Equation of state and spin crossover of (Mg,Fe)O at high pressure, with implications for explaining topographic relief at the core-mantle boundary

Abstract

Iron-bearing periclase is thought to represent a significant fraction of Earth's lower mantle. However, the concentration of iron in (Mg,Fe)O is not well constrained at all mantle depths. Therefore, understanding the effect of iron on the density and elastic properties of this phase plays a major role in interpreting seismically observed complexity in the deep Earth. Here we examine the high-pressure behavior of polycrystalline (Mg,Fe)O containing 48 mol% FeO, loaded hydrostatically with neon as a pressure medium. Using X-ray diffraction and synchrotron Mössbauer spectroscopy, we measure the equation of state to about 83 GPa and hyperfine parameters to 107 GPa at 300 K. A gradual volume drop corresponding to a high-spin (HS) to low-spin (LS) crossover is observed between ~45 and 83 GPa with a volume drop of 1.85% at 68.8(2.7) GPa, the calculated spin transition pressure. Using a newly formulated spin crossover equation of state, the resulting zero-pressure isothermal bulk modulus K 0T,HS for the HS state is 160(2) GPa with a Kʹ 0T,HS of 4.12(14) and a V 0,HS of 77.29(0) Å 3. For the LS state, the K 0T,LS is 173(13) GPa with a Kʹ 0T,LS fixed to 4 and a V 0,LS of 73.64(94) Å 3. To confirm that the observed volume drop is due to a spin crossover, the quadrupole splitting (QS) and isomer shift (IS) are determined as a function of pressure. At low pressures, the Mössbauer spectra are well explained with two Fe 2+-like sites. At pressure between 44 and 84, two additional Fe 2+-like sites with a QS of 0 are required, indicative of low-spin iron. Above 84 GPa, two low-spin Fe 2+-like sites with increasing weight fraction explain the data well, signifying the completion of the spin crossover. To systematically compare the effect of iron on the equation of state parameters for (Mg,Fe)O, a spin crossover equation of state was fitted to the pressure-volume data of previous measurements. Our results show that K 0,HS is insensitive to iron concentration between 10 to 60 mol% FeO, while the spin transition pressure and width generally increases from about 50-80 and 2-25 GPa, respectively. A key implication is that iron-rich (Mg,Fe)O at the core-mantle boundary would likely contain a significant fraction of high-spin (less dense) iron, contributing a positive buoyancy to promote observable topographic relief in tomographic images of the lowermost mantle.

Pressure-induced spin-state transition of iron in magnesiowüstite (Fe,Mg)O

“…In addition, we obtain a substantial change in the HS-LS transition range, from ∼50-140 GPa in FeO to 30-88 GPa in (Fe,Mg)O with Mg x = 0.75. This indicates that the HS-LS transition width decreases with Mg x, in agreement with recent experiments [29,88].…”

Elastic Softening of (Mg0.8Fe0.2)O Ferropericlase Across the Iron Spin Crossover Measured at Seismic Frequencies

“…The work on (Mg 0.94 Fe 0.06 )O (Crowhurst et al, ), however, suggests a smaller pressure interval for the transition. These differences may be the result of different iron content (e.g., Persson et al, ; Solomatova et al, ) or caused by non‐hydrostatic effects (e.g., Lin et al, ; Marquardt & Miyagi, ). We note that our results rely on diffraction from one lattice plane only and may be particularly sensitive to non‐hydrostaticity.…”

“…Experimental measurements and computations suggest that this spin crossover leads to a marked decrease of compressional wave velocities in ferropericlase over a pressure range of ~20 GPa at 300 K (Crowhurst et al, ; Marquardt, Speziale, Reichmann, Frost, & Schilling, ; Wentzcovitch et al, ; Yang et al, ) with wide‐ranging implications for the interpretation of seismic observations in the lower mantle (Cammarano et al, ; Lin et al, ; Wu & Wentzcovitch, ). Low‐spin Fe 2+ has a smaller ionic radius than high‐spin ferrous iron (Shannon, ), and the reduction of wave velocities is a consequence of the enhanced compressibility of the ferropericlase structure in the pressure region where the octahedrally coordinated Fe 2+ ions change their electronic spin state (Lin et al, ; Speziale et al, ; Marquardt, Speziale, Reichmann, Frost, & Schilling, ; Solomatova et al, ). Most direct measurements of compressional wave velocities and the softening of the bulk modulus have been performed on samples with iron contents X Fe ≤ 0.08, where X Fe = Fe/(Mg + Fe) (Crowhurst et al, ; Yang et al, ).…”

“…In several other studies, the bulk modulus softening was indirectly inferred from a decrease of unit cell volumes of ferropericlase observed in static X‐ray diffraction experiments. Even though these studies generally agree on the qualitative effects of the iron spin crossover on the compressibility, a quantification of the bulk modulus softening is based on few experimental measurements throughout the spin transition region and requires assumptions to be made (Lin et al, ; Fei, Zhang, et al, ; Speziale et al, ; Marquardt, Speziale, Reichmann, Frost, & Schilling, ; Komabayashi et al, ; Mao et al, ; Solomatova et al, ).…”

Thermodynamic estimation the compressibility of ferropericlase under high pressure

“…1 To date, the elastic properties of (Mg 1−x Fe x )O ferropericlase and its two end members (MgO and FeO) have been studied up to the core-mantle boundary conditions by means of in-situ synchrotron-based X-ray diffraction. [2][3][4][5][6][7][8][9] However, the large scattering of the measured bulk moduli of the MgO-FeO solid solution was observed to be most likely caused by the use of different pressure-medium conditions, 10 as well as the occurrence of phase transitions, a spin transition, and defect clustering as a function of the FeO component. [11][12][13] Therefore, the effect of the FeO content on the compressibility of (Mg 1−x Fe x )O remains ambiguous.…”