δD and δ18O systematics in geothermal fluids, Olkaria Geothermal system, Kenya

Abstract

The δD and δ18O systematics of geothermal fluids, Olkaria Kenya, were studied. Samples were collected from well discharges, both liquid and vapour phases collected at the same temperature and pressure, and analysed for major elemental and δD and δ18O composition. The measured discharge fluids ranged in δD and δ18O composition from -3.7 to +16.1‰ and -1.45 to +3.18‰, respectively. From these data, the reservoir chemical and isotope composition was reconstructed with δD and δ18O ratios from -2.1 to +14.6‰ and -0.84 to +2.61‰, respectively. According to boiling modelling, large δD and δ18O fractionations can occur from the reservoir to sampling at surface, and these changes exceed the range of observed δD and δ18O variability for fluids. Accurate reconstruction of reservoir and source δD and δ18O values requires, therefore, careful sampling, analysis and geochemical and isotope modelling. Based on the composition of the reservoir δD and δ18O and the local rainwater, surface water, lake water and non-thermal groundwater, and consideration of total geothermal fluid discharge and annual precipitation, it is concluded that the geothermal reservoir fluids at Olkaria are predominantly meteoric origin locally and from the surrounding mountains. Upon water-rock interaction, the geothermal waters are further modified, shifting the δ18O ratios to higher values.