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Synthesis, Elasticity, and Spin State of an Intermediate MgSiO3-FeAlO3 Bridgmanite: Implications for Iron in Earth's Lower Mantle

Journal Article · · Journal of Geophysical Research. Solid Earth
DOI:https://doi.org/10.1029/2020jb019964· OSTI ID:1774255
 [1];  [2];  [3];  [4];  [5];  [6];  [7];  [8];  [8];  [9]
  1. Univ. of Michigan, Ann Arbor, MI (United States); Univ. of Hawaii at Manoa, Honolulu, HI (United States). Hawai'i Inst. of Geophysics and Planetology
  2. Univ. of Michigan, Ann Arbor, MI (United States); Michigan State Univ., East Lansing, MI (United States)
  3. Univ. of Hawaii at Manoa, Honolulu, HI (United States). Hawai'i Inst. of Geophysics and Planetology; China Univ. of Geosciences, Wuhan (China). Gemmological Inst.
  4. Carnegie Inst. of Science, Argonne, IL (United States). High Pressure Collaborative Access Team (HPCAT); Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
  5. Univ. of Chicago, Argonne, IL (United States). Center for Advanced Radiation Sources (CARS)
  6. Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS); Univ. of Illinois at Urbana-Champaign, IL (United States); Univ. of Alabama, Birmingham, AL (United States)
  7. Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
  8. Univ. of Hawaii at Manoa, Honolulu, HI (United States). Hawai'i Inst. of Geophysics and Planetology
  9. Univ. of Michigan, Ann Arbor, MI (United States)
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Fe-Al-bearing bridgmanite may be the dominant host for ferric iron in Earth's lower mantle. Here we report the synthesis of (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3 bridgmanite (FA50) with the highest Fe3+-Al3+ coupled substitution known to date. X-ray diffraction measurements showed that at ambient conditions, the FA50 adopted the LiNbO3 structure. Upon compression at room temperature to 18 GPa, it transformed back into the bridgmanite structure, which remained stable up to 102 GPa and 2,600 K. Fitting Birch-Murnaghan equation of state of FA50 bridgmanite yields V0 = 172.1(4) Å3, K0 = 229(4) GPa with K0' = 4(fixed). Furthermore, the calculated bulk sound velocity of the FA50 bridgmanite is ~7.7% lower than MgSiO3 bridgmanite, mainly because the presence of ferric iron increases the unit-cell mass by 15.5%. This difference likely represents the upper limit of sound velocity anomaly introduced by Fe3+-Al3+ substitution. X-ray emission and synchrotron Mössbauer spectroscopy measurements showed that after laser annealing, ~6% of Fe3+ cations exchanged with Al3+ and underwent the high- to low-spin transition at 59 GPa. The low-spin proportion of Fe3+ increased gradually with pressure and reached 17–31% at 80 GPa. Since the cation exchange and spin transition in this Fe3+-Al3+-enriched bridgmanite do not cause resolvable unit-cell volume reduction, and the increase of low-spin Fe3+ fraction with pressure occurs gradually, the spin transition would not produce a distinct seismic signature in the lower mantle. However, it may influence iron partitioning and isotopic fractionation, thus introducing chemical heterogeneity in the lower mantle.

Research Organization:
Argonne National Laboratory (ANL), Argonne, IL (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES); USDOE National Nuclear Security Administration (NNSA); National Science Foundation (NSF); National Aeronautic and Space Administration (NASA)
Grant/Contract Number:
AC02-06CH11357; FG02-94ER14466
OSTI ID:
1774255
Alternate ID(s):
OSTI ID: 1639149
Journal Information:
Journal of Geophysical Research. Solid Earth, Journal Name: Journal of Geophysical Research. Solid Earth Journal Issue: 7 Vol. 125; ISSN 2169-9313
Publisher:
American Geophysical UnionCopyright Statement
Country of Publication:
United States
Language:
English

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Related Subjects

58 GEOSCIENCES
bridgmanite
cation exchange
equation of state
ferric iron
lower mantle
spin transition