Hotspots have geochemical fingerprints distinct from mid-ocean ridges. Approximately half of catalogued hotspots are obviously hotter than the ambient mantle. This is compatible with buoyant plumes rising from the lowermost mantle, entraining a distinct geochemical reservoir, perhaps housed in the Low Shear Velocity Provinces (LLSVPs). However, the nature of LLSVPs themselves and the dynamical relationship between plumes and LLSVPs remain hotly debated. In the spectrum of LLSVPs, two end-members exist: purely thermal plume clusters and fixed, rigid, thermochemical piles.

We address the link between hotspots and LLSVPs from a new perspective, using state-of-the-art 3-D optical visualization in very viscous fluids, which serve as analogs for the two end-member cases. We perform a series of experiments in a 40.5cm x 40.5 cm x 27.5 cm plexiglass tank, with corn syrup heated from below. With the reconstruction of the full 4-D velocity and temperature field, as well as Lagrangian analysis, we can track the plumes with unprecedented detail. I will show how rigid thermochemical piles are a better explanation for LLSVPs as they can explain the buoyancy flux of global hotspots to the first order. I further discuss the entrainment of LLSVP material and the implications for the source of the distinct geochemical signatures.