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Geochemical and Isotopic Characterization of Volcanic and Geothermal Fluids Discharged from the Ecuadorian Volcanic

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Geochemical and Isotopic Characterization of Volcanic and Geothermal Fluids Discharged from the Ecuadorian Volcanic Geofluids (2010) doi: 10.1111/j.1468-8123.2010.00315.x Geochemical and isotopic characterization of volcanic and geothermal fluids discharged from the Ecuadorian volcanic arc S. INGUAGGIATO1 ,S.HIDALGO2 ,B.BEATE3 AND J. BOURQUIN2 1Istituto Nazionale di Geofisica e Vulcanologia – Sezione di Palermo, Via Ugo La Malfa, Palermo Italy; 2Instituto Geofı´sico – Escuela Polite´cnica Nacional, Apartado, Quito, Ecuador; 3Facultad de Geologı´a – Escuela Polite´cnica Nacional, Quito, Ecuador ABSTRACT The Ecuadorian Quaternary volcanic arc is characterized by about 60 volcanoes many of which are active or potentially active. This volcanic activity is the result of the subduction processes of the Nazca Plate beneath the north-western part of South America. The geochemical signature of the discharged fluids from these vol- canic systems gives an important contribution to the comprehension of the subduction processes in the South-American region. In this work, we present the first systematic geochemical characterization of dis- charged fluids from the entire Ecuadorian volcanic arc on the basis of the chemical and isotopic composition of 56 samples of thermal and cold waters, as well as 32 dissolved and 27 bubbling gases collected from north to south across the arc. The isotopic composition of waters reveals a mainly meteoric origin, while the chemistry of the dissolved gases is characterized by He and CO2 contents, 2–3 orders of magnitude higher than the air saturated water values, which implies very active gas–water interaction processes with deep flu- ids. Moreover, both dissolved and bubbling gases’ isotopic signature shows a wide compositional range, with 13 helium ranging between 0.1 and 7.12 R ⁄ Ra and carbon ranging from )1.75 to )10.50& d C(TDIC). Such iso- topic features may be related to the presence of at least two distinct end-member sources: the mantle and the crust. Finally, this geochemical study clearly reveals the two distinct geographic parts of the arc, showing different isotopic characteristics of fluids for the Quaternary active volcanism, (north of 2°S), and for the inac- tive arc, (south of 2°S). Key words: carbon isotope, ecuador volcanoes, helium isotope, nitrogen isotope, thermal waters Received 12 April 2010; accepted 7 September 2010 Corresponding author: Salvatore Inguaggiato, Istituto Nazionale di Geofisica e Vulcanologia – Sezione di Palermo, Via Ugo La Malfa, 153 – 90146 Palermo, Italy. Email: [email protected]. Tel: +39 091 680 9435. Fax: +39 091 680 9449. Geofluids (2010) Moreover, the collection of fumarolic gases in the summit INTRODUCTION is difficult and potentially unsafe considering the dimen- The Ecuadorian Quaternary volcanic arc is characterized by sions of the volcanic edifices, their high altitudes at least 60 volcanoes (Hall & Beate 1991). At least 10 of (>4500 masl) and their potential explosive activity. these volcanoes experienced Holocene eruptions, indicating On the other hand, the Ecuadorian volcanic arc repre- that they are potentially active; four of them, i.e. Pichincha, sents a great potential for harnessing geothermal energy in Tungurahua, Sangay and Reventador, are currently erupt- this country, although appraisal of all geothermal prospects ing or have erupted during the last 15 years. Interestingly, is still at an early stage. most of these Quaternary volcanic edifices display associated In this framework, several geothermal areas along the hydrothermal systems; fluids related to these hydrothermal Ecuadorian Andes have been investigated to provide a pre- sources are the only surface manifestation that can be easily liminary geochemical characterization of the fluids, to dis- accessed to provide information about the volcanic activity. criminate the hydrothermal reservoirs and to identify the Ó 2010 Blackwell Publishing Ltd 2 S. INGUAGGIATO et al. (A) (B) 85°W 80° 75° 5°N 1 Cerro Negro 78°00'W de Mayasquer 100 km 2 Chiles 1°00'N 50 km 3 Chalpatán GSC 150 km 4 Potrerillos 1 2 5 Chiltazón Tu 100 km 6 Soche 7 Iguán 5 4 3 8 Azufral 7 6 5–7 cm per year 9 Mangus 10 8 Fig. 1b Colombia 10 Pilavo 11 11 Yanaurcu 12 13 9 0° 12 Huanguillaro Ib 13 Pulumbura 14 16 14 Cotacachi Carnegie 15 Cuicocha 15 17 Ecuador 19 Ridge 16 Imbabura 18 17 Cubilche 23 21 18 Cusín 20 24 22 19 Cushnirumi 0°00' 20 Mojanda- 28 25 Fuya Fuya 29 26 Peru 21 Cayambe Q 30 Grijalva F.Z. 22 Reventador 23 Pululahua 27 31 5°S 24 Casitagua 32 33 34 37 36 35 (C) 38 25 Pambamarca Colombia 41 26 Puntas Pacific N Ocean IV 39 27 Chacana 40 42 28 R. Pichincha 0° 44 29 G. Pichincha Quito La 43 30 Ilaló 45 31 Pan de Azúcar WC 1°00' Te 32 Atacazo Amazonian 46 33 Pasochoa Pacific Basin 47 34 Antisana Coast 35 Sumaco 36 Sincholagua 2°S Guayaquil EC 48 Am 37 Corazón 49 50 38 Rumiñahui 39 Santa Cruz 52 51 Pu 40 Illiniza 41 Cotopaxi Peru G 53 i 42 Chalupas 54 43 Quilindaña R ii 44 Quilotoa 4°S 45 Chinibano 46 Putzalagua 55 iii 47 Sagoatoa 150 km 48 Carihuairazo 2°00'S iv 49 Puñalica 50 Huisla 80° W 78° W 76° W v 51 Chimborazo 52 Tungurahua 53 Igualata Ma 54 Altar 79°00' 55 Sangay Fig. 1. (A) Geodynamical setting of the Ecuadorian Arc (modified from Gutscher et al., 1999). (B) Ecuadorian volcanoes distribution map from (Hall & Beate 1991). Numbers indicate the different volcanic edifices; i = Quaternary volcanoes, ii = proximal deposits, iii = distal deposits, iv = caldera rim, v = tectonic alignments. (C) Schematic map of the geomorphological ⁄ geological zones of Ecuador (modified from Aspden et al. 1992a,b). WC = Western Cordillera, EC = Eastern Cordillera and IV = Interandean Valley. isotopic signature of volcanic and geothermal fluids (mainly feature of this subduction system is the presence of the He, C, N). Carnegie Ridge, which is the product of the uninter- rupted interaction of the Gala´pagos hot spot and the Cocos-Nazca Spreading Centre (Sallare`s and Charvis, GEODYNAMIC CONTEXT 2003) (Fig. 1A). In Ecuador, magmatism results from the subduction of The subducting Carnegie Ridge is covered by a 400– the Nazca Plate (>22 My in age; Lonsdale 1978; Lons- 500 m thick sedimentary blanket consisting of carbonate dale & Klitgord 1978) beneath the North Andean Block, sediments (Michaud et al. 2005). Sediments facing the an independent block located in the north-western part northern Ecuadorian coast line are composed by siliceous of South America (Pennington 1981; Kellogg & Vega nanofossil ooze, chalk and limestone (Hein & Yeh 1983). 1995; Witt et al. 2006). The average rate of convergence The Ecuadorian Quaternary volcanic arc is limited to the ) is around 58 mm year 1 with an almost E–W direction south at 2°S and comprises at least 60 volcanic edifices that (Trenkamp et al. 2002). An interesting geomorphological are distributed in three different domains following a Ó 2010 Blackwell Publishing Ltd Geochemistry of Ecuadorian volcanic fluids 3 roughly NNE trench orientation: (i) The Volcanic Front, Bubbling gases were sampled using stopcock bottles and where the volcanoes are built over the Western Cordillera for- Giggenbach bottles (filled with NaOH 4 M and pre-evacu- mations, (ii) the Main Arc, which includes the Interandean ated in laboratory). Gas samples were analysed for the Valley volcanoes and the Eastern Cordillera volcanoes and chemical and isotopic composition (He, C and N). (iii) the Back Arc volcanoes emplaced in the headwaters of Dissolved gases were sampled and analysed according to the Amazonian basin (Fig. 1B). the method described by Capasso & Inguaggiato (1998), The nature and age of the basement of these three volca- which is based on the equilibrium partition of gas species nic domains are very different, changing from oceanic bas- between a liquid and a gas phase after the introduction of alts, dioritic intrusions and volcano-clastic deposits below a host gas (Ar) into the sample. Dissolved gases were the Volcanic Front (Goosens & Rose 1973; Lebrat et al. analysed using a Perkin Elmer 8500 gas-chromatograph 1987; Cosma et al. 1998; Reynaud et al. 1999; Lapierre equipped with a 4-m-long Carbosieve S II column and Ar et al. 2000; Hughes & Pilatasig 2002; Luzieux et al. 2006) as the carrier gas. He, H2,O2,N2 and CO2 were measured to older and geochemically more mature continental forma- by means of a TCD detector, while CH4 and CO were tions consisting of metasedimentary, igneous and metamor- determined through a FID detector coupled with a meth- phic rocks under the Main Arc (Aspden & Litherland 1992; anizer. Analyses of the dissolved He isotopic composition Aspden et al. 1992b; Litherland et al. 1994) (Fig. 1C). were performed using the methodology proposed by South of 2°S, there is no active volcanism, but there are Inguaggiato & Rizzo (2004). impressive remnants of the Miocene volcanic activity, which The determination of the helium isotopic composition are mainly characterized by andesitic to rhyolitic products was carried out on a static vacuum mass spectrometer (GVI- cropping out as ignimbrites, lava flows and lava domes (La- Helix SFT) built for the simultaneous detection of 3He and venu et al. 1992; Beate et al. 2001). This old and highly 4He ion beam, to reduce the analytical error down to very 3 4 eroded volcanic arc, known as the Saraguro arc, is the host low values (an average of ±0.05 Ra). The He ⁄ He ratios to several porphyry and epithermal ore deposits. have been corrected for the atmospheric contamination on the basis of their 4He ⁄ 20Ne ratios (Sano & Wakita 1985). Values are reported as R ⁄ Ra values (where Ra is equal to SAMPLING AND ANALYTICAL ) 1.39 · 10 6). The d13C of total dissolved inorganic carbon METHODOLOGIES 18 (TDIC) and the d OofH2O of spring waters were analy- On the basis of previous knowledge (Beate & Salgado sed by a Finnigan Delta Plus mass spectrometer.
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