Sound Velocities and Density of (Mg0.65, Fe0.35)O ferropericlase up to 1.4 Mbar

The Earth’s lower mantle accounts for more than half of the mass and volume of the planet. Iron is the most abundant transition-metal in the lower mantle, and it is hosted mainly by silicate perovskite, post-perovskite and ferropericlase. The spin-pairing transitions of iron from high-spin (HS) to low-spin (LS) state in ferropericlase have been recognized to affect the density and sound velocities of this second most abundant lower mantle phase (e.g. Speziale et al., 2005; Lin et al., 2005; Lin et al., 2006; Crowhurst et al., 2008; Marquardt et al., 2009; Zhuravlev et al., 2009). High-pressure and high-temperature physical properties of ferropericlase (Mg,Fe)O with various iron content are of significant importance for interpreting the seismological structure of the lower mantle in terms of compositional variations and for geodynamic modeling (e.g. Bower et al., 2009). We have determined the elastic and vibrational properties of (Mg_0.65Fe_0.35)O up to 1.4 Mbar at 300 K using nuclear resonant inelastic x-ray scattering (NRIXS) and in situ X-ray diffraction (XRD) measurements in diamond anvil cell at Sector 3 of Advanced Photon Source, Argonne National Laboratory. (Mg_0.65Fe_0.35)O contains the amount of iron within the estimated range expected for a “pyrolite” lower mantle (e.g. Westrenen et al., 2005; Sinmyo et al., 2008). From the low-energy region of the partial phonon density of states (PDoS), in situ XRD measurements, and our separate equation-of-state study with smaller pressure steps, we derived its shear and compressional velocities up to 1.4 Mbar. From integration of the PDoS, we also obtained Fe-weighted vibrational parameters as a function of pressure for this ferropericlase. Our new data provide fundamental new knowledge about the effects of spin transition and iron content on the density and sound velocities of ferropericlase in a previously uncharted pressure-composition sector. We will synthesize these effects by combining our results with those of other iron-poor and iron-rich ferropericlases and discuss the implications for seismological observations and compositional variations in the lower mantle. (References: Speziale, S., Milner, A., Lee, V. E., et al. (2005) Proc. Natl. Acad. Sci. U. S. A. 102, 17918-17922. Lin, J.-F., Struzhkin, V. V., Jacobsen, S. D., et al. (2005) Nature 436, 377-380. Lin, J.-F., Jacobsen, S. D., Sturhahn, W., et al. (2006) Geophys. Res. Lett. 33, L22304. Crowhurst, J., Brown, J., Goncharov, A., & Jacobsen, S. (2008) Science 319, 451-453. Marquardt, H., Speziale, S., Reichmann, H. J., et al. (2009) Science 324, 224-226. Zhuravlev, K., Jackson, J., Wolf, A., et al. (2009) Physics and Chemistry of Minerals 37, 465-474. Bower, D. J., Gurnis, M., Jackson, J. M., & Sturhahn, W. (2009) Geophys. Res. Lett. 36, L10306. Westrenen, W., Li, J., Fei, Y., et al. (2005) Phys. Earth Planet. Inter. 151, 163-176. Sinmyo, R., Hirose, K., Nishio-Hamane, D., et al. (2008) J. Geophy. Res. - Solid Earth 113, B11204.)