Oceanic hotspots are defined by age-progressive chains of volcanoes that form by partiallymelting plumes that upwell from the deep mantle. Much attention has been given to studying 1) plumeridge interactions, 2) plume-slab interactions, and 3) the generation of large igneous provinces (LIPs) by melting the heads of upwelling mantle plumes. By contrast, much less attention has been given to the consequences for the passage of an old oceanic LIP (formed by an ancient plume) over the top of a young, actively upwelling plume. We refer to this phenomenon as “plume-plateau” interaction, and we argue this is an important geologic phenomenon. At ~60 Ma, absolute plate motion models for the Pacific show that the Ontong Java Plateau (OJP) moved over the top of the Samoa and Rarotonga plumes. This would have resulted in partial suppression of plume melting under the exceptionally thick OJP lithosphere, which is supported by the paucity of seamounts atop the OJP along the modeled hotspot tracks for these two plumes. Thus, the seamounts atop the OJP offer a natural laboratory for studying the compositions of
low-degree melts sampled by hotspot lavas, which may have extreme EM signatures. While a small number of seamounts on top of the OJP have been sampled, none of the seamounts that lie on or near Samoa and Rarotonga hotspot tracks have been sampled by dredging, ROV dives, drilling, or otherwise. This presents an important seagoing opportunity to provide the first critical tests examining how two active plumes—Samoa and Rarotonga—melt when overtopped by an ancient LIP.