6=T

2−6:61 ð2Þ

where T is the temperature in Kelvin, Eq. (1) from Seal et al. (2000) andEq. (2) the best t of the data from Halas and Pluta (2000) and Zeebe(2010) (see Boschetti, 2013 for details). SO4

2− is the main sulfate species

at the outpourings of the coastal hot springs,howeverHSO4−would be the

prevailing oxidized sulfur species in the deep hydrothermal vent, proba-bly originated by the disproportionation of magmatic SO2 (after Mülleret al., 2013):

103

lnαðHSO−

4 –H2OÞ ¼ 1:852106=T

2−0:288 ð3Þ

The approaching of the Obock spring with the highest estimatedtemperature to the intersection of the HSO4

−

–H2O equilibrium lines

sustains this hypothesis (Fig. 8C). In fact, the obtained temperaturefrom the intersection of Eqs. (1) and (3) occurs at T = 207 °C,which is in line with the temperature of 210 °C estimated by Sanjuan

(2010) and our maximum temperature of 200 °C estimated by thegeothermometrical modeling.

5. Discussion

It has been reported that the uids resulting from seawater–basalt

interaction at temperature higher than 100 °C have been completelydepleted of magnesium and sulfate (Bowers et al., 1988; Mottl andHolland, 1978; Tomasson and Kristmannsdottir, 1972). Moreover,straight lines were obtained between points of hot springs and point

of sea water when plotting normalized concentrations of elements tochloride (Mg/Cl, SO4/Cl, K/Cl, Ca/Cl, SiO2/Cl) versus concentration of Li/Cl (Fig. 9). Therefore, one may extrapolate the Mg and SO4 concentra-tions to zero, which corresponds to the maximum interaction between

seawater and basalt. The Mg/Cl and SO4/Cl straight lines get zero for thesame Li/Cl value of 3.228 × 10−4 (Fig. 9A). The silicate concentration of the hot end-member (e.g. Li/Cl = 3.228 × 10−4) was estimated about212 mg/L (Fig. 9B). When the value obtained for dissolved silica is used

as quartz geothermometer (Verma, 2001), it gives a geothermal reservoirtemperature of about 187 °C, which is in good agreement with that ob-tained from the chemical geothermometer (172–191 °C, Table 2) aswell as from multiple mineral equilibrium approach (180–200 °C, Fig. 7).

As stated above, the δ34S(SO4) compositions of Obock thermalwaters are similar to that of the present-day seawater. These results

rule out any evolution of the seawater–basalt system that would not

Fig. 7. Diagrams showing the change in calculated saturation indices (log Q/K) of various minerals as a function of temperature (Obock-HS2 sample). The temperature range with the

convergence of saturation indices to zero is assumed to indicate the temperature of deep thermal reservoir for OBGS. It should be noted that similar diagrams were obtained for other

Obock beach hot springs.

Table 4

Isotopic data of the hot springs and seawater from Obock.

Sample δ18O(H2O)

(‰ vs

V-SMOW)

δ2H(H2O)

(‰ vs

V-SMOW)

3H

(T.U.)

δ34S(SO4)

(‰ vs

V-CDT)

δ18O(SO4)

(‰ vs

V-SMOW)

Obock-HS1 0.7 3.0 b0.6 20.9 9.1

Obock-HS2 0.6 1.6 b0.6 21.2 8.7

Obock-HS3 0.6 2.9 b0.6 21.1 8.9

Obock-HS6 0.3 2.9 b0.6 21.3 8.9

Seawater 1.1 5.2 1.3 20.8 9.1


http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-


10/13

yet attain the equilibrium (e.g. no end-member depleted of sulfate andmagnesium). Indeed,when the diminution of SO4 and Mgisduetoa de-

gree of advancement of the reaction without reaching the nal state, itshould remain SO4 in the solution that would present therefore34S/32S ratio higher than that of seawater because of isotopic fractioning(Camo et al., 1991), which is not the case in Obock coastal geothermal

eld.In the other hand, whenthe diminution of SO4 istheresultof a blend

betweenan end-member, depletedof Mg and SO4, and the surroundingseawater which mix each other during ascent, the 34S/32S ratio of the

end-member is similar to that of the seawater, which is the case of the

present study (Table 4).

The Obock region was dened as the nal termination of a riftsegment (Manighetti, 1993). In addition, Manighetti (1993) revealed

the occurrence of hydrothermalism in the Obock ridge. Therefore, thefollowing model was proposed for the Obock coastal geothermal eld(Fig. 10): the magma chamber of the submarine volcano located be-neath the Obock ridge could constitute the main heat source of the

Obock geothermal system. Seawater would inltrate through the ba-salts fractures to reach the geothermal reservoir. In this later, seawatercould react with the basalt bedrock at the temperature of the geother-mal reservoir (197± 10°C) for a relatively long time so that the equilib-

rium can be reached. Therefore, the seawater–basalt interaction in the

geothermal reservoir could produce a geothermal uid which were

5

6

7

8

9

10

11

12

3 4 5 6 7 8 9 10 11 12 13

Atlantis II

Obock

Temperature (°C)

Mean Ocean

Gulf of Aden

Red Sea

seawater

3

1 0 l n α

6 210 /T(kelvin)

S

O

- H O

4

2

2 -

H S O

- H O

4

2

-

S O

- d i s

p

2

C a

S O

- H O

4

2

2 0 6 0 3 0 0

2 6 0

2 2 0

1 8 0

1 4 0

1 0 0

6.0

6.5

7.0

7.5

8.0

?

8.5

9.0

9.5

10.0

20.0 20.5 21.0 21.5 22.0

Atlantis II

A t l a

n t i s

I I

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

SMOW

2

1 8

δ H

= 6 x δ O

Gulf of Adenseawater

18δ O(H O) ( vs V-SMOW)2 ‰

2 δ

H ( H O ) (

v s V - S M O W )

2

‰

34δ S(SO ) ( vs V-CDT)4 ‰

1 8

δ

O ( S O ) (

v s V - S M O W )

4

‰

L M W L

Obockseawater

Obockseawater

A

B

C

Fig. 8. Isotopediagrams of thehot springs(squares)and seawater (diamonds) from Obock.A: Water isotope data arecomparedwith theLocalMeteoricWaterLine (LMWL; Fonteset al.,

1980), the seawater from the Gulf of Aden (Craig, 1969; Ganssen and Kroon, 1991), the Craig's line δ2H(H2O)= 6 δ18O(H2O) relating surface seawaters of the area (Craig, 1969) and the

Atlantis II hydrothermal brine from the Red-Sea Rift (Pierret et al., 2001). B: The sulfate isotope values are compared with Red-Sea seawater sampled at different depth (triangles) and

Atlantis II hydrothermal brine(Zierenberg and Shanks, 1986). C: Sulfate–wateroxygen isotope fractionation vs. temperature(after Boschetti, 2013). Graysquares andarea depictthe cal-

culated fractionation usingthe obtainedδ18O(SO4)and δ18O(H2O)isotope data andthetemperatureat thesampling site ofthe Obock'shotsprings (Table4) and AtlantisII (Zierenberg and

Shanks, 1986), respectively. Red squares and yellowarea are thesame samples, but usingthe temperature calculatedby geothermometers. Horizontalarrows representconductive and/or

mixing cooling. Please note that the δ

18

O(SO4) data of Zierenberg and Shanks (1986) were recalculated considering the value of the mean present-day seawater δ

18

O(SO4) = +8.6‰

(Boschetti, 2013; Boschetti and Iacumin, 2005).



11/13

depleted of magnesium and sulfate. Then, during their ascent by

convection through basalt fractures, the geothermal uid is mixed in

varying degrees with cold seawater which, after an additional possible

mixing with freshwater, give rise to coastal hot springs of Obock city

(Fig. 10). This model could explain the fact that the thermal waters in

the presentstudy are partially equilibrated at the outpourings. Likewise,

A

B

C

Fig. 9. A: Mg/Cl and SO4/Cl versus Li/Cl; B: SiO2/Cl versus Li/Cl; C: Ca/Cl and K/Cl versus Li/Cl.



12/13

the lower amount of chloride in hot springs than in seawater suggests

the dilution by freshwater. This is further supported by the Obockregional piezometric map which shows that the meteoric water owtoward the sea (Fig. 2). However, one cannot preclude any phase sepa-ration process that could be responsible of the lower concentration of

chloride in Obock hot springs than in seawater as reported for some

submarine hydrothermal sources (Bischoff and Rosenbauer, 1989).In a recent pre-feasibility study, a geophysical survey of the Obock

geothermal prospect, based on electro telluric, magnetotelluric and

gravimetric measurements, was conducted by the CERD (2013). Thesegeophysical investigations reveal a geothermal anomaly in an areanear the coast of Obock city. Therefore, these results support the con-

ceptual model proposed for Obock beach geothermal manifestations(Fig. 10).

6. Conclusions and perspectives

The reservoir temperatures in the Obock beach geothermalsystem were assessed using a number of geothermometry techniques.

Chemical geothermometers (i.e. the Na/K, Na–K–Ca, SiO2) and multiple

mineral equilibrium approaches estimate the temperatures of the deepgeothermal reservoir at 197 ± 10 °C, which is also conrmed by theapplication of the 18O/16O isotope fractionation α(HSO4

−

–H2O).A conceptual model was proposed for the Obock beach geothermal

system. During their ascent through fractures, the geothermal uid ismixed in varying degrees with cold seawater which, after an additionalpossible mixing with small amounts of fresh water, give rise to coastalhot springs of Obock city.

Similarly to the Atlantis II hydrothermal water from the RedSea, thehot waters from Obock have δ34S(SO4) values very near to that of thepresent-day seawater. Moreover, the magnesium and sulfate depletionof the water are consistent with a uid having interacted with hot

axial-rift basalts.However, future investigations, such as offshore geophysical

surveys, should be undertaken to locate the Obock geothermal reservoir.

The conceptual model proposed here clearly indicates that deep deviated

drilling should be used to intercept the Obock geothermal reservoir.

Acknowledgments

This research work wasnancially supported by the Centre d'Etudes

et de Recherche de Djibouti (CERD). We are grateful to Abdi AbdillahiDjibril and Samaleh Idriss for their assistances in the eld works. Wewould like to thank the Editor-in-Chief, Prof. Benedetto De Vivo, andan anonymous reviewer for their very constructive comments that

substantially improved the manuscript.

References

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Aquater, 1982. Etude technico-économique de pré-faisabilité géothermique. Rapportspécique — Nouvelles perspectives de développement intégré avec les ressourcesgéothermiques (République de Djibouti, 90 pp.).

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Arnórsson,S., Gunnlaugsson, E., Svavarsson, H., 1983. The chemistry of geothermal watersin Iceland III: chemical geothermometry in geothermal investigations. Geochim.Cosmochim. Acta 47, 567–577.

Awaleh,M.O., Hoch, F.B., Soubaneh, Y.D., Boschetti, T., Egueh, N.M., Elmi, S.A., 2015. The geo-thermal resources of the Republic of Djibouti — II: geochemical study of the Lake Abhegeothermal eld. J. Geochem. Explor. (submitted for publication).

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Barberi, F., Ferrara, G., Santacroce, R., Varet, J., 1975. Structural evolution of the Afar triple junction. In: Pilger, A., Rosler, A. (Eds.), Afar Depression of Ethiopia, Inter-Union Com-mission on Geodynamics, Scientic Report 14, E. Schweizerbart'scheVerlagsbuchhandlung (Nägele u Obermiller), Stuttgart, pp. 38–54.

Bethke, C.M., 2008. Geochemical and Biogeochemical Reaction Modeling. Cambridge Uni-versity Press, New York.

Bischoff, J.L., Rosenbaeur, R.J., 1989. Salinity variations in submarine hydrothermal sys-tems by layered double diffusive convection. J. Geol. 97, 613–623.

Boschetti, T., 2013. Oxygen isotope equilibrium in sulphate–water systems: a revision of geothermometric applications in low-enthalpy systems. J. Geochem. Explor. 124,

92–100.

Fig. 10. Simplied sketch of the conceptual model of the Obock geothermal system.


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