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Effects of composition and pressure on electronic states of iron in bridgmanite

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Published/Copyright: June 30, 2020
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American Mineralogist
From the journal American Mineralogist Volume 105 Issue 7

Abstract

Electronic states of iron in the lower mantle’s dominant mineral, (Mg,Fe,Al)(Fe,Al,Si)O3 bridgmanite, control physical properties of the mantle including density, elasticity, and electrical and thermal conductivity. However, the determination of electronic states of iron has been controversial, in part due to different interpretations of Mössbauer spectroscopy results used to identify spin state, valence state, and site occupancy of iron. We applied energy-domain Mössbauer spectroscopy to a set of four bridgmanite samples spanning a wide range of compositions: 10–50% Fe/total cations, 0–25% Al/total cations, 12–100% Fe3+/total Fe. Measurements performed in the diamond-anvil cell at pressures up to 76 GPa below and above the high to low spin transition in Fe3+ provide a Mössbauer reference library for bridgmanite and demonstrate the efects of pressure and composition on electronic states of iron. Results indicate that although the spin transition in Fe3+ in the bridgmanite B-site occurs as predicted, it does not strongly affect the observed quadrupole splitting of 1.4 mm/s, and only decreases center shift for this site to 0 mm/s at ~70 GPa. Thus center shift can easily distinguish Fe3+ from Fe2+ at high pressure, which exhibits two distinct Mössbauer sites with center shift ~1 mm/s and quadrupole splitting 2.4–3.1 and 3.9 mm/s at ~70 GPa. Correct quantification of Fe3+/total Fe in bridgmanite is required to constrain the efects of composition and redox states in experimental measurements of seismic properties of bridgmanite. In Fe-rich, mixed-valence bridgmanite at deep-mantle-relevant pressures, up to ~20% of the Fe may be a Fe2.5+ charge transfer component, which should enhance electrical and thermal conductivity in Fe-rich heterogeneities at the base of Earth’s mantle.

Keywords: Bridgmanite; Mössbauer spectroscopy; iron oxidation state; lower mantle

† Present address: Applied Physics Department, Soreq Nuclear Research Center (NRC), Yavne 81800, Israel.

‡ Present address: Cavendish Laboratory, University of Cambridge, Cambridge, CB3 OHE, U.K.

Acknowledgments

The authors thank Susanne Seitz for assistance with microprobe analysis of starting materials at the University of Lausanne. We thank Richard Gaal for assistance with gas loading at the EPFL and Sergey Tkachev for assistance at the APS. Use of the COMPRES-GSECARS gas loading system and the APS offline Mössbauer laboratory facilities were supported by COMPRES under NSF Cooperative Agreement EAR-1606856 and by GSECARS through NSF Grant EAR-1634415 and DOE Grant DE-FG02-94ER14466. 57Co-source Mössbauer spectroscopy was performed at the APS beamline 3-ID Mössbauer Laboratory.

  1. Funding

    This research used resources of the Advanced Photon Source, a U. S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We thank the European Synchrotron Radiation Facility (Grenoble, France) for provision of synchrotron radiation at beamlines ID18 for synchrotron Mössbauer spectroscopy, and beamlines ID09 and ID27 for synchrotron X‑ray diffraction. Laser heating was performed at beamline ID24 of the ESRF with assistance from Innokenty Kantor. S.M. Dorfman acknowledges funding support from the Marie Heim-Vögtlin program of the Swiss National Science Foundation through project PMPDP2_151256 and the National Science Foundation project EAR-1664332. E. Greenberg thanks the Israel Science Foundation (ISF) for their support (Grant No. 1189/14).

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Received: 2019-10-01
Accepted: 2020-01-16
Published Online: 2020-06-30
Published in Print: 2020-07-28

© 2020 Walter de Gruyter GmbH, Berlin/Boston

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  1. Experimental determination of the solubility constant of kurnakovite, MgB3O3(OH)5·5H2O
  2. Elastic properties of majoritic garnet inclusions in diamonds and the seismic signature of pyroxenites in the Earth’s upper mantle
  3. Establishing a protocol for the selection of zircon inclusions in garnet for Raman thermobarometry
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  5. High-pressure silica phase transitions: Implications for deep mantle dynamics and silica crystallization in the protocore
  6. The Cr-Zr-Ca armalcolite in lunar rocks is loveringite: Constraints from electron backscatter diffraction measurements
  7. Effects of composition and pressure on electronic states of iron in bridgmanite
  8. Ti diffusion in feldspar
  9. Radiation-induced defects in montebrasite: An electron paramagnetic resonance study of O– hole and Ti3+ electron centers
  10. New IR spectroscopic data for determination of water abundances in hydrous pantelleritic glasses
  11. Experimental investigation of the effect of nickel on the electrical resistivity of Fe-Ni and Fe-Ni-S alloys under pressure
  12. An experimental approach to examine fluid-melt interaction and mineralization in rare-metal pegmatites
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https://doi.org/10.2138/am-2020-7309
Keywords for this article
Bridgmanite; Mössbauer spectroscopy; iron oxidation state; lower mantle

Articles in the same Issue

  1. Experimental determination of the solubility constant of kurnakovite, MgB3O3(OH)5·5H2O
  2. Elastic properties of majoritic garnet inclusions in diamonds and the seismic signature of pyroxenites in the Earth’s upper mantle
  3. Establishing a protocol for the selection of zircon inclusions in garnet for Raman thermobarometry
  4. Reversely zoned plagioclase in lower crustal meta-anorthosites: An indicator of multistage fracturing and metamorphism in the lower crust
  5. High-pressure silica phase transitions: Implications for deep mantle dynamics and silica crystallization in the protocore
  6. The Cr-Zr-Ca armalcolite in lunar rocks is loveringite: Constraints from electron backscatter diffraction measurements
  7. Effects of composition and pressure on electronic states of iron in bridgmanite
  8. Ti diffusion in feldspar
  9. Radiation-induced defects in montebrasite: An electron paramagnetic resonance study of O– hole and Ti3+ electron centers
  10. New IR spectroscopic data for determination of water abundances in hydrous pantelleritic glasses
  11. Experimental investigation of the effect of nickel on the electrical resistivity of Fe-Ni and Fe-Ni-S alloys under pressure
  12. An experimental approach to examine fluid-melt interaction and mineralization in rare-metal pegmatites
  13. Crystal-chemistry of sulfates from the Apuan Alps (Tuscany, Italy). VI. Tl-bearing alum-(K) and voltaite from the Fornovolasco mining complex
  14. Letter
  15. First-principles modeling of X-ray absorption spectra enlightens the processes of scandium sequestration by iron oxides
  16. Effects of the dissolution of thermal barrier coating materials on the viscosity of remelted volcanic ash
  17. New Mineral Names
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