1 The Significance of Seismic Wavespeed Minima and Thermal Maxima

16 17 It is widely assumed that the boundary layer above the core is the source of intraplate volcanoes 18 such as Hawaii, Samoa and Yellowstone and that the sub-plate boundary layer at the top of the 19 mantle is thin and entirely subsolidus. In fact, this layer is thicker and has higher expansivity, 20 buoyancy and insulating power than the lower one, and may have higher potential temperatures. The 21 observed seismic structure of the low-velocity zone (LVZ) including attenuation, anisotropy, sharp 22 boundaries and a reduction of both compressional and shear moduli can be taken as strong evidence 23 for the ubiquitous presence of melt in the upper mantle. If the LVZ contains as little as 1-2% melt 24 then it is the most plausible and accessible source for mid-plate magmas; deeply rooted active 25 upwellings are unnecessary. The upper boundary layer is also the most plausible source of ancient 26 isotopic signatures of these magmas and their inclusions. 27 28