Seismic evidence that the source of the Iceland hotspot lies at the core-mantle boundary

Although Morgan(1) proposed in 1971 that hotspots such as Iceland were the result of hot, rising mantle plumes, it is still debated whether plumes originate from a thermal boundary just above the core-mantle boundary or at the base of the upper mantle(2). Although seismic evidence of plumes in the upper mantle is accumulating(3), narrow plume conduits in the deep mantle have yet to be detected. Details of plume formation in the lower mantle have therefore remained largely unconstrained(4). Here, however, we present seismic evidence for the presence of a localized patch of material with ultra-low seismic wave speed, located at the coremantle boundary beneath the Iceland hotspot, and propose that this zone represents the hot, partially molten source region of the Iceland mantle plume. Through the modelling of seismic waveforms, we constrain the seismic velocity structure at this patch of the core-mantle boundary using a numerical-analytical interfacing code(5) designed to reproduce the complex interference of shear-wave phases transmitted through, and refracted at, the boundary(6). Although this structure is difficult to constrain precisely, our preferred model consists of a dome which is 250 km wide, 40 km high and contains P- and S-wave velocity (wave-speed) reductions of 10% and 30%, respectively.