Geochemical and isotopic study of the Menengai geothermal field, Kenya.

Abstract

Results of chemical and isotopic investigations of geothermal fluids within the Menengai high temperature geothermal field are presented. The isotopic composition of lakes, boreholes and springs in the Kenyan rift valley, from Lake Naivasha in the South to Lake Baringo in the North are also presented and discussed. The Menengai area is mainly composed of trachytic lavas traversed by northnorthwest and north-northeast trending fracture zones with the Ol‘Rongai and Molo Tectono volcanic axes, within which there are several eruptive centres. Geochemical samples were collected in the year 2012 from the exploration/production geothermal wells in Menengai and analysed for chemical constituents in GDC and KenGen geochemical laboratories and for stable isotopes of oxygen and hydrogen at the Institute of Earth Sciences, University of Iceland.

In addition, samples for isotope measurements were collected from lakes and cold water shallow boreholes, as well as cold and hot springs and analysed at the Institute of Earth Sciences, University of Iceland. The results indicate that the Menengai thermal fluids are of meteoric origin as they plot close to the local evaporation water line, however mixing of Lake Nakuru and groundwater is seen both in the isotopic and chemical composition of the fluids. In well MW-05 the fluid is richer in 18O, either due to more intense water-rock interaction, or boiling at higher temperature than in wells MW-01 and MW-04. The isotopic composition of the lakes has varied considerably with time, due to changing rainfall patterns, lake water levels and the amount of evaporation over the years. The shallow and cold springs are all of meteoric origin, as they plot in a cluster slightly more enriched than indicated by the intersection between the local rainwater and evaporation lines. Very little geographical difference is observed for the cold groundwater samples. Fluids discharged by wells MW-01 and MW-04 are sodium bicarbonate in nature. The calculated temperatures from solute geothermometers are somewhat lower than measured temperatures. Flashing of the fluids during discharge causes boiling which results in cooling and lowering of the boiling temperature and hence high CO2 in both MW-01 and MW-04. High CO2 could also be originating from a degassing magmatic body.