Earth and Planetary Interior Physics Laboratory, Meiji University

We aim to infer the structure of Earth’s interior by determining the properties of materials at high pressure and high temperature and comparing those data with geophysical observations. Our work includes the development of in situ electrical-conductivity measurements using diamond-anvil cells (DACs), as well as synchrotron X-ray measurements at large-scale facilities. Through these approaches, we seek to understand the present-day internal structure and chemical composition of the Earth.

For example, by measuring the electrical conductivity of NaCl-bearing aqueous fluids at high pressure and high temperature and comparing the results with geophysical observations, we clarified the circulation pathway of seawater in subduction zones (Fig. 1, upper right; Sinmyo & Keppler 2017). We have also measured the electrical conductivity of bridgmanite, a major mineral of the lower mantle, and used the data to estimate the chemical composition of the lower mantle (Fig. 1, lower right; Sinmyo et al. 2014). Those results suggested that the lower mantle may have a composition similar to that of the upper mantle. In addition, in situ measurements of the electrical conductivity of newly discovered iron oxide phases stable only at high pressure have led us to propose that heterogeneity in oxygen fugacity inside the Earth may be detectable through conductivity observations (Maitani et al. 2022).

Fig. 1. Constraining the structure of Earth’s interior

Another major goal of the laboratory is to constrain evolutionary processes inside Earth by determining stable phase relations, element partitioning, and valence states of transition elements, and by comparing these results with geological and geochemical evidence. We generate high-pressure and high-temperature conditions using diamond-anvil cells and large-volume presses, and then analyze recovered samples by X-ray diffraction, multiple spectroscopic techniques, and transmission electron microscopy. By determining stable phase relations, volatile cycles, and chemical partitioning between coexisting materials, we seek to understand the material evolution of Earth over 4.6 billion years.

Transmission electron microscopy allows us to determine crystal structure, chemical composition, and valence states of ultrafine minerals recovered from high-pressure and high-temperature conditions on the nanometer scale (Sinmyo et al. 2011, 2013). We also determine crystal structures under extreme conditions through X-ray diffraction and various spectroscopic methods (Fig. 2). For example, when highly oxidized materials are transported deep into the lower mantle, previously unknown iron oxide phases become stable and the eutectic temperature decreases markedly (Sinmyo et al. 2016, 2019). These findings indicate that redox state and volatile cycling strongly influence the evolution of Earth’s deep interior.

Fig. 2. Understanding the evolution of deep-Earth materials

The deep interior of the Earth is an extreme environment, reaching pressures up to 3.6 million atmospheres and temperatures of about 6,000 °C. Reproducing such conditions experimentally is always technically challenging. We therefore continue to develop improved experimental methods to obtain more reliable data under extreme conditions. Our current efforts include refinement of laser-heating techniques for diamond-anvil cells and advancement of comparatively new resistive-heating methods.

In an internally resistive-heated diamond-anvil cell, an electrical conductor is connected to the outside of the cell and heated by applying a voltage (Fig. 3a–c). Compared with widely used laser heating, this method is expected to provide more stable and homogeneous heating. Using this technique, we succeeded in generating pressure–temperature conditions exceeding previous world records by more than a factor of two, and we constrained the thermal structure of Earth’s core (Sinmyo et al. 2019). We have also developed a DAC system specialized for fluids and succeeded in measuring the in situ electrical conductivity of subducting seawater at high pressure and high temperature, which had previously been difficult to achieve (Fig. 3d; Sinmyo & Keppler 2017).

Fig. 3. Development of high-pressure and high-temperature experimental methods