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Title: Single-crystal equations of state of magnesiowüstite at high pressures

Solid solutions of (Mg,Fe)O with high iron enrichment may be an important component of ultralow-velocity zones at Earth’s core-mantle boundary. However, to date there have been few high-precision studies on the elastic properties of these materials. In this study we present results on the compression of (Mg 0.22Fe 0.78)O magnesiowüstite in both neon and helium pressure media using single-crystal diffraction to ~55 GPa. In addition, our sample was characterized by time-domain synchrotron Mössbauer spectroscopy at ambient pressure using an extended time range that resulted in vastly improved energy resolution. The combination of these high-resolution techniques tightly constrains the presence of a defect-structure component at room pressure due to 4.7 mol% tetrahedrally-coordinated ferric iron, resulting in a renormalized composition of (Mg 0.215Fe 0.7620.023)O. Both high-pressure diffraction datasets are well described by a 3rd-order Birch-Murnaghan equation of state. The best fit-parameters for a crystal with cubic structure in helium are K 0T = 148(3) GPa, K' 0T = 4.09(12), and V 0 = 78.87(6) Å 3. Increasing differential stress in the neon-containing sample chamber was correlated with increasing apparent distortion of the initially cubic unit cell, requiring a lower-symmetry hexagonal cell to fit the data above ~20 GPa. For fit equationsmore » of state, we determine the pressure-dependent correlation ellipses for the equation of state parameters and compare with previously published single-crystal diffraction data from (Mg,Fe)O crystals in a helium medium. We make two main observations from the datasets using a helium pressure medium: K 0T decreases as a function of increasing iron content from periclase to wüstite and K' 0T is consistent with an approximately constant value of 4.0 that is independent of iron content, at least up to (Mg,Fe)O containing ~78 mol% FeO. Finally, in combination with previously reported thermal parameters, we compute the density of Mw78 at core-mantle boundary conditions and discuss the implications.« less
Authors:
 [1] ;  [1] ;  [1] ;  [2] ;  [3] ;  [3]
  1. California Inst. of Technology (CalTech), Pasadena, CA (United States)
  2. Univ. of Hawaii, Honolulu, HI (United States)
  3. Argonne National Lab. (ANL), Argonne, IL (United States)
Publication Date:
Grant/Contract Number:
AC02-06CH11357
Type:
Accepted Manuscript
Journal Name:
American Mineralogist
Additional Journal Information:
Journal Volume: 102; Journal Issue: 8; Journal ID: ISSN 0003-004X
Publisher:
Mineralogical Society of America
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; magnesiowüstite; single-crystal diffraction; elasticity; equations of state; Mössbauer 19 spectroscopy; high pressure
OSTI Identifier:
1416148

Finkelstein, Gregory J., Jackson, Jennifer M., Sturhahn, Wolfgang, Zhang, Dongzhou, Alp, E. Ercan, and Toellner, Thomas S.. Single-crystal equations of state of magnesiowüstite at high pressures. United States: N. p., Web. doi:10.2138/am-2017-5966.
Finkelstein, Gregory J., Jackson, Jennifer M., Sturhahn, Wolfgang, Zhang, Dongzhou, Alp, E. Ercan, & Toellner, Thomas S.. Single-crystal equations of state of magnesiowüstite at high pressures. United States. doi:10.2138/am-2017-5966.
Finkelstein, Gregory J., Jackson, Jennifer M., Sturhahn, Wolfgang, Zhang, Dongzhou, Alp, E. Ercan, and Toellner, Thomas S.. 2017. "Single-crystal equations of state of magnesiowüstite at high pressures". United States. doi:10.2138/am-2017-5966. https://www.osti.gov/servlets/purl/1416148.
@article{osti_1416148,
title = {Single-crystal equations of state of magnesiowüstite at high pressures},
author = {Finkelstein, Gregory J. and Jackson, Jennifer M. and Sturhahn, Wolfgang and Zhang, Dongzhou and Alp, E. Ercan and Toellner, Thomas S.},
abstractNote = {Solid solutions of (Mg,Fe)O with high iron enrichment may be an important component of ultralow-velocity zones at Earth’s core-mantle boundary. However, to date there have been few high-precision studies on the elastic properties of these materials. In this study we present results on the compression of (Mg0.22Fe0.78)O magnesiowüstite in both neon and helium pressure media using single-crystal diffraction to ~55 GPa. In addition, our sample was characterized by time-domain synchrotron Mössbauer spectroscopy at ambient pressure using an extended time range that resulted in vastly improved energy resolution. The combination of these high-resolution techniques tightly constrains the presence of a defect-structure component at room pressure due to 4.7 mol% tetrahedrally-coordinated ferric iron, resulting in a renormalized composition of (Mg0.215Fe0.762⟂0.023)O. Both high-pressure diffraction datasets are well described by a 3rd-order Birch-Murnaghan equation of state. The best fit-parameters for a crystal with cubic structure in helium are K0T = 148(3) GPa, K'0T = 4.09(12), and V0 = 78.87(6) Å3. Increasing differential stress in the neon-containing sample chamber was correlated with increasing apparent distortion of the initially cubic unit cell, requiring a lower-symmetry hexagonal cell to fit the data above ~20 GPa. For fit equations of state, we determine the pressure-dependent correlation ellipses for the equation of state parameters and compare with previously published single-crystal diffraction data from (Mg,Fe)O crystals in a helium medium. We make two main observations from the datasets using a helium pressure medium: K0T decreases as a function of increasing iron content from periclase to wüstite and K'0T is consistent with an approximately constant value of 4.0 that is independent of iron content, at least up to (Mg,Fe)O containing ~78 mol% FeO. Finally, in combination with previously reported thermal parameters, we compute the density of Mw78 at core-mantle boundary conditions and discuss the implications.},
doi = {10.2138/am-2017-5966},
journal = {American Mineralogist},
number = 8,
volume = 102,
place = {United States},
year = {2017},
month = {8}
}