skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Iron-rich perovskite in the Earth;s lower mantle

Abstract

The equations of state of perovskite with (Mg{sub 0.75},Fe{sub 0.25})SiO{sub 3} and MgSiO{sub 3} compositions have been investigated by synchrotron X-ray diffraction up to 130 GPa at 300 K in diamond anvil cells. Here we show that the addition of 25% Fe in MgSiO{sub 3} perovskite increases its density and bulk sound velocity (V{phi}) by 4-6% and 6-7%, respectively, at lower-mantle pressures. Based on concurrent synchrotron X-ray emission and Moessbauer spectroscopic studies of the samples, the increase in V{phi} and density can be explained by the occurrence of the low-spin Fe3+ and the extremely high-quadrupole component of Fe{sup 2+}. Combining these experimental results with thermodynamic modeling, our results indicate that iron-rich perovskite can produce an increase in density and a value of V{phi} that is compatible with seismic observations of reduced shear-wave velocity in regions interpreted as dense, stiff piles in the lower mantle. Therefore, the existence of the Fe-rich perovskite in the lower mantle may help elucidate the cause of the lower-mantle large low-shear-velocity provinces (LLSVPs) where enhanced density and V{phi} are seismically observed to anti-correlate with the reduced shear wave velocity.

Authors:
; ; ; ; ; ; ;  [1];  [2];  [2];  [2];  [2];  [2]
  1. (NIU)
  2. (
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
Sponsoring Org.:
FOREIGNNSFDOE - BASIC ENERGY SCIENCES
OSTI Identifier:
1027657
Resource Type:
Journal Article
Resource Relation:
Journal Name: Earth Planet. Sci. Lett.; Journal Volume: 309; Journal Issue: (3-4) ; 09, 2011
Country of Publication:
United States
Language:
ENGLISH
Subject:
58 GEOSCIENCES; DIAMONDS; EQUATIONS OF STATE; PEROVSKITE; SHEAR; SIMULATION; SYNCHROTRONS; THERMODYNAMICS; VELOCITY; X-RAY DIFFRACTION

Citation Formats

Mao, Z., Lin, J.F., Scott, H.P., Watson, H.C., Prakapenka, V.B., Xiao, Y., Chow, P., McCammon, C., Bayreuth), CIW), UC), Texas), and Indiana). Iron-rich perovskite in the Earth;s lower mantle. United States: N. p., 2013. Web. doi:10.1016/j.epsl.2011.06.030.
Mao, Z., Lin, J.F., Scott, H.P., Watson, H.C., Prakapenka, V.B., Xiao, Y., Chow, P., McCammon, C., Bayreuth), CIW), UC), Texas), & Indiana). Iron-rich perovskite in the Earth;s lower mantle. United States. doi:10.1016/j.epsl.2011.06.030.
Mao, Z., Lin, J.F., Scott, H.P., Watson, H.C., Prakapenka, V.B., Xiao, Y., Chow, P., McCammon, C., Bayreuth), CIW), UC), Texas), and Indiana). 2013. "Iron-rich perovskite in the Earth;s lower mantle". United States. doi:10.1016/j.epsl.2011.06.030.
@article{osti_1027657,
title = {Iron-rich perovskite in the Earth;s lower mantle},
author = {Mao, Z. and Lin, J.F. and Scott, H.P. and Watson, H.C. and Prakapenka, V.B. and Xiao, Y. and Chow, P. and McCammon, C. and Bayreuth) and CIW) and UC) and Texas) and Indiana)},
abstractNote = {The equations of state of perovskite with (Mg{sub 0.75},Fe{sub 0.25})SiO{sub 3} and MgSiO{sub 3} compositions have been investigated by synchrotron X-ray diffraction up to 130 GPa at 300 K in diamond anvil cells. Here we show that the addition of 25% Fe in MgSiO{sub 3} perovskite increases its density and bulk sound velocity (V{phi}) by 4-6% and 6-7%, respectively, at lower-mantle pressures. Based on concurrent synchrotron X-ray emission and Moessbauer spectroscopic studies of the samples, the increase in V{phi} and density can be explained by the occurrence of the low-spin Fe3+ and the extremely high-quadrupole component of Fe{sup 2+}. Combining these experimental results with thermodynamic modeling, our results indicate that iron-rich perovskite can produce an increase in density and a value of V{phi} that is compatible with seismic observations of reduced shear-wave velocity in regions interpreted as dense, stiff piles in the lower mantle. Therefore, the existence of the Fe-rich perovskite in the lower mantle may help elucidate the cause of the lower-mantle large low-shear-velocity provinces (LLSVPs) where enhanced density and V{phi} are seismically observed to anti-correlate with the reduced shear wave velocity.},
doi = {10.1016/j.epsl.2011.06.030},
journal = {Earth Planet. Sci. Lett.},
number = (3-4) ; 09, 2011,
volume = 309,
place = {United States},
year = 2013,
month =
}
  • The lower mantle is dominated by a magnesium- and iron-bearing mineral with the perovskite structure. Iron has the ability to adopt different electronic configurations, and transitions in its spin state in the lower mantle can significantly influence mantle properties and dynamics. However, previous studies aimed at understanding these transitions have provided conflicting results. Here we report the results of high-pressure (up to 110 GPa) and high-temperature (up to 1,000 K) experiments aimed at understanding spin transitions of iron in perovskite at lower-mantle conditions. Our Moessbauer and nuclear forward scattering data for two lower-mantle perovskite compositions demonstrate that the transition ofmore » ferrous iron from the high-spin to the intermediate-spin state occurs at approximately 30 GPa, and that high temperatures favour the stability of the intermediate-spin state. We therefore infer that ferrous iron adopts the intermediate-spin state throughout the bulk of the lower mantle. Our X-ray data show significant anisotropic compression of lower-mantle perovskite containing intermediate-spin ferrous iron, which correlates strongly with the spin transition. We predict spin-state heterogeneities in the uppermost part of the lower mantle associated with sinking slabs and regions of upwelling. These may affect local properties, including thermal and electrical conductivity, deformation (viscosity) and chemical behaviour, and thereby affect mantle dynamics.« less
  • The electronic spin state of iron in lower mantle perovskite is one of the fundamental parameters that governs the physics and chemistry of the most voluminous and massive shell in the Earth. We present experimental evidence for spin-pairing transition in aluminum-bearing silicate perovskite (Mg,Fe)(Si,Al)O{sub 3} under the lower mantle pressures. Our results demonstrate that as pressure increases, iron in perovskite transforms gradually from the initial high-spin state toward the final low-spin state. At 100 GPa, both aluminum-free and aluminum-bearing samples exhibit a mixed spin state. The residual magnetic moment in the aluminum-bearing perovskite is significantly higher than that in itsmore » aluminum-free counterpart. The observed spin evolution with pressure can be explained by the presence of multiple iron species and the occurrence of partial spin-paring transitions in the perovskite. Pressure-induced spin-pairing transitions in the perovskite would have important bearing on the magnetic, thermoelastic, and transport properties of the lower mantle, and on the distribution of iron in the Earth's interior. The lower mantle constitutes more than half of the Earth's interior by volume (1), and it is believed to consist predominantly (80-100%) of (Mg,Fe)(Si,Al)O{sub 3} perovskite (hereafter called perovskite), with up to 20% (Mg,Fe)O ferropericlase (2). The electronic spin state of iron has direct influence on the physical properties and chemical behavior of its host phase. Hence, knowledge on the spin state of iron is important for the interpretation of seismic observations, geochemical modeling, and geodynamic simulation of the Earth's deep interior (3, 4). Crystal field theory (4, 5) and band theory (6) predicted that a high-spin to low-spin transition would occur as a result of compression. To date, no experimental data exist on the spin sate of iron in Al-bearing perovskite. To detect possible spinpairing transition of iron in perovskite under the lower mantle conditions, we measured the x-ray emission spectra of an Al-bearing perovskite sample to 100 GPa. For comparison, a parallel measurement was also carried out on an Al-free perovskite sample.« less
  • We investigated phase relations in pyrolite at -33--40 GPa and -1800--2150 K by in situ X-ray diffraction using Kawai-type apparatus with sintered diamond anvils. The results demonstrated that MgSiO{sub 3}-rich orthorhombic perovskite (mpv), CaSiO{sub 3}-rich cubic perovskite (cpv) and (Mg,Fe)O ferropericlase (fp) are the stable phases in pyrolite bulk composition at the conditions corresponding to the lower mantle. However, chemical analyses of a run product recovered from -34 GPa by an analytical transmission electron microscope showed the coexistence of metallic iron particles with mpv, fp, and SiO{sub 2}-rich amorphous phase. Also, Fe/Mg partitioning coefficient between mpv and fp was foundmore » to be 0.66(31), which is consistent with previous results for pyrolite bulk composition at 26--30 GPa and -1900 K. These results indicate that iron-rich metallic particles can exist in the lower mantle as a stable phase to the depth of at least -900 km.« less
  • Pressure-induced electronic spin-pairing transitions of iron and associated effects on the physical properties have been reported to occur in the lower-mantle ferropericlase, silicate perosvkite, and perhaps in post silicate perovskite at high pressures and room temperature. These recent results are motivating geophysicists and geodynamicists to reevaluate the implications of spin transitions on the seismic heterogeneity, composition, as well as the stability of the thermal upwellings of the Earth's lower mantle. Here we have measured the spin states of iron in ferropericlase and its crystal structure up to 95 GPa and 2000 K using a newly constructed X-ray emission spectroscopy andmore » diffraction with the laser-heated diamond cell. Our results show that an isosymmetric spin crossover occurs over a pressure-temperature range extending from the upper part to the lower part of the lower mantle, and low-spin ferropericlase likely exists in the lowermost mantle. Although continuous changes in physical and chemical properties are expected to occur across the spin crossover, the spin crossover results in peculiar behavior in the thermal compression and sound velocities. Therefore, knowledge of the fraction of the spin states in the lower-mantle phases is thus essential to correctly evaluate the composition, geophysics, and dynamics of the Earth's lower mantle.« less