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

TL;DR

This study finds that a deep zone beneath Hawai’i is rich in iron, which may influence how mantle plumes form and behave.

Contribution

The paper provides new mineralogical and seismic evidence for iron-rich composition in a large ultralow-velocity zone beneath Hawai’i.

Findings

The RS/P ratio of Hawai’i’s mega-ULVZ is constrained to 1 to 1.3 using seismic data.

Iron-rich magnesiowüstite matches the seismic observations and explains the ULVZ’s properties.

Iron enrichment may increase thermal conductivity and drive localized mantle plume upwelling.

Abstract

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.

Hawai’i’s volcanic roots trace back to dense, iron-rich relics deep within Earth’s mantle.

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