Lithospheric thickness beneath the southern Kenya Rift: implications from basalt geochemistry

Abstract.

Geochemical data are reported for samples from the flanks and floor of the southern Kenya Rift Valley in the Lake Magadi area, and from two central volcanoes located within the rift valley. Rift lavas include samples of Singaraini and Ol Tepesi basalts on the eastern flank, Kirikiti basalts from the western flank, and plateau trachytes from the rift valley floor. Central volcano samples are from Ol Esayeiti and Lenderut located on the eastern flank. The rift basalts are mildly ne-normative, moderately evolved (Mg#=0.39–0.62) alkali basalts and show an overall range in differentiation. Incompatible trace element abundances are moderately elevated (Nb=17–51; Zr=93–274; La=17–55 ppm) and show strongly coherent variations and constant inter-element ratios (e.g. Zr/Nb=4.2–5.5; Nb/Ta=17.5±0.4; (La/Sm)_n=7.3±1.1); isotope ratios are restricted in range (⁸⁷Sr/⁸⁶Sr=0.70393–0.70436; ¹⁴³Nd/¹⁴⁴Nd=0.51272–0.51280; ²⁰⁶Pb/²⁰⁴Pb=19.87–19.92; ²⁰⁷Pb/²⁰⁴Pb=15.68–15.70; ²⁰⁸Pb/²⁰⁴Pb=39.56–39.71). Central volcano lavas are more alkaline in character and include basanite (Ol Esayeiti; Mg# >60) and hawaiite to benmoreite (Lenderut; Mg#=0.48–0.38). Incompatible element ratio are similar to those of the rift basalts, although the chondrite normalised REE patterns are steeper (La/Sm)_n=17.4±1.2). ⁸⁷Sr/⁸⁶Sr (0.70358, 0.70391), ¹⁴³Nd/¹⁴⁴Nd (0.51280, 0.51267), ²⁰⁶Pb/²⁰⁴Pb (19.96,20.17), ²⁰⁷Pb/²⁰⁴Pb (15.66,15.76) and ²⁰⁸Pb/²⁰⁴Pb (39.80,40.00) ratios of Ol Esayeiti basanites are similar to the rift basalts, whereas the Lenderut lavas have unusually low¹⁴³Nd/¹⁴⁴Nd (0.512388–0.512453) ratios for their ⁸⁷Sr/⁸⁶Sr (0.70370–0.70481) ratios, and distinctly less radiogenic and variable Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb=17.93–19.01; ²⁰⁷Pb/²⁰⁴Pb=15.43–15.58; ²⁰⁸Pb/²⁰⁴Pb=37.91–39.14). An integrated model is developed in which the geochemical signature of the lavas is attributed to variable degrees of melting to depths within the garnet stability field, and in the presence of residual amphibole. The stability fields of these phases in P–T space indicates that the lavas must have formed within the sub-continental lithosphere rather than within the underlying ambient asthenosphere or a rising mantle plume. The subcontinental lithospheric mantle must therefore extend to a depth of at least 75 km beneath the Lake Magadi area, which contrasts with recent gravity models for the area, which infer that lithospheric mantle is absent beneath this section of the southern Kenya Rift.