Mapping the Hawaiian Plume Conduit with Converted Seismic Waves Xueqing Li, Rainer Kind (GFZ Potsdam) The volcanic edifice of the Hawaiian islands and seamounts, as well as the surrounding area of shallow sea floor known as the Hawaiian swell, are believed to result from the passage of the oceanic lithosphere over a mantle hotspot. Although geochemical and gravity observations indicate the existence of a mantle plume beneath Hawaii, no direct seismic evidence for such a plume in the upper mantle has yet been found. Our analyses of the P--to--S converted seismic phases indicate a zone of very low S--wave velocity (smaller than 4~km/s) starting at a depth of 130--140km beneath the central part of the Island of Hawaii and extending deeper into the upper mantle. The upper mantle transition zone (410--660~km depth) appears to be thinned by up to 40--50km to the south--southwest of the Island of Hawaii. We interpret these observations as localized effects of the Hawaiian plume conduit in the asthenosphere and the mantle transition zone with excess temperature of about 300$^\circ$C. Large variations in the transition-zone thickness suggest a lower-mantle origin of the Hawaiian plume similar to the Iceland plume, but our results indicate a 100$^\circ$C higher temperature for the Hawaiian plume.