Figure 3 . Segregation velocity (Vm ) as function of grain size and melt fraction in the tubules model (a) and olivine-ref model (b). Contours with different Vmare shown with numbers. The blue line indicates the boundary whered = ds , above which mantle is disequilibrium with the core forming melts. ds~√Dt , where D is the diffusion coefficient andt is the melts residence time, which equals to the mantle thickness divided by Vm . The calculation uses D ~10-18 [Holzapfel et al. , 2005]. t is the time melt needs to travel through a 3000 km thick lower mantle.