Crustal origin for peralkaline rhyolites from Kenya: Evidence from U-series disequilibria and Th-isotopes

The Olkaria complex is a recent, peralkaline rhyolite field in the Kenya Rift Valley close to the axial region of the Kenya Dome. U-Th disequilibrium was measured by alpha-spectrometry in whole rocks and mineral separates from seven geographically and compositionally distinct groups (centres) of rhyolites. Forty-three whole-rock samples show variable Th/U ratios (2.8-6.2) and a large range of (U-238/Th-232) ratios (0.5-1.1); 79% of the rhyolites share, U excess. Rocks from some centres Plot entirely to the left or right of the equiline, whereas some centres es straddle if. internal isochrons give U-Th ages of between 1.46(-2.1)(+2.2) and 36.2(-2.6)(+2.2) ka (2 sigma) respectively for the Gorge Farm centre and 50.5(-7.3)(+7.9) ka for the Broad Acres centre. These ages, interpreted as phenocryst crystallization ages, are older than eruption ages by 10(3)-10(4) yr. The youngest centre displays (Ra-226/Th-230) > 1, indicating that Ra-Th fractionation has taken place <8000 yr bp. There is a positive correlation between (Ra-226/Th-230) ratios for rocks of the youngest centre, indicating that the Ra enrichment and the U enrichment probably occurred during the same event. Closed-system fractionation of observed mineral phases cannot alone explain the U-series disequilibria although it may have contributed to other compositional features of the rhyolites. The degree of U enrichment is related to major and trace element variations in the rhyolites as a whole, in that there is a correlation with peralkalinity and thus with incompatible trace element abundances, in particular Nb, Rb and Zr. There is a good correlation between (U-238/Th-230) and pre-eruptive F contents but not pre-eruptive H2O contents, consistent with previous suggestions that the rhyolites formed by halogen-fluxed melting of the crust. Compositional variations between groups of rhyolites are related to heterogeneity of the crustal source rocks, degree of partial melting (and thus residual mineralogy) and the composition and abundance of metasomatic fluids.