Tungsten isotopes provide important constraints on the ocean-island basalt (OIB) source regions. Recent analyses of mu W-182 in modern basalts with high He-3/He-4 originating from the core-mantle boundary region reveal two distinct features: positive mu W-182 in Phanerozoic flood basalts indicating the presence of primordial reservoir, and negative mu W-182 in modern OlBs. One possibility to produce large variations in mu W-182 is interaction between the mantle and outer core. Here, we report grain boundary diffusion of W in lower mantle phases. High pressure experimental results show that grain boundary diffusion of W is fast and strongly temperature dependent. Over Earth's history, diffusive transport of W from the core to the lowermost mantle may have led to significant modification of the W isotopic composition of the lower mantle at length scales exceeding one kilometer. Such grain boundary diffusion can lead to large variations in mu W-182 in modern basalts as a function of the distance of their source regions from the core mantle boundary. Modern oceanic island basalts from Hawaii, Samoa and Iceland exhibit negative mu W-182 and likely originated from the modified isotope region just above the core-mantle boundary, whereas those with positive mu W-182 could be derived from the thick Large Low Shear Velocity Provinces (LLSVPs) far from the core-mantle boundary (CMB). When highly-oxidized slabs accumulate at the CMB oxidizing the outer core at the interface, a large W flux with negative mu W-182 can be added to the silicate mantle. As a result, the source region of the mu W-182 would be effectively modified to a negative mu W-182. (C) 2019 Elsevier B.V. All rights reserved.