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Title: Elasticity of Ferropericlase across the Spin Crossover in the Earth’s Lower Mantle

Authors:
; ; ; ;  [1]
  1. Texas
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
Sponsoring Org.:
National Science Foundation (NSF)
OSTI Identifier:
1234739
Resource Type:
Journal Article
Resource Relation:
Journal Name: Scientific Reports; Journal Volume: 5; Journal Issue: 2015
Country of Publication:
United States
Language:
ENGLISH

Citation Formats

Yang, Jing, Tong, Xinque, Lin, Jung-Fu, Okuchi, Takuo, and Tomioka, Naotaka. Elasticity of Ferropericlase across the Spin Crossover in the Earth’s Lower Mantle. United States: N. p., 2016. Web. doi:10.1038/srep17188.
Yang, Jing, Tong, Xinque, Lin, Jung-Fu, Okuchi, Takuo, & Tomioka, Naotaka. Elasticity of Ferropericlase across the Spin Crossover in the Earth’s Lower Mantle. United States. doi:10.1038/srep17188.
Yang, Jing, Tong, Xinque, Lin, Jung-Fu, Okuchi, Takuo, and Tomioka, Naotaka. 2016. "Elasticity of Ferropericlase across the Spin Crossover in the Earth’s Lower Mantle". United States. doi:10.1038/srep17188.
@article{osti_1234739,
title = {Elasticity of Ferropericlase across the Spin Crossover in the Earth’s Lower Mantle},
author = {Yang, Jing and Tong, Xinque and Lin, Jung-Fu and Okuchi, Takuo and Tomioka, Naotaka},
abstractNote = {},
doi = {10.1038/srep17188},
journal = {Scientific Reports},
number = 2015,
volume = 5,
place = {United States},
year = 2016,
month = 6
}
  • Knowing the elasticity of ferropericlase across the spin transition can help explain seismic and mineralogical models of the lower-mantle including the origin of seismic heterogeneities in the middle to lowermost parts of the lower mantle1–4. However, the effects of spin transition on full elastic constants of ferropericlase remain experimentally controversial due to technical challenges in directly measuring sound velocities under lower-mantle conditions1–5. Here we have reliably measured both V P and V S of a single-crystal ferropericlase ((Mg 0.92,Fe 0.08)O) using complementary Brillouin Light Scattering and Impulsive Stimulated Light Scattering coupled with a diamond anvil cell up to 96 GPa.more » The derived elastic constants show drastically softened C 11 and C 12 within the spin transition at 40–60 GPa while C 44 is not affected. The spin transition is associated with a significant reduction of the aggregate V P/V S via the aggregate V P softening because V S softening does not visibly occur within the transition. Based on thermoelastic modelling along an expected geotherm, the spin crossover in ferropericlase can contribute to 2% reduction in V P/V S in a pyrolite mineralogical model in mid lower-mantle. Our results indicate that the middle to lowermost parts of the lower-mantle would exhibit enhanced seismic heterogeneities due to the occurrence of the mixed-spin and low-spin ferropericlase.« less
  • The effects of the spin transition on the physics and chemistry of the lower-mantle ferropericlase and perovskite have been suggested to have significant consequences on the seismology and geochemistry. Knowing the elasticity of ferropericlase across the spin transition can help explain seismic and mineralogical models of the lower-mantle including the origin of seismic heterogeneities in the middle to lowermost parts of the lower mantle. However, full elastic tensors of ferropericlase within the spin transition and in the low-spin state remains experimentally controversial due to technical challenges in directly measuring sound velocities under lower-mantle conditions. Here we have reliably measured bothmore » VP and VS of a single-crystal ferropericlase ((Mg 0.92,Fe 0.08)O) using complementary Brillouin Light Scattering and Impulsive Stimulated Light Scattering coupled with diamond anvil cell (DAC) up to 96 GPa. The derived elastic tensors show drastically softened C 11 and C 12 within the spin transition at 40-60 GPa while C 44 is not affected. Based on thermoelastic modelling along an expected geotherm, the spin crossover in ferropericlase can contribute to elastic abnormalities at mid lower-mantle conditions. Furthermore, our results imply the middle to lowermost parts of the lower mantle would exhibit enhanced seismic heterogeneities due to the occurrence of the mixed-spin and low-spin ferropericlase.« less
  • The thermal equation of state of ferropericlase [(Mg{sub 0.75}Fe{sub 0.25})O] has been investigated by synchrotron X-ray diffraction up to 140 GPa and 2000 K in a laser-heated diamond anvil cell. Based on results at high pressure-temperature conditions, the derived phase diagram shows that the spin crossover widens at elevated temperatures. Along the lower-mantle geotherm, the spin crossover occurs between 1700 km and 2700 km depth. Compared to the high-spin state, thermoelastic modeling of the data shows a {approx}1.2% increase in density, a factor of two increase in thermal expansion coefficient over a range of 1000 km, and a maximum decreasemore » of 37% and 13% in bulk modulus and bulk sound velocity, respectively, at {approx}2180 km depth across the spin crossover. These anomalous behaviors in the thermoelastic properties of ferropericlase across the spin crossover must be taken into account in order to understand the seismic signatures and geodynamics of the lower mantle.« less
  • Deciphering the origin of seismic heterogeneity has been one of the major challenges in understanding the geochemistry and geodynamics of the deep mantle. Fully anisotropic elastic properties of constituent minerals at relevant pressure-temperature conditions of the lower mantle can be used to calculate seismic heterogeneity parameters in order to better understand chemically and thermally induced seismic heterogeneities. In this study, the single-crystal elastic properties of ferropericlase (Mg0.94Fe0.06)O were measured using Brillouin spectroscopy and X-ray diffraction at conditions up to 50 GPa and 900 K. The velocity-density results were modeled using third-order finite-strain theory and thermoelastic equations along a representative geothermmore » to investigate high pressure-temperature and compositional effects on the seismic heterogeneity parameters. Our results demonstrate that from 660 to 2000 km, compressional wave anisotropy of ferropericlase increased from 4% to 9.7%, while shear wave anisotropy increased from 9% to as high as 22.5%. The thermally induced lateral heterogeneity ratio (RS/P = ∂lnVS/∂lnVP) of ferropericlase was calculated to be 1.48 at ambient pressure but decreased to 1.43 at 40 GPa along a representative geotherm. The RS/P of a simplified pyrolite model consisting of 80% bridgmanite and 20% ferropericlase was approximately 1.5, consistent with seismic models at depths from 670 to 1500 km, but showed an increased mismatch at lower mantle depths below ~1500 km. This discrepancy below mid-lower mantle could be due to either a contribution from chemically induced heterogeneity or the effects of the Fe spin transition in the deeper parts of the Earth's lower mantle.« less
  • Recent high-pressure studies have shown that an electronic spin transition of iron in ferropericlase, an expected major phase of Earth's lower mantle, results in changes in its properties, including density, incompressibility, radiative thermal conductivity, electrical conductivity, and sound velocities. To understand the rheology of ferropericlase across the spin transition, we have used in situ radial X-ray diffraction techniques to examine ferropericlase, (Mg{sub 0.83},Fe{sub 0.17})O, deformed non-hydrostatically in a diamond cell up to 81 GPa at room temperature. Compared with recent quasi-hydrostatic studies, the range of the spin transition is shifted by approximately 20 GPa as a result of the presencemore » of large differential stress in the sample. We also observed a reduction in incompressibility and in the unit cell volume of 3% across the spin transition. Our radial X-ray diffraction results show that the {l_brace}0 0 1{r_brace} texture is the dominant lattice preferred orientation in ferropericlase across the spin transition and in the low-spin state. Viscoplastic self-consistent polycrystal plasticity simulations suggest that this preferred orientation pattern is produced by {l_brace}1 1 0{r_brace}<1-10> slip. Analyzing our radial X-ray diffraction patterns using lattice strain theory, we evaluated the lattice d-spacings of ferropericlase and Mo as a function of the {psi} angle between the compression direction and the diffracting plane normal. These analyses give the ratio between the uniaxial stress component (t) and the shear modulus (G) under constant stress condition, which represents a proxy for the supported differential stress and elastic strength. This ratio in the mixed-spin and low-spin states is lower than what is expected from previous studies of high-spin ferropericlase, indicating that the spin transition results in a reduced differential stress and elastic strength along with the volume reduction. The influence of the spin transition on the differential stress and strength of ferropericlase is expected to be less dominant across the wide spin transition zone at high pressure-temperature conditions relevant to the lower mantle.« less