East African Rift System (EARS) Plume Structure: Insights from Quaternary Mafic Lavas of Turkana, Kenya

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Abstract
Quaternary mafic lavas from Lake Turkana (northern Kenya) provide information on processes operating beneath the East African Rift in an area of anomalous lithospheric and crustal thinning. Inferred depths of melting beneath Turkana (15–20 km) are shallower than those recorded elsewhere along the rift, consistent with the anomalously thin crustal section. The mafic lavas have elevated incompatible trace element contents when compared with mid-ocean ridge basalts, requiring an enrichment event in the source region. Basalts with low Sr isotopic ratios (∼0·7030) have high 143Nd/144Nd (>0·5129) and 206Pb/204Pb values (∼19·4) and incompatible trace element abundances that indicate derivation from a sub-lithospheric mantle source region. Quaternary mafic rocks with 10–15 wt % MgO record contributions from a mantle plume that is isotopically similar to the deep mantle source region for global hotspots. These Turkana basalts have isotope and incompatible trace element ratios that overlap with those of Quaternary mafic lavas from the Red Sea, the western Gulf of Aden, and northern Kenya, interpreted as being derived from mixtures of plume and ambient mantle sources. The Turkana data imply a common and long-lived mantle plume composition beneath both the Ethiopia and Kenya domes. This scenario is supported by tomographic results indicating a discontinuous thermal and chemical anomaly that originates in the deep mantle beneath southern Africa, and is also consistent with the seismically determined shallow mantle structure beneath East Africa.