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

摘要

Trace element and radiogenic isotope data have been measured on Oligocene flood basalts from the northwestern Ethiopian plateau. Our aim was to investigate and identify the nature of mantle and crustal sources involved in the genesis of this huge volume of pre-rift basalts to constrain the interaction between the Afar mantle plume and the lithosphere at the onset of continental break-up. The three magma types previously identified on this plateau display contrasting geochemical signatures. The Low-Ti magma type (LT) basalts display a strong and variably developed lithospheric signature characterized by relative depletions in Nb, Ta, Th, and Rb and peaks at Ba and Pb compared to oceanic baslats. The High-Ti magma type basalts (HT2) display much more homogeneous compositions and have ocean island basalt-like trace element signatures, whereas HT1 basalts exhibit intermediate compositions between those of the two other groups. In contrast to the wide range of trace element compositions, Sr, Nd, and Pb isotope ratios display limited variations (~(87)Sr/~(86)Sr=0.70304-0.70429; ~(143)Nd/~(144)Nd=0.51271-0.51289; ~(206)Pb/~(204)Pb=18.00-18.86). Correlations among isotopic and trace element ratios provide evidence for the involvement of various mantle and crustal components in the petrogenesis of these flood basalts. Two distinct mantle components are involved in the genesis of the LT and HT2 extreme magma types. The HT2 basalts were derived from an ocean island basalt-like mantle component (~(87)Sr/~(86)Sr approx 0.704; ~(143)Nd/~(144)Nd approx 0.51295; ~(206)Pb/~(204)Pb approx 18.8) that corresponds to the initial material of the Afar mantle plume. By contrast, the LT basalts result from the melting of a more depleted mantle component (~(87)Sr/~(86)Sr approx 0.7033; ~(143)Nd/~(144)Nd approx 0.5130; ~(206)Pb/~(204)Pb approx 18.6), either intrinsic to the plume itself or entrained in the Afar plume head during its ascent. Correlations of incompatible trace element and isotopic ratios with differentiation indices indicate that the more or less pronounced lithospheric signature of the Ethiopian flood basalts was acquired by crustal contamination of the magmas during their variable residence time in the lower and upper crust. The effects of crustal contamination are much more evident in the lT basalts because of their much less enriched initial characteristics.