Two mantle plumes beneath the east African rift system: Sr, Nd and Pb isotope evidence from Kenya Rift basalts

Major and trace element and radiogenic isotope ratios (Sr, Nd and Pb) are presented for a suite of Neogene to Recent basalts (MgO > 4 wt%) from the axial regions of the Kenya Rift. Samples have compositions ranging from hypersthenenormative basalt through alkali basalt to basanite and are a subset of a larger database in which compositions extend to nephelinite. A broadly negative correlation between Zr/Nb (< 2-7) and Ce/Y (1-8) indicates derivation from a garnet-bearing mantle source region as a result of < 3% melting. Isotope ratios in basalts from the axial regions of the Kenya Rift have Nd-143/Nd-144 = 0.51300 to 0.51255, Sr-87/Sr-86 0.7030 to 0.7055 and Pb-206/Pb-204 < 18 to > 20, broadly similar to values from OIB. The Kenya Rift cuts through basement of different ages and aspects of the composition of mafic magmas reflect the anisotropy of the underlying lithosphere, Specifically, those basalts from that part of the rift underlain by the Tanzanian craton (TC) have higher Ce/Y and lower Zr/Nb ratios than those erupted through the Panafrican Mozambique belt (MB) implying an origin either at greater depth or from a more trace element-enriched source region. Samples erupted through the zone of reactivated craton margin (RCM) share the characteristics of mafic lavas from both the craton and the mobile belt. MB samples have Nd-143/Nd-144 = 0.5130-0.5127, Sr-87/Sr-86 = 0.7030-0.7035 and Pb-206/Pb-204 = 18.3-19.8, defining a steep negative trend on the Nd-Sr diagram and plotting close to the NHRL on conventional Pb isotope diagrams. By contrast TC and RCM samples have Nd-143/Nd-144 = 0.5124-0.51275, Sr-87/Sr-86 0.7035-0.7056 and Pb-206/Pb-204 = 17.6-21.2, defining flat-lying arrays on Nd-Sr plots and a much greater scatter and spread on Pb isotope diagrams, with many analyses plotting above the NHRL. Both groups of analyses trend towards a common end member on a plot of Nd-143/Nd-144 against Sr-87/Sr-86, at Nd-143/Nd-144 similar to 0.51275 and Sr-87/Sr-86 similar to 0.7035. These values are suggested to reflect the isotopic characteristics of the sub-lithospheric Kenyan mantle, inferred to be the Kenya mantle plume. Comparison with data from Afar suggest that the Kenya plume is distinct from the Afar plume, implying that the east African Rift is underlain by at least two distinct mantle plumes. Eocene and Oligocene basalts from southern Ethiopia bear a closer resemblance to the Kenyan basalts than to those from Afar and the Ethiopian plateau, suggesting that the Kenya plume has been active for at least 45 Ma. Migration of magmatism from southern Ethiopia southwards is consistent with the northeastward migration of the African plate over the Kenya plume over the past 50 My. (C) 2000 Elsevier Science B.V. All rights reserved.