Deep mantle plumes and convective upwelling beneath the Pacific Ocean

Earth's mantle is thought to convect as a whole, with material flow across the upper mantle phase transitions of the mineral olivine at 410 and 660. km depth. However, the details of convection, especially mantle upwelling and plumes, are not well constrained. Here, we study seismic shear wave reflections from the underside of these temperature and composition dependent phase boundaries, which we resolve to be relatively flat beneath most of the Pacific, except under subduction regions and volcanic hotspots. The phase boundaries are closer together beneath the Hawaiian hotspot and also in a larger region of the South Pacific that is flanked by a number of volcanic hotspots. This region overlies the southern part of the large-scale low shear velocity province in the lowermost mantle. A large plume head or cluster of several plumes originating in the lowermost mantle and impinging upon the South Pacific 660. km discontinuity is consistent with observed phase boundary topography and subduction patterns. This feature may be related to large volume volcanic eruptions, such as the Cretaceous Ontong Java Plateau flood basalts, which have been proposed to originate in the South Pacific.