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Equation of state and spin crossover of (Mg,Fe)O at high pressure, with implications for explaining topographic relief at the core-mantle boundary

Solomatova, Natalia V. and Jackson, Jennifer M. and Sturhahn, Wolfgang and Wicks, June K. and Zhao, Jiyong and Toellner, Thomas S. and Kalkan, Bora and Steinhardt, William M. (2016) Equation of state and spin crossover of (Mg,Fe)O at high pressure, with implications for explaining topographic relief at the core-mantle boundary. American Mineralogist, 101 (5). pp. 1084-1093. ISSN 0003-004X. doi:10.2138/am-2016-5510.

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Iron-bearing periclase is thought to represent a significant fraction of Earth’s lower mantle. However, the concentration of iron in (Mg,Fe)O is not well constrained at all mantle depths. Therefore, understanding the effect of iron on the density and elastic properties of this phase plays a major role in interpreting seismically observed complexity in the deep Earth. Here we examine the high-pressure behavior of polycrystalline (Mg,Fe)O containing 48 mol% FeO, loaded hydrostatically with neon as a pressure medium. Using X-ray diffraction and synchrotron Mössbauer spectroscopy, we measure the equation of state to about 83 GPa and hyperfine parameters to 107 GPa at 300 K. A gradual volume drop corresponding to a high-spin (HS) to low-spin (LS) crossover is observed between ~45 and 83 GPa with a volume drop of 1.85% at 68.8(2.7) GPa, the calculated spin transition pressure. Using a newly formulated spin crossover equation of state, the resulting zero-pressure isothermal bulk modulus K_(0T,HS) for the HS state is 160(2) GPa with a K′_(0T,HS) of 4.12(14) and a V_(0,HS) of 77.29(0) Å^3. For the LS state, the K_(0T,LS) is 173(13) GPa with a K′_(0T,LS) fixed to 4 and a V_(0,LS) of 73.64(94) Å^3. To confirm that the observed volume drop is due to a spin crossover, the quadrupole splitting (QS) and isomer shift (IS) are determined as a function of pressure. At low pressures, the Mössbauer spectra are well explained with two Fe^(2+)-like sites. At pressure between 44 and 84, two additional Fe^(2+)-like sites with a QS of 0 are required, indicative of low-spin iron. Above 84 GPa, two low-spin Fe^(2+)-like sites with increasing weight fraction explain the data well, signifying the completion of the spin crossover. To systematically compare the effect of iron on the equation of state parameters for (Mg,Fe)O, a spin crossover equation of state was fitted to the pressure-volume data of previous measurements. Our results show that K_(0,HS) is insensitive to iron concentration between 10 to 60 mol% FeO, while the spin transition pressure and width generally increases from about 50–80 and 2–25 GPa, respectively. A key implication is that iron-rich (Mg,Fe)O at the core-mantle boundary would likely contain a significant fraction of high-spin (less dense) iron, contributing a positive buoyancy to promote observable topographic relief in tomographic images of the lowermost mantle.

Item Type:Article
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URLURL TypeDescription
Solomatova, Natalia V.0000-0002-2331-3427
Jackson, Jennifer M.0000-0002-8256-6336
Sturhahn, Wolfgang0000-0002-9606-4740
Zhao, Jiyong0000-0002-0777-3626
Additional Information:© 2016 Mineralogical Society of America. Manuscript received July 24, 2015; Manuscript accepted December 24, 2015; First Published on May 02, 2016. Manuscript handled by Martin Kunz. We thank E.E. Alp and W. Bi for the isomer shift measurement of the reference stainless steel foil. We are thankful to NSF-EAR-CAREER-0956166, NSF CSEDI-EAR-1161046, and COMPRES, which partially supports operations at Sector 3 (APS), the Mössbauer Laboratory (APS), and Beamline 12.2.2 (ALS). Ambient X‑ray diffraction experiments at 11-BM of APS were made possible by Saul Lapidus and Lynn Ribaud. Microprobe analyses at Caltech were partially funded by MRSEX Program of the NSF under DMR-0080065. Ruby fluorescence measurements for the SMS experiments were conducted at GSE-CARS. Use of the Advanced Photon Source is supported by the U.S. DOE, Office of Science (DE-AC02-06CH11357). The Advanced Light Source is supported by the U.S. DOE, Office of Science (DE-AC02-05CH11231). We thank two anonymous reviewers for their thoughtful comments.
Funding AgencyGrant Number
Consortium for Materials Properties Research in Earth Sciences (COMPRES)UNSPECIFIED
Department of Energy (DOE)DE-AC02-06CH11357
Department of Energy (DOE)DE-AC02-05CH11231
Subject Keywords:(Mg,Fe)O, ferropericlase, spin crossover, equation of state, X‑ray diffraction, synchrotron Mössbauer spectroscopy, lower mantle, ultralow-velocity zones
Issue or Number:5
Record Number:CaltechAUTHORS:20160708-114514140
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Official Citation:Equation of state and spin crossover of (Mg,Fe)O at high pressure, with implications for explaining topographic relief at the core-mantle boundary Natalia V. Solomatova, Jennifer M. Jackson, Wolfgang Sturhahn, June K. Wicks, Jiyong Zhao, Thomas S. Toellner, Bora Kalkan, William M. Steinhardt American Mineralogist May 2016, 101 (5) 1084-1093; DOI: 10.2138/am-2016-5510
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:68925
Deposited By: Tony Diaz
Deposited On:08 Jul 2016 23:33
Last Modified:11 Nov 2021 04:06

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