Available online at www.sciencedirect.com
Procedia Earth and Planetary Science 7 (2013) 822 – 825
Water Rock Interaction [WRI 14]
The Theistareykir geothermal field, NE Iceland. Isotopic characteristics and origin of circulating fluids Á.E. Sveinbjornsdottira, H. Ármannssonb, M. Ólafssonb, F. Óskarssonb, S. Markússonc, S. Magnusdottira* a
Institute of Earth Sciences, University of Iceland. Sturlugata 7, IS-101 Reykjavik, Iceland b Iceland GeoSurvey (ÍSOR), Grensásvegur 9, IS-108 Reykjavik, Iceland c National Power Compnay, Háaleitisbraut 68, IS-103 Reykjavik, Iceland
Abstract The Theistareykir high temperature field in NE Iceland seems to be complex in terms of both inflow and structure, as reflected in the division of the area into several subfields. Oxygen and hydrogen isotopes in water and steam condensate from wells are reported. Some differences can be seen between the Theistareykir well fields, but the recharge is in all cases non-local in origin. The isotopic composition of some of the thermal waters is anomalously depleted in 2H, by about 35‰, compared to precipitation anywhere in Iceland today. The isotopes therefore suggest that the thermal water contains a component of past precipitation under a colder climate. The oxygen shift due to water-rock interaction is up to 6.5 ‰. The isotopic signature of the Theistareykir thermal water is compared to that from nearby low temperature fields within the westernmost part of the Northern Neovolcanic Zone. © 2013 Published by Elsevier B.V. Open © 2012The TheAuthors. Authors. Published by Elsevier B.V.access under CC BY-NC-ND license. Selection and/or peer-review under responsibility of the Organizing and Scientific Committee of WRI 14of– 2013 Selection and/or peer-review under responsibility of Organizing and Scientific Committee WRI 14 - 2013 Keywords: Theistareykir; Iceland; geothermal field; stable water isotopes; origin of fluid; Pre-Holocene component; oxygen shift.
1. Introduction The Northern Volcanic Zone of Iceland extends from the centre of Iceland to the Öxarfjördur Bay in the north. It consists of five NNE striking left-stepping en echelon volcanic systems. The 70-80 km long and 7-8 km wide Theistareykir fissure swarm is the westernmost (Figure 1a). It is characterized by large normal faults (N22°E) with maximum displacement of 200-300 meters and rift fissures. To the north, the fault and fissure systems meet the NW strike slip Húsavík-Flatey Fault System (HFF) [1]. The high
* Corresponding author. Tel.: +354-525-4800; fax: +354-552-8911. E-mail address:
[email protected].
1878-5220 © 2013 The Authors. Published by Elsevier B.V. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibility of the Organizing and Scientific Committee of WRI 14 – 2013 doi:10.1016/j.proeps.2013.03.171
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temperature geothermal activity is connected to recent magma intrusions. The most recent volcanic activity in the area (Theistareykir lava) occurred some 2500 years ago [2].
Fig. 1. (a) Location of the Theistrareykir high temperature area is shown by the Theistareykir-square. The fissure swarm within the Theistareykir volcanic system and the Húsavík-Flatey Fault System (HFF) are also shown as well as the locations of the low temperature areas at Hafralækur, Laugar, Hveravellir and Húsavík; (b) Locations and directions of drillholes within the Theistareykir-square in Figure 1a. The division of the area into 5 sub-fields (A, B, C, D, E) are also shown.
In the years 2002 to 2008 seven deep wells were drilled into the Theistareykir field, with depths ranging from 1723m to 2799m. In the autumn of 2011 two additional deep wells were drilled into the area. The maximum rock temperature in 5 of the wells exceeded 300°C, with a maximum temperature of 380°C. However, geochemical thermometers show lower temperatures, generally in the range 270– 300°C, which seems to reflect the measured temperatures of the major feed zones. The Theistareykir thermal area has been divided into 5 subareas (A, B, C, D, E) (Figure 1b) on the basis of geology and geochemistry of fumaroles [3]. Isotopic composition of fumaroles confirmed this division [4]. Wells 1, 4, 5 and 5B are drilled from the same pad located in area C. Well 1 is drilled horizontally into the Theistareykjagrundir field, wells 5 and 5B into area D (Tjarnarás field (fault)) and well 4 to the south into the Bæjarfjall area. Well 2 is located in area D (Tjarnarás) and drilled horizontally. Wells 3, 6 and 7 are drilled from the same pad, within area A (Ketilfjall). Well 3 is drilled horizontally, well 6 is drilled to the WNW into the northern part of area C and well 7 is drilled to the NE, as shown in Figure 1b. 2. Isotopic characteristics 2.1 H and O isotopes The isotopic composition of the thermal fluids in Theistareykir, both for the water and steam condensate are shown in Figure 2a. The discharges from wells 1, 4 and 5B have been relatively stable with time whereas the samples from the other pad vary considerably as the first samples from the wells still reflect a component of the drilling fluid and it is only the most recent samples that reflect the true deep thermal
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fluid. Shallow groundwater in the Theistareykir area used as drilling fluid has been estimated at -85‰ and -12‰ in δ2H and δ18O respectively [4]. The deep fluid discharges as calculated from the steam and water isotopic composition in accord with the steam/water ratio of the sample at the well head are given in Figure 2b. The calculations are based on measured discharge enthalpy and the reservoir temperature as estimated by geothermometry. The wells discharging from area D (Tjarnarás; wells 2, 5 and 5B) give the most 2H enriched isotopic composition (103 to -105‰) and about 2‰ oxygen shift. Bæjarfjall and the southern part of area C (wells 1 and 4) are slightly more depleted with discharging deep fluids with 2H -112 and -108‰ respectively. The hydrogen isotope ratio for well 7 is -126‰ and the oxygen shift is 6.5‰. The most depleted fluids are observed in area A (Ketilfjall) (-134.8‰, well 3) and the northern part of area C (Theistareykjagrundir) (-141‰, well 6). Oxygen shifts for these waters are 4.5‰ and 5.5‰ respectively. On Figure 2b the isotopic composition of thermal fluids from nearby low geothermal fields within the westernmost part of the Northern Neovolcanic Zone (Hafralækur, Laugar, Hveravellir) are given as well as for Húsavík to the NW of Theistareykir. The locations of these areas are shown on Figure 1a. The thermal waters from the areas are all depleted in 2H and 18O isotopes compared to local precipitation. The δ2H in the water from Hafralækur has been measured as depleted as -142.6‰ [6], i.e. very similar to the deep discharge from the northern part of area C. This is more depleted than any precipitation in Iceland under the present climate regime and therefore suggests a component of past precipitation from a colder climate. δ18O in the thermal waters to the west of Theistareykir (Hafralækur, Laugar, Hveravellir) are not affected by water-rock interaction like the Theistareykir thermal water and the waters within the Húsavík field to the NW.
Fig. 2. (a) Isotopic composition of water (closed symbols) and steam (open symbols) for the Theistareykir wells. The local groundwater and the WMWL [5] are also shown; (b) Isotopic composition of the total discharge of most recent samples from the wells. The fluids from Hafralækur, Laugar, Hveravellir, and Húsavík are shown for comparison.
2.2 Origin of fluid and conceptual model of groundwater flow Earlier studies from fumaroles indicated a deep thermal inflow of -100‰ and -12‰ for δ2H and δ18O respectively. This was interpreted to mean that the thermal water either originated from a long distance away in the ice-cap in south-central Iceland or to be more local remnants of past precipitation under a colder climate, which was thought less likely [4]. The present results show that the deep thermal inflow is more depleted, ranging from about -105‰ to about -140‰ in δ2H. The most depleted precipitation in Iceland under the present climate regime is from the northern part of the ice cap in central Iceland, -106‰ [6]. According to Arnason (1976) [6] the annual mean temperature decreases by 1°C if δ2H decreases by
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about 6‰. Therefore the most depleted thermal water at Theistareykir suggests a cooling of at least 7°C, far more than the last 12000 years, the Holocene, has experienced. The Theistareykir thermal water or at least part of it must therefore originate as past precipitation from the last glaciation. With reference to the isotopic results it is suggested that area A and the northern part of area C are closest to the source of the deep inflow into the Theistareykir high temperature field. The fluid probably originates from the south of the area and has a component of pre-Holocene precipitation. Considerable oxygen shift is observed for the deep undisturbed flow. The discharges of wells within C south (Theistareykjagrundir south), D (Tjarnarás) and Bæjarás are more mixed with local and more recent precipitation. The fluid from well 7 is distinctly different from that of the other wells both in chemistry and oxygen isotopic composition [7]. Further sampling is needed to fully understand how it fits into the conceptual model of groundwater flow in the area. It is suggested that the origin of the waters in Hafralækur, Laugar and Hveravellir is the same as that of the deep inflow into Theistareykir, though the waters from Laugar and Hveravellir are more mixed with local and more recent precipitation. These waters however have not experienced high enough temperatures to result in an oxygen shift. Though the temperature is not higher within the Húsavík thermal field (max 94°C) the most depleted water there shows an oxygen shift of about 3‰. It is therefore suggested that this water may in part be effluent water from the Theistareykir field.
Acknowledgement Many thanks to Gunnlaugur M. Einarsson who helped generating Figure 1b and Asta Rut Hjartardottir for allowing us to use tectonic data on the southern part of the Theystareykir fissure swarm in Figure 1a.
References [1] Magnúsdóttir S, Brandsdóttir B.Tectonics of the Theistareykir fissure swarm. Jökull 2011; 61: 65-79. [2] Saemundsson K. The geology of Theistareykir (In Icelandic). IcelandGeoSurvey report, ÍSOR-07270 2007; 23 pp. [3] Ármannsson H, Gíslason G, Torfason H. Surface exploration of the Theistareykir high-temperature geothermal area, Iceland, with special reference to the application of geochemical methods. Appl Geochem 1986; 1: 47-64. [4] Darling G, Ármannsson H. Stable isotope aspects of fluid flow in the Krafla, Námafjall and Theistareykir geothermal systems of northeast Iceland. Chem Geol 1989; 76: 197-213. [5] Craig H. Isotopic variations in meteoric waters. Science 1961; 133: 1702-1703. [6] Árnason B. Groundwater systems in Iceland traced by deuterium. Societas Scientiarium Islandica 1976; 42: 236pp. [7] Óskarsson F, Ármannsson H, Ólafsson M, Sveinbjornsdottir ÁE, Markússon S. The Theistareykir geothermal field, NE-Iceland. Fluid chemistry and production properties. Proceeding WRI-14. Water-Rock Interaction 2013; this volume.
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