Isotopic and trace element signatures of Ethiopian flood basalts: Evidencefor plume-lithosphere interactions

Trace element and radiogenic isotope data have been measured on Oligocene f lood basalts from the northwestern Ethiopian plateau. Our aim was to invest igate and identify the nature of mantle and crustal sources involved in the genesis of this huge volume of pre-rift basalts to constrain the interacti on between the Afar mantle plume and the lithosphere at the onset of contin ental break-up. The three magma types previously identified on this plateau display contrasting geochemical signatures. The Low-Ti magma type (LT) bas alts display a strong and variably developed lithospheric signature charact erized by relative depletions in Nb, Ta, Th, and Rb and peaks at Ba and ph compared to oceanic basalts. The High-Ti magma type basalts (HT2) display m uch more homogeneous compositions and have ocean island basalt-like trace e lement signatures, whereas HT1 basalts exhibit intermediate compositions be tween those of the two other groups. In contrast to the wide range of trace element compositions, Sr, Nd, and Pb isotope ratios display limited variat ions (Sr-87/Sr-86 = 0.70304-0.70429; Nd-143/Nd-144 = 0.51271-0.51298; Pb-20 6/Pb-204 = 18.00-18.86). Correlations among isotopic and trace element rati os provide evidence for the involvement of various mantle and crustal compo nents in the petrogenesis of these flood basalts. Two distinct mantle compo nents are involved in the genesis of the LT and HT2 extreme magma types. Th e HT2 basalts were derived from an ocean island basalt-like mantle componen t (Sr-87/Sr-86 similar to 0.704; Nd-143/Nd-144 similar to 0.51295; Pb-206/P b-204 similar to 18.8) that corresponds to the initial material of the Afar mantle plume. By contrast, the LT basalts result from the melting of a mor e depleted mantle component (Sr-87/Sr-86 similar to 0.7033; Nd-143/Nd-144 s imilar to 0.5130; Pb-206/Pb-204 similar to 18.6), either intrinsic to the p lume itself or entrained in the Afar plume head during its ascent. Correlat ions of incompatible trace element and isotopic ratios with differentiation indices indicate that the more or less pronounced lithospheric signature o f the Ethiopian flood basalts was acquired by crustal contamination of the magmas during their variable residence time in the lower and upper crust. T he effects of crustal contamination are much more evident in the LT basalts because of their much less enriched initial characteristics. Copyright (C) 1999 Elsevier Science Ltd.