Source controls on sulfur abundance and isotope fractionation in hydrothermal fluids in the Olkaria geothermal field, Kenya

Abstract Sulfur is among the major components in volcanic hydrothermal fluids and may derive from multiple sources. However, processes such as boiling, magma degassing, fluid-rock interaction and associated redox reactions often obscure the origin of sulfur in the system. The multiple sulfur isotope system is a powerful tool to unravel the processes that affect isotope ratios of sulfur in geothermal systems. Here, we utilized concentrations and multiple isotope ratios (32S/33S/34S/36S) of sulfur from well discharge from geothermal fields at Olkaria, SE Kenya, to investigate the hydrothermal processes and to quantify the sources of sulfur to the system. Fluid sampling temperatures ranged from 157 to 204 °C, and the sampled fluids had low Cl concentrations (119 to 1267 ppm Cl) with both liquid water and vapor being present. The concentration of H2S and SO4 in the water phase of the well discharges ranged from 0.53 to 68.0 ppm and from 4.7 to 108 ppm, respectively. In the vapor phase, H2S was the only observed sulfur compound with concentrations between 1.18 and 214 μmol mol−1. In the water phase, δ34S values for H2S and SO4 ranged from −1.2 to +4.0‰ and +0.7 to +7.7‰, respectively. The δ34S values of H2S in the vapor phase ranged from +2.0 to +11.3‰. The Δ33S values for H2S in the water and vapor phase were found to be between −0.017 and +0.021‰ and −0.042 and +0.008‰, respectively, and for SO4 in the water phase between −0.024 and + 0.004‰. Sulfur isotopes of H2S in liquid phase were depleted in 34S as a result of depressurization boiling. Sulfur isotope ratios as well as the results of geochemical modelling suggest that sulfide in the fluids dominantly originated from rock leaching where insignificant amounts of sulfur was derived from direct degassing of a magmatic source. Sulfate primarily derived from the oxidation of H2S to SO4 upon fluid ascent to the surface. Based on the geochemical model results and measured sulfur concentrations, the annual natural H2S emission rate at Olkaria was estimated to range from 320 to 510 t H2S yr−1. These comparatively low sulfur emissions and the lack of significant sulfur sources (e.g., magma or seawater) imply that Olkaria could represent an ideal re-injection site for anthropogenic H2S even if sulfur sequestration rates would be low.