Open system alkaline magmatism in northern Kenya: Evidence from U-series disequilibria and radiogenic isotopes

U-series activity ratios, Sr-Nd-Pb isotopic ratios and major and trace element compositions have been determined on young basalts (<10 ka) and trachytes from the volcano Emuruangogolak in the Kenya Rift Valley. The basalts are mildly alkaline and are associated with small volumes of hawaiite. The mafic rocks are characterised by high (Th-230/Th-232) (greater than or equal to 1.06) with low (U-238/Th-230) ratios (less than or equal to 0.72). They have variable incompatible trace element ratios (e.g. Zr/Nb, Ba/Zr), indicating that they represent a number of magmatic lineages. The trachytes, which comprise both comenditic and pantelleritic varieties, have significantly lower (Th-230/Th-232) ratios than the basalts, with clear differences between pantelleritic and comenditic types. The (U-238/Th-230) ratios in the pantellerites range from less, to greater, than 1. The variations in composition and isotopic diversity must represent different sources for the trachytes. Internal isochrons for the trachytes give U-Th ages of 14 to 40 ka, similar to single crystal laser fusion 40Ar/39Ar ages from sanidine phenocrysts (16-38 ka) for the same rocks. Post-crystallisation residence times of the trachytes were very short, implying relatively rapid movement of trachyte from magma chamber to the surface. Variations in the initial (Th-230/Th-232)(o) ratios (0.69-1.14) of both basalts and trachytes indicate that Emuruangogolak has erupted a large range of isotopically diverse magmas over a very short period of time (38 ka), from conduits closely spaced around the summit of the volcano.