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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

Modelled seismic velocities and VP/VS ratio of ferropericlase ((Mg0.92Fe0.08)O) along an expected lower-mantle geotherm.(A) Aggregate compressional and shear wave velocities. These results are calculated from the single-crystal elastic constants. (B) Calculated VP/VS ratio. Solid lines: modelled seismic parameters with uncertainties shown as grey areas; black dashed lines: modelled parameters for the high-spin state; green dotted lines: theoretical VP and VS values of ferropericlase ((Mg0.875Fe0.125)O)25; crosses: experimental results with 17% iron14. PREM seismic parameters are plotted as blue dotted dashed lines for comparison6; The magenta lines are the modelled velocity profiles assuming that the lower mantle is composed of 20% ferropericlase (fp8) and 80% bridgmanite (Pv)14.
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f4: Modelled seismic velocities and VP/VS ratio of ferropericlase ((Mg0.92Fe0.08)O) along an expected lower-mantle geotherm.(A) Aggregate compressional and shear wave velocities. These results are calculated from the single-crystal elastic constants. (B) Calculated VP/VS ratio. Solid lines: modelled seismic parameters with uncertainties shown as grey areas; black dashed lines: modelled parameters for the high-spin state; green dotted lines: theoretical VP and VS values of ferropericlase ((Mg0.875Fe0.125)O)25; crosses: experimental results with 17% iron14. PREM seismic parameters are plotted as blue dotted dashed lines for comparison6; The magenta lines are the modelled velocity profiles assuming that the lower mantle is composed of 20% ferropericlase (fp8) and 80% bridgmanite (Pv)14.

Mentions: To understand the effects of the spin transition on the elasticity of ferropericlase at relevant P-T conditions of the lower mantle2230, we have modelled the elastic constants of ferropericlase along an expected lower-mantle geotherm up to approximately 125 GPa using thermoelastic models and a previously reported spin crossover diagram2325. The geotherm profile at the core-mantle boundary conditions has not been considered in our modelling here. The thermal EoS parameters of ferropericlase with 25 at% iron in a previous experimental report are linearly scaled back for the compositional effects of FeO in MgO to construct the spin crossover diagram for our ferropericlase with 8 at% iron at high P-T (See SI for details). Our modeled results show that the spin crossover of ferropericlase with 8 at% iron occurs between 65 and 105 GPa along the geotherm. The temperature derivatives of the elastic constants for pure MgO31 are combined with our high-pressure elasticity results (Table S1 and Table S2) to account for the high P-T effects of FeO solid solution for the HS and LS states (Fig. S9). These modelled results show that the effects of the spin crossover on the elastic and seismic parameters along an expected adiabatic geotherm remain profound, even though the spin crossover is broadened by high temperatures (Fig. 4 and Fig. S9). In particular, C11 and C12 exhibit 15% and 60% maximum reduction, respectively, within the spin crossover at approximately 85 GPa that corresponds to 1900 km in depth. The VP anisotropy increases to 18.6% at ~85 GPa within the spin transition (a 23% increase in the anisotropy as compared to the extrapolated HS state reference), while the pressure-dependent VS anisotropy is lower than that of the HS counterpart in the LS state (Fig. S9 B). Compared to the HS state reference, the aggregate VP decreases by 10% while the VP/VS ratio drops by 13% within the spin crossover (Fig. 4). On the other hand, the aggregate VP and VS profiles of the LS ferropericlase are significantly higher than that of their HS state counterparts (Fig. 4A). Using the HS state as the reference, we have calculated the deviations of a number of seismic parameters across the spin crossover along an expected mantle geotherm. These results show that the spin crossover produces VP and VS velocities and anisotropies, VP/VS ratio, and Poisson’s ratio that vary as a function of the low-spin fraction as compared to the extrapolated HS state. Specifically, the VP, VP/VS ratio, and Poisson’s ratio are significantly reduced within the spin crossover, whereas the LS state exhibits enhanced VP and VS velocities as well as reduced VP/VS and Poisson’s ratio (Fig. S10).


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)

Modelled seismic velocities and VP/VS ratio of ferropericlase ((Mg0.92Fe0.08)O) along an expected lower-mantle geotherm.(A) Aggregate compressional and shear wave velocities. These results are calculated from the single-crystal elastic constants. (B) Calculated VP/VS ratio. Solid lines: modelled seismic parameters with uncertainties shown as grey areas; black dashed lines: modelled parameters for the high-spin state; green dotted lines: theoretical VP and VS values of ferropericlase ((Mg0.875Fe0.125)O)25; crosses: experimental results with 17% iron14. PREM seismic parameters are plotted as blue dotted dashed lines for comparison6; The magenta lines are the modelled velocity profiles assuming that the lower mantle is composed of 20% ferropericlase (fp8) and 80% bridgmanite (Pv)14.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Modelled seismic velocities and VP/VS ratio of ferropericlase ((Mg0.92Fe0.08)O) along an expected lower-mantle geotherm.(A) Aggregate compressional and shear wave velocities. These results are calculated from the single-crystal elastic constants. (B) Calculated VP/VS ratio. Solid lines: modelled seismic parameters with uncertainties shown as grey areas; black dashed lines: modelled parameters for the high-spin state; green dotted lines: theoretical VP and VS values of ferropericlase ((Mg0.875Fe0.125)O)25; crosses: experimental results with 17% iron14. PREM seismic parameters are plotted as blue dotted dashed lines for comparison6; The magenta lines are the modelled velocity profiles assuming that the lower mantle is composed of 20% ferropericlase (fp8) and 80% bridgmanite (Pv)14.
Mentions: To understand the effects of the spin transition on the elasticity of ferropericlase at relevant P-T conditions of the lower mantle2230, we have modelled the elastic constants of ferropericlase along an expected lower-mantle geotherm up to approximately 125 GPa using thermoelastic models and a previously reported spin crossover diagram2325. The geotherm profile at the core-mantle boundary conditions has not been considered in our modelling here. The thermal EoS parameters of ferropericlase with 25 at% iron in a previous experimental report are linearly scaled back for the compositional effects of FeO in MgO to construct the spin crossover diagram for our ferropericlase with 8 at% iron at high P-T (See SI for details). Our modeled results show that the spin crossover of ferropericlase with 8 at% iron occurs between 65 and 105 GPa along the geotherm. The temperature derivatives of the elastic constants for pure MgO31 are combined with our high-pressure elasticity results (Table S1 and Table S2) to account for the high P-T effects of FeO solid solution for the HS and LS states (Fig. S9). These modelled results show that the effects of the spin crossover on the elastic and seismic parameters along an expected adiabatic geotherm remain profound, even though the spin crossover is broadened by high temperatures (Fig. 4 and Fig. S9). In particular, C11 and C12 exhibit 15% and 60% maximum reduction, respectively, within the spin crossover at approximately 85 GPa that corresponds to 1900 km in depth. The VP anisotropy increases to 18.6% at ~85 GPa within the spin transition (a 23% increase in the anisotropy as compared to the extrapolated HS state reference), while the pressure-dependent VS anisotropy is lower than that of the HS counterpart in the LS state (Fig. S9 B). Compared to the HS state reference, the aggregate VP decreases by 10% while the VP/VS ratio drops by 13% within the spin crossover (Fig. 4). On the other hand, the aggregate VP and VS profiles of the LS ferropericlase are significantly higher than that of their HS state counterparts (Fig. 4A). Using the HS state as the reference, we have calculated the deviations of a number of seismic parameters across the spin crossover along an expected mantle geotherm. These results show that the spin crossover produces VP and VS velocities and anisotropies, VP/VS ratio, and Poisson’s ratio that vary as a function of the low-spin fraction as compared to the extrapolated HS state. Specifically, the VP, VP/VS ratio, and Poisson’s ratio are significantly reduced within the spin crossover, whereas the LS state exhibits enhanced VP and VS velocities as well as reduced VP/VS and Poisson’s ratio (Fig. S10).

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