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ARTICLES

A Geochemical Approach for the Evaluation of the Geo-thermalPotential in Yemen

ABSTRACT

Thermal springs, fumaroles and boiling water pools from conti-nental Yemen have been investigated for chemical and isotopic compositions. Whatever the emergence, all the water discharg-es have an isotopic signature of meteoric origin. Springs seeping out from high altitudes in the central volcanic plateau show a prevalent Na-HCO3-composition, clearly affected by an anoma-lous flux of CO2 deriving from the active hydrothermal systems located in the Jurassic Amran Group limestone sequence, likely underlying the 2000-3000 m thick volcanic suite and/or the Cre-taceous Tawilah Group sandstone. All the CO2-rich gas samples have a δ13C-CO2 signature that falls in the range of mantle CO2 (-3<δ13C<-7 ‰ V-PDB). The relatively high 3He/4He (1<R/Ra<3.2) ratios measured in all the CO2-rich springs and also some mixed N2-CO2 gas vents in the far east Hadramaut region support the presence of mantle magmas and related hydrothermal systems residing at the crust level in sev-eral areas of Yemen. This well agrees with the presence of Qua-ternary magmatic activity along the Gulf of Aden as well as the inland Quaternary volcanic fields.Liquid- and gas-geothermometry and geological considerations suggest that three areas (Al Lisi, Al Makhaya and Damt) inside the Yemen volcanic plateau may have promising perspectives for the future development of geothermal energy in Yemen.

Copyright © 2009. The Yemen Geological Society. All Rights Reserved.

Manuscript Received March 2009, Revised May 2009, Accepted June, Press ver-

sion proofread by authors December 2009

AUTHORS

Yemen Geoscience Bulletin 1 (1/2), p. 1-19, San’a, December 2009

Minissale Angelo, Mattash Mohamed, Vaselli Orlando, Tassi Franco, Al-Ganad Ismail, Selmo Enrico, Shawki Nasr, Tedes-co Dario, Poreda Robert, Ad-Dukhain Mohamed and Hazzae Mohamed

ANGELO MINISSALE ~ Institute of Geoscience and Earth Resource, Florence, Italy, [email protected] Angelo Minissale Senior fluid geochemist and Head of the Section of Florence (Italy) of the Institute of Geosciences and Earth Resources of the National Research Council of Italy. Expert in geothermal and volcanic fluids in different geodynamic environments, presently working on paleoclimate reconstruction using travertine deposits.

MOHAMED MATTASH ~ Geological Survey and Mineral Resources, Sana’a, Yemen, mattashma@yahoo,comMohamed Mattash Senior Geologist, Expert in volcanic activity and geothermal resources of Yemen. Presently working on mineral resources of the sedimentary cover in Hadramawt and Al-Mahrah Governorate.

ORLANDO VASELLI ~ Department of Earth Sciences, University of Florence (Italy), [email protected] Vaselli Associate Professor in Geochemistry and Head of the Department of Earth Science of Florence. After studying the mantle geochemistry from 1990 to 1997 (Birkbeck College, University of London) he moved to the field of fluid geochemistry in volcanic and geothermal environments. Presently he is involved in the volcanic surveillance of some Italian and Latin American volcanoes.

FRANCO TASSI ~ Department of Earth Sciences, University of Florence (Italy), [email protected] Tassi Researcher Assistant at the Department of Earth Sciences, University of Florence (Italy) and responsible of the Laboratory of Fluid Geochemistry. Ph.D in 2004 on the geochemistry of organic compound in volcanic and geothermal gas discharges. He has set up one of the most important laboratories of gas analysis at international level. He is involved in many projects regarding the Italian (Vulcano, Naples, Panarea, Etna) and Costa Rica, Nicaragua, Chile, Colombia volcanoes.

Yemen Geoscience Bulletin 1 (1/2) 2009 Minissale et al., A Geochmical Approach2

INTRODUCTION

Yemen is located in one of the most active plate boundar-ies of the World, i.e. the triple junction made up by the Gulf of Aden, the Red Sea and the Eastern African Rift System. Setting aside some active volcanic islands in southern Red Sea, abundant Oligocene-to-Quaternary flood alkaline ba-salt trap series (YTS) occupies the central-western sector of Yemen for about 50,000 km2. Recent volcanic activity is anyway present in several continental areas: i) along the tectonically active Gulf of Aden, ii) inside the central el-evated areas of the Yemen Trap Plateau (YTP), from north

ملخص

العيون الحارة واألبخرة وكذلك برك المياه المتميزة بدرجة الغليان في اليمن تمت دراستها والبحث فيها ألغراض معرفة مركباتها الكيميائية والنظائرية. بصرف النظر عن نوع الوحدات الصخرية فإن المياه الحارة التي تصعد عبرها التي الحارة العيون األمطار. لمياه يعود أصل ذات نظائرية بصمات تحمل تنبع من المناطق المرتفعة في الهضبة البركانية تبين في معظمها تركيبة ثاني غاز من مكثف بتيار بوضوح تأثرت أنها حيث الصوديوم, بيكربونات أكسيد الكربون المشتق من أنظمة المياه الحارة النشطة التي تمر عبر تتابع تأتي تحت والتي الجوراسي للعصر التابعة لمجموعة عمران الجيري الحجر ما يقرب من ألفين إلى ثالثة آالف متر من سماكة البراكين و/أو تحت صخور

الحجر الرملي لمجموعة الطويلة التابعة للعصر الطباشيري.نظير بصمة لديها الكربون أكسيد ثاني بغاز الغنية الغازية العينات كل الكربون من ثاني أكسيد الكربون والذي يقع في مدى ثاني أكسيد الكربون العالية نسبيًا الهيليوم )δ13 C‹-7%V-PDB<-3( نسب الوشاح مـن القادم بغاز الغنية العيون كل في قياسها تم التي )3He/4He )1<R/Ra<3.2التي يختلط فيها المخاريط أو الفوهات وأيضًا بعض الكربون أكسيد ثاني ثاني أكسيد الكربون مع النيتروجين في منطقة حضرموت تدعم وجود أصل صهاري من الوشاح وما يتعلق به من أنظمة حرمائية تصعد إلى مستويات

في القشرة في مناطق مختلفة من اليمن.الرباعي على طول خليج عدن هذا يفسر بوجود نشاط صهاري من العصر وكذلك الحقول البركانية ذات األصل الرباعي في المناطق الداخلية. التقييم الحراري للسوائل والغازات واالعتبارات الجيولوجية كلها تدعم بأن هناك ثالثة مناطق ذات أهمية جيوحرارية واعدة وهي اللسي في ذمار ومش الكافر بإب ودمت في الضالع وباإلمكان تطويرها مستقبليا كمصادر جيوحرارية مأمولة

في اليمن.

ISMAIL AL-GANAD ~ Geological Survey and Mineral Resources Board, Sana’a, Yemen.Ismail Al-Ganad Senior geologist and Chairman of the Geological Survey and Mineral Resources Board since 1996.

ENRICO MARIA SELMO ~ Department of Earth Sciences of the University of Parma (Italy), [email protected] Maria Selmo is a technician with research duties. His main interests are mainly devoted to the isotope geochimistry oxygen, hydrogen and carbon in natural waters.

SHAWKI NASR ~ Department of Geology University of Taiz, Yemen,[email protected] Nasr Associate Professor in structure geology of the Department of Geology of the University of TaizHe obtained his M.Sc. and Ph.D in Azerbaijan

DARIO TEDESCO ~ Department of Environmental Sciences of the Second University of Naples, (Italy) [email protected] Tedesco is Associate Professor in Geochemistry and Volcanology. His interests are mainly related to study the isotopic composition of volcanic and natural fluids. He is a consultant for the United Nations-OCHA (Office for the Coordination of Humanitarian Affaire). He recently worked for the UN as volcanologist in the midst of the eruption of Nyiragongo volcano (D.R. Congo). He is member of the UN-scientific commette for volcanic and seismic risks in the Virunga region. He has taken care the volcanic part for the “Contingency and Master Plan (evacuation plan) for the city of Goma.

3Yemen Geoscience Bulletin 1 (1/2) 2009 Minissale et al., A Geochmical Approach

of Sana’a to south of Ta’iz and iii) near Marib (McCombe et al., 1994). Anomalous heat flow in Yemen likely started 40 Ma ago and reached its maximum about 12 Ma (Elf Aquitaine, 1990). The present geothermal gradient in the Red Sea region is still quite anomalous, varying from 49 to 77 °C/km. Being Yemen a developing country, the assessment of the geothermal potentiality, in terms of alternative energy, has to be considered a useful contribution for the future devel-opment of the country itself. Only occasionally, the geo-thermal potential of Yemen has been investigated (ELC-Electroconsult, 1982; BRGM, 1985; Dowgiallo, 1986; Fara et al., 1999) despite the fact that a large number of ther-mal springs, and even volcanic fumaroles discharges from several localities (Mattash et al., 2005) is present.In this study the chemical and isotopic data from about 100 cold and thermal water discharges (thermal springs and cold wells) and some gas emissions and fumaroles, sampled in two campaigns carried out in January 2001 and December 2002, are discussed. This study was re-alized thanks to the Geological Survey and Mineral Re-sources Board of Yemen, the Italian Council for Research (CNR-Institute of Geosciences and Earth Resources of Florence) and UNESCO, the latter for having funded an International School (by A.M, O.V. and M.M.) on Geother-mics held in December 2002 in Sana’a. This work is part of a paper published in Applied Geochemistry (Minissale et al., 2007).

GEOLOGY OF YEMEN

The geological architecture of Yemen is founded on large outcrops of Archean-Proterozoic metamorphic basement blocks belonging to the Arabian Shield (NW-Basement Block and Central Basament Block in Figure 1), covered by rocks of Ordovician, Permian, Jurassic, Cretaceous, Tertiary, and Quaternary age (Beydoun, 1966; Robertson Group, 1992; Mattash, 1994; Beydoun et al., 1998; As-Saruri, 1999) and whose geological characterization can be found in Mattash (1994).The basement blocks, transected by the NW-SE Ram-lat As Sab’atayn Graben (Figure 1), are bordered to the south and to the west by Tertiary-to-present day Gulf of

ROBERT J. POREDA ~ Department of Earth and Environmental Sciences of the University of Rochester. pozedu@earth. rochester.eduRobert J. Poreda Full Professor at the Department of Earth and Environmental Sciences of the University of Rochester. He completed his Ph.D in 1983 on the isotopic composition of noble gases in volcanic systems. He has spent the past 25 years researching the noble gas signature in the earth’s oceans, atmosphere, crust and mantle.

MOHAMED AD-DUKHAIN ~ Geological Survey and Mineral Resources Board, Sana’a, Yemen.Mohamed Ad-Dukhain has his B. Sc. degree from the University of Sanaa faculty of Science, Earth and Environmental Sciences Section and he is now M. Sc. Student and Geothermal Project Manager, Geological Survey and Mineral Resources Board.

MOHAMED HAZZAE ~ Geological Survey and Mineral Resources Board, Sana’a, Yemen.Mohamed Hazzae has his B. Sc. degree from the University of Sanaa faculty of Science, Earth and Environmental Sciences Section and he is now Ph.D Student and Geologist at the Geological Survey and Mineral Resources Board.

ACKNOWLEDGEMENTS

The Geological Survey and Mineral Resources Board of Yemen is greatly thanked for providing transportation facilities during the sampling campaign of January 2001. The Geothermal office of UNESCO in New York and Mrs Marnell Dikson (CNR-Institute of Geosciences and Earth Resources of Pisa, Italy) are really thanked for having allowed the second gas sampling campaign on December 2002.


Aden and Red Sea rift systems, the latter two being part of the triple junction system located between the NE Africa (Afar) and the Arabian Peninsula. In continental Yemen the magmat-ic activity started during the Eocene, with the most intense volcanic activity occurring in the Oligocene-Early Miocene (31.6-15 Ma; e.g. Greenwood and Bleakley, 1967; Davidson and Rex, 1980; Capaldi et al., 1983; 1987a; Strojex-port 1988; Manetti et al., 1991; Mattash, 1994; Al-Kadasi et al., 1999; Menzies et al., 2001).The rifting activity produced the YTS, which is constituted by 25-30 Ma old isolated basal-tic continental flood volcanism in Sa’dah area (Figure1) and relatively younger and much more abundant and thick sequences south of Sana’a (Chiesa et al., 1989). Post-rifting vol-canic activity resumed in central and southern Yemen at 10 and 5 Ma ago in coincidence with the formation of the western Indian Ocean (10 Ma) and the Red Sea (5 Ma) and proceeded, discontinuously, up to the Quaternary, at sev-eral localities inside the YTP, in Marib area and

along the Gulf of Aden (Neumann van Padang, 1963).The YTP is bounded to the west by Tertiary granitic intrusions related to the tensile tecton-ics, the uplift of the Afro-Arabian dome and the lithosphere thinning (Capaldi et. al., 1987b) and by thick (up to 4000 m) Miocene-Pliocene sedi-mentary formations of the Tihama plain (Figure 1). To the north, Mesozoic clastic and platform carbonate successions of the Amran Group (Jurassic) crop out extensively. Such carbon-ate units are commonly present in Yemen, es-pecially at the boundary of grabens (i.e. west of Sab’atayn Graben) and, when buried by impermeable formations, can be considered a potentially good aquiclude. The ESE boundary of the YTP is occupied by Precambrian forma-tions (mostly gneisses and granites) belonging to the Arabian-Nubian shield (central basement block; Figure1). Sporadically, they crop out up to Mukallà to the east, along the Gulf of Aden, and also north of Sana’a in the Sa’dah area (Menzies et al., 2001).

Figure 1. Schematic geological map of Yemen with location of sampling sites.


From a hydrogeological point of view, the YTP, from Sana’a to Ta’iz, is characterized by relative high precipitation when compared to the latitude (up to 800 mm/y; Dequin, 1976). This meteoric water infiltrates inside the more elevated areas of YTP feeding several local aquifers hosted in intra-mountain basins, such as those in Sana’a, Dhamar, and Ad Dhala (Farquharson et al., 1996). Although rainfall in coastal areas is less than 80 mm/y, the main regional aquifers are in-deed those located along the coasts, such as the aquifer hosted in the Tihama and the Aden plains (Mc Combe et al., 1994). Very likely, these coastal aquifers are fed by precipitation on the YTP and drained southwards and westwards by

wadies, characterized by fast run-off, especially along the Great Western Escarpment (Dowgial-lo, 1986) bordering to the west of YTP.

CHEMICAL AND ISOTOPICCOMPOSITIONOF WATER SAMPLES

The location of the sampling site is reported in Figure 1, whereas the main components in so-lution have been plotted in the Langelier-Lud-wig (1942) diagram of Figure2. Thermal sam-ples are divided into 9 groups according to the area of emergence (Hajjah, Sana’a, Dhamar, Ibb, the South, Hadramawt, Damt, AlJawf and

Figure 2. Langelier-Ludwig (1942) diagram for the samples investigated


Ta’iz in Figure1), whereas cold samples con-stitute a separate homogeneous group. The area named “South” refers to a large region lo-cated to the S-SE of the YTP (Lahj-Abyan dis-tricts), and includes thermal springs prevalently emerging from metamorphic formations, either Proterozoic or Archean in age.Only few cold samples have the typical low salinity Ca-HCO3 composition (Figure2), com-monly characterizing, at least in temperate regions, ground waters. There are other cold samples with a more saline signature, typical of arid regions (chot-type), with either Na-Cl or Ca-SO4 composition. Generally speaking, sa-linity values vary from as low as 273 (# 92 from

Hajjah area) up to more than 83,000 (# 96, Bir Ali crater lake) mg/L. In spite of the limited number of cold samples used for characterizing different areas, by mov-ing clockwise from the Ca-SO4 corner (left bot-tom) to the Na-HCO3 corner (top right), several evolutionary trends can be recognized for the thermal springs of Yemen. The first one is iden-tified by the thermal samples from the Hadra-maut and the two springs emerging N-NW of AlJawf. All these samples have a mixed Ca(Na)-SO4(Cl) composition, likely deriving from the dissolution of sabkha-related gypsum and/or halite along their hydrological circuits. The second trend (Hajjah-Ta’iz) is typical of thermal

Figure 3. Binary diagram of Na vs. Cl for the samples investigated


springs circulating in crystalline formations and/or geological units characterized by the pres-ence of halite (marine evaporites) or connate marine waters. They prevalently emerge in the NW part of the studied area (N of Hajjah) but also south of Ta’iz. The third (Sana’a), the fourth (Ibb) and the fifth (Dhamar and Damt) trends, which include the thermal springs emerging in the YTP, are characterized, with respect to the Hajjah trend, by a progressive increase in the Na-HCO3 component with respect to the more usual Na-Cl ocean-type (SW) trend.To better define this Na-HCO3 increase in trends 3, 4 and 5, with respect to the Hajjah springs, all samples have been plotted in the Na-Cl binary plot of Figure3. All Hajjah thermal samples and those from Ta’iz and Hadramawt areas, as well as many cold samples, fall in the area delimited by the seawater-meteoric water (Cl/Na=1.2 in meq/L) dilution line and the ha-lite dissolution (Cl/Na=1.0 in meq/L) lines. On the other hand, Dhamar, Damt, Ibb and Sana’a thermal discharges and some samples from the South lie on the right side of the Cl/Na=1 line. Furthermore, two samples are located far away from most of the studied waters: i) # 35, that emerges isolated along the Great Escarpment, at the western edge of the YTP (Figure 1), has an unusual marked Ca-Cl component and ii) # 96 that is from the Quaternary volcanic crater of Bir Ali (SE of the Balhaf graben; Figure1). In this latter case it has to be considered that, al-though the crater rim is at 103 m a.s.l., the lake, affected by strong evaporation processes, is likely connected with the nearby Indian Ocean, as testified by the Cl/Na=1.2 ratio. At Al Lisi and Isbil, NE of Dhamar, the conden-sate from three low-fux fumaroles emerging at 82 and 88.6 (Al Lisi) and 43 (Isbil) °C, respec-tively, are quite acidic (pH: 3.1, 4.32 and 3.42, respectively), possibly due to the presence of acidic species in the gas phase such as H2SO4 (possibly from oxidation of H2S) in # 3 and 4,

and HNO3 in # 5. However, anthropic influence cannot be excluded for the Isbil sample, this spot being commonly used for Turkish baths by local people. The analytical δ values (in ‰ SMOW) have been plotted in the usual δD-δ18O diagram (Fig-ure 4), where the Global Meteoric Water Line (GMWL, Craig, 1961), the isotopic composition of the Standard Mean Oceanic Water (SMOW) and that of a single rain event collected in De-cember 2002 near Ta’iz at an elevation of 2300 m, were also reported. A good alignment of both cold and thermal springs along the GMWL, up to -3 ‰ δ18O values is observed. From this val-ue up to the +5.2 ‰ δ18O of the partially evapo-rated Bir Ali crater lake (# 96), many samples have likely undergone evaporative processes, along more or less parallel lines (Figure 4). Sev-eral cold samples from rivers or wadies (# 16: Tuban River, # 33: Wadi Al-Barh, # 56: Wadi Ar-Raboa’, # 94: Wadi Al-Meer, # 95: Wadi Laa’h) are isotopically heavier than sea water, and that all of them plot in the CaSO4 and/or in the Na-Cl sectors of the Langelier-Ludwig diagram (# 16, 33, 93, 94, 95, 96; Figure 2), possibly in relation to strong evaporation of running waters and salt-bearing soil (halite and gypsum) dis-solution. The two isotopically lighter cold springs (# 89 and 90) in Figure 4 are located east of the YTP in the Marib area (Figure1) at lower elevation with respect to the YTP springs, clearly suggesting a “continental effect”, likely caused by fraction-ation of feeding meteoric waters after crossing the YTP. In spite of their isotopic lightness, in the δ18O-elevation diagram in Figure 5 they are not indeed the more elevated samples, and this clearly excludes that they have only undergone topographic isotopic fractionation. Moreover, all thermal samples from Hajjah area, Hadramawt, Shabwahh and two springs from south of Ta’iz discharge at low altitude (<500 m). They have an isotopic composition of mother rainfalls fed


by precipitation occurring at least at elevations higher than 1000-1500 m, with a quite vertical drop underground before the emergence. This well agrees with the fact that the main precipi-tation areas in Yemen (>800 mm/y) are highly concentrated in the central-southern areas of the YTP, east of Ta’iz and Ibb (Dequin, 1976).

Chemical and isotopiccomposition of gas samples

The main components CO2-N2-CH4 have been plotted in the relative ternary diagram in the top left-hand side of Figure 6 where three groups of samples can be recognized: i) CO2-dominat-ed gases, associated to springs located in the

YTP and one spring in the nearby Hajjah area; ii) N2-dominated gases, from some springs emerging in the southern part of Yemen: near Ta’iz, in the Lahj district and at Shabwahh and iii) N2-CO2 mixed gases that discharge in the far east Hadramawt area. The presence of the first group suggests that the HCO3- enrichment shown by the thermal YTP water samples that plot close to the Na-HCO3 corner of Figure 2 and right to the Cl/Na = 1 line of the Na-Cl dia-gram of Figure3 is likeldue to high pCO2 in so-lution. There are no gas emissions dominated by CH4, whose maximum contents are those in the Hadramawt samples, although never ex-ceeding 7000 µmol/mol.

Figure 4. δ18O-δD diagram for the samples from Yemen


In the same Figure 6 a more meaningful Ar-He(x10)-N2(/100) triangular diagram (Giggen-bach et al., 1983) is shown where: i) pure air (N2/Ar=83), ii) exolution from air-saturated wa-ters (ASW) at 20 °C (N2/Ar=38), iii) gases from andesitic-type magma chambers (N2/Ar=800) and iv) gases typically generated in the crust, characterized by a progressive enrichment of radiogenic helium-4 with time, up to the He cor-ner, are reported. Setting aside samples from Hajjah and Sana’a, both located along the Great West Escarpment and characterized by a slight “N2-excess” of likely organic origin, all the re-maining gases are confined in the gray crustal trending triangle delimited by the position of air,

ASW and the pure crust end-members.The δ13C values of CO2 (in ‰ vs. PDB) suggest that in those gases seeping inside the YTP, CO2 has a prevalent mantle-related value, their δ13C values being between -4.0 to -8.0 ‰. On the other side, CO2 from the South and from the Hadramaut has a lighter isotopic signature (-10 <δ13C< -15 ‰). The light isotopic composition of C-CO2 in N2-rich gas samples well agrees with their relatively higher concentration in CH4 pointing, once again, to a shallow organic source of both CH4 (and light hydrocarbons) and CO2 in gases outside the YTP.The different helium isotopic ratios, from 0.03 to 3.21 R/Ra (where R is the 3He/4He ratio measured in the sample and Ra is that of the

Figure 5. δ18O -elevation diagram for the samples from Yemen


air, 1.39x10-6) can be related to a two-member mixing process. A crustal source region is re-lated to the N2-rich Anchab gas, seeping out in the central basement block and characterized by a low crustal ratio (R=0.025) with only 0.4% of juvenile helium. The second end-member, whose contribution varies from 12 to 40 %, is related to a deep source (mantle) is present in all the remaining gases from Yemen, Damt (R/Ra=3.2) and Al Rawdhah (R/Ra=2.57) in Had-ramaut being the sites where the highest R/Ra ratios were measured.

Origin of components andgas-water-rock interactionin Yemen

The oxygen and hydrogen isotopic composi-tion indicates that the natural Yemen water discharges have a meteoric origin, indepen-dently by their outlet temperature and salinity. The Langelier-Ludwig classificative diagram of Figure 2 suggests the usual Na-Cl ocean trend of deep circulating waters in silicate rocks only for the thermal springs NNW of Hajjah, SSW of Ta’iz and a few of the South, and an evaporite-ocean type (Na(Ca)-Cl(SO4)) mixed trend for those discharging from Hadramawt and some of those bordering the Great Escarpment Fault. The Sana’a, Ibb and Dhamar-Damt thermal samples, all located in the central part of the YTP and all seeping out from relatively high al-titudes, have a clear increase of the Na-HCO3 component.

Figure 6. Double CO2-CH4-N2 and Ar-He(x10)-N2(/100) diagrams for the gas samples from Yemen


By considering the δ13C-CO2 values of CO2-rich gas samples Al Makhaya, Annamajah and Damt (-6.3, -7.0 and -3.8 ‰ V-PDB, respec-tively), an inorganic origin from the mantle is suggested. Although these values are indeed not typical of δ13C values for CO2 deriving from limestone thermo-metamorphism (δ13C val-ues clustering around 0 ‰ PDB), they could represent the mixing of three end-members, as observed in other tectonically active ar-eas (Minissale et al., 1997) where deep CO2 (mantle) mixes with a crustal component (CO2 deriving from alteration of the Amran limestone sequence) and a shallower soil-originated CO2 (δ13C <-20) at which an atmospheric CO2 (δ

13C = -7.0) can eventually also be added. On the contrary, CO2 from the South and Hadramaut has a clear shallower organic signature (δ13C < -11 ‰ V-PDB).

From a regional point of view, uprising of deep CO2 can also be envisaged by calculating the partial pressure of CO2 (pCO2) in the thermal springs whose iso-distribution map in thermal spring and well waters is shown in Figure 7. A large anomalous area in the YTP is recognized, likely due to a strong contribution of deep CO2 in the volcanic plateau centered at Al Lisi, Al Makhaya and Damt with respect to other sedi-mentary or metamorphic areas of Yemen. The deep origin of CO2 is also confirmed by the relatively high 3He/4He ratio measured in the Damt gas vent (R/Ra = 3.2). Relative high R/Ra ratios were also measured in gas samples outside the YTP, such as the N2-rich gas phase associated to the thermal springs of Kirsh (R/Ra = 1.21) and at Al Rawdhah (R/Ra = 2.57) in the far east of Yemen (Mukalla area). The uprising of deep CO2 into the thermal

Figure 7. Isodistribution map of calculated pCO2 (-log(pCO2)) in the thermal springs of Yemen


aquifer(s) hosted inside the YTP and its conse-quent acidic solution affects the final pH of ther-mal aquifers, by contrasting the natural basic evolution of pH during mineral water-rock inter-action due to the typical alkaline hydrolysis of silicates. The top left pH-pCO2 diagram of Fig-ure 8 shows this effect, more pronounced in the high pCO2 Dhamar and Damt samples, which have, in most cases, an associated CO2-rich gas phase (pCO2>0). Several thermal discharges are saturated in calcite [SI(saturation index)>0, where SI=log(IAP(calcite)/KT(calcite))], being SI(calcite) inversely proportional to the pCO2 of the solution (top right of Figure 8), but mostly

directly related on the pH of the solution (bot-tom left of Figure 8). The HCO3- and CO3

2- con-centration in solution strictly depends on the pH and, therefore, SI(calcite) is proportional to pH and pCO2 if the concentration of HCO3- ions is due to high pCO2 in solution. The correlation between HCO3- and pCO2 values (bottom right of Figure 8) suggests that only a few samples from the central metamorphic basement block have HCO3- deriving from other sources to ob-tain more insights on how the CO2 flux is re-lated to the emergence temperature of springs, a pCO2-temperature correlation diagram is shown in Figure 9. All thermal springs from Ye-

Figure 8. Multiple pH-pCO2 (top left), pCO2-S.I.(calcite) (top right), pH-S.I.(calcite) (bottom left), HCO3-pCO2 (bot tom right) diagrams for the spring samples of Yemen (see text)


men lie inside a triangle where, on the left side, relatively low temperature springs are located, though affected by progressive CO2 inflow. They have undergone cooling during rising to the surface by: i) conduction with the forma-tions hosting the relative aquifers and/or ii) be-cause of mixing with colder shallower aquifers perching into the thermal rising ducts. By con-trast, on the top right-hand side of the diagram are positioned the two steam condensates from Al Lisi, where the gas flux was too low to be sampled (but certainly have a pCO2>0), and the boiling CO2-bubbling springs emerg-ing at Al Makhaya. These areas, comparing with all other thermal emergences of Yemen,

must be considered the less affected by both shallow thermal re-equilibration in liquid phase and mixing processes. Although not as high as in the Damt gases, the helium isotopic ra-tio at Al Makhaya (R/Ra=0.93) still accounts for the presence of a mantle component (R>0.2; O’Nions and Oxburg, 1988) in the gas phase of 10-15 %. Several springs aligned along a straight line delimited by springs at almost boil-ing temperature and high pCO2 and spring from the same Dhamar area (Figure 9).

GEOTHERMOMETRY

The application of geothermometric techniques

Figure 9. Binary diagram of calculated pCO2 of thermal springs versus emergence tem-peratures of springs


in liquid and gas phases is always a difficult task, especially when performed on a country scale. Liquid phase geothermometers are com-monly divided into i) those that re-equilibrate relatively fast at decreasing temperature during fluid uprising toward surface, such as the silica geothermometers (Fournier and Rowe 1966), and ii) those that record the deep parameters also at lower temperature, such as the Na/K gothermometer in its various formulations (see the review of Fournier, 1991). The technique proposed by Giggenbach (1986; 1988), that combines in a single Na-K-Mg ternary diagram the graphical representation of a fast re-equili-brating (K/Mg0.5) geothermometer (Giggenbach

et al., 1983) and the widely used Na/K geother-mometer in the version proposed by Giggen-bach (1991) has been applied to the Yemen water samples. This ternary diagram (Figure 10) is based on the fact that fluids circulating in hydrothermal systems interact with primary minerals (e.g. plagioclases or pyroxenes) that are altered to secondary minerals, such as muscovite, chlorite…etc. The relative Mg2+, Na+ and K+ concentrations (as mg/L) in the hydrothermally-derived solu-tions after water-rock interaction, are fixed by the thermodynamic parameters, mainly tem-perature, of alteration reactions occurring with-in the hydrothermal system. This is the mean-

Figure 10. Ternary K/100-Na/1000-Mg0.5 diagram (after Giggenbach, 1988) for the samples investigated (see text)


ing of the “full equilibrium” line shown in the Na/1000-K/100-Mg0.5 diagram (Figure 10). In the same figure the “immature waters” line rep-resents the trend of natural solutions at shal-low conditions that, at increasing time, after having initially solubilized preferentially Mg2+ from the more easily alterable mafic minerals, will increase the concentration in Na+ and K+ along the ocean Na/K ratio. The area named “partial equilibrium-dilution” refers to intermedi-ate conditions between simple rock dissolution at low temperatures and hydrothermal condi-tions at higher temperatures. Figure 10 shows that most thermal water samples in Yemen lies near the Mg2+ corner, half way between the full equilibrium and the immature waters lines. K/Mg temperatures (bottom axis) are always be-low 130 °C, whereas the Na/K geothermometer (left side) range 170-230 °C.In spite of the high deep temperature suggest-ed by the K/Na geothermometer, all samples are clearly positioned far from any certain hy-drothermal equilibrium. The position of sam-ples closer to the right ascending branch of the full equilibration curve suggests that re-equil-ibration and/or dilution play a relevant role in the final cationic composition of most thermal springs. For this reason K/Mg temperature (al-ways lower than 130 °C) must be considered more realistic. Similarly, it is possible to inter-pret the isotopic composition of oxygen-18 in the δD-δ18O diagram of Figure 4 that points to the absence of any real high-temperature shift for oxygen, indirectly suggesting again to a prevalence of shallow components with re-spect to the uprising of hydrothermal 18O-shift-ed solutions. The position in Figure 10 of the Al Lisi condensate along the immature waters line suggests that steam derives from secondary boiling of shallow water. This, indirectly, points to a very high local thermal gradient able to va-porize even shallow groundwater.Gas phase geothermometers are generally less affected by re-equilibration and/or mixing, if the

source is not too far from the surface. More-over, typical components of the atmosphere, such as Ar and Ne, are common shallow con-taminants of deep rising gases and they can be used as reference components, to evaluate the meaning of the more typical hydrothermal geo-indicator gases, such as H2, CO2 and CH4. In Figure 11 all gas samples have been plotted in a log(CH4/CO2)-log(H2/Ar) diagram (Giggen-bach, 1991), where equilibrium lines for: i) gas compositions buffered by the low temperature calcite-anhydrite pair and ii) gas compositions buffered by the Fe(II)O-Fe(III)O1.5 pair in liquid and vapor phases (Giggenbach, 1987), in a typical hydrothermal environment, are report-ed in the range 100-400 °C. The figure shows that gas compositions from hydrothermal sys-tems located between the full equilibrium line for liquid-dominated, two phases and vapor-dominated 200-400 °C systems, are complete-ly missing in Yemen. As already observed for liquid phases (Figure 10), all gas compositions plotted in the diagram seem to be far from any deep equilibrated high-temperature conditions. Nevertheless, two different groups of gases can be recognized. The first one, on the top left side of the diagram, characterized by rela-tive higher CH4 content and lower H2, includes samples from the South of Yemen and the east-ern Hadramaut region. The second one, with relatively lower CH4 and higher H2, pertains to those gases emerging from the YTP (Dhamar, Ibb, Damt). Although quite re-equilibrated at relative lower temperature, the second group suggests relative higher deep temperature of the source in the range 150-200 °C. Being the relative thickness of the volcanic YTP of at least 2000-3000 m and considering that at Al Makhaya and Al Lisi there are boiling mud-pools and fumaroles, respectively, we might consider the base of the volcanic products as the main reservoir source of rising fluids inside the YTP, at likely temperature of 150-200 °C. Considering that mixing and re-equilibration af-


fects the chemical composition of both gas and liquid phases, such a guessed temperature has to be considered as the minimum temperature existing at that depth.

CONCLUSIONS

The chemical-physical features of the many thermal springs, gas vents and fumaroles in Yemen, as well the large number of emergenc-es suggest an active tectonics that, according to the mantle 3He signature at several places, is present not only in the volcanic YTP and sur-rounding areas, but also spread in other regions,

e.g. in the Hadramaut region. This is in line with the presence of active basaltic volcanism at several places inside the YTP (Dhamar, Al Lisi, Damt, Harra of Arhab, Jabal el-Marha) as well as along the Aden coast ((Harra es-Sawâd or Shuqra, Bir Ali, and elsewhere in Hadramaut and Al Mahra, to the far east) and the occur-rences of magmas residing at shallow depth.There is an anomalous regional CO2 flux inside the YTP likely coinciding with the most thermally anomalous Quaternary volcanic areas around the town of Dhamar. Such anomalous flow, al-ready delimited by the calculated pCO2 iso-dis-tribution map of the thermal springs of Yemen

Figure 11. Geothermometric interpretation of gases from Yemen using the(CH4/CO2)- log(H2/Ar) diagram (after Giggenbach, 1991)


(Figure 7), is confirmed by other solutes, such as fluorides. In Figure12 the F- iso-distribution map in the thermal springs of Yemen is shown. By considering that fluoride (like CO2) can be of magmatic-hydrothermal origin and is one important component of volcanic fumaroles, its evident anomaly centered in the Dhamar-Damt areas, strongly support that traces of a residual volcanic activity are present in the area. As a matter of fact high F- concentrations are report-ed in waters from other areas of the African-Arabian triple junction (e.g. Gizaw, 1996).According to this further evidence, besides of the measured values of the 3He/4He and 13C/12C-CO2 isotopic ratios, the areas of Al Lisi, Al Makhaya and Damt can be considered of very high geothermal potentiality, and this in spite of the fact that such hypothesis is scarce-ly supported by geothermometric calculation, possibly because fluids circulating at depth do not attain a thermodynamic equilibrium and/or are re-equilibrated at lower temperature during

their uprising. As a further possible economic resource, shallow explorative wells at Damt could be drilled to verify the possibility to pro-duce and sell CO2 for the Yemen industry.

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