Seismological evidence for a mid-mantle discontinuity beneath Hawaii and Iceland

Receiver functions derived from body waves recorded in Iceland and on the Hawaiian Islands reveal a seismic velocity discontinuity at about 1050 km depth beneath the two regions of presumed mantle upwelling. The waveforms of the converted phases from the mid-mantle discontinuity indicate a velocity increase with depth. The lack of consistent scattering found in recent systematic searches for seismic boundaries in the middle mantle argues against a globally continuous, sub-horizontal mineralogical phase transition near 1000 km depth. The sense of the velocity change at the mid-mantle discontinuity beneath the two hotspots is consistent with geodynamic models in which the mantle has distinct chemical reservoirs and the material beneath the compositional boundary in these areas is more silicon-rich than the overlying mantle, although the possibility of a yet-unknown phase transition particular to mantle plume material cannot be ruled out.