SEISMIC EVIDENCE FOR A LOWER-MANTLE ORIGIN OF THE ICELAND PLUME

Iceland, one of the most thoroughly investigated hotspots(1-3), is gen erally accepted to be the manifestation of an upwelling mantle plume(4 ). Yet whether the plume originates from the lower mantle or from a co nvective instability at a thermal boundary layer between the upper and lower mantle near 660 km depth(5.6) remains unconstrained. Tomographi c inversions of body-wave delay times show that low seismic velocities extend to at least 400 km depth beneath central Iceland(7,8), but can not resolve structure at greater depth. Here we report lateral variati ons in the depths of compressional-to-shear wave conversions at the tw o seismic discontinuities marking the top and bottom of the mantle tra nsition zone beneath Iceland. We find that the transition zone is 20 k m thinner than in the average Earth(9) beneath central and southern Ic eland, but is of normal thickness beneath surrounding areas, a result indicative of a hot and narrow plume originating from the lower mantle .