Seismic Constraints on a Double-Layered Asymmetric Whole-Mantle Plume Beneath Hawai'i

It is generally accepted that mantle plumes are responsible for hotspot chains and as such provide insight to mantle convection processes. Among all the hotspots, the Hawaiian chain is a characteristic example that has been extensively explored. However, many questions remain. If a plume does exist beneath the Hawaiian chain, what is the shape, size, and orientation of the plume conduit? To what extent can the seismic structure of the plume be mapped? Can we see a continuous plume conduit extending from the lower to the upper mantle? At what depth do melting processes occur? Here, we combine constraints from three data sets (body waves, ballistic surface waves, and ambient noise) to create 3D images of the velocity structure beneath the Hawaiian islands from a depth of ~800km to the surface. We use data from the Hawaiian Plume Lithosphere Undersea Melt Experiment (PLUME), which was a network of four‐component broadband ocean bottom seismometers that had a network aperture of ~1000km. Our multiphase 3D model results indicate there is a large deep‐rooted low‐velocity anomaly rising from the lower mantle. At transition zone depths the conduit is located to the southeast of Hawai‘i. A 2% S‐wave anomaly is observed in the core of the plume conduit around 700km depth, which, once corrected for damping effects, suggests a 200–250°C temperature anomaly assuming a thermal plume. In the upper mantle, there is a horizontal plume “pancake” at shallow depths beneath the oceanic lithosphere, and there is also a second horizontal low‐ velocity layer in the 250 to 410km depth range beneath the island chain. This second layer is only revealed after surface wave phase velocity data are incorporated into the inversion scheme to improve the constraints on the structure in the upper ~200km. We suggest this feature is a deep eclogite pool (DEP), an interpretation consistent with geodynamic modeling [Ballmer et al., 2013]. The model also shows reduced lithospheric velocities compared to the typical ~100 Myr old lithosphere, implying lithospheric rejuvenation by the plume. In addition, a shallow (~20km) low‐velocity anomaly is observed southeast of the Island of Hawai‘i. This suggests a newly modified lithosphere, as might be expected in the location of an emerging new island in the Hawaiian chain