Sound Velocities and Equations of State for Lower Mantle Phases: Implications for Deep Mantle Structures

We will present recent nuclear resonant scattering and x-ray diffraction measurements on iron-bearing phases and their application towards our understanding of deep mantle structures. Specifically, we will present measurements on (Mg,Fe)O and bridgmanite-structured (Mg,Fe)SiO₃ with their relevance to ultra-low velocity zones and large piles in the core-mantle boundary region. The nuclear resonant inelastic x-ray scattering method provides specific vibrational information and in combination with x-ray diffraction data permits the determination of sound velocities and thermodynamic parameters. The nuclear resonant forward scattering method, also called synchrotron Mössbauer spectroscopy, provides hyperfine interactions between the resonant nucleus and electronic environment like isomer shifts and quadrupole splittings, which provide information on valence and spin states. These methods are complementary in terms of providing important information necessary to understand Earth’s interior. For example, accurate determination of the sound velocities and equations of state of deep Earth materials combined with seismic observations and dynamic modeling is essential for understanding the radial and lateral distributions of minerals. Also, knowledge of iron valences and spin states of minerals are relevant to our understanding of transport properties, mechanical behavior, and element partitioning, all of which control Earth’s internal dynamics.