Effect of iron content on electrical conductivity of ringwoodite, with implications for electrical structure in the transition zone

Takashi Yoshino, Tomoo Katsura

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38 Citations (Scopus)

Abstract

Electrical conductivity of ringwoodite with various iron contents [Fe/(Fe + Mg) = 0.09, 0.2 and 0.3] was measured at pressure (20 GPa) and temperature (up to 1900 K) conditions of the lower part of the mantle transition zone in a Kawai-type multi-anvil apparatus. The conductivity increased with increasing total iron content. All electrical conductivity data were fitted to the formula of electrical conductivity σ = σ0 XFe exp(-H/kT), where σ0 is the pre-exponential term, XFe is the mole fraction of iron content in the Mg site, H is the activation enthalpy, k is the Boltzmann constant and T is absolute temperature. The activation enthalpy becomes higher at a certain temperature. At high temperatures, the activation enthalpy decreased from 1.44 to 0.92 eV with increasing total Fe content. At low temperatures less than 1000 K, the activation enthalpy also decreases from 1.15 to 0.74 eV with total Fe content. Dependence of the activation enthalpy on Fe content suggests that the dominant mechanism of charge transport is Fe2+-Fe3+ hopping (small polaron). Recent electrical conductivity-depth profiles of the transition zone beneath the Pacific Ocean obtained from the electromagnetic induction study shows that the conductivity values between 520 and 660 km depths may be explained by ringwoodite with Fe/(Fe + Mg) = 0.10. On the other hand, assuming a normal geotherm, conductivity values beneath the continent or stable craton are considerably lower than those of ringwoodite with Fe/(Fe + Mg) = 0.10. Taking into consideration results from the global seismic tomographic studies, relatively low conductivity in these regions can be explained by the existence of a cooler region compared with the surrounding mantle, rather than the presence of iron-poor ringwoodite, or a combination of both.

Original languageEnglish
Pages (from-to)3-9
Number of pages7
JournalPhysics of the Earth and Planetary Interiors
Volume174
Issue number1-4
DOIs
Publication statusPublished - May 2009

Fingerprint

ringwoodite
enthalpy
transition zone
electrical conductivity
iron
conductivity
electrical resistivity
activation
Earth mantle
mantle
temperature
low conductivity
cratons
Pacific Ocean
continents
craton
anvils
coolers
effect
induction

Keywords

  • Electrical conductivity
  • Iron content
  • Ringwoodite
  • Transition zone
  • Upper mantle

ASJC Scopus subject areas

  • Geophysics
  • Space and Planetary Science
  • Physics and Astronomy (miscellaneous)
  • Astronomy and Astrophysics

Cite this

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title = "Effect of iron content on electrical conductivity of ringwoodite, with implications for electrical structure in the transition zone",
abstract = "Electrical conductivity of ringwoodite with various iron contents [Fe/(Fe + Mg) = 0.09, 0.2 and 0.3] was measured at pressure (20 GPa) and temperature (up to 1900 K) conditions of the lower part of the mantle transition zone in a Kawai-type multi-anvil apparatus. The conductivity increased with increasing total iron content. All electrical conductivity data were fitted to the formula of electrical conductivity σ = σ0 XFe exp(-H/kT), where σ0 is the pre-exponential term, XFe is the mole fraction of iron content in the Mg site, H is the activation enthalpy, k is the Boltzmann constant and T is absolute temperature. The activation enthalpy becomes higher at a certain temperature. At high temperatures, the activation enthalpy decreased from 1.44 to 0.92 eV with increasing total Fe content. At low temperatures less than 1000 K, the activation enthalpy also decreases from 1.15 to 0.74 eV with total Fe content. Dependence of the activation enthalpy on Fe content suggests that the dominant mechanism of charge transport is Fe2+-Fe3+ hopping (small polaron). Recent electrical conductivity-depth profiles of the transition zone beneath the Pacific Ocean obtained from the electromagnetic induction study shows that the conductivity values between 520 and 660 km depths may be explained by ringwoodite with Fe/(Fe + Mg) = 0.10. On the other hand, assuming a normal geotherm, conductivity values beneath the continent or stable craton are considerably lower than those of ringwoodite with Fe/(Fe + Mg) = 0.10. Taking into consideration results from the global seismic tomographic studies, relatively low conductivity in these regions can be explained by the existence of a cooler region compared with the surrounding mantle, rather than the presence of iron-poor ringwoodite, or a combination of both.",
keywords = "Electrical conductivity, Iron content, Ringwoodite, Transition zone, Upper mantle",
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TY - JOUR

T1 - Effect of iron content on electrical conductivity of ringwoodite, with implications for electrical structure in the transition zone

AU - Yoshino, Takashi

AU - Katsura, Tomoo

PY - 2009/5

Y1 - 2009/5

N2 - Electrical conductivity of ringwoodite with various iron contents [Fe/(Fe + Mg) = 0.09, 0.2 and 0.3] was measured at pressure (20 GPa) and temperature (up to 1900 K) conditions of the lower part of the mantle transition zone in a Kawai-type multi-anvil apparatus. The conductivity increased with increasing total iron content. All electrical conductivity data were fitted to the formula of electrical conductivity σ = σ0 XFe exp(-H/kT), where σ0 is the pre-exponential term, XFe is the mole fraction of iron content in the Mg site, H is the activation enthalpy, k is the Boltzmann constant and T is absolute temperature. The activation enthalpy becomes higher at a certain temperature. At high temperatures, the activation enthalpy decreased from 1.44 to 0.92 eV with increasing total Fe content. At low temperatures less than 1000 K, the activation enthalpy also decreases from 1.15 to 0.74 eV with total Fe content. Dependence of the activation enthalpy on Fe content suggests that the dominant mechanism of charge transport is Fe2+-Fe3+ hopping (small polaron). Recent electrical conductivity-depth profiles of the transition zone beneath the Pacific Ocean obtained from the electromagnetic induction study shows that the conductivity values between 520 and 660 km depths may be explained by ringwoodite with Fe/(Fe + Mg) = 0.10. On the other hand, assuming a normal geotherm, conductivity values beneath the continent or stable craton are considerably lower than those of ringwoodite with Fe/(Fe + Mg) = 0.10. Taking into consideration results from the global seismic tomographic studies, relatively low conductivity in these regions can be explained by the existence of a cooler region compared with the surrounding mantle, rather than the presence of iron-poor ringwoodite, or a combination of both.

AB - Electrical conductivity of ringwoodite with various iron contents [Fe/(Fe + Mg) = 0.09, 0.2 and 0.3] was measured at pressure (20 GPa) and temperature (up to 1900 K) conditions of the lower part of the mantle transition zone in a Kawai-type multi-anvil apparatus. The conductivity increased with increasing total iron content. All electrical conductivity data were fitted to the formula of electrical conductivity σ = σ0 XFe exp(-H/kT), where σ0 is the pre-exponential term, XFe is the mole fraction of iron content in the Mg site, H is the activation enthalpy, k is the Boltzmann constant and T is absolute temperature. The activation enthalpy becomes higher at a certain temperature. At high temperatures, the activation enthalpy decreased from 1.44 to 0.92 eV with increasing total Fe content. At low temperatures less than 1000 K, the activation enthalpy also decreases from 1.15 to 0.74 eV with total Fe content. Dependence of the activation enthalpy on Fe content suggests that the dominant mechanism of charge transport is Fe2+-Fe3+ hopping (small polaron). Recent electrical conductivity-depth profiles of the transition zone beneath the Pacific Ocean obtained from the electromagnetic induction study shows that the conductivity values between 520 and 660 km depths may be explained by ringwoodite with Fe/(Fe + Mg) = 0.10. On the other hand, assuming a normal geotherm, conductivity values beneath the continent or stable craton are considerably lower than those of ringwoodite with Fe/(Fe + Mg) = 0.10. Taking into consideration results from the global seismic tomographic studies, relatively low conductivity in these regions can be explained by the existence of a cooler region compared with the surrounding mantle, rather than the presence of iron-poor ringwoodite, or a combination of both.

KW - Electrical conductivity

KW - Iron content

KW - Ringwoodite

KW - Transition zone

KW - Upper mantle

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