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Title: Spin Transition of Iron in the Earth's Lower Mantle

Abstract

Electronic spin-pairing transitions of iron and associated effects on the physical properties of host phases have been reported in lower-mantle minerals including ferropericlase, silicate perovskite, and possibly in post-perovskite at lower-mantle pressures. Here we evaluate current understanding of the spin and valence states of iron in the lower-mantle phases, emphasizing the effects of the spin transitions on the density, sound velocities, chemical behavior, and transport properties of the lower-mantle phases. The spin transition of iron in ferropericlase occurs at approximately 50 GPa but likely turns into a wide spin crossover under lower-mantle temperatures. Current experimental results indicate a continuous nature of the spin crossover in silicate perovskite at high pressures, but which valence state of iron undergoes the spin crossover and what is its associated crystallographic site remain uncertain. The spin transition of iron results in enhanced density, incompressibility, and sound velocities, and reduced radiative thermal conductivity in the low-spin ferropericlase, which should be considered in future geophysical and geodynamic modeling of the Earth's lower mantle. Our evaluation of the experimental and theoretical pressure-volume results shows that the spin crossover of iron results in a density increase of 3-4% in ferropericlase containing 17-19% FeO. Here we have modeled the densitymore » and bulk modulus profiles of ferropericlase across the spin crossover under lower-mantle pressure-temperature conditions and showed how the ratio of the spin states of iron affects our understanding of the state of the Earth's lower mantle.« less

Authors:
;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
964106
Report Number(s):
UCRL-JRNL-231256
Journal ID: ISSN 0031-9201; PEPIAM; TRN: US200922%%100
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of the Earth and Planetary Interiors, vol. 170, no. 3-4, February 3, 2008, pp. 248-259; Journal Volume: 170; Journal Issue: 3-4
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; EVALUATION; IRON; PEROVSKITE; PHYSICAL PROPERTIES; SILICATES; SIMULATION; SPIN; THERMAL CONDUCTIVITY; TRANSPORT; VALENCE

Citation Formats

Lin, J, and Tsuchiya, T. Spin Transition of Iron in the Earth's Lower Mantle. United States: N. p., 2007. Web.
Lin, J, & Tsuchiya, T. Spin Transition of Iron in the Earth's Lower Mantle. United States.
Lin, J, and Tsuchiya, T. Wed . "Spin Transition of Iron in the Earth's Lower Mantle". United States. doi:. https://www.osti.gov/servlets/purl/964106.
@article{osti_964106,
title = {Spin Transition of Iron in the Earth's Lower Mantle},
author = {Lin, J and Tsuchiya, T},
abstractNote = {Electronic spin-pairing transitions of iron and associated effects on the physical properties of host phases have been reported in lower-mantle minerals including ferropericlase, silicate perovskite, and possibly in post-perovskite at lower-mantle pressures. Here we evaluate current understanding of the spin and valence states of iron in the lower-mantle phases, emphasizing the effects of the spin transitions on the density, sound velocities, chemical behavior, and transport properties of the lower-mantle phases. The spin transition of iron in ferropericlase occurs at approximately 50 GPa but likely turns into a wide spin crossover under lower-mantle temperatures. Current experimental results indicate a continuous nature of the spin crossover in silicate perovskite at high pressures, but which valence state of iron undergoes the spin crossover and what is its associated crystallographic site remain uncertain. The spin transition of iron results in enhanced density, incompressibility, and sound velocities, and reduced radiative thermal conductivity in the low-spin ferropericlase, which should be considered in future geophysical and geodynamic modeling of the Earth's lower mantle. Our evaluation of the experimental and theoretical pressure-volume results shows that the spin crossover of iron results in a density increase of 3-4% in ferropericlase containing 17-19% FeO. Here we have modeled the density and bulk modulus profiles of ferropericlase across the spin crossover under lower-mantle pressure-temperature conditions and showed how the ratio of the spin states of iron affects our understanding of the state of the Earth's lower mantle.},
doi = {},
journal = {Physics of the Earth and Planetary Interiors, vol. 170, no. 3-4, February 3, 2008, pp. 248-259},
number = 3-4,
volume = 170,
place = {United States},
year = {Wed May 23 00:00:00 EDT 2007},
month = {Wed May 23 00:00:00 EDT 2007}
}
  • Here, the spin state of Fe(II) and Fe(III) at temperatures and pressures typical for the Earth's lower mantle is discussed. We predict an extended high-spin to low-spin crossover region along the geotherm for Fe-dilute systems depending on crystal-field splitting, pairing energy, and cooperative interactions. In particular, spin transitions in ferromagnesium silicate perovskite and ferropericlase, the dominant lower mantle components, should occur in a wide temperature-pressure range. We also derive a gradual volume change associated with such transitions in the lower mantle. The gradual density changes and the wide spin crossover regions seem incompatible with lower mantle stratification resulting from amore » spin transition.« less
  • High-pressure x-ray diffraction of (Mg{sub 0.8}Fe{sub 0.2})O at room temperature reveals a discontinuity in the bulk modulus at 40 ({+-}5) GPa, similar pressure at which an electronic spin-pairing transition of Fe{sup 2+} is also observed. In the x-ray diffraction experiments the transition is completed only at 80 GPa, possibly reflecting lack of equilibration. Combining recent measurements, we document anomalies in the compression curve of Mg-rich magnesiowuestites that are manifestations of the spin transition. The best fit to a third order Birch-Murnaghan equation for the low-spin phase of magnesiowuestite with 17-20 mol% FeO yields bulk modulus K{sub T0} = 190 ({+-}150)more » GPa, pressure derivative ({partial_derivative}K{sub T}/{partial_derivative}){sub T0} = 4.6 ({+-}2.7) and unit-cell volume V{sub 0} = 71 ({+-}5) {angstrom}{sup 3}, consistent with past estimates of the ionic radius of octahedrally-coordinated low-spin Fe{sup 2+} in oxides. A sharp spin transition at lower-mantle depths between 1100 and 1900 km (40-80 GPa) would cause a unit-cell volume decrease ({Delta}{nu}{sub {phi}}) of 3.7 ({+-}0.8) to 2.0 ({+-}0.2) percent and bulk sound velocity increase ({Delta}{nu}{sub {phi}}) of 8.1 ({+-}6-1.7) percent ({nu}{sub {phi}} = {radical}K{sub s}/{rho}). Even in the absence of a visible seismic discontinuity, we expect the Fe-spin transition to imply a correction to current compositional models of the lower mantle, with up to 10 mol percent increase of magnesiowuestite being required to match the seismological data.« less
  • Mineral properties in Earth's lower mantle are affected by iron electronic states, but representative pressures and temperatures have not yet been probed. Spin states of iron in lower-mantle ferropericlase have been measured up to 95 gigapascals and 2000 kelvin with x-ray emission in a laser-heated diamond cell. A gradual spin transition of iron occurs over a pressure-temperature range extending from about 1000 kilometers in depth and 1900 kelvin to 2200 kilometers and 2300 kelvin in the lower mantle. Because low-spin ferropericlase exhibits higher density and faster sound velocities relative to the high-spin ferropericlase, the observed increase in low-spin (Mg,Fe)O atmore » mid-lower mantle conditions would manifest seismically as a lower-mantle spin transition zone characterized by a steeper-than-normal density gradient.« less
  • No abstract prepared.