We used three-dimensional inverse scattering of core-reflected shear waves for large-scale, high-resolution exploration of Earth's deep interior (D″) and detected multiple, piecewise continuous interfaces in the lowermost layer (D″) beneath Central and North America. With thermodynamic properties of phase transitions in mantle silicates, we interpret the images and estimate in situ temperatures. A widespread wave-speed increase at 150 to 300 kilometers above the coremantle boundary is consistent with a transition from perovskite to postperovskite. Internal D″ stratification may be due to multiple phase-boundary crossings, and a deep wave-speed reduction may mark the base of a postperovskite lens about 2300 kilometers wide and 250 kilometers thick. The core-mantle boundary temperature is estimated at 3950 ± 200 kelvin. Beneath Central America, a site of deep subduction, the D″ is relatively cold (ΔT = 700 ± 100 kelvin). Accounting for a factor-of-two uncertainty in thermal conductivity, core heat flux is 80 to 160 milliwatts per square meter (mW m ^–2 ) into the coldest D″ region and 35 to 70 mW m ^–2 away from it. Combined with estimates from the central Pacific, this suggests a global average of 50 to 100 mW m ^–2 and a total heat loss of 7.5 to 15 terawatts.

Raman spectra of Fe₂O₃ were measured to 62 GPa in a diamond anvil cell. All group theoretically predicted Raman-active phonon modes were detected to 54 GPa. In addition, some high-pressure spectra show an IR-active E_u mode (~660 cm⁻¹), possibly induced by surface defects or stress. This mode is related by a factor of two to a mode at 1320 cm⁻¹. The assignment of the 1320 cm⁻¹ mode has been controversial (two-magnon scattering or two-phonon scattering), and our observation supports the phonon assignment. All Raman-active phonons show nonlinear pressure-induced shifts. The mode Grüneisen parameters and their logarithmic volume dependences for two low-frequency phonons, A_1g and E_g, become negative and infinite, respectively, near 50 GPa as a result of the instability of the corundum structure at this pressure. Using the observed Raman-active phonons together with acoustic phonons previously measured by ultrasonics, and Kieffer’s model, we calculate the phonon contribution to the thermodynamic parameters of hematite. Comparison with experimentally measured values allows an estimation of an upper bound to the magnon contribution to the heat capacity at ambient pressure. This increases continuously above the Morin temperature and reaches a maximum at the Néel temperature (~37%). The Raman spectra change dramatically at pressures greater than 54 GPa as indicated by the appearance of new peaks and a significant increase in background. Although direct structural analysis is not possible due to the low signal-to-background ratio and the lack of polarization information, we were able to examine the consistency of our Raman observation with the corundum-to-perovskite phase transformation using the results for an analog system: MgSiO₃ ilmenite (ordered corundum type) and perovskite. This analysis shows that observed new features in Fe₂O₃ Raman spectra may not be consistent with the GdFeO₃ perovskite structure.