Significance: Presence of iron in Earth’s lower mantle may critically affect its physical properties and thermochemical structures. However, its effects on thermal conductivity and dynamics of deep mantle remain unknown. We studied lattice thermal conductivity of lower-mantle ferropericlase to 120 GPa using ultrafast optics. We observed an enhanced iron substitution effect in the low-spin iron-rich ferropericlase, with thermal conductivity that significantly drops across spin transition. Combined with bridgmanite data, we provided a self-consistent radial profile of lower-mantle thermal conductivity, which is dominated by pressure, temperature, and iron effects and shows a twofold increase throughout the lower mantle. If ultralow velocity zones are hot and strongly enriched in iron, their exceptionally low thermal conductivity will delay their cooling, influencing lowermost mantle dynamics.

Earth’s core is composed of iron (Fe) alloyed with light elements, e.g., silicon (Si). Its thermal conductivity critically affects Earth’s thermal structure, evolution, and dynamics, as it controls the magnitude of thermal and compositional sources required to sustain a geodynamo over Earth’s history. Here we directly measured thermal conductivities of solid Fe and Fe–Si alloys up to 144 GPa and 3300 K. 15 at% Si alloyed in Fe substantially reduces its conductivity by about 2 folds at 132 GPa and 3000 K. An outer core with 15 at% Si would have a conductivity of about 20 W m⁻¹ K⁻¹, lower than pure Fe at similar pressure–temperature conditions. This suggests a lower minimum heat flow, around 3 TW, across the core–mantle boundary than previously expected, and thus less thermal energy needed to operate the geodynamo. Our results provide key constraints on inner core age that could be older than two billion-years.

Complex seismic, thermal, and chemical features have been reported in Earth's lowermost mantle. In particular, possible iron enrichments in the large low shear‐wave velocity provinces (LLSVPs) could influence thermal transport properties of the constituting minerals in this region, altering the lower mantle dynamics and heat flux across core‐mantle boundary (CMB). Thermal conductivity of bridgmanite is expected to partially control the thermal evolution and dynamics of Earth's lower mantle. Importantly, the pressure‐induced lattice distortion and iron spin and valence states in bridgmanite could affect its lattice thermal conductivity, but these effects remain largely unknown. Here we precisely measured the lattice thermal conductivity of Fe‐bearing bridgmanite to 120 GPa using optical pump‐probe spectroscopy. The conductivity of Fe‐bearing bridgmanite increases monotonically with pressure but drops significantly around 45 GPa due to pressure‐induced lattice distortion on iron sites. Our findings indicate that lattice thermal conductivity at lowermost mantle conditions is twice smaller than previously thought. The decrease in the thermal conductivity of bridgmanite in mid‐lower mantle and below would promote mantle flow against a potential viscosity barrier, facilitating slabs crossing over the 1000 km depth. Modeling of our results applied to LLSVPs shows that variations in iron and bridgmanite fractions induce a significant thermal conductivity decrease, which would enhance internal convective flow. Our CMB heat flux modeling indicates that while heat flux variations are dominated by thermal effects, variations in thermal conductivity also play a significant role. The CMB heat flux map we obtained is substantially different from those assumed so far, which may influence our understanding of the geodynamo.

Significance: Thermal conductivity of mantle minerals is critical for controlling the temperature profile and dynamics of the mantle and subducting slabs. However, the effect of hydration on lattice thermal conductivity remains poorly understood. We studied lattice thermal conductivity of olivine (Mg _0.9 Fe _0.1 ) ₂ SiO ₄ (Fo90) to 15 GPa using ultrafast optics. The thermal conductivity of hydrous Fo90 with ∼7,000 wt ppm water is 2 times smaller than its anhydrous counterpart at transition zone pressures. Modeling thermal structure of a subducting slab shows that the hydration-reduced thermal conductivity in the oceanic crust further decreases the temperature within the subducting slab, which substantially lowers the olivine−wadsleyite transformation rate and extends the metastable olivine to greater depths. Such an effect could enhance water transportation to the transition zone.

Seismic anomalies observed in Earth's deep mantle are conventionally considered to be associated with thermal and compositional anomalies, and possibly partial melt of major lower‐mantle phases. However, through deep water cycle, impacts of hydrous minerals on geophysical observables and on the deep mantle thermal state and geodynamics remain poorly understood. Here we precisely measured thermal conductivity of δ‐(Al,Fe)OOH, an important water‐carrying mineral in Earth's deep interior, to lowermost mantle pressures at room temperature. The thermal conductivity varies drastically by twofold to threefold across the spin transition of iron, resulting in an exceptionally low thermal conductivity at the lowermost mantle conditions. As δ‐(Al,Fe)OOH is transported to the lowermost mantle, its exceptionally low thermal conductivity may serve as a local thermal insulator, promoting high‐temperature anomalies and the formation of partial melt and thermal plumes at the base of the mantle, strongly influencing thermo‐chemical profiles in the region and fate of Earth's deep water cycle.

Complex intra-molecular interactions and the hydrogen-bonding network in H₂O-volatile mixtures play critical roles in many dynamics processes in physical chemistry, biology and Earth and planetary sciences. We used high pressure Raman spectroscopy to study the pressure evolution of vibrational frequencies and bonding behavior in H₂O-CH₃OH mixtures. We found that the presence of low CH₃OH content in H₂O increases the transition pressure where water crystallizes to ice VI, but does not significantly change the pressure where ice VI transforms to ice VII. Furthermore, the stiffening rates of C-H stretching frequencies dω/dP in CH₃OH significantly decrease upon the crystallization of water and the softening rates of the O-H stretching frequencies of ice VII are suppressed over a narrow pressure range, after which the frequencies of these modes shift with pressure in ways similar to pure CH₃OH and ice VII, respectively. Such complex pressure evolution of Raman frequencies along with pronounced variations in Raman intensities of CH₃OH within the sample and the hysteresis of the water-ice VI phase transition suggest pressure-induced segregation of low content CH₃OH from ice VII. These findings indicate the significant influence of volatiles on the crystallization of sub-surface ocean and thermal evolution within large icy planets and satellites.