| Abstract |
Paka volcano is characterised by intense and widespread geothermal surface activity manifested in the form of fumaroles, hot grounds and hydrothermally altered rocks. It is located within the Kenya Rift Valley which is endowed with geothermal resource potential. This study seeks to reinterpret the results of a geochemical survey carried out by Clarke, Woodhall, and Darling (1990), Dunkley, Smith, Allen, and Darling (1993) and (Darling, Griesshaber, Andrews, Armannsson, & O'Nions, 1995). The main aims of this undertaking were to; (i) investigate the source of fluids in this volcano and their relationship with the tectonic assessment of the volcano; (ii) assess and evaluate the isotopic geothermometry temperatures; (iii) estimate the contributions of the three major CO2 sources along convergent plate margins, i.e., the mantle, organic sediments, and marine limestones. The results of 3He/4He show that all the samples have values in excess of 1, indicating enrichment to varying degrees in mantle He, and the maximum ratio of 8.0 is comparable typical MORB values. The stable isotopes indicate that there is insufficient variation in the local groundwaters to account for all of the steam condensates by a simple process of primary steam separation from these waters. Fumarole 163 and 165 are more enriched in heavy isotopes than the local ambient groundwaters. The ƒÔ2H and ƒÔ18O values of the steam condensate of most the samples are in reasonable agreement with the theoretical isotope compositions expected for the vapors separated through single-step boiling either at 250-100¢XC approximately from the undiluted deep geothermal liquid or at 310-250¢XC approximately from the diluted deep geothermal liquid. Isotopic geothermometry temperatures of samples 163 and 166 give 350 and 312¢XC, respectively, adopting the fractionation factors of Horita (2001), and 340 and 310¢XC, respectively, accepting the fractionation factors of Bottinga (1969). Despite all these the gases show a clear mantle origin. |