Stable isotopes of hydrothermal minerals as tracers for geothermal fluids in Iceland
Introduction
The neo-volcanic zone of Iceland is a unique natural laboratory for studying fluid-rock interaction in magma-hydrothermal systems. There are more than twenty high-temperature (>200 °C) geothermal systems associated with central volcanoes and fissure swarms that provide an opportunity to study, in situ, geochemical interaction between basaltic rocks (magmatic products of the Iceland mantle plume and the sub-aerial extension of the Mid-Atlantic spreading ridge [MAR]) and aqueous electrolyte solutions at elevated temperatures (Arnórsson, 1978, Arnórsson, 1995). Critical for the application of these natural laboratories toward understanding chemical mass transfer in magma-hydrothermal systems is a well-characterized source of hydrothermal fluids. The extent of chemical alteration of both the geothermal fluid and surrounding rock can only be quantified when starting rock and fluid properties are known. Further, from an economic viewpoint, fluid source is critical to understanding recharge rates for a producing geothermal field, and potential hazards such as fluid acidity or scaling of pipes during production.
In Iceland, the hydrogen isotope ratio of geothermal fluids have traditionally been used as a tracer for fluid source, based on early studies (e.g. Craig et al., 1956) indicating that there was generally no discernable difference between dD of thermal fluids and that of local meteoric water. Árnason (1977) measured the dD of meteoric waters throughout Iceland, and used the resultant contour map of his analyses to interpolate the origin of thermal waters that did not have a dD similar to the local mean values of precipitation. This interpretation requires three major assumptions: (1) dDWATER remains constant with time for a given region, (2) the dD of groundwater does not change due to water-rock reactions as is observed in the d18O of groundwater and (3) input of non-meteoric fluids (such as seawater or magmatic fluids) is at most a few per cent and thus cannot significantly affect the dD of the meteorically-sourced thermal waters.
More recently, thorough investigation of two of the high-temperature geothermal regions in Iceland, facilitated through exploration activities of the Iceland Deep Drilling Project (IDDP), demonstrates that one or more of these assumptions may be invalid for any given geothermal reservoir. The IDDP, which intends to increase both the economic and scientific utility of geothermal systems within Iceland's neo-volcanic zone by sampling supercritical fluids at depths of 4–5 km has piloted two experimental deep drill holes in the Reykjanes and Krafla geothermal systems (Fig. 1; Elders and Fridleifsson, 2010, Fridleifsson and Elders, 2005). In Reykjanes, a pre-existing drill hole (RN-17) was extended to 3.1 km before collapse of the upper wall rock prevented further downward drilling. The Krafla geothermal system is the site of drill hole IDDP-1, which was drilled in Spring 2009. Before reaching supercritical reservoir conditions, IDDP-1 intercepted magma at a depth of 2104 m (Elders et al., 2011). Their selection as sites for the first two IDDP drill holes, and the continued interest in deep drilling in these areas, warranted a detailed analysis of the isotopic properties of alteration minerals formed through fluid-rock interaction in Reykjanes and Krafla in order to better assess the hydrogeologic character of these regions (Pope, 2011, Pope et al., 2009).
Hydrogen and oxygen isotope compositions of geothermal fluids in these systems have been monitored since the 1970s (Árnason, 1977, Arnórsson, 1978, Arnórsson, 1995, Darling and Ármannsson, 1989, Ólafsson and Riley, 1978) and their dD values do not show a straightforward relationship to either local meteoric fluids or to an obvious distal meteoric source. This is because the hydrologic properties of both systems deviate from the assumptions Árnason (1977) required for using dDFLUID as a tracer, but in different ways. Here we review and augment the isotopic results of Pope (2011) and Pope et al. (2009), and show that alone, deuterium may not be an effective independent tracer of geothermal fluids. However, by combining data on major and isotopic fluid chemistry with hydrogen and oxygen isotope properties of alteration minerals, we not only overcome uncertainties relating to fluid source of individual systems, but we can develop a spatially and temporally detailed record of variations in fluid source, composition and interaction with surrounding host rocks in discrete geothermal environments. Finally, we apply our improved understanding of what variables influence the isotopic record of fluid-mineral interaction preserved in hydrothermal epidote in active geothermal systems by presenting new isotope data from an extinct geothermal system located within the ca. 5–6 Ma old Geitafell central volcano (Fig. 1a), where direct fluid analyses are not possible.
Section snippets
Reykjanes
The Reykjanes geothermal system is located on the southwestern tip of the Reykjanes Peninsula, on the landward extension of the Mid-Atlantic Ridge (Fig. 1a). Like other high-temperature systems in Iceland, it is composed of highly fractured basalt lavas and hyaloclastites that have been intruded by shallow dikes and sills. Mafic intrusions are more abundant with increasing depth (Franzson et al., 2002, Kristmannsdóttir, 1983). Fracturing and faulting in the region is due to extension along a
Methods
The stability of epidote in hydrothermal systems is a sensitive function of temperature, permeability and fluid composition (Árnason and Bird, 1992). We can use the hydrogen and oxygen isotope compositions of hydrothermal epidote sampled from the drill cuttings of active geothermal wells as an indicator of the isotopic composition of fluids from which they precipitated, and thus determine how they may vary temporally or spatially. We characterize the isotopic properties of the geothermal fluids
Results and discussion
Oxygen and hydrogen isotope compositions of geothermal epidote and the hydrothermal fluid that would be in equilibrium with these epidotes (Epi-forming fluids) at present-day downhole temperatures are presented in Fig. 2 for the Reykjanes, Krafla and Geitafell geothermal systems (data is from Pope et al., 2009, Pope, 2011, and this study, respectively). These results are compared with the composition of seawater, meteoric water (along the meteoric water line; Craig, 1961), and modern local
Conclusions
The hydrogeologic characteristics of the Reykjanes and Krafla geothermal systems represent two extremes of Icelandic high-temperature geothermal systems. The Reykjanes geothermal system is a seawater-dominated system that has been active since the Pleistocene, and has undergone extensive hydrothermal alteration; enough so that modern fluids are chemically modified through diffusional exchange with relict hydrous alteration minerals. In contrast, the Krafla geothermal system occurs in an
Acknowledgements
This research was carried out in collaboration with the Iceland Deep Drilling Project, an international research consortium developing 5-km deep drill holes in Iceland's geothermal systems to investigate geologic processes in the supercritical region of actively spreading margins. NSF grant number NSF EAR 0506882 provided funding for this project to D.K. Bird. We also acknowledge funding provided by Stanford University School of Earth Sciences’ Allan C. Cox Professorship to S. Arnórsson, and by
References (66)
- et al.
Gas changes in the Krafla geothermal system, Iceland
Chemical Geology
(1989) Hydrothermal systems in Iceland traced by deuterium
Geothermics
(1977)Geothermal systems in Iceland: structure and conceptual models – I. High-temperature areas
Geothermics
(1995)- et al.
Processes controlling the distribution of boron and chlorine in natural waters in Iceland
Geochimica et Cosmochimica Acta
(1995) - et al.
The use of gas chemistry to evaluate boiling processes and initial steam fractions in geothermal reservoirs with an example from the Olkaria Field, Kenya
Geothermics
(1990) - et al.
A new technique for determining equilibrium hydrogen isotope fractionation factors using the ion microprobe: application to the epidote-water system
Geochimica et Cosmochimica Acta
(1999) - et al.
Stable isotopic aspects of fluid flow in the Krafla, Namafjall and Theistareykir geothermal systems of northeast Iceland
Chemical Geology
(1989) - et al.
The Iceland Deep Drilling Project: a search for deep unconventional geothermal resources
Geothermics
(2005) - et al.
18O-depleted rocks from the Tertiary complex of the Isle of Mull, Scotland
Earth and Planetary Science Letters
(1976) - et al.
Liquid–vapor fractionation of oxygen and hydrogen isotopes of water from the freezing to the critical temperature
Geochimica et Cosmochimica Acta
(1994)
Oxygen isotope geochemistry of the amphiboles: isotope effects of cation substitutions in minerals
Geochimica et Cosmochimica Acta
The closing of a seaway: ocean water masses and global climate change
Earth and Planetary Science Letters
Oxygen isotopic fractionation in the system quartz-albite-anorthite-water
Geochimica et Cosmochimica Acta
Oxygen isotope fractionation between zoisite and water
Geochimica et Cosmochimica Acta
Geochemical studies on the thermal brine from Reykjanes (Iceland)
Chemical Geology
Isotopic constraints on ice age fluids in active geothermal systems: Reykjanes, Iceland
Geochimica et Cosmochimica Acta
Description and interpretation of the composition of fluid and alteration mineralogy in the geothermal system at Svartsengi, Iceland
Geochimica et Cosmochimica Acta
A laser-based microanalytical method for in situ determination of oxygen isotope ratios of silicates and oxides
Geochimica et Cosmochimica Acta
A rapid method for determination of hydrogen and oxygen isotope ratios from water and hydrous minerals
Chemical Geology
Oxygen isotope activities and concentrations in aqueous salt solutions at elevated temperature: consequences for isotope geochemistry
Earth Planetary Science Letters
Isotopic evidence on environments of geothermal systems
Handbook of Environmental Isotope Geochemistry
Exploration and development of the Krafla geothermal area
Jökull
Formation of zoned epidote in hydrothermal systems
Major element chemistry of the geothermal sea-water at Reykjanes and Svartsengi, Iceland
Mineralogical Magazine
Fluid-fluid interaction in geothermal systems
Reviews in Mineralogy and Geochemistry
Silicic magma petrogenesis in Iceland by remelting of hydrothermally altered crust based on oxygen isotope diversity and disequilibria between zircon and magma with implications for MORB
Terra Nova
Geologic field studies of the Miki Fjord Area, East Greenland
Bulletin of the Geological Society of Denmark
Calc-silicate mineralization in active geothermal systems
Economic Geology
Epidote in geothermal systems
Reviews in Mineralogy and Geochemistry
Calculation of fractionation factors for carbon and oxygen isotope exchange in the system calcite-carbon dioxide-water
The Journal of Physical Chemistry
Imprint of meteoric water on the stable isotope compositions of igneous and secondary minerals, Kap Edvard Holm Complex, East Greenland
Contributions to Mineralogy and Petrology
Contrasting serpentinization processes in the eastern Central Alps
Contributions to Mineralogy and Petrology
Isotopic variations in meteoric waters
Science
Cited by (23)
A microanalytical oxygen isotopic and U-Th geochronologic investigation and modeling of rhyolite petrogenesis at the Krafla Central Volcano, Iceland
2021, Journal of Volcanology and Geothermal ResearchCitation Excerpt :The transition during drilling from the ~300 °C hydrothermal regime to ~930 °C occurred over ~30 m (Eichelberger, 2020). Triple oxygen isotope data suggest that this low δ18O (+3.1‰) rhyolite was formed predominantly by assimilation of partial melts from hydrothermally altered and isotopically depleted basaltic crust into intruding basaltic magma (Zakharov et al., 2019), with altering waters tracing back to the last glacial (Pope et al., 2014; Zakharov et al., 2019). This magma provides a rare glimpse into the active upper crustal petrogenetic factory (Eichelberger et al., 2020).
Impact of fluid-rock interaction on water uptake of the Icelandic crust: Implications for the hydration of the oceanic crust and the subducted water flux
2020, Earth and Planetary Science LettersCitation Excerpt :δD values of altered basalts from Krafla which is fed by meteoric water (e.g., Stefánsson et al., 2017) ranged from −125 to −96‰ (Fig. 1). In contrast, at Reykjanes where the geothermal system is fed by seawater (e.g., Pope et al., 2014), the corresponding δD values ranged from −80 to −59‰. At Surtsey, a geothermal system also fed by seawater, δD values of altered basalts were similar to those obtained from Reykjanes and ranged from −78 to −46‰.