Origin of geothermal waters and subsurface temperature estimates in the southern lowlands of Iceland

Abstract

Interpretation of the isotopic and chemical content of natural waters in the Southern Lowlands of Iceland reveals that the origin of the waters for the low-temperature geothermal systems in the area is related to their geographical and topographical positions relative to the sea and submergence of the lowlands at the end of the last glaciation. The low-temperature thermal waters that are located at relatively low altitudes are composed of present-day precipitation, seawater from the end of the last glaciation and also meteoric water from the same time. Linear correlation between chloride and boron in spring waters from the Geysir high-temperature geothermal field indicate that warm spring waters are formed by the mixing of cold water and undegassed deep hot water. The same correlation also indicates mixing of cold ground and geothermal waters in the eastern and northwestern low-temperature thermal systems. All thermal waters have more negative δ2H values than the local precipitation. Oxygen shift from the global meteoric line indicates strong (extensive) water-rock interaction for the non-mixed waters but is limited in the case of mixed waters. Subsurface temperature estimates from silica and Na-K geothermometers indicate that they are unreliable in the cold surface waters and mixed waters from the Geysir area due to lack of equilibrium. The chalcedony and Na-K temperature estimates in the low-temperature systems are comparable but the Na-K geothermometer gives a higher estimate for temperatures of less than 100°C. The Na-K geothermometer gives lower temperature estimates for the un-mixed waters of the Geysir area, probably due to base-exchange reactions involving clays that remove K from the solution. Quartz equilibrium temperatures are, therefore, considered to be more reliable.