Compilation & interpretation of rock geochemical data for the Longonot & Greater Olkaria volcanic centres

187 analyses of lavas and pyroclastics are listed and displayed as bivariant TAS and trace element plots. Compositions at Longonot are slightly oversaturated peralkaline trachytes, while Nb/Zr ratios show a single trend indicating a single source fractionating trachytic magma chamber. Subordinate lavas from the Northern Plain and crater plot as trachytes and benmoreites respectively but other work has shown these to be mixtures of the typical Longonot trachyte with basalt. Absolute values of incompatable trace elements (ICE) Nb,Zr,Rb,Th & U - vary widely, being highest in pyroclastics associated with caldera collapse, surge horizons early plinian beds. ICE contents fall, both during indevidual plinian eruptions, and from early to late in the plinian sequence, indicating magma chamber zonation. Olkaria compositions are usually alkali rhyolites (comendites) with a tendency for older events to be trachytic. Mixed lavas, having intermediate compositions, and basalts are present north of Olkaria (Ndabibi area). On the plains to the south of Olkaria basaltic looking rocks include mugearites. ICE content varies widely in comendites, increasing from domes through lavas to pumice paralleling the presumed volatile content. Highest values are found in pumice from surge beds. Nb/Zr & Th/Zr ratios are distinct for different phases of Olkaria activity with the ring fracture related volcanism (03) having highest Nb,U & Th/Zr ratios and highest absolute contents of these elements (<4000ppm Zr, l00Oppm Nb, 250ppm Th & 43ppm U). The different ratios exhibited by different phases of volcanism indicate accessing of separate magma batches which may result from different episodes of partial melting. U & Th contents at Olkaria are higher than at Longonot & Suswa indicating a greater radioactive heating potential.