Mapping the Hawaiian plume conduit with converted seismic waves

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 beli eved to result from the passage of the oceanic lithosphere over a mantle ho tspot(1-3). Although geochemical and gravity observations indicate the exis tence of a mantle thermal plume beneath Hawaii(4-6), no direct seismic evid ence for such a plume in the upper mantle has yet been found. Here we prese nt an analysis of compressional-to-shear (P-to-S) converted seismic phases, recorded on seismograph stations on the Hawaiian islands, that indicate a zone of very low shear-wave velocity (< 4 km s(-1)) starting at 130-140 km depth beneath the central part of the island of Hawaii and extending deeper into the upper mantle. We also find that the upper-mantle transition zone (410-660 km depth) appears to be thinned by up to 40-50 km to the south-sou thwest of the island of Hawaii. We interpret these observations as localize d effects of the Hawaiian plume conduit in the asthenosphere and mantle tra nsition zone with excess temperature of similar to 300 degrees C. Large var iations in the transition-zone thickness suggest a lower-mantle origin of t he Hawaiian plume similar to the Iceland plume(7), but our results indicate a 100 degrees C higher temperature for the Hawaiian plume.