Geodynamic Evolution of the East African Superplume: Insights From Volcanism in the Western Turkana Basin

There is a consensus that volcanism along the East African Rift System (EARS) is related to plume activities. However, because of our limited knowledge of the local lithospheric mantle, the dynamics of the plume are poorly constrained by magma chemistry. The Turkana Basin is one of the best places to study plume-related volcanism because the lithospheric mantle there is unusually thin. New Ar-Ar geochronology and geochemical data on lavas from western Turkana show that Eocene volcanics have relatively low Pb-206/Pb-204 (<19.1) and high epsilon Nd (>3.78). Their relatively high Ba/Rb (35-78) ratios suggest contributions from the shallow lithospheric mantle. Oligo-Miocene Turkana volcanics have HIMU- and EMI- type enriched mantle signatures with overall lower Ba/Rb ratios, which is consistent with partial melting of plume material. Pliocene and younger Turkana volcanics have low Ba/Rb and Sr-Nd-Pb isotope ratios that resemble those of Ethiopian volcanics with elevated He-3/He-4 ratios. This temporal variation can be reconciled with a layered plume model where an outer layer of ancient recycled oceanic crust and sediment overlies more primitive lower mantle material. Beneath Ethiopia, the outer layer of the plume is either missing or punctured by the delamination of the thicker overlying lithospheric mantle at ca. 30 Ma, an event that would have facilitated the rapid upwelling of the inner portion of the plume and triggered the Ethiopian flood volcanism. The outer layer of the plume may be thicker in the southern EARS, which could explain the occurrence of young HIMU- and EMI-type volcanics with primordial noble gas signatures.