Local variability in the 410-km mantle discontinuity under a hotspot

Observations of the natural variability in the structure of seismic discontinuities in the Earthʹs mantle can be used to infer spatial variations in temperature and composition. Here I present a high-resolution analysis of converted seismic waves from the olivine–wadsleyite phase transition beneath Rarotonga, Cook Islands. New advances in seismic signal processing permit the determination of local variations in phase transition thickness in addition to variations in absolute depth. I find that the depth and thickness of the phase transition vary systematically underneath the island, and further that temperature alone is insufficient to explain these observations. A chemical anomaly is required, consistent with the presence of a plume in the mantle.