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Elasticity of Ferropericlase across the Spin Crossover in the Earth's Lower Mantle.

Yang J, Tong X, Lin JF, Okuchi T, Tomioka N - Sci Rep (2015)

Bottom Line: The spin transition is associated with a significant reduction of the aggregate VP/VS via the aggregate VP softening because VS 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 VP/VS in a pyrolite mineralogical model in mid lower-mantle.Our results imply 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712, USA.

ABSTRACT
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, 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 conditions. Here we have reliably measured both VP and VS of a single-crystal ferropericlase ((Mg0.92,Fe0.08)O) using complementary Brillouin Light Scattering and Impulsive Stimulated Light Scattering coupled with a diamond anvil cell up to 96 GPa. The derived elastic constants show drastically softened C11 and C12 within the spin transition at 40-60 GPa while C44 is not affected. The spin transition is associated with a significant reduction of the aggregate VP/VS via the aggregate VP softening because VS 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 VP/VS in a pyrolite mineralogical model in mid lower-mantle. Our results imply 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.

No MeSH data available.


Related in: MedlinePlus

Elasticity of single-crystal ferropericlase (Mg0.92Fe0.08)O as a function of pressure at 300 K.(A) Compressional and shear wave velocities along the [100] and [110] crystallographic axes as a function of pressure. Compressional wave velocities were measured using the ISS technique, while shear wave velocities were measured using the BLS technique. Open circles: experimental data; solid lines: modelled velocity profiles using thermoelastic equations (see SI for details). (B) Elastic constants (Cij) as a function of pressure. Open circles: Cij directly derived from measured compressional and shear wave velocities via Christoffel’s equations; solid lines: modelled Cij profiles. Vertical dashed lines are plotted to guide the eyes for the high-spin (HS), mixed-spin (MS; HS + LS), and low-spin (LS) regions, respectively (see Supplementary Fig. S2 for details).
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f2: Elasticity of single-crystal ferropericlase (Mg0.92Fe0.08)O as a function of pressure at 300 K.(A) Compressional and shear wave velocities along the [100] and [110] crystallographic axes as a function of pressure. Compressional wave velocities were measured using the ISS technique, while shear wave velocities were measured using the BLS technique. Open circles: experimental data; solid lines: modelled velocity profiles using thermoelastic equations (see SI for details). (B) Elastic constants (Cij) as a function of pressure. Open circles: Cij directly derived from measured compressional and shear wave velocities via Christoffel’s equations; solid lines: modelled Cij profiles. Vertical dashed lines are plotted to guide the eyes for the high-spin (HS), mixed-spin (MS; HS + LS), and low-spin (LS) regions, respectively (see Supplementary Fig. S2 for details).

Mentions: P-V relations of single-crystal ferropericlase ((Mg0.92, Fe0.08)O) in the (100) platelet were measured using synchrotron X-ray diffraction up to 91 GPa at room temperature in a DAC. These results are used to evaluate the EoS parameters, the width of the spin transition, and the fraction of the high-spin (HS) and low-spin (LS) states in ferropericlase2023 (Fig. S1 and Fig. S2) (See Methods and SI for details of the experiments and modelling). Analysis of the measured P-V curve shows that the spin transition occurs over pressures ranging between 40 GPa and 60 GPa, and is associated with a density increase of 1.2% (0.1%). The derived isothermal bulk modulus at ambient conditions (KT0 ) and its pressure derivative (KT0′) are: KT0 = 152.5 (2.4) and KT0′ = 4.1 (0.2) for the HS state, and KT0 = 161.6 (7.1) with a fixed KT0′ of 4 for the LS state, consistent with previous studies27 (Fig. S3). The single-crystal platelet was also used for simultaneously measuring VS in the BLS experiments and VP in the ISS experiments along principle [100] and [110] crystallographic axes up to 96 GPa in the Mineral Physics Laboratory of The University of Texas at Austin (Figs 1 and 2); at relatively lower pressures, the VP and VS velocities of the platelet were also measured as a function of the azimuthal angles in order to confirm the orientation of the platelets and to further assure the reliability of our measurements as compared with previous studies (Fig. S4). Together with P-V results from XRD measurements, the measured VP and VS velocities of single-crystal ferropericlase permit direct derivations of the full elastic constants (C11, C12, C44) at high pressures via Christoffel’s equations (Fig. 2). Using the Eulerian finite-strain theory28 and a thermoelastic model for the cubic system25, we have modelled the elastic constants within the spin transition using formulations reported previously25 (See SI for details). Specifically, the elastic compliances Sij of the crystal across the spin transition are given by:


Elasticity of Ferropericlase across the Spin Crossover in the Earth's Lower Mantle.

Yang J, Tong X, Lin JF, Okuchi T, Tomioka N - Sci Rep (2015)

Elasticity of single-crystal ferropericlase (Mg0.92Fe0.08)O as a function of pressure at 300 K.(A) Compressional and shear wave velocities along the [100] and [110] crystallographic axes as a function of pressure. Compressional wave velocities were measured using the ISS technique, while shear wave velocities were measured using the BLS technique. Open circles: experimental data; solid lines: modelled velocity profiles using thermoelastic equations (see SI for details). (B) Elastic constants (Cij) as a function of pressure. Open circles: Cij directly derived from measured compressional and shear wave velocities via Christoffel’s equations; solid lines: modelled Cij profiles. Vertical dashed lines are plotted to guide the eyes for the high-spin (HS), mixed-spin (MS; HS + LS), and low-spin (LS) regions, respectively (see Supplementary Fig. S2 for details).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4664863&req=5

f2: Elasticity of single-crystal ferropericlase (Mg0.92Fe0.08)O as a function of pressure at 300 K.(A) Compressional and shear wave velocities along the [100] and [110] crystallographic axes as a function of pressure. Compressional wave velocities were measured using the ISS technique, while shear wave velocities were measured using the BLS technique. Open circles: experimental data; solid lines: modelled velocity profiles using thermoelastic equations (see SI for details). (B) Elastic constants (Cij) as a function of pressure. Open circles: Cij directly derived from measured compressional and shear wave velocities via Christoffel’s equations; solid lines: modelled Cij profiles. Vertical dashed lines are plotted to guide the eyes for the high-spin (HS), mixed-spin (MS; HS + LS), and low-spin (LS) regions, respectively (see Supplementary Fig. S2 for details).
Mentions: P-V relations of single-crystal ferropericlase ((Mg0.92, Fe0.08)O) in the (100) platelet were measured using synchrotron X-ray diffraction up to 91 GPa at room temperature in a DAC. These results are used to evaluate the EoS parameters, the width of the spin transition, and the fraction of the high-spin (HS) and low-spin (LS) states in ferropericlase2023 (Fig. S1 and Fig. S2) (See Methods and SI for details of the experiments and modelling). Analysis of the measured P-V curve shows that the spin transition occurs over pressures ranging between 40 GPa and 60 GPa, and is associated with a density increase of 1.2% (0.1%). The derived isothermal bulk modulus at ambient conditions (KT0 ) and its pressure derivative (KT0′) are: KT0 = 152.5 (2.4) and KT0′ = 4.1 (0.2) for the HS state, and KT0 = 161.6 (7.1) with a fixed KT0′ of 4 for the LS state, consistent with previous studies27 (Fig. S3). The single-crystal platelet was also used for simultaneously measuring VS in the BLS experiments and VP in the ISS experiments along principle [100] and [110] crystallographic axes up to 96 GPa in the Mineral Physics Laboratory of The University of Texas at Austin (Figs 1 and 2); at relatively lower pressures, the VP and VS velocities of the platelet were also measured as a function of the azimuthal angles in order to confirm the orientation of the platelets and to further assure the reliability of our measurements as compared with previous studies (Fig. S4). Together with P-V results from XRD measurements, the measured VP and VS velocities of single-crystal ferropericlase permit direct derivations of the full elastic constants (C11, C12, C44) at high pressures via Christoffel’s equations (Fig. 2). Using the Eulerian finite-strain theory28 and a thermoelastic model for the cubic system25, we have modelled the elastic constants within the spin transition using formulations reported previously25 (See SI for details). Specifically, the elastic compliances Sij of the crystal across the spin transition are given by:

Bottom Line: The spin transition is associated with a significant reduction of the aggregate VP/VS via the aggregate VP softening because VS 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 VP/VS in a pyrolite mineralogical model in mid lower-mantle.Our results imply 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712, USA.

ABSTRACT
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, 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 conditions. Here we have reliably measured both VP and VS of a single-crystal ferropericlase ((Mg0.92,Fe0.08)O) using complementary Brillouin Light Scattering and Impulsive Stimulated Light Scattering coupled with a diamond anvil cell up to 96 GPa. The derived elastic constants show drastically softened C11 and C12 within the spin transition at 40-60 GPa while C44 is not affected. The spin transition is associated with a significant reduction of the aggregate VP/VS via the aggregate VP softening because VS 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 VP/VS in a pyrolite mineralogical model in mid lower-mantle. Our results imply 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.

No MeSH data available.


Related in: MedlinePlus