Effects of iron spin transition on the electronic structure, thermal expansivity and lattice thermal conductivity of ferropericlase: a first principles study

Recent progress in the study on phonon heat transport property of Earth's lower mantle minerals

The lattice thermal conductivities (κlat) of Earth's lower mantle (LM) minerals is a crucial parameter in the study of deep Earth dynamics and its determination is also one of the grand challenges in condensed matter physics. Here, we review recent progress on theoretical and experimental studies for theκlatunder high pressure (P) and high temperature (T) condition up to 150 GPa and 4000 K. After the critical parameters necessary to obtain converged values of theκlatare summarized, the theoreticalκlatof the LM minerals, determined through various computational methodologies, is compiled along with experimental findings. Although significant scattering is found in the experimental results at LMP,T, the quantum anharmonic lattice dynamics theory combined with the phonon Boltzmann transport theory demonstrates a clear relationship in theκlatof the end-member LM phases, MgO, MgSiO3bridgmanite (Brg) and post-perovskite (PPv),κlatMgO>>κlatPPv>κlatBrg, and a discontinuous change in theκlatby ∼20%-50% expected across the Brg-PPv transition. Knowledge on the additional but geophysically important factors, such as the effects of iron solid solution, isotopic mass difference, and higher order crystal anharmonicity are also summarized in detail. Current problems and future perspectives are finally mentioned.

The influence of δ-(Al,Fe)OOH on seismic heterogeneities in Earth's lower mantle

The high-pressure phases of oxyhydroxides (δ-AlOOH, ε-FeOOH, and their solid solution), candidate components of subducted slabs, have wide stability fields, thus potentially influencing volatile circulation and dynamics in the Earth's lower mantle. Here, we report the elastic wave velocities of δ-(Al,Fe)OOH (Fe/(Al + Fe) = 0.13, δ-Fe13) to 79 GPa, determined by nuclear resonant inelastic X-ray scattering. At pressures below 20 GPa, a softening of the phonon spectra is observed. With increasing pressure up to the Fe3+ spin crossover (~ 45 GPa), the Debye sound velocity (vD) increases. At higher pressures, the low spin δ-Fe13 is characterized by a pressure-invariant vD. Using the equation of state for the same sample, the shear-, compressional-, and bulk-velocities (vS, vP, and vΦ) are calculated and extrapolated to deep mantle conditions. The obtained velocity data show that δ-(Al,Fe)OOH may cause low-vΦ and low-vP anomalies in the shallow lower mantle. At deeper depths, we find that this hydrous phase reproduces the anti-correlation between vS and vΦ reported for the large low seismic velocity provinces, thus serving as a potential seismic signature of hydrous circulation in the lower mantle.