Nuclear Resonant Scattering Studies of Iron-rich (Mg,Fe)O at High Pressures: Insights into the Core-Mantle Boundary Region

(Mg,Fe)O periclase is one of the main constituents of Earth's lower mantle. Iron-poor (Mg1-xFex)O, with x < 40, is of interest because it has been found to be in equilibrium with phase assemblages consisting of CaSiO3-perovskite and (Mg,Fe)(Si,Al)O3-perovskite under shallow lower mantle conditions (e. g. Irifune 1994). Iron-rich (Mg,Fe)O, where x > 40, may exist in the deep lower mantle, as the preferential partitioning of iron into that phase has been observed in the presence of perovskite (Auzende 2008; Sinmyo 2008). Further enrichment may occur in the deepest regions due to interactions with the liquid outer core. Iron-rich (Mg,Fe)O is also of interest for its changes in magnetic states under pressure. Although several measurements have been made to determine the acoustic wave velocities of (Mg,Fe)O, only a few have been made on iron-rich (Mg,Fe)O and none at pressures above 50 GPa. We present nuclear resonant scattering (NRS) studies on (Mg0.15Fe0.85)O to 135 GPa monitoring the 57Fe-weighted phonon density of states (PDOS) using nuclear resonant inelastic x-ray scattering (NRIXS) and magnetic state using synchrotron Mössbauer spectroscopy (SMS). Using the low energy region of the PDOS at each pressure, the Debye sound velocity (VD) is determined. With known density and VD, the seismically relevant compressional (VP) and shear (VS) are calculated. A softening of VS was detected at pressures below 30 GPa, above which the magnetic transition to the magnetically-ordered state was observed. The softening behavior of VS associated with the paramagnetic to magnetically-ordered transition is consistent with other studies on iron-rich (Mg,Fe)O and FeO using ultrasonic interferometry (Jacobsen et al. 2004) and NRIXS (Struzhkin et al. 2001). We will discuss the wave velocities obtained in our study in the context of Earth's core-mantle boundary region. References: Auzende, A-L., et al. (2008), EPSL, 269, 164-174. Irifune, T (1994), Nature, 370, 131-133. Jacobsen, S., et al. (2004), PNAS, 101, 5867-5871. Sinmyo, R., et al. (2008), JGR, 113, B11204. Struzhkin, V. V., et al. (2001), GRL, 31, L06, 620.