Title: "δD AND δ18O SYSTEMATICS IN GEOTHERMAL FLUIDS, 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
ounding mountains. Upon water-rock interaction, the geothermal waters
are further modified, shifting the δ¹⁸O ratios to higher values.