We use mantle xenoliths brought to the surface by alkaline lavas to determine the chemical and physical properties of the metasomatized lithospheric mantle that contribute to the earliest rifting stage in East Africa. Our results help to interpret the seismic tomographic images in terms of vein and inclusions proportions in the lithospheric mantle. We focus on mantle xenoliths from the in-rift Pello Hill volcano in the North Tanzanian Divergence (NTD). These xenoliths reveal the presence of refractory mantle harzburgites and dunites with coarse granular to porphyroclastic textures and 6–80 % of diopside, phlogopite and amphibole-bearing veins and phlogopite-rich hornblendite xenolith. The presence of calc-potassic and FeO, TiO2-rich veins, and mineral equilibria of olivine and pyroxenes indicate that fluid/melt-rock interactions occurred at depth from 40 km to 80–90 km, and indicate the presence of a high-temperature isotherm beneath the NTD (T = 1040–1200°C). We computed the seismic properties of the mantle xenoliths with different proportions, compositions, and geometric distributions of crystallized and fluid-filled veins. Compared to vein-free peridotites, for crystallized vein-bearing xenoliths, the velocity is lowered by 2–4 % to 28–37 % for Vp and by 2–3 % to 25–29 % for Vs for 6 % to 60 % veins, respectively. For fluid-filled inclusions, hydrous melt lens-shape inclusions are the most effective parameter to reduce P velocity, compared to dry or 2.5 %–CO2 peridotitic melt. A comparison with seismic tomography velocities allows us to discuss the current state of the lithospheric mantle. The best agreement obtained between P teleseismic tomography (Vp anomalies between −9 % and −15 %) and vein-bearing peridotites (depth 40–90 km) corresponds to 12–25 % of crystallized veins or 8–15 % for fluid filled-veins for a vertical foliation and transtensional strain regime in the mantle lithosphere beneath the NTD.