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Evaluating the Role of Iron-Rich (Mg,Fe)O in Ultralow Velocity Zones

Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA
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Minerals 2019, 9(12), 762; https://doi.org/10.3390/min9120762
Received: 12 November 2019 / Revised: 4 December 2019 / Accepted: 6 December 2019 / Published: 8 December 2019
(This article belongs to the Special Issue Mineral Physics—In Memory of Orson Anderson)
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, K0T = 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. View Full-Text
Keywords: (Mg,Fe)O; ultralow velocity zones; core–mantle boundary; equations of state; finite strain analysis; inverse model; X-ray diffraction; synchrotron Mössbauer spectroscopy (Mg,Fe)O; ultralow velocity zones; core–mantle boundary; equations of state; finite strain analysis; inverse model; X-ray diffraction; synchrotron Mössbauer spectroscopy
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Dobrosavljevic, V.V.; Sturhahn, W.; Jackson, J.M. Evaluating the Role of Iron-Rich (Mg,Fe)O in Ultralow Velocity Zones. Minerals 2019, 9, 762.

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