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

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 in this paper, 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 ofmore » 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.« less
Authors:
 [1] ;  [1] ;  [1] ;  [2] ;  [3] ;  [3] ;  [4] ;  [5]
  1. California Inst. of Technology (CalTech), Pasadena, CA (United States). Seismological Lab.
  2. California Inst. of Technology (CalTech), Pasadena, CA (United States). Seismological Lab.; Princeton Univ., NJ (United States). Dept. of Geosciences
  3. Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
  4. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Advanced Light Source (ALS)
  5. California Inst. of Technology (CalTech), Pasadena, CA (United States). Seismological Lab.; Harvard Univ., Cambridge, MA (United States). Earth and Planetary Sciences
Publication Date:
Grant/Contract Number:
AC02-06CH11357; AC02-05CH11231
Type:
Accepted Manuscript
Journal Name:
American Mineralogist
Additional Journal Information:
Journal Volume: 101; Journal Issue: 5; Journal ID: ISSN 0003-004X
Publisher:
Mineralogical Society of America
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org:
National Science Foundation (NSF); Consortium for Materials Properties Research in Earth Sciences (COMPRES); University of Chicago - Center for Advanced Radiation Sources (CARS); USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES
OSTI Identifier:
1430073
Alternate Identifier(s):
OSTI ID: 1454445

Solomatova, Natalia V., Jackson, Jennifer M., Sturhahn, Wolfgang, Wicks, June K., Zhao, Jiyong, Toellner, Thomas S., Kalkan, Bora, and Steinhardt, William M.. Equation of state and spin crossover of (Mg,Fe)O at high pressure, with implications for explaining topographic relief at the core-mantle boundary. United States: N. p., Web. doi:10.2138/am-2016-5510.
Solomatova, Natalia V., Jackson, Jennifer M., Sturhahn, Wolfgang, Wicks, June K., Zhao, Jiyong, Toellner, Thomas S., Kalkan, Bora, & Steinhardt, William M.. Equation of state and spin crossover of (Mg,Fe)O at high pressure, with implications for explaining topographic relief at the core-mantle boundary. United States. doi:10.2138/am-2016-5510.
Solomatova, Natalia V., Jackson, Jennifer M., Sturhahn, Wolfgang, Wicks, June K., Zhao, Jiyong, Toellner, Thomas S., 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". United States. doi:10.2138/am-2016-5510. https://www.osti.gov/servlets/purl/1430073.
@article{osti_1430073,
title = {Equation of state and spin crossover of (Mg,Fe)O at high pressure, with implications for explaining topographic relief at the core-mantle boundary},
author = {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.},
abstractNote = {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 in this paper, 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 K0T,HS for the HS state is 160(2) GPa with a K'0T,HS of 4.12(14) and a V0,HS of 77.29(0) Å3. For the LS state, the K0T,LS is 173(13) GPa with a K'0T,LS fixed to 4 and a V0,LS of 73.64(94) Å3. 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 Fe2+-like sites. At pressure between 44 and 84, two additional Fe2+-like sites with a QS of 0 are required, indicative of low-spin iron. Above 84 GPa, two low-spin Fe2+-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 K0,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.},
doi = {10.2138/am-2016-5510},
journal = {American Mineralogist},
number = 5,
volume = 101,
place = {United States},
year = {2016},
month = {5}
}