Grain boundary diffusion of W in lower mantle phase with implications for isotopic heterogeneity in oceanic island basalts by core-mantle interactions

Tungsten isotopes provide important constraints on the ocean-island basalt (OIB) source regions. Recent analyses of μ¹⁸²W in modern basalts with high ³He/⁴He originating from the core-mantle boundary region reveal two distinct features: positive μ¹⁸²W in Phanerozoic flood basalts indicating the presence of primordial reservoir, and negative μ¹⁸²W in modern OIBs. One possibility to produce large variations in μ¹⁸²W 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 μ¹⁸²W 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 μ¹⁸²W and likely originated from the modified isotope region just above the core-mantle boundary, whereas those with positive μ¹⁸²W 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 μ¹⁸²W can be added to the silicate mantle. As a result, the source region of the OIB would be effectively modified to a negative μ¹⁸²W.