Application of radium isotopes to determine crustal residence times of hydrothermal fluids from two sites on the Reykjanes Peninsula, Iceland

Abstract

Radium isotopes were used to determine the crustal residence times of hydrothermal fluids from two geothermal wells (Svartsengi and Reykjanes) from the Reykjanes Peninsula, Iceland. The availability of rock samples from the subsurface (to depths of 2400 m) allowed direct comparison of the radium isotopic characteristics of the fluids with those of the rocks within the high temperature and pressure reaction zone. The ²²⁶Ra activity of the Svartsengi fluid was ∼one-fourth of the Reykjanes fluid and the ²²⁸Ra/²²⁶Ra ratio of the Svartsengi fluid was ∼twice that of Reykjanes. The fluid isotopic characteristics were relatively stable for both sites over the 6 years (2000–2006) of the study. It was determined, using a model that predicts the evolution of the fluid ²²⁸Ra/²²⁶Ra ratio with time, that both sites had fluid residence times, from the onset of high temperature water–rock reaction, of less than 5 years. Measurement of the short-lived ²²⁴Ra and ²²³Ra allowed estimation of the recoil input parameter used in the model. The derived timescale is consistent with results from similar studies of fluids from submarine systems, and has implications for the use of terrestrial systems in Iceland as an exploited energy resource.

Introduction

The submarine ridge system that winds through 60,000 km of the deep ocean floor can be described as an “extended volcano” which produces new oceanic crust on a global scale. Hydrothermal systems within the ridge environment have been the focus of much study since their discovery at the Galapagos Spreading Center in 1977 (Corliss et al., 1979) and there remains considerable interest in the potential impact of these systems on global chemistry and biology. There was early recognition that seafloor hydrothermal systems impact the ocean chemistry of numerous elements (e.g. Edmond et al., 1979, Edmond et al., 1982) and more recently, discovery of microbiological communities within the ocean crust has suggested the potential of this environment to harbor an extensive “subsurface biosphere” (e.g. Gold, 1992, Deming and Baross, 1993).

One of the keys to understanding the fluid flow, chemistry, and the associated microbial life within these hydrothermal systems is knowledge of the convective path length and crustal residence time of the circulating fluid. The reaction of seawater with basalt within the hydrothermal systems at mid-ocean spreading centers produces significant change in the chemical and isotopic character of seawater. Among other factors (Von Damm et al., 1985) such as temperature, extent of phase separation, rock type, degree of prior alteration (i.e. the maturity of the system) that control the composition of the exiting high temperature fluids, the convective path length and crustal residence time determine the extent of phase equilibrium and timescales over which components required for sustaining microbial life can be synthesized. Where such systems manifest themselves subaerially, and are exploited for energy as in Iceland, knowledge of residence time allows insight into aquifer lifetime and recovery.

Over the past twenty years, techniques have been developed for estimating these timescales by utilizing naturally occurring radioisotopes analyzed in hydrothermal material sampled from the seafloor (Kadko et al., 1985/1986, Turekian and Cochran, 1986, Kadko and Moore, 1988, Grasty et al., 1988, Stakes and Moore, 1991, Kadko, 1996, Kadko and Butterfield, 1998). Similar methods have also been applied to continental thermal areas displaying diverse chemical and physical properties (e.g. Zukin et al., 1987, Hammond et al., 1988, Clark and Turekian, 1990, Sturchio et al., 1993). For marine systems, these techniques are based on comparison of the isotopic character of the venting fluids with that of the subsurface rocks with which they react at high temperature and pressure. This emphasizes the importance of appropriate rock samples to accurately assess initial isotopic ratios in the high temperature water–rock reaction zone. However, in submarine studies, the subsurface is inaccessible, and the necessary use of seafloor rock samples (e.g. Kadko and Butterfield, 1998) has relied on the assumption that they represent the isotopic character of rocks greater than 1 km depth.

Here, we apply these techniques to two hydrothermal systems on the Reykjanes Peninsula in Iceland, a subaerial portion of the Mid-Atlantic Ridge. These systems are dominated by seawater flow, but are far more accessible than submarine systems with ready availability of both fluid and importantly, sub-surface rock substrate samples. The availability of subsurface samples for comparison to fluids represents a major advance over similar studies in submarine ridge investigation. Additionally the geology, geochemistry and mineralogy of Iceland have been well studied. These considerations allow methods used on the seafloor to be applied to Icelandic systems and be refined and tested for further application to seafloor studies.