Geochemical interperetation of thermal water and gas samples from Krýsuvík, Iceland and Alid, Eritrea

Krýsuvik, Iceland and Alid, Eritrea are two geothermal fields located within plate tectonic rift zones; an environment characterized by high heat-flow. In this paper, geochemical data from these two systems are reviewed and interpreted. The objective of this study is to explore the potential of geochemical methods to predict the reservoir temperatures and evaluate equilibrium conditions for the two fields. Eighteen silica and cation geothermometers were used to calculate temperatures for 19 fluid samples from Krýsuvik. Temperatures calculated by various solute geothermometers were generally higher than measured temperatures and the discrepancy between measured and calculated temperatures increased with decreasing temperature. This is indicative of mixing between ground water and geothermal fluids. Mixing was further established by a SiO2-enthalpy mixing model that predicted two reservoirs for Krýsuvik with temperatures equal to 270oC and 203°C. The agreement of seven geothermometers was not very good as predicted temperatures commonly varied by about 70oC for individual samples. However, the discrepancy between the gas geothermometers was more or less constant for different samples, i.e., most of the geothermometers predicted similar differences between the hot and cold samples. The results of the H2 geothermometer was generally close to the average of other gas geothermometers, ranging between 193 and 270°C with an average value of 238°C in good agreement with measured temperatures and results of solute geothermometers. A map showing isothermal contour lines based on gas geothermometer temperatures of the Krýsuvik area reveals two anomalous semi-linear features, one E-W trending near Krýsuvik, and another NE-SW trending near Trolladyngja. The Alid water samples are characterized by low pH and high sulphate content, typical of steam heated surface waters. As a result, solute geothermometers do not represent temperatures of the geothermal reservoir. This is reflected by low quartzgeothermometer temperatures (80-224oC) compared to the gas-thermometer temperatures that range between 163 and 364°C with an average of 272oC. The results indicate that the geothermal reservoir at Alid is vapour-dominated and surface thermal waters are derived from shallow, steam heated ground water reservoirs.