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

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 Ra-226 activity of the Svartsengi fluid was similar to one-fourth of the Reykjanes fluid and the Ra-228/Ra-226 ratio of the Svartsengi fluid was similar to 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 Ra-228/Ra-226 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 Ra-224 and Ra-223 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. (C) 2007 Elsevier Ltd. All rights reserved.