Elsevier

Geothermics

Volume 49, January 2014, Pages 99-110
Geothermics

Stable isotopes of hydrothermal minerals as tracers for geothermal fluids in Iceland

https://doi.org/10.1016/j.geothermics.2013.05.005Get rights and content

Highlights

Abstract

The Reykjanes and Krafla geothermal systems, located within the active rift zone of Iceland, are both potential venues for exploitation of deep supercritical fluids by the Iceland Deep Drilling Project (IDDP). An essential aspect of properly characterizing geochemical and hydrologic processes occurring at supercritical depths is establishing the source, composition and evolution of geothermal fluids. Traditionally, hydrogen isotopes of thermal fluids are used to determine their source. We show that for these, and likely many other Icelandic geothermal systems, analyzing fluid dD is not sufficient alone. Rather, d18O and dD of hydrothermal minerals in conjunction with geochemical characteristics of extant geothermal fluids are necessary to characterize the source and geologic evolution of geothermal reservoir fluids. Here we review results from existing drill holes in the Reykjanes and Krafla geothermal systems to depths of =3 km, and explore the utility of using stable isotopes in alteration minerals such as epidote to assess the hydrogeology of extinct volcano-hydrothermal systems by presenting new data from the Geitafell fossil hydrothermal system in southeast 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

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