Izumi Mashino

Silicate melt has been proposed to exist near the base of the mantle and has been invoked to explain seismic and electrical conductivity anomalies observed near the core‐mantle boundary; however, its presence and stability under lowermost‐mantle conditions remain uncertain. Here we report high‐pressure electrical conductivity measurements of Fe ²⁺ ‐ and Fe ³⁺ ‐bearing pyroxene glasses (Fe ³⁺ /ΣFe ≈ 0.5), used as analogs of silicate melts, up to megabar pressures at room temperature. At lower pressures, conductivity increases with pressure and iron content, consistent with electron‐hole hopping between Fe ²⁺ and Fe ³⁺ . Above 77–85 GPa, all samples show a marked conductivity decrease, suggesting a pressure‐induced spin transition of Fe ³⁺ . Previous studies have suggested that lower‐mantle silicate melts may be enriched in Fe ³⁺ , and iron spin‐state changes may modify iron partitioning between silicate melts and coexisting crystalline phases. Therefore, the Fe ³⁺ spin transition inferred here may affect the evolution of deep‐mantle melts through pressure‐dependent changes in iron partitioning behavior.

High-pressure spectroscopic measurements of ε-FeOOH were conducted up to ~ 65 GPa at room temperature in diamond anvil cells. The pressure evolution of the Raman vibrational modes confirms that a hydrogen-bond-symmetrization-induced phase transition from P 2 ₁ nm to Pnnm occurs at ~ 18 GPa. Infrared (IR) spectroscopic measurements suggest that the Pnnm phase has a disordered hydrogen state, and no spectroscopic evidence for fully centered hydrogen bonds is observed within the investigated pressure range. Above ~ 45 GPa, Fe ³⁺ in ε-FeOOH undergoes a high-spin to low-spin transition as indicated by a reduction of the unit cell volume, together with reductions in IR transmitted and Raman signals. These results demonstrate that ε‑FeOOH preserves a disordered hydrogen‑bond configuration up to at least 45 GPa, whereas δ-AlOOH transforms to a centered hydrogen-bond configuration at ~ 18 GPa. This compositional contrast suggests that Fe‑bearing oxyhydroxides follow a distinct evolution of hydrogen bonding under compression, providing insight into hydrogen behavior in deep Earth materials.

The behavior of iron-bearing magnesium carbonate (Mg,Fe)CO3 at high pressures has significant implications for deep carbon cycling in Earth’s mantle, as this composition more realistically reflects the carbonates expected in the Earth’s deep interior. In this study, we have investigated high-pressure vibrational properties of Fe-bearing magnesite using synchrotron Mössbauer, infrared (IR), and Raman spectroscopies in diamond anvil cells up to ∼71 GPa at room temperature. Above 29 GPa, Raman and IR spectra reveal signatures of pressure-induced lattice distortion, including splitting of lattice-coupled (T) and in-plane bending (ν4) modes of internal CO32- vibrations, changes in the pressure dependence of the wavenumbers of both Raman and IR modes, and a variation in the L/ν4 mode intensity ratio. While lattice distortion has been proposed in pure MgCO3, even minor Fe substitution alters the response of internal modes. This result suggests that low Fe content can modify the local bonding environment, which may influence the high-pressure and -temperature stability of the MgCO3-FeCO3 solid solution. Mössbauer spectra show that a high-spin to low-spin transition occurs between ∼45–50 GPa in Fe-bearing magnesite. Raman and IR measurements display discontinuous shifts in both lattice (L) and internal vibrational (ν1 and ν4) modes across the spin transition. Previous studies on the MgCO3 endmember have reported additional lattice distortion near 50 GPa, close to the pressure at which the spin transition occurs in Fe-rich compositions. The relative changes in the Grüneisen parameters (Δγ) of the lattice mode across the spin transition for Fe-containing compositions and the further distortion for MgCO3 endmember exhibit a linear dependence on Fe content, whereas those of the internal modes show a second-order, non-linear trend, particularly for the ν4 mode. Both lattice and internal modes may reflect the combined effect of lattice distortion and the spin transition, but the two cannot be distinguished from the behavior of the lattice mode. While lattice distortions have been discussed for the Mg-endmember, they have not been reported for Fe-bearing or Fe-rich compositions, possibly because the lattice distortions may be masked by the dominant effects of the spin transition, particularly in Fe-rich compositions. Since lattice modes are expected to primarily govern elastic wave propagation and the contribution of internal-mode variations is limited, the Fe-content dependence of sound velocities after the spin transition is expected to be approximately linear. This suggests that Fe-bearing carbonates may lower seismic wave velocities in the deep mantle where they are sufficiently abundant.

The subducting crustal materials serve as a crucial channel for transporting water to the lower mantle. Recent experimental studies suggest that crustal materials such as basaltic crust can be a main water carrier and reservoir playing an important role on water cycling in the lower mantle. Despite being a primary mineral in crustal materials, the water solubility of calcium ferrite-type (CF) phase and its stability are unclear yet. A recent phase relation study of hydrous basalts showed Na-depletion in lower-mantle minerals, suggesting the presence of fluid possibly with high Na concentration and the absence of CF phase along the low-temperature slab geotherms, where Al-rich hydrous phase H and ferropericlase appear instead. These phases could consequently produce Na-depleted CF phase when reaching the dehydration temperature of Al-rich hydrous phase H. In this study, we investigated the stability and water solubility of CF-type MgAl₂O₄, which is a main CF component in a hydrous basalt, in water-bearing systems at 26–32 GPa and 1200–1900 °C using a Kawai-type multi-anvil press. Our results indicate that the stability of the CF phase is strongly influenced by water content in the system. Water contents of recovered CF phases estimated by Fourier-transform infrared spectroscopy show a limited variation between 73 and 87 ppm wt at a pressure of 26 GPa and temperatures of 1500–1900 °C. We suggest that CF phase could not be a primary water carrier at lower mantle depths. This emphasizes contributions of hydrous aluminous silica minerals to Earth’s deep water cycling and heterogeneous structures in the lower mantle due to the strong water partitioning to this phase compared with other constituent minerals.

We have conducted high‐pressure electrical conductivity and Mössbauer spectroscopic measurements of peridotite glass as an analog of silicate melts. We observed the shoulder feature in the Mössbauer spectra above 60 GPa due to the emergence of the new Fe ²⁺ component, which could be associated with the change of the iron partitioning coefficient between solid and melt observed in previous melting experiments. The change in the trend of the electrical conductivity profile has been observed at around ∼83 GPa, suggesting the increase of the relative abundance of the new component. The pressure dependence of hyperfine parameters indicates the spin transitions of both Fe ²⁺ and Fe ³⁺ occur above 100 GPa, which is likely related to the structural change of the glass. Our results suggest that the spin state changes of iron lead to further densification of the silicate melts at the bottom of the mantle.

The electrical conductivity of pure and Al/Fe‐bearing phase E was measured up to 950 K at 15 GPa using a complex impedance spectroscopy. Pure phase E shows comparable conductivity to that of phase D, and a few orders of magnitude higher than that of phase A and super‐hydrous phase B. Al‐bearing phase E does not exhibit a conductivity difference, while a certain amount of incorporated Fe prominently increases its electrical conductivity by a factor of 4. Unlike the sole substitution 2Al³⁺→Mg²⁺+Si⁴⁺ in phase D and H, H⁺ is likely involved in the substitution. Proton conduction is the dominant conduction mechanism, while small polaron conduction becomes dominant with increasing Fe content. Phase E in subducted slabs at depth of the upper transition zone cannot explain the high electrical conductivity anomalies beneath the Philippine Sea or Northeast China. Other mechanisms such as dehydration of hydrous minerals is needed to account for them.

We have conducted electrical conductivity measurements of FeCO₃ siderite under high pressure up to 63 GPa in order to understand the nature and effect of iron spin transition and its influence on the geophysical properties of siderite, which is an end-member of major carbonate minerals. The results from Raman and Mössbauer spectroscopic measurements show that the high- to low-spin transition of iron occurs at around 50 GPa in agreement with previous studies. A sharp decrease of the electrical conductivity was also observed at around 50 GP, which is associated with the spin transition in iron. Although the stability of FeCO₃ siderite may be limited under high-temperature conditions along with the mantle geotherm, solid solutions in the MgCO₃-FeCO₃ system, Mg_1-xFe_xCO₃, could be stable up to the pressure-temperature condition of the lowermost mantle. The pressure-temperature range of the spin transition in Mg_1-xFe_xCO₃ is narrower than those of the major lower mantle minerals, ferropericlase and bridgmanite, and thus the drop of the electrical conductivity induced by the spin transition could be clearer under lower mantle conditions. Therefore, the existence of Mg_1-xFe_xCO₃ may affect the observed heterogeneity of electrical conductivity in the mid-lower mantle.

We have conducted electrical conductivity measurements of FeCO₃ siderite under high pressure up to 62 GPa in order to understand the nature and effect of iron spin transition and its influence on the geophysical properties of siderite, which is an end-member of major carbonate minerals. The results from Raman and Mössbauer spectroscopic measurements show that the high- to low-spin transition of iron occurs at around 50 GPa. A sharp decrease of the electrical conductivity was also observed at around 50 GP, which is associated with the iron spin transition. Although the stability of FeCO₃ siderite may be limited under high-temperature conditions along with the mantle geotherm, solid solutions in the MgCO₃-FeCO₃ system, Mg_1-xFe_xCO₃, could be stable up to the pressure-temperature condition of the lowermost mantle. The pressure-temperature range of the iron spin transition of Mg_1-xFe_xCO₃ would be narrower than those of the major lower mantle minerals of ferropericlase and bridgmanite, and thus the drop of the electrical conductivity induced by the spin transition could be clearer under the lower mantle conditions. Therefore, the existence of Mg_1-xFe_xCO₃ may affect the observed heterogeneity of electrical conductivity in the mid-lower mantle.

We conducted in situ high‐pressure acoustic‐wave velocity measurements of Fe²⁺‐bearing MgSiO₃ glass up to 158 GPa by Brillouin scattering spectroscopy to clarify the effect of iron on the elasticity and structural evolution of silicate melts in the lower mantle. The change in trend of the V_S profile, likely induced by the structural transition of Si‐O coordination number from 6 to 6+ proposed in previous studies of silicate glasses, was confirmed to be located at ∼106 GPa. Given the iron contents of partial melts derived from a pyrolitic or chondritic mantle, the transition pressure would be at around 84–97 GPa, which is well within the lowermost‐mantle pressure regime. Our data show the substitution of 12 mol% Fe in MgSiO₃ glass decreases the V_S by ∼5.5%. This implies that iron affects the buoyancy relations between melts/crystals and the melts at the lowermost mantle will have the higher coordination number than 6.