Equation of State of FeO

Wustite, Fe(1-x)O, is an important endmember of (Mg,Fe)O in the Earth’s lower mantle. The Earth’s core contains several weight percent of an unknown light element; if this element is oxygen, then wustite may also be a significant component of the core. Therefore the high pressure, high temperature behavior of FeO, including its equation of state and phase diagram, is essential knowledge for understanding the properties and evolution of Earth’s deep interior. We performed X-ray diffraction measurements on wustite+iron mixtures at beamline 13-ID-D of the GSECARS sector of the Advanced Photon Source, using a laser-heated diamond anvil cell to achieve simultaneous high pressures and temperatures (90 to 155 GPa and up to 3100 K). Pressures were determined from the lattice parameters of hcp iron and of NaCl, the insulator and pressure medium. Temperatures were determined by spectroradiometry. To confirm the accuracy of our temperature measurements, we verified the location of the hcp-fcc transition in Fe [1]. The wustite in our experiments was mixed with metallic iron to produce stoichiometric FeO at high pressures and temperatures [2]. We have determined the thermal equation of state of B1-FeO to 155 GPa, combining our new P-V-T data with earlier data [2, 3]. We obtain a bulk modulus K = 149.4 GPa, its pressure derivative K’ = 3.60, and Gruneisen parameter 1.41. The updated equation of state produces a high pressure Fe-FeO oxygen fugacity buffer curve that is only slightly different than that previously reported [2] over the range of overlapping conditions (less than 0.2 log units at 100 GPa), but with more significant differences at higher pressures such as those achieved in the present study. We have also determined an equation of state of B8-FeO, with parameters V = 11.997 cc/mol, K = 137.8 GPa, Gruneisen parameter 1.73, and a fixed K’ = 4. The subsolidus phase boundary between the B1 and B8 structures obtained in this study is broadly consistent with [3], confirming that the stable phase of FeO has the B1 structure at P-T conditions relevant to the Earth’s deep interior. We have also compared our results for endmember FeO to previous studies of (Mg,Fe)O with varying iron contents, to determine the pressure dependence of the excess volume of mixing in the MgO-FeO system. Our results for the equation of state for B1-FeO can be compared to the equation of state of hcp-Fe [4] to match the seismologically determined density of the outer core. Assuming a core-mantle boundary temperature of 4000 +/- 500 K and a 1-2% density decrease upon melting, the amount of oxygen in the outer core required to match PREM at the core-mantle boundary is 7.7 +/- 1.1 weight percent, under the simplifying assumption of a purely Fe-O outer core. [1] Komabayashi & Fei (2010) JGR 115, B03202. [2] Campbell et al. (2009) EPSL 286, 556-564. [3] Ozawa et al. (2010) PEPI 179, 157-163. [4] Dewaele et al. (2006) PRL 97, 215504.