Limits...
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

Aggregate bulk and shear moduli KS and G, aggregate velocities, elastic anisotropies and aggregate VP/VS ratio of ferropericlase (Mg0.92Fe0.08)O at high pressure and 300 K.(A) Adiabatic bulk and shear modulus from Voigt-Reuss-Hill average; (B) Aggregate compressional VP and shear wave velocities VS, where  and ; (C) Compressional and shear wave anisotropy as a function of pressure; dashed lines are the extrapolated anisotropies for the HS state that are plotted for comparison; (D) aggregate VP/VS ratio. Grey shaded areas represent uncertainties calculated from standard error propagations using the experimentally derived elastic constants. Vertical dashed lines are plotted to guide the eyes for the HS, MS, and LS regions, respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4664863&req=5

f3: Aggregate bulk and shear moduli KS and G, aggregate velocities, elastic anisotropies and aggregate VP/VS ratio of ferropericlase (Mg0.92Fe0.08)O at high pressure and 300 K.(A) Adiabatic bulk and shear modulus from Voigt-Reuss-Hill average; (B) Aggregate compressional VP and shear wave velocities VS, where and ; (C) Compressional and shear wave anisotropy as a function of pressure; dashed lines are the extrapolated anisotropies for the HS state that are plotted for comparison; (D) aggregate VP/VS ratio. Grey shaded areas represent uncertainties calculated from standard error propagations using the experimentally derived elastic constants. Vertical dashed lines are plotted to guide the eyes for the HS, MS, and LS regions, respectively.

Mentions: where V is the volume, nLS is the LS fraction, σi and σj are the i th and jth stress component, respectively, in the Voigt notation, and G is the Gibbs free energy. In this modelling, the low-spin fraction (nLS) and the unit cell volume (V) derived from the equation of state, and the elastic constants (Cij) are used to constrain the elastic compliances Sij according to the relationship between Cij and Sij (See SI for details). To further obtain the pressure-dependent EoS parameters for the HS and LS states, respectively, the elastic constants and the aggregate bulk and shear moduli as a function of pressure are derived by fitting the results to the third-order Eulerian finite-strain equation of state (Figs 2 and 3).


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)

Aggregate bulk and shear moduli KS and G, aggregate velocities, elastic anisotropies and aggregate VP/VS ratio of ferropericlase (Mg0.92Fe0.08)O at high pressure and 300 K.(A) Adiabatic bulk and shear modulus from Voigt-Reuss-Hill average; (B) Aggregate compressional VP and shear wave velocities VS, where  and ; (C) Compressional and shear wave anisotropy as a function of pressure; dashed lines are the extrapolated anisotropies for the HS state that are plotted for comparison; (D) aggregate VP/VS ratio. Grey shaded areas represent uncertainties calculated from standard error propagations using the experimentally derived elastic constants. Vertical dashed lines are plotted to guide the eyes for the HS, MS, and LS regions, respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: Aggregate bulk and shear moduli KS and G, aggregate velocities, elastic anisotropies and aggregate VP/VS ratio of ferropericlase (Mg0.92Fe0.08)O at high pressure and 300 K.(A) Adiabatic bulk and shear modulus from Voigt-Reuss-Hill average; (B) Aggregate compressional VP and shear wave velocities VS, where and ; (C) Compressional and shear wave anisotropy as a function of pressure; dashed lines are the extrapolated anisotropies for the HS state that are plotted for comparison; (D) aggregate VP/VS ratio. Grey shaded areas represent uncertainties calculated from standard error propagations using the experimentally derived elastic constants. Vertical dashed lines are plotted to guide the eyes for the HS, MS, and LS regions, respectively.
Mentions: where V is the volume, nLS is the LS fraction, σi and σj are the i th and jth stress component, respectively, in the Voigt notation, and G is the Gibbs free energy. In this modelling, the low-spin fraction (nLS) and the unit cell volume (V) derived from the equation of state, and the elastic constants (Cij) are used to constrain the elastic compliances Sij according to the relationship between Cij and Sij (See SI for details). To further obtain the pressure-dependent EoS parameters for the HS and LS states, respectively, the elastic constants and the aggregate bulk and shear moduli as a function of pressure are derived by fitting the results to the third-order Eulerian finite-strain equation of state (Figs 2 and 3).

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