Seismic and mineralogical evidence for an iron-rich mega–ultralow-velocity zone beneath Hawai’i

Mantle plumes beneath major oceanic hot spots appear to be rooted in unusually large structures near the core-mantle boundary, which have markedly reduced seismic wave speeds. The origin of these large ultralow-velocity zones (ULVZs), referred to as mega-ULVZs, remains uncertain partly because of lack of constraints on the relative reduction in shear versus compressional wave speeds (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 the RS/P of its mega-ULVZ to 1 to 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.