Implications for the origin of Hawaiian volcanism from a converted wave analysis of the mantle transition zone

The debate over the origin of intraplate volcanism has been ongoing since the discovery of age-progression at oceanic “hotspots.” The causes of such anomalous volcanic activity have been attributed to either deep-seated thermal plumes in the convecting mantle or shallower causes such as lithospheric structure and deformation or localized mantle flow. Data from the Hawaiian Plume–Lithosphere Undersea Melt Experiment (PLUME) have made it possible to provide detailed images of upper mantle heterogeneity beneath the Hawaiian Islands with much greater resolution than previous experiments. Using receiver function analysis, we determine the depth to and topography along the mantle transition zone discontinuities. Our results indicate that the 410-km discontinuity deepens from northwest to southeast beneath the Hawaiian Islands, corresponding to an average thermal anomaly of ∼325 K located beneath and southeast of the Big Island of Hawaii. In general, temperatures remain elevated by at least ∼150 K directly beneath the Big Island, as far as 200 km away from the hottest measurement inferred from the discontinuity structure. Heterogeneity at these length scales raises questions about rheology, convective circulation, and melt transport in the mantle. Our results also robustly indicate the presence of a low-velocity converter in the deep upper mantle, which carries additional implications for melt transport, volatile budget, and chemical composition of the hotspot source.