Mantle plumes beneath major oceanic hotspots appear to be rooted in unusually large structures near the core-mantle boundary, which have dramatically reduced seismic wave speeds. The origin of these large ultralow-velocity zones, referred to as mega-ULVZs, remains uncertain partly due to lack of constraints on the relative reduction in shear vs. compressional wavespeeds (RS/P). This ratio can give clues into the compositional makeup of the mega-ULVZs. Through joint seismic analysis of core-diffracted P- and S-waves beneath Hawai'i, we constrain RS/P of its mega-ULVZ to 1–1.3. Mineralogical modeling reveals that iron enrichment via solid iron-rich magnesiowüstite ((Mg,Fe)O) matches this seismic constraint, independent of modeled ULVZ thickness. Enrichment of metallic iron-rich magnesiowüstite likely enhances the thermal conductivity of mega-ULVZs and provides a mechanism to drive localized plume upwelling. Higher reported RS/P values for smaller ULVZs near subduction zones may therefore indicate different processes at play controlling ULVZ formation across the diverse core-mantle boundary landscape.