Abstract
The composition of ultralow velocity zones (ULVZs) remains an open question, despite advances in both seismology and experimental work. We investigate the hypothesis of iron-rich (Mg,Fe)O (magnesiowüstite) as a cause of ULVZ seismic signatures. We report new quasi-hydrostatic X-ray diffraction measurements to constrain the equation of state of (Mg0.06Fe0.94)O with fit parameters V0 = 9.860 ± 0.007 Å3, K0T = 155.3 ± 2.2 GPa, K’0T = 3.79 ± 0.11, as well as synchrotron Mössbauer spectroscopy measurements to characterize the high-pressure magnetic and spin state of magnesiowüstite. We combine these results with information from previous studies to calculate the elastic behavior at core–mantle boundary conditions of magnesiowüstite, as well as coexisting bridgmanite and calcium silicate perovskite. Forward models of aggregate elastic properties are computed, and from these, we construct an inverse model to determine the proportions of magnesiowüstite that best reproduce ULVZ observations within estimated mutual uncertainties. We find that the presence of magnesiowüstite can explain ULVZ observations exhibiting 1:2 VP:VS reduction ratios relative to the Preliminary Reference Earth Model (PREM), as well as certain 1:3 VP:VS reductions within estimated uncertainty bounds. Our work quantifies the viability of compositionally distinct ULVZs containing magnesiowüstite and contributes to developing a framework for a methodical approach to evaluating ULVZ hypotheses.
Highlights
The boundary layer separating the iron-dominant liquid outer core from the silicate-rich mantle is a region of great complexity, where extreme contrasts in material properties promote the persistence of multiscale structural heterogeneities (e.g., [1,2,3])
Advances in seismological observation over the past several decades have provided increasing evidence for the existence of ultralow velocity zones (ULVZs)—Small (
Having calculated sound velocities and densities for three iron-rich compositions of (Mg,Fe)O at the pressure–temperature conditions of the core–mantle boundary (CMB), we investigated the likelihood of the presence of this material in ULVZs given the range of seismic observations
Summary
The boundary layer separating the iron-dominant liquid outer core from the silicate-rich mantle is a region of great complexity, where extreme contrasts in material properties promote the persistence of multiscale structural heterogeneities (e.g., [1,2,3]). Despite the centrality of this region in the development of the solid Earth system, many open questions remain regarding the characteristics, origins, and dynamic interactions of observed heterogeneities, such as large thermochemical piles, ultralow velocity zones, subducted former oceanic material, and small seismic scatterers. Less than 20% of the core–mantle boundary (CMB) has been explored by seismic studies investigating ULVZs, yet the existing observations of ULVZs reveal a large variety of elastic properties, locations, and structural geometries (e.g., [7]). Whether all ULVZs share common characteristics and/or origins and what dynamical links may exist between them and other multi-scale features like slabs or slab debris, large thermochemical piles, and upwelling mantle plumes is the subject of current multidisciplinary research
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