Iron In Ferropericlase Research Articles

Entropic broadening of the spin-crossover pressure in ferropericlase

The pressure-induced spin crossover of iron in ferropericlase, the Fe-bearing MgO, is a key to understanding the seismological observations in the lower mantle. However, the experimentally measured spin-crossover pressure zone (SCPZ) shows large variations, which disagree with the theoretically predicted values that are generally less than half of experimental ones. Here, we resolve this outstanding controversy by revealing the critical role of Fe distribution in MgO in broadening the Fe SCPZ, using comprehensive first-principles calculations combined with cluster expansion approach and Monte Carlo simulations. By employing a large supercell containing up to $\ensuremath{\sim}{10}^{6}$ atoms, we derive the spin-crossover pressures as functions of Fe concentration for different Fe distributions. We determine a critical temperature of ${T}_{c}\ensuremath{\sim}900\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, below which Fe segregation (clustering) occurs, in accordance with the thermodynamic phase diagram. Above ${T}_{c}$, an entropy-driven randomized Fe distribution creates large variations in Fe local environments, which in turn broadens the SCPZ, such as 17.5 GPa for 25 mol % Fe-bearing MgO in good agreement with experiments. Therefore, the broad SCPZ, rendering a smooth change of ferropericlase mechanical properties during the spin crossover, should be mainly caused by entropy, consistent with the high-temperature state of the lower mantle.

Iron isotopic fractionation in mineral phases from Earth's lower mantle: Did terrestrial magma ocean crystallization fractionate iron isotopes?

Iron is the most abundant transition metal in the Earth's interior, yet considerable uncertainties remain as to why mantle-derived rocks have diverse iron isotopic compositions. In particular, the isotopic fractionation behavior of iron in the lower-mantle minerals bridgmanite and ferropericlase are largely unexplored. The reason is that it is challenging to study isotopic fractionation at the high pressures relevant to the deep mantle. Here we report in situ measurements of the mean force constants of iron bonds in these minerals pressurized in diamond anvil cells using the technique of nuclear resonant inelastic X-ray scattering (NRIXS). We find that the transition from high- to low-spin iron in ferropericlase ((Mg0.75Fe0.25)O) at approximately 60 GPa drastically stiffens its iron bonds in the low-spin state. The mean force constant of iron bonds in both Fe-bearing and (Fe,Al)-bearing bridgmanite exhibits softening by 21% at approximately 40–60 GPa, which seems to be associated with changes in the iron local environment during the transition from low to high quadrupole splitting states. These results indicate that in the lower mantle, low-spin ferropericlase is enriched in heavy iron isotopes relative to bridgmanite and metallic iron by +0.15‰ and +0.12‰, respectively. Based on these results, we investigate whether terrestrial magma ocean crystallization could have fractionated iron isotopes. We conclude that this process cannot be responsible for the heavy iron isotope enrichment measured in terrestrial basalts.

Critical behavior of Mg1–xFexO at the pressure-induced iron spin-state crossover

We present a high-pressure study of $\mathrm{M}{\mathrm{g}}_{1--x}\mathrm{F}{\mathrm{e}}_{x}\mathrm{O}$ (ferropericlase, Fp) single crystals $0.04(1)\ensuremath{\le}x\ensuremath{\le}0.33(1)$ with a focus on ferrous iron spin-state crossover and the material behavior preceding it. Using Vegard's law and highly accurate high-pressure single-crystal experimental data, we extract the lattice parameter ${a}_{\mathrm{FeO}}^{\mathrm{HS}}$ of the ``FeO high spin lattice contribution'' in the MgO-FeO solid solution. We find that ferropericlases with a wide range of compositions share the same critical parameter ${a}_{C}$ (defined as the minimum ${a}_{\mathrm{FeO}}^{\mathrm{HS}}$ after which spin crossover starts). Furthermore, we discuss the effect of composition on spin crossover in ferrous iron in ferropericlase in the limits of low and moderate concentrations of $\mathrm{F}{\mathrm{e}}^{2+}$.

Spawning superplumes from the midmantle: The impact of spin transitions in the mantle

AbstractThe formation of large‐scale upwellings with lateral extents of several hundreds of kilometers, reaching up to ∼10,000 km or more, still remains a hotly debated topic. Some seismic imaging studies based on high‐resolution data suggest that the main superplumes underneath Africa and South‐central Pacific are clusters, composed of several individual plumes rather than being a single large mantle upwelling. The iron spin transition in the lower mantle minerals may present a new idea on the origin and the formation of such superplumes, notably sourcing such features in the midmantle. Stagnation of both cold sinking slabs and hot rising plumes can be caused by density and viscosity variation due to the spin transition in iron in ferropericlase (Fp) and a possible spin‐dependent bulk modulus hardening in bridgmanite silicate perovskite (Pv). This process produces intermittent downward spin transition‐induced midmantle avalanches (SIMMA) of the cold sinking flow as well as upward spin transition‐induced midmantle superplume avalanches (SIMMSA) of the rising hot plumes, triggered at the spin transition‐induced thermal boundary layer at around 1600 km depth. Our high‐resolution axi‐symmetric models reveal that the hot upwellings, trapped below ∼1600 km depth, can suddenly penetrate into the upper levels in the mantle and spread laterally for hundreds of kilometres. Owing to the upward penetration of the midmantle‐rooted superplumes, as broad as ∼1500 km across, a large amount of heat can be delivered to the upper mantle and base of the lithosphere with implications for large volcanic episodes.

First-principles calculations of elasticity of minerals at high temperature and pressure

The elasticity of minerals at high temperature and pressure (PT) is critical for constraining the composition and temperature of the Earth’s interior and understand better the deep water cycle and the dynamic Earth. First-principles calculations without introducing any adjustable parameters, whose results can be comparable to experimental data, play a more and more important role in investigating the elasticity of minerals at high PT mainly because of (1) the quick increasing of computational powers and (2) advances in method. For example, the new method reduces the computation loads to one-tenth of the traditional method with the comparable precise as the traditional method. This is extraordinarily helpful because first-principles calculations of the elasticity of minerals at high PT are extremely time-consuming. So far the elasticity of most of lower mantle minerals has been investigated in detail. We have good idea on the effect of temperature, pressure, and iron concentration on elasticity of main minerals of the lower mantle and the unusual softening in bulk modulus by the spin crossover of iron in ferropericlase. With these elastic data the lower mantle has been constrained to have 10–15 wt% ferropericlase, which is sufficient to generate some visible effects of spin crossover in seismic tomography. For example, the spin crossover causes that the temperature sensitivity of P wave at the depth of ~1700 km is only a fraction of that at the depth of ~2300 km. The disruptions of global P wave structure and of P wave image below hotspots such as Hawaii and Iceland at similar depth are in consistence with the spin crossover effect of iron in ferropericlase. The spin crossover, which causes anomalous thermodynamic properties of ferropericlase, has also been found to play a control role for the two features of the large low shear velocity provinces (LLSVPs): the sharp edge and high elevation up to 1000 km above core-mantle boundary. All these results clearly suggest the spin crossover of iron in the lower mantle. The theoretical investigations for the elasticity of minerals at the upper mantle and water effect on elasticity of minerals at the mantle transition zone and subducting slab have also been conducted extensively. These researches are critical for understanding better the composition of the upper mantle and water distribution and transport in the Earth’s mantle. Most of these were static calculations, which did not include the vibrational (temperature) effect on elasticity, although temperature effect on elasticity is basic because of high temperature at the Earth’s interior and huge temperature difference between the ambient mantle and the subducting slab. Including temperature effect on elasticity of minerals should be important future work. New method developed is helpful for these directions. The elasticity of iron and iron-alloy with various light elements has also been calculated extensively. However, more work is necessary in order to meet the demand for constraining the types and amount of light elements at the Earth’s core.

Spin state transition and partitioning of iron: Effects on mantle dynamics

Experimental studies at pressure and temperature conditions of the Earth's lower mantle have shown that iron in ferropericlase (Fp) and in Mg-silicate perovskite (Pv) undergoes a spin state transition. This electronic transition changes elastic and transport properties of lower mantle minerals and can play an important role in mantle convection. Here we focus on the geodynamic effect of the spin-induced density modifications caused by the volume collapse of Fp and by the variation of Fe partitioning (KPv–Fp) between Fp and Pv. Since KPv–Fp behavior strongly depends on alumina content, we explore two end-member compositions, one Al-bearing (with 4.7 wt% Al2O3 in Pv) and the other Al-free. We use the theoretical model by Sturhahn et al. (2005) to calculate the spin configuration of Fp over a range of pressure–temperature conditions, and use experimental results to model Fe partitioning. We then apply the Mie–Grüneisen–Debye equation of state to obtain the density of the mineral assemblages. The calculated amplitude of the density change across the spin state transition is less than 1%, consistent with experiments by Mao et al. (2011); our density profiles differ from PREM by less than 1.5%. The spin-induced density variations are included in a three dimensional convection code (Stag3D) for a compressible mantle. We find small temperature differences between models with and without spin state transitions, since over billions of years the relative temperature difference is less than 50 K. However the relative RMS vertical velocity difference is up to 15% for an Al-free system, but only less than 6% for an Al-bearing system.

Optical properties of (Mg0.97, Fe0.03)O ferropericlase under the pressure of the Earth’s lower mantle

The optical-absorption and refractive-index properties of (Mg0.97, Fe0.03)O ferropericlase crystals without and with Mg and O ionic divacancy point-defect under the pressure of the Earth’s lower mantle are investigated using the first-principles calculations. Optical-absorption data show that the perfect-crystal results are similar to the predictions from the crystal-field theory:the pressure-induced spin transition of iron in ferropericlase causes a large blue-shift in its optical-absorption spectrum, leaving the near-infrared region transparent. However, when there are point defects in ferropericlase, the calculated optical-absorption results are completely inconsistent with predictions from the crystal-field theory, the spin transition causes the enhancement in the optical absorption in the near-infrared region. Refractive-index data of defect crystal indicate that the effects of pressure, wavenumber, and spin-transition on the high-pressure refractive-index of (Mg0.97, Fe0.03)O ferropericlase are obvious, but perfect-crystal results show that those effects should be relatively weak. The ～15%-20% iron-bearing ferropericlase is currently considered as an important mineral in the Earth’s lower mantle. Due to similar characteristics of the observed high-pressure optical-absorption spectrum in ferropericlase with different iron content, we suggest that:(1) the above-mentioned calculated results is conducive to the understanding of high-pressure optical properties of lower-mantle ferropericlase and the exploring of the origin of discrepancies in its high-pressure optical-absorption spectrum between experiment and crystal-field theory; (2) the high-pressure optical-absorption spectrum measurements may be a good approach for probing iron spin state.

Spin crossover in ferropericlase and velocity heterogeneities in the lower mantle.

Deciphering the origin of seismic velocity heterogeneities in the mantle is crucial to understanding internal structures and processes at work in the Earth. The spin crossover in iron in ferropericlase (Fp), the second most abundant phase in the lower mantle, introduces unfamiliar effects on seismic velocities. First-principles calculations indicate that anticorrelation between shear velocity (VS) and bulk sound velocity (Vφ) in the mantle, usually interpreted as compositional heterogeneity, can also be produced in homogeneous aggregates containing Fp. The spin crossover also suppresses thermally induced heterogeneity in longitudinal velocity (VP) at certain depths but not in VS. This effect is observed in tomography models at conditions where the spin crossover in Fp is expected in the lower mantle. In addition, the one-of-a-kind signature of this spin crossover in the RS/P (∂ ln VS/∂ ln VP) heterogeneity ratio might be a useful fingerprint to detect the presence of Fp in the lower mantle.

Effect of anomalous compressibility on Fe diffusion in ferropericlase throughout the spin crossover in the lower mantle

The transport of iron in the deep earth is important for understanding chemical equilibration between deep earth phases and between the core and mantle. However, iron transport modeling is complicated by its changes in spin state with pressure and temperature. Recent studies (Saha et al., 2011; Ammann et al., 2011) calculated the diffusivity of iron in ferropericlase as a function of iron's spin state but did not include the coupling of iron migration energetics to the observed anomalous bulk modulus softening associated with the spin transition (Wentzcovitch et al., 2009; Crowhurst et al., 2008). Here we calculate the diffusivity of iron in ferropericlase throughout the lower mantle using a model for both iron's spin and the anomalous bulk modulus softening to provide a complete model of iron diffusion in ferropericlase. Including the bulk modulus softening increases iron diffusivities by a factor of thirty relative to values without this effect at some depths on the geotherm.

Effect of iron content on electrical conductivity of ferropericlase with implications for the spin transition pressure

[1] The pressure-induced electronic spin transition of iron in ferropericlase was investigated as a function of iron content in ferropericlase by in situ electrical conductivity measurement. The electrical conductivity of ferropericlase, (Mg1-x,Fex)O (x = 0.07, 0.10, 0.13, 0.17, 0.24), was measured up to 53 GPa and 600 K using the Kawai-type multianvil apparatus equipped with sintered diamond anvils. At pressures up to 25 GPa, the electrical conductivity of ferropericlase generally increases with increasing pressure, and both the activation energy and activation volume of ferropericlase decrease with increasing iron content. For the samples with x = 0.07 and 0.10, the electrical conductivity shows a slight initial decrease and becomes constant between 25 and 40 GPa, upon which it increases slightly as the pressure increases. For the samples with higher iron content, the electrical conductivity constantly increases with pressure over the investigated pressure range. If these changes in the electrical conductivity are due to the isosymmetric high- to low-spin transition of iron in ferropericlase, this conductivity change suggests that the spin transition pressure significantly decreases with decreasing iron content in ferropericlase. Because the amount of iron in ferropericlase that coexists with the Al-bearing perovskite seems to be less than that in the Al-free perovskite, the influence of the iron partitioning between perovskite and ferropericlase by the spin transition appears in a pressure range of about 30–40 GPa in the lower mantle of the Earth.

Ferrous iron diffusion in ferro-periclase across the spin transition

We present first-principle calculations on absolute diffusion rates of high- and low-spin ferrous iron in ferro-periclase. The diffusivity of high- and low-spin iron depends on the physical conditions (pressure and temperature), iron concentration and the value of the chosen Hubbard U. We also find that low-spin iron swaps back to high-spin during migration. Our results show that throughout the Earth's lower mantle, iron diffuses at a faster rate than magnesium (up to an order of magnitude). Thus, the effect of the spin transition only has a small impact on the rheology of periclase in the Earth's mantle. Using reasonable vacancy concentrations, we find that ferro-periclase is much weaker than MgSiO 3-perovskite throughout the mantle. We conclude, therefore, that ferro-periclase controls the viscosity of the mantle in regions where it becomes interconnected, such as regions of high strain around slabs or near plumes. At very high pressures, such as in Super-Earths, low-spin iron diffusion is expected to become much slower than magnesium, eventually making ferro-periclase more viscous than pure periclase.

Effect of composition, structure, and spin state on the thermal conductivity of the Earth's lower mantle

The change in electronic structure of iron at high pressures to spin-paired states in ferropericlase, silicate perovskite, and post-perovskite may have a profound influence on the thermal conductivity of the lower mantle. Here, we present optical absorption data for lower mantle minerals to assess the effect of composition (including iron oxidation state), structure, and iron spin state on radiative heat transfer. We confirm that the presence of ferric iron in ferropericlase strongly affects the optical properties, while the effect of the spin-pairing transition may be more secondary. We also show that post-perovskite exhibits larger optical absorption in the near infrared and visible spectral ranges than perovskite which may have a profound effect on the dynamics the lowermost mantle. We present preliminary results from measurements of the phonon thermal conductivity of perovskite at 125 GPa using a pulsed laser heating technique. The available data suggest a larger value than what previously estimated, although the uncertainty is large.

Spin-State Crossover of Iron in Lower-Mantle Minerals: Results of DFT+U Investigations

The lower mantle is the largest layer of the Earth. Pressures and temperatures there vary from 23 GPa to 135 GPa and ~1,900 K to 4,000 K. Aluminous perovskite, Al-Mg1− x Fe x SiO3, and ferropericlase, Mg1− x Fe x O, are the most important phases of the Earth’s lower mantle, comprising ~62 vol% and ~35 vol% of this region, respectively. The remaining consists of CaSiO3-perovskite, according to the pyrolitic compositional model (Ringwood 1982). Silicate perovskite transforms into another polymorph, post-perovskite, at conditions expected to occur near the D″ discontinuity in the deep slower mantle, i.e., 2,500 K and 125 GPa (Murakami et al. 2004; Oganov and Ono 2004; Tsuchiya et al. 2004; Wentzcovitch et al. 2006). The spin-state crossover (also referred to as “spin pairing transition” or “spin(-state) transition”) of iron in ferropericlase under pressure was observed in 2003 by X-ray emission spectroscopy (XES) (Badro et al. 2003). In the following year a similar phenomenon was also identified in iron-bearing perovskite (Mg1− x Fe x SiO3) by the same technique (Badro et al. 2004). This phenomenon had been predicted for decades (Fyfe 1960; Gaffney and Anderson 1973; Ohnishi 1978; Sherman 1988, 1991; Sherman and Jansen 1995; Cohen et al. 1997) but the pressure conditions were challenging for this type of experiment and it took several decades to observe it. The implications of this phenomenon for the properties of this region, or of the entire planet, are yet to be understood. Today, theoretical studies are making decisive contributions to this problem. This article reviews the main theoretical results on the spin-state crossover in the lower mantle phases. Spin changes in strongly correlated oxides and silicates under pressure is the type of problem that …

Deformation of lower-mantle ferropericlase (Mg,Fe)O across the electronic spin transition

Recent high-pressure studies have shown that an electronic spin transition of iron in ferropericlase, an expected major phase of Earth's lower mantle, results in changes in its properties, including density, incompress- ibility, radiative thermal conductivity, electrical conduc- tivity, and sound velocities. To understand the rheology of ferropericlase across the spin transition, we have used in situ radial X-ray diffraction techniques to examine ferro- periclase, (Mg0.83,Fe0.17)O, deformed non-hydrostatically in a diamond cell up to 81 GPa at room temperature. Compared with recent quasi-hydrostatic studies, the range of the spin transition is shifted by approximately 20 GPa as a result of the presence of large differential stress in the sample. We also observed a reduction in incompressibility and in the unit cell volume of 3% across the spin transition. Our radial X-ray diffraction results show that the {0 0 1} texture is the dominant lattice preferred orientation in ferropericlase across the spin transition and in the low-spin state. Viscoplastic self-consistent polycrystal plasticity simulations suggest that this preferred orientation pattern is produced by {1 1 0}\1-10( slip. Analyzing our radial X-ray diffraction patterns using lattice strain theory, we evaluated the lattice d-spacings of ferropericlase and Mo as a function of the w angle between the compression direc- tion and the diffracting plane normal. These analyses give the ratio between the uniaxial stress component (t) and the shear modulus (G) under constant stress condition, which represents a proxy for the supported differential stress and elastic strength. This ratio in the mixed-spin and low-spin states is lower than what is expected from previous studies of high-spin ferropericlase, indicating that the spin transi- tion results in a reduced differential stress and elastic strength along with the volume reduction. The influence of the spin transition on the differential stress and strength of ferropericlase is expected to be less dominant across the wide spin transition zone at high pressure-temperature conditions relevant to the lower mantle.

Element partitioning between magnesium silicate perovskite and ferropericlase: New insights into bulk lower-mantle geochemistry

In this study, we investigated iron–magnesium exchange and transition-metal trace-element partitioning between magnesium silicate perovskite (Mg,Fe)SiO 3 and ferropericlase (Mg,Fe)O synthetised under lower-mantle conditions (up to 115 GPa and 2200 K) in a laser-heated diamond anvil cell. Recovered samples were thinned to electron transparency by focused ion beam and characterized by analytical transmission electron microscopy (ATEM) and nanometer-scale secondary ion mass spectroscopy (nanoSIMS). Iron concentrations in both phases were obtained from X-ray energy dispersive spectroscopy measurements and nanoSIMS. Our results are the first to show that recently reported spin-state and phase transitions in the lower mantle directly affect the evolution of Fe–Mg exchange between both phases. Mg-perovskite becomes increasingly iron-depleted above 70–80 GPa possibly due to the high spin–low spin transition of iron in ferropericlase. Conversely, the perovskite to post-perovskite transition is accompanied by a strong iron enrichment of the silicate phase, ferropericlase remaining in the Fe-rich phase though. Nanoparticles of metallic iron were observed in the perovskite-bearing runs, suggesting the disproportionation of ferrous iron oxide, but were not observed when the post-perovskite phase was present. Implications on the oxidation state of the Earth and core segregation will be discussed. Transition trace-element (Ni, Mn) concentrations (determined with the nanoSIMS) show similar trends and could thus be used to trace the origin of diamonds generated at depth. This study provides new results likely to improve the geochemical and geophysical models of the Earth's deep interiors.

Spin transition of iron in the Earth's lower mantle

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 and room temperature but 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 and electrical conductivity in the low-spin ferropericlase, which should be considered in future geophysical and geodynamic modeling of the Earth's lower mantle. In addition, a reduction in sound velocities within the spin transition is recently reported. Our evaluation of the experimental and theoretical pressure–volume results shows that the spin crossover of iron results in a density increase of 2–4% in ferropericlase containing 17–20% FeO. Here we have modeled the density and bulk modulus profiles of ferropericlase across the spin crossover under lower-mantle pressure–temperature conditions and shown how the ratio of the spin states of iron affects our understanding of the state of the Earth's lower mantle.

Electrical conductivity of the lower‐mantle ferropericlase across the electronic spin transition

Electrical conductivity of the lower‐mantle ferropericlase‐(Mg0.75,Fe0.25)O has been studied using designer diamond anvils to pressures over one megabar and temperatures up to 500 K. The electrical conductivity of (Mg0.75,Fe0.25)O gradually rises by an order of magnitude up to 50 GPa but decreases by a factor of approximately three between 50 to 70 GPa. This decrease in the electrical conductivity is attributed to the isosymmetric high‐spin to low‐spin transition of iron in ferropericlase. That is, the electronic spin transition of iron results in a decrease in the mobility and/or density of the charge transfer carriers in the low‐spin ferropericlase. The activation energy of the low‐spin ferropericlase is 0.27 eV at 101 GPa, consistent with the small polaron conduction (electronic hopping, charge transfer). Our results indicate that low‐spin ferropericlase exhibits lower electrical conductivity than high‐spin ferropericlase, which needs to be considered in future geomagnetic models for the lower mantle.

Comment on “Spin crossover in (Mg,Fe)O: A Mössbauer effect study with an alternative interpretation of x-ray emission spectroscopy data”

Electronic spin-pairing transition of iron in ferropericlase-(Mg,Fe)O has been recently studied with x-ray emission and M\"ossbauer spectroscopies under high pressures. While these studies reported a high-spin to low-spin transition of iron to occur at pressures above approximately $50\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, the width of the observed transition varies significantly. In particular, Kantor et al. [Phys. Rev. B 73, 100101(R) (2006)] reported that the transition in $({\mathrm{Mg}}_{0.8},{\mathrm{Fe}}_{0.2})\mathrm{O}$ occurs over a pressure range of approximately $50\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ in high-pressure M\"ossbauer measurements. To account for the discrepancy in the transition pressure, Kantor et al. reanalyzed the x-ray emission spectra by Lin et al. [Nature (London) 436, 377 (2005)] using a simple spectral decomposition method and claimed that x-ray emission measurements are also consistent with a spin crossover of iron at high pressures. Here, we show that the proposed fitting method is inadequate to describe the x-ray emission spectrum of the low-spin $\mathrm{Fe}{\mathrm{S}}_{2}$ and would result in an erroneous satellite peak $(K{\ensuremath{\beta}}^{\ensuremath{'}})$ intensity, leading to an artificial high-spin component and, consequently, to invalid conclusions regarding the width of the pressure-induced transition in ferropericlase. Furthermore, we show that an energy decrease of $\ensuremath{\sim}1.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in the $K{\ensuremath{\beta}}_{1,3}$ main peak was observed across the spin transition in ferropericlase, which can be used as an additional line of evidence for the electronic spin-pairing transition of iron in (Mg,Fe)O.

Iron partitioning in Earth's mantle: toward a deep lower mantle discontinuity.

We measured the spin state of iron in ferropericlase (Mg0.83Fe0.17)O at high pressure and found a high-spin to low-spin transition occurring in the 60- to 70-gigapascal pressure range, corresponding to depths of 2000 kilometers in Earth's lower mantle. This transition implies that the partition coefficient of iron between ferropericlase and magnesium silicate perovskite, the two main constituents of the lower mantle, may increase by several orders of magnitude, depleting the perovskite phase of its iron. The lower mantle may then be composed of two different layers. The upper layer would consist of a phase mixture with about equal partitioning of iron between magnesium silicate perovskite and ferropericlase, whereas the lower layer would consist of almost iron-free perovskite and iron-rich ferropericlase. This stratification is likely to have profound implications for the transport properties of Earth's lowermost mantle.