Evolution of the lithospheric mantle beneath the East African Rift in Tanzania and its potential signatures in rift magmas

New and published whole-rock major-element contents of xenolithic peridotites, combined with mineral trace-element, Sr-87/Sr-86, Nd-143/Nd-144, and He-3/He-4 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-mu (HIMU)-like Sr-87/Sr-86 (0.7029), Nd-143/Nd-144 (0.51286), and He-3/He-4 (5.9), which may reflect 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 SiO2, and remarkably homogeneous Sr-87/Sr-86 (0.7034-0.7035) and Nd-143/Nd-144 (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 and Eledoi), 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 Sr-87/Sr-86 and Nd-143/Nd-144 values of the clinopyroxene from these samples can be used to define 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 Sr-87/Sr-86 versus Nd-143/Nd-144 plot, indicating highly variable time-integrated rare earth element (REE) patterns and low time-integrated Rb/Sr. Lashaine peridotites range to much higher Sr-87/Sr-86 at a given Nd-143/Nd-144, and several plot in the right quadrants in the Sr-87/Sr-86 versus Nd-143/Nd-144 diagram, suggesting the influence of a (subducted?) Archean upper continental crust component on the lithospheric mantle beneath Lashaine. Their variable whole-rock SiO2 and high Na2O contents, and clinopyroxene with high Sr/Y, low Sm/Nd, and variable Zr/Sm are consistent with this interpretation. Silicate lavas from the eastern branch of the East African Rift show increasingly evolved Sr and Nd isotope composition from north to south and hence increasing input of ancient metasomatized lithosphere ("EM1" and "EM2" components), similar to that beneath Lashaine and Labait, and well outside the suggested range in isotope compositions of the heterogeneous Kenya plume (Sr-87/Sr-86 = 0.7029-0.7036; Nd-143/Nd-144 = 0.51275-0.51286). In the western branch, the "anomalous" Sr signature identified in Lashaine peridotites is prominent in silicate lavas and may indicate that the lithospheric mantle beneath that area was similarly enriched during ancient subduction. By contrast, the Sr-Nd isotope systematics of carbonatites reflect EM1 but not EM2 inputs, suggesting that such melts in the East African Rift neither derive from nor have interacted with subduction-modified mantle regions.