There are three primary components of the mantle convection system: 1) subduction where one tectonic plate (slab) descends into the Earth’s interior along an oceanic trench, 2) plate spreading at mid-ocean ridges where the majority of seafloor is created, and 3) buoyant upwelling, or mantle plumes, where hot and less dense materials rise from the deep earth to the surface to create volcanoes like the Hawaiian islands. Locations where these three systems interact—including the Tonga oceanic trench, Lau back-arc spreading centers, and Samoa volcanic islands—provide unique opportunities to investigate these competing mechanisms. In this study, we use the juxtaposition of the Samoan plume adjacent to the northern edge of the Tonga-Lau subduction system as a unique natural laboratory to investigate mantle flow in a slab-plume setting, and to understand how the distinctive compositional and thermal characteristics of plume materials are distributed.