New and published whole-rock major-element contents of xenolithic peridotites, combined with mineral trace-element, 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, and 3 He/ 4 He isotopic compositions, are used to unravel the metasomatic history of lithospheric mantle sampled by volcanic pipes in the Tanzanian section of the East African Rift. The deepest portion of the mantle beneath Labait (craton margin) exhibits high-μ (HIMU)-like 87 Sr/ 86 Sr (0.7029), 143 Nd/ 144 Nd (0.51286), and 3 He/ 4 He (5.9), which may refl ect the plume in this region. Within the Mozambique belt, recent calcio-carbonatite melt metasomatism has overprinted the mantle lithosphere signature beneath Olmani, leading to high whole-rock Ca/Al and low SiO 2 , and remarkably homogeneous 87 Sr/ 86 Sr (0.7034-0.7035) and 143 Nd/ 144 Nd (0.51281-0.51283) of clinopyroxenes. Identical Sr and Nd isotope values are also reported for clinopyroxenes from peridotite xenoliths from the northern portion of the Gregory Rift in Tanzania (Pello Hill andEledoi), which have a strong rift magma overprint. The silicate and carbonatite metasomatic melts are likely to be related to recent plume-derived magmatism of the East African Rift, and thus 87 Sr/ 86 Sr and 143 Nd/ 144 Nd values of the clinopyroxene from these samples can be used to defi ne the rift isotopic signature beneath northern Tanzania. Some mantle regions beneath Lashaine and Labait escaped the recent rift-related overprint and have highly variable Sr-Nd isotope systematics. Labait clinopyroxenes show a near-vertical array on a 87 Sr/ 86 Sr versus 143 Nd/144Nd plot, indicating highly variable time-integrated rare earth element (REE) patterns and low time-integrated Rb/Sr. Lashaine peridotites range to much higher 87 Sr/ 86 Sr at a given 143 Nd/ 144 Nd, and several plot in the right quadrants in the 87 Sr/ 86 Sr versus 143Nd/144Nd diagram, suggesting the influence of a (subducted?) Archean upper continental crust component on the lithospheric mantle beneath Lashaine. Their variable whole-rock SiO 2 and highNa 2 O contents, and clinopyroxene with high Sr/Y, low Sm/Nd, and variable Zr/Sm are consistent with this interpretation.