ABSTRACT

In this study, we review recent seismological and geochemical studies of the hotspots in the South Pacific superswell. Extensive studies on global-scale seismic tomography have revealed the presence of slow seismic-velocity anomalies in the lower mantle beneath the superswell region although the size and vertical extent of these anomalies are not well constrained. In the last decade, regional seismic networks deployed on islands and the seafloor on the superswell have enabled detailed imaging of the mantle structure. Results show that the low-velocity superplume extended from the core-mantle boundary to a depth of 1000 km, and the low-velocity narrow plumes extend from the top of the superplume toward the South Pacific hotspots. Geochemical studies have suggested that dehydrated subducted oceanic crust is involved in the formation of the HIMU lavas, but the trace element and isotopic composition of HIMU lavas cannot be explained by the inclusion of this crust. Recent studies have revealed that the oceanic crust should sink into the lowermost mantle and melt, which would metasomatize the ambient mantle to form the HIMU reservoir. We present a model incorporating a thermochemical superplume and secondary narrow plumes generated from the superplume; our model can explain seismic structure, geochemical heterogeneities of ocean island basalts in the superswell region, age progression of the South Pacific hotspots, and massive eruption and formation of large oceanic plateaus in the Cretaceous period.