Yoshiyuki Okuda

Water in Earth’s mantle plays a crucial role in shaping geological and geochemical processes. While previous electrical conductivity measurements on major mantle minerals have constrained water content in the Earth’s mantle down to the mantle transition zone, the water content of the lower mantle remains experimentally unconstrained. This study measures the electrical conductivity of hydrous (Fe,Al)-bearing bridgmanite, synthesized with Fe and Al compositions representative of peridotitic lithology, at pressures up to 47 GPa and temperatures up to 2210 K. The results demonstrate that proton conduction is the predominant charge transport mechanism in hydrous (Fe,Al)-bearing bridgmanite under the investigated pressure-temperature conditions. Water incorporation markedly enhances the electrical conductivity of bridgmanite, increasing it by approximately an order of magnitude relative to that of dry samples. Extrapolation of the experimental data suggests that bridgmanite in the lower mantle contains less than 40–70 wt. ppm H ₂ O near the top and at mid-mantle depths. These findings offer insight into potential hidden water reservoirs in Earth’s lower mantle.

Electromagnetic sounding coupled with experimental measurements of electrical conductivity (EC) can provide valuable constraints on the thermochemical state of Earth's lower mantle. Here, we report a new set of experimentally determined EC values of pyrolite, a candidate composition of the lower mantle, between 25 and 80 GPa at room temperature and high temperature between 1,200 and 2,300 K using a laser‐heated diamond anvil cell combined with impedance spectroscopy. To maintain and better constrain the geometry of the experimental assembly, we used focused ion beam‐cut discs of a pre‐synthesized pyrolitic sample. Our results reveal a monotonic increase in EC with pressure, in contrast to earlier measurements that suggested a spin‐transition‐induced conductivity drop in the mid‐lower mantle. We also identify intrinsic voltage‐ and pressure‐related time‐dependent behavior that are likely expressions of nonlinear grain‐boundary conduction and stress‐driven relaxation processes, respectively. To our knowledge, this is the first report of such behavior in geological materials and may represent a previously unrecognized source of uncertainty in earlier high‐pressure EC data sets. Finally, we compare predictions based on our experimental findings against the radial EC structure derived from analysis of 10 years of satellite magnetic data from the ESA Swarm mission, with improved resolution in the lower mantle down to 2,200 km. We find that the measured EC of pyrolite is consistent with the geophysically inferred conductivity structure.

Carbon and hydrogen speciation and cycling in planetary interiors fundamentally shapes planetary structure and evolution. However, the behaviors of C-H system under extreme pressure-temperature (P-T) conditions across diverse planetary redox conditions remains poorly constrained. Through high P-T experiments spanning ice giant to terrestrial planet conditions, we investigated the C-H-O speciation, transport mechanisms, and implications for volatile budgets in differentiated bodies throughout the solar system and beyond. Our laser-heated diamond anvil cell experiments combined with X-ray diffraction up to 108 GPa and 3,410 K revealed pronounced melting-point depression of diamond in the presence of H2O, demonstrating that carbon speciation and phase stability in deep planetary interiors—from ice giants like Neptune and Uranus to carbon-rich exoplanets—are strongly influenced by volatile interactions. Multi-anvil press experiments on carbon-saturated Fe-Si alloys (4-27 wt.% Si) at 5-20 GPa constrained carbon solubility variations with silicon conent in the Fe-Si-C liquids. This relationship reveals how redox conditions controls carbon partitioning during core-mantle differentiation in reduced planetary environments like Mercury, potentially driving carbon exsolution and transport from metallic to silicate reservoirs to form distinct diamond layer on top of core-mantle boundary. For more oxidized and hydrated planetary mantle, such as the Earth’s mantle, our high P-T experiments examining the reaction of iron carbides (Fe3C, Fe7C3) with hydrous minerals such as brucite produce elemental carbon, in form of diamond, and (Mg,Fe)O, demonstrating that redox reactions between reduced carbon-bearing phases and hydrous minerals can generate diamond and redistribute carbon during magma ocean crystallization and slab-mantle interactions. These findings illuminate the speciation and cycling of C-H under varying redox conditions controls volatile budgets and distribution across planetary bodies, influences the core-mantle-crust carbon cycling through diverse planetary processes in our solar system and exoplanetary systems.

Electrical conductivity (EC) is a key physical property of minerals and rocks that constrains the composition and structure of Earth's deep interior. Theoretical studies predict that the CaCl ₂ ‐type hydrous Al‐bearing SiO ₂ phase, present in subducted crustal materials, becomes superionic—where protons are no longer bonded to specific oxygen atoms but instead become mobile within the SiO ₂ lattice—under high‐pressure and high‐temperature conditions of the lower mantle. The enhancement of the EC upon such superionic transition has not been experimentally verified yet. Here, we measured the EC of Al‐bearing SiO ₂ containing 1,750 ppm H ₂ O at pressures up to 82 GPa and temperatures up to 2610 K by employing a recently developed technique designed for measuring transparent materials. Results demonstrate a sudden increase in EC to approximately 10 S/m at temperatures of 1,100–2,200 K, depending on pressure. This is several to 10 times higher than the conductivity of the surrounding shallow to mid‐lower mantle and is consistent with a transition to the superionic state. If hydrous SiO ₂ is substantially weaker than other coexisting phases and thus forms an interconnected film in subducted mid‐oceanic ridge basalt (MORB) crust, the EC of the bulk MORB materials is significantly enhanced by superionic SiO ₂ to ∼1,800 km depth, which may explain the high EC anomalies observed at subduction zones underneath northeastern China. The observed EC anomalies can be matched by the EC of subducted MORB materials containing Al‐bearing SiO ₂ with a water content of approximately 0.2 wt%, providing insights into deep H ₂ O circulation and mantle distribution.

Electrical conductivity (EC) is one of the important physical properties of minerals and rocks that can be used to characterize the composition and structure of the deep interior of the Earth.Theoretical studies have predicted that the CaCl2-type hydrous Al-bearing SiO2 phase, present in subducted crustal materials, becomes superionic-meaning that protons are no longer bonded to a specific oxygen atom but instead become mobile within the SiO2 lattice-under high-pressure and high-temperature conditions corresponding to the lower mantle. The enhancement of the EC upon such superionic transition has not been experimentally verified yet. Here, we measured the EC of Al-bearing SiO2 containing 1750 ppm H2O at pressures up to 82 GPa and temperatures up to 2610 K by employing a recently developed technique designed for measuring transparent materials. Results demonstrate a sudden increase in EC to approximately 10 S/m at temperatures of 1100-2200 K, depending on pressure, which is several to ten times higher than that of the surrounding shallow to middle part of the lower mantle, which is attributed to a transition to the superionic state. If hydrous SiO2 is substantially weaker than other coexisting phases and thus forms an interconnected film in subducted MORB crust, the EC of the bulk MORB materials is significantly enhanced by superionic SiO2 in the lower mantle up to ~1800 km depth, which may explain the high EC anomalies observed at subduction zones underneath northeastern China. The observed EC anomalies can be matched by the EC of subducted MORB materials containing Al-bearing SiO2 with a water content of approximately 0.2 wt%, providing insights into the deep H2O circulation and distribution in the Earth's mantle.

We developed a near-infrared focused heating system (IRrDAC) for deformation experiments using a rotational diamond anvil cell. This study reports the results of annealing tests on silver and antigorite conducted at SPring-8 (BL47XU) using the IRrDAC system. The experimental results demonstrated the melting of silver and the dehydration of antigorite, confirming the capability of this system. The reproducible relationships between temperature and input power were also confirmed. The IRrDAC system enables deformation experiments at pressures equivalent to those of the lower mantle under homogeneous and stable temperatures and is expected to contribute to the understanding of deep Earth rheology.

The magnetic fields of terrestrial planets are created by core convection. Molten silicate mantles could also generate magnetic fields through their convective motion, known as a silicate dynamo. Recent computational studies have suggested that silicate melts may exhibit high electrical conductivity (EC) at temperatures above 4000 K due to strong electronic conduction, which could activate a silicate dynamo. We determined the EC of dense molten MgSiO ₃ up to 71 GPa and 4490 K by static compression experiments. It jumped by one order of magnitude upon melting, but 57(27) S/m at 4490 K is much lower than previous predictions, suggesting that molten MgSiO ₃ carries charge via ions rather than predicted electronic conduction. Nevertheless, the strong temperature dependence of the ionic conductivity found in this study suggests that super‐Earths’ hotter magma ocean with larger‐scale convection could power a dynamo that drives magnetic fields, which plays key roles in sustaining planetary surface environments.

The externally heated diamond anvil cell (EHDAC) conducts high pressure and temperature experiments with spatial uniformity and temporal stability. These are conventionally combined with various spectroscopies and x-ray diffraction measurements. EHDAC techniques perform Joule heating on a heater placed close to or directly in contact with diamond anvils. However, the electrical wiring and heater required for Joule heating complicate EHDAC setups, hindering easy access for the measurement of physical properties. This study proposes an EHDAC technique using laser- instead of Joule-heating. We successfully achieved temperatures reaching 900 K by applying heat to diamond anvils through laser-heating of the gaskets with thermally insulating anvil seats. To test this setup, we measured the melting temperature of H₂O ice VII, which was consistent with previous studies. We also measured the high-pressure and temperature impedance of H₂O VII and verified the capability of electrical resistivity measurements in this setup. This technique allows various physical property measurements owing to its simple setup required for externally laser-heated diamond anvil cell experiments. The unique characteristics of this heating technique are that (1) no heaters or wiring are required, (2) it exhibits the most efficient heating among EHDAC studies, (3) it maintains the DAC body at room temperature, and (4) diamond anvils do not detach from anvil seats after the EHDAC experiment. This method significantly simplifies the experimental setup, which allows much easier access to various physical property measurements using an EHDAC.

The electrical conductivity (EC) of minerals found on Earth and throughout the solar system is a fundamental transport property that is used to understand various dynamical phenomena in planetary interiors. High-pressure and high-temperature (P–T) EC measurements are also an important tool for observing phase transitions. Impedance measurements can accurately measure the EC of a nonmetallic sample. In previous measurements under static conditions using a laser-heated diamond-anvil cell (LHDAC), only direct current resistance is measured, but this method overestimates the bulk sample resistance. Moreover, the previous methodology could only be applied to nontransparent samples in an LHDAC using infrared lasers, limiting the range of measurable composition. To the best of our knowledge, no in situ high-P–T EC measurements of transparent materials have been reported using LHDAC techniques. We developed a novel impedance measurement technique under high-P–T conditions in an LHDAC that applies to transparent samples. As a validation, we measured the EC of Mg_0.9Fe_0.1SiO₃ bridgmanite up to 51 GPa and 2000 K and found that the results are consistent with those of previous studies. We also measured the EC values of sodium chloride to compare with those of previous studies, as well as those of cubic boron nitride and zirconia cement to quantify how well they insulate under high P–T conditions. This is the first report of the impedance and EC measurements of transparent minerals in an LHDAC, which allows the measurement of Fe-poor/-free materials, including the major constituents of the interiors of gas giants and icy planets, under extreme conditions.