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Silicate perovskite-type phases are the most abundant constituent inside our planet and are the predominant minerals in Earth’s lower mantle more than 660 kilometers below the surface. Magnesium-rich perovskite is a major lower mantle phase and undergoes a phase transition to post-perovskite near the bottom of the mantle. Calcium-rich perovskite is proportionally minor but may host numerous trace elements that record chemical differentiation events. The properties of mantle perovskites are the key to understanding the dynamic evolution of Earth, as they strongly influence the transport properties of lower mantle rocks. Perovskites are expected to be an important constituent of rocky planets larger than Mars and thus play a major role in modulating the evolution of terrestrial planets throughout the universe.

The viscosity structure of Earth's deep mantle affects the thermal evolution of Earth, the ascent of mantle plumes, settling of subducted oceanic lithosphere, and the mixing of compositional heterogeneities in the mantle. Based on a reanalysis of the long-wavelength nonhydrostatic geoid, we infer viscous layering of the mantle using a method that allows us to avoid a priori assumptions about its variation with depth. We detect an increase in viscosity at 800- to 1200-kilometers depth, far greater than the depth of the mineral phase transformations that define the mantle transition zone. The viscosity increase is coincident in depth with regions where seismic tomography has imaged slab stagnation, plume deflection, and changes in large-scale structure and offers a simple explanation of these phenomena.

The Thin Melt Line Pronounced and persistent variations in the speed of seismic waves as they travel through Earth's interior imply the presence of a physical or chemical boundary layer. Near the lithosphere-asthenosphere boundary, these seismic discontinuities are often patchy and their origins are disputed. By analyzing a set of short-period seismic waves across the Pacific Ocean, Schmerr (p. 1480 ; see the Perspective by Kawakatsu ) relates the intermittency of this seismic discontinuity to a layer of partial melt below the oceanic lithosphere. The melt may arise from a number of geodynamic processes, including upwellings from the mantle at hot spots like Hawaii, or small-scale thermal convection in the mantle.

Mantle convection and plate tectonics are driven by the heat flow from Earth's core to the surface. The radiative contribution to heat transport is usually assumed to be negligible. Here, we report the near-infrared and optical absorption spectra of silicate perovskite, the main constituent of the lower mantle, to 125 gigapascals. Silicate perovskite remains quite transparent up to the pressures at the core-mantle boundary. Estimates of radiative thermal conductivity derived from these spectra approach 10 watts meter –1 kelvin –1 at lowermost mantle conditions, implying that heat conduction is dominated by radiation. However, the increase in radiative conductivity with temperature ( T ) is less pronounced than expected from a T 3 dependency.

Seismic studies indicate that beneath some regions the 520-kilometer seismic discontinuity in Earth's mantle splits into two separate discontinuities (at ∼500 kilometers and ∼560 kilometers). The discontinuity near 500 kilometers is most likely caused by the (Mg,Fe) 2 SiO 4 β-to-γ phase transformation. We show that the formation of CaSiO 3 perovskite from garnet can cause the deeper discontinuity, and by determining the temperature dependence for this reaction we demonstrate that regional variations in splitting of the discontinuity arise from variability in the calcium concentration of the mantle rather than from temperature changes. This discontinuity therefore is sensitive to large-scale chemical heterogeneity. Its occurrence and variability yield regional information on the fertility of the mantle or the proportion of recycled oceanic crust.

We report seismic evidence for the transportation of water into the deep mantle in the subduction zone beneath northeastern Japan. Our data indicate that water is released from the hydrated oceanic crust at shallow depths (< ∼100 kilometers) and then forms a channel of hydrated mantle material on top of the subducting plate that is the pathway for water into the deep mantle. Our result provides direct evidence that shows how water is transported from the ocean to the deep mantle in a cold subduction zone environment.

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.

In situ x-ray diffraction measurements of MgSiO 3 were performed at high pressure and temperature similar to the conditions at Earth's core-mantle boundary. Results demonstrate that MgSiO 3 perovskite transforms to a new high-pressure form with stacked SiO 6 -octahedral sheet structure above 125 gigapascals and 2500 kelvin (2700-kilometer depth near the base of the mantle) with an increase in density of 1.0 to 1.2%. The origin of the D″ seismic discontinuity may be attributed to this post-perovskite phase transition. The new phase may have large elastic anisotropy and develop preferred orientation with platy crystal shape in the shear flow that can cause strong seismic anisotropy below the D″ discontinuity.

The equilibrium iron isotope fractionation between lower mantle minerals and metallic iron at core-mantle boundary conditions can be evaluated from the high-pressure 57 Fe partial vibrational density of states determined by synchrotron inelastic nuclear resonant x-ray scattering spectroscopy using a diamond anvil. Ferropericlase [(Mg,Fe)O] and (Fe,Mg)SiO 3 –post-perovskite are enriched in heavy iron isotopes relative to metallic iron at ultrahigh pressures, as opposed to the equilibrium iron isotope fractionation between these compounds at low pressure. The enrichment of Earth and Moon basalts in heavy iron isotopes relative to those from Mars and asteroid Vesta can be explained by the equilibrium iron isotope fractionation during the segregation of Earth's core and the assumption that Earth was already differentiated before the Moon-forming “giant impact.”

Stacks of global seismic data reveal the time and distance dependence of high-frequency (1-hertz) precursors to the seismic phase PKKP . Synthetic seismogram modeling shows that scattering from random small-scale topography at the PKKP core-mantle – boundary reflection point generates precursory arrivals similar to those seen in the data. Models with a root mean square core-mantle–boundary topography of 250 to 350 meters and correlation length of 7 to 10 kilometers explain the main features of the data. However, a systematic range-dependent misfit between observed and predicted precursor power suggests that inner core scattering may contribute to the precursors.

Three source regions show evidence of a low-velocity layer that is less than 15 kilometers thick on top of the core-mantle boundary and require about a 3:1 ratio of shear-to-compressional velocity reduction, which is consistent with partial melt. Layer thickness is correlated with travel time residuals of the seismic phases that are most sensitive to the lowermost mantle velocity. These observations suggest that the layer is thinned beneath downwellings but is present everywhere along the core-mantle boundary. Low viscosity accompanying partial melt can localize the upwelling of warmed mantle, making the low-velocity layer a plausible source of mantle plumes.

Strong, scattered reflections beyond 8 degrees (8°) offset are characteristic features of all high-resolution seismic sections from the continents. The reflections identify a low-velocity zone below approximately 100 kilometers depth beneath generally stratified mantle. This zone may be caused by partial melting, globally initiated at equal depth in the continental mantle. Solid state is again attained at the Lehmann discontinuity in cold, stable areas, whereas the zone extends to near the 400-kilometer discontinuity in hot, tectonically active areas. Thus, the depth to the Lehmann discontinuity may be an indicator of the thermal state of the continental mantle.

A persistent reversal in the expected polarity of the initiation of vertically polarized shear waves that graze the D″ layer (the layer at the boundary between the outer core and the lower mantle of Earth) in some regions starts at the arrival time of horizontally polarized shear waves. Full waveform modeling of the split shear waves for paths beneath the Caribbean requires azimuthal anisotropy at the base of the mantle. Models with laterally coherent patterns of transverse isotropy with the hexagonal symmetry axis of the mineral phases tilted from the vertical by as much as 20° are consistent with the data. Small-scale convection cells within the mantle above the D″ layer may cause the observed variations by inducing laterally variable crystallographic or shape-preferred orientation in minerals in the D″ layer.

Understanding the boundary between Earth9s rock mantle and the molten core is one of the key challenges in geoscience. In his Perspective, Garnero discusses recent geophysical discoveries that are forcing the adoption of a new paradigm for studying the core-mantle boundary. Long thought to be a simple division between solid silicate rock and liquid iron alloy, the core-mantle boundary is now emerging as an active, complex, and heterogeneous region of Earth9s interior.

We applied global waveform tomography to model radial anisotropy in the whole mantle. We found that in the last few hundred kilometers near the core-mantle boundary, horizontally polarized S -wave velocities ( V SH ) are, on average, faster (by ∼1%) than vertically polarized S -wave velocities ( V SV ), suggesting a large-scale predominance of horizontal shear. This confirms that the D ″ region at the base of the mantle is also a mechanical boundary layer for mantle convection. A notable exception to this average signature can be found at the base of the two broad low-velocity regions under the Pacific Ocean and under Africa, often referred to as “superplumes,” where the anisotropic pattern indicates the onset of vertical flow.

In contrast to the upper mantle, the lower mantle of the Earth is elastically nearly isotropic, although its dominant constituent mineral [(Mg,Fe)SiO 3 perovskite] is highly anisotropic. On the basis of high-temperature experiments on fabric development in an analog CaTiO 3 perovskite and the elastic constants of MgSiO 3 perovskite, the seismic anisotropy was calculated for the lower mantle. The results show that absence of anisotropy is strong evidence for deformation by superplasticity. In this case, no significant transient creep is expected in the lower mantle and the viscosity of the lower mantle is sensitive to grain size; hence, a reduction in grain size will result in rheological weakening.

The lower mantle, lying between ~670- and ~2890-km depth, comprises most of the rocky portion of Earth. Convection processes within this region transfer heat from the iron core upward, advecting the heat flow that drives the near-surface tectonic engine. As many of the largest volcanic events appear to be correlated with seismic features in the deep mantle ( 1 ), the deep lower mantle may also represent an occasional scourge of our surface environment. In this issue, two sets of challenging experiments yield new pictures for how different deep seismic anomalies might be generated. On page 892, Andrault et al. ( 2 ) examine the melting temperature of oceanic crust (basalt) to core-mantle boundary (CMB) pressures and temperature, and use that to explain the genesis of areas with ultralow seismic velocities near the CMB. On page 877, Zhang et al. ( 3 ) report the startling discovery of a new, iron-rich silicate phase that may be a major component of the lowermost ~700 km of Earth's mantle.

Laboratory experiments document that liquid iron reacts chemically with silicates at high pressures (above 2.4 x 10 to the 10th Pa) and temperatures. In particular, (Mg,Fe)SiO3 perovskite, the most abundant mineral of earth's lower mantle, is expected to react with liquid iron to produce metallic alloys (FeO and FeSi) and nonmetallic silicates (SiO2 stishovite and MgSiO3 perovskite) at the pressures of the core-mantle boundary, 14 x 10 to the 10th Pa. The experimental observations, in conjunction with seismological data, suggest that the lowermost 200 to 300 km of earth's mantle, the D-double-prime layer, may be an extremely heterogeneous region as a result of chemical reactions between the silicate mantle and the liquid iron alloy of earth's core. The combined thermal-chemical-electrical boundary layer resulting from such reactions offers a plausible explanation for the complex behavior of seismic waves near the core-mantle boundary and could influence earth's magnetic field observed at the surface.

Thermal convection experiments in a rapidly rotating hemispherical shell suggest a model in which the convection in Earth's liquid outer core is controlled by a thermally heterogeneous mantle. Experiments show that heterogeneous boundary heating induces an eastward flow in the core, which, at a sufficiently large magnitude, develops into a large-scale spiral with a sharp front. The front separates the warm and cold regions in the core and includes a narrow jet flowing from the core-mantle boundary to the inner-core boundary. The existence of this front in the core may explain the Pacific quiet zone in the secular variation of the geomagnetic field and the longitudinally heterogeneous structure of the solid inner core.

Forward modeling of differential travel times of phases sensitive to lowermost mantle beneath the central Pacific reveals lateral heterogeneity that is higher in amplitude than predicted by tomographic models. A broad zone of low S velocity (−4 percent with respect to standard models), which may correspond to the base of a thermal “plume,” narrows and is deflected as it extends to about 1000 kilometers above the core-mantle boundary. To the east of this zone, a localized region of fast S velocity (+5 percent) suggests strong heterogeneity or anisotropy related to the presence of high pressure and temperature assemblages, which may or may not involve core material. Its presence could also explain the observation of precursors to core reflected phases in this region.

Experiments at high pressure and temperature to determine the dihedral angle of core melts in lower mantle phases yielded a value of approximately 71 degrees for perovskite-dominated matrices. This angle, although greater than the 60 degrees required for completely efficient percolation, is considerably less than the angles observed in mineral matrices at upper mantle pressure-temperature conditions in experiments. In other words, molten iron alloy can flow much more easily in lower mantle mineralogies than in upper mantle mineralogies. Accordingly, although segregation of core material by melt percolation is probably not feasible in the upper mantle, core formation by percolation may be possible in the lower mantle.

Short- and long-period precursors of the PKP phase were used to study an ultra-low velocity zone (ULVZ) near the core-mantle boundary beneath the Western Pacific. Synthetic seismograms were computed from a hybrid method, which handles seismic wave propagation through two-dimensional complex structures. Long-period precursors were explained by Gaussian-shaped ULVZs of 60 to 80 kilometers height with P velocity drops of at least 7 percent over 100 to 300 kilometers. Short-period precursors suggest the presence of smaller scale anomalies accompanying these larger Gaussian-shaped structures. These fine structures may be areas of partial melt caused by vigorous small-scale convection or the instability of a thermal boundary layer at the mantle's base, or both.

The lower mantle extends from 660 to 2890 km below the surface of the Earth. The rocks and minerals of the deep mantle are not accessible in nature, except those occurring infrequently as inclusions in diamond. However, they can be synthesized and examined at the relevant high pressure and temperature conditions in the laboratory. Recent such experimental investigations, as well as theoretical calculations, have suggested that the properties of lower-mantle minerals vary with increasing depth much more than was previously thought. On page 193 of this issue, Irifune et al. ( 1 ) report that iron (Fe) partitioning between the two main lower-mantle constituents, iron–magnesium silicate perovskite (Pv) and iron–magnesium oxide (ferropericlase, Fp), indeed changes in a natural mantle composition for conditions corresponding to depths below 1100 km. The results have profound implications for predicting the properties and dynamics of the deep mantle.

Analyses of x-ray-diffraction measurements on (Mg,Fe)SiO(3) perovskite and (Mg,Fe)O magnesiowüstite at simultaneous high temperature and pressure are used to determine pressure-volume-temperature equations of state and thermoelastic properties of these lower mantle minerals. Detailed comparison with the seismically observed density and bulk sound velocity profiles of the lower mantle does not support models of this region that assume compositions identical to that of the upper mantle. The data are consistent with lower mantle compositions consisting of nearly pure perovskite (85 percent), which would indicate that the Earth's mantle is compositionally, and by implication, dynamically stratified.