Mantle plume and lithosphere contributions to basalts from southern Ethiopia

Major, trace element and radiogenic isotope analyses are presented for a suite of basaltic rocks from two transects across the Main Ethiopian Rift in southern Ethiopia. The northern transect crosses the MER within the limits of the Ethiopian Plateau, regarded as the topographic expression of the underlying Afar mantle plume, while the southern transect crosses the Turkana Depression to the south of the plateau. The samples are generally transitional between tholeiitic and alkaline, although, in the northern samples, there is a tendency for compositions to become more alkaline and enriched in incompatible elements with time. Sr-87/Sr-86 ratios vary between 0.7032 and 0.7043, Nd-143/Nd-144 from 0.51261 to 0.51287 and Pb-206/Pb-204 from 18.11 to 20.28, and there is a marked tendency for Pb-206/(204)pb to increase and Zr/Nb to decrease with time in the northern samples. Pb-206/Pb-204,and Zr/Nb show a marked negative correlation, which is interpreted as reflecting mixing between plume-derived small melt fractions with high Pb-206/Pb-204 and low Zr/Nb and larger melt fractions from the mantle lithosphere with low Pb-206/Pb-204 and high Zr/Nb. Plume contributions to the pre-rift basalts in the northern transect are < 30% but increase in the syn-rift basalts, which are predominantly plume derived. All but the most recent alkali basalts from the southern transect are predominantly lithosphere derived. Extrapolating these results to the whole Ethiopian Province suggests that melt production rates in the mantle plume increased from < 0.01 km(3)a(-1) in the pre-rift to similar to 0.03 km(3)a(-1) in the syn-rift basalts, which is similar to that of plume-related ocean islands. These results imply that lithospheric thinning was achieved initially by thermo-mechanical erosion by the underlying plume and subsequently by tectonic extension during rifting.