DATE2026.06.10 #Press Releases

Discovery of Seafloor Rocks Deep Inside Earth

Disclaimer: machine translated by DeepL which may contain errors.

Experiments, theoretical calculations, and seismic observations reveal that subducted oceanic plates may reach the core–mantle boundary

Summary 

A research group led by Associate Professor Ryosuke Shinmyo and graduate student Shuhou Maitani (then a graduate student) of the School of Science and Technology, Meiji University; Senior Researcher Saori Kawaguchi-Imada of the Japan Synchrotron Radiation Research Institute (JASRI) (then; currently Project Associate Professor at Kyoto University); Associate Professor Kenji Kawai, graduate student Rei Sato (then), and graduate student Keisuke Otsuru (then) of the Graduate School of Science, The University of Tokyo; Senior Researcher Hiroshi Sakuma and Senior Researcher Shigeru Suehara of the National Institute for Materials Science (NIMS); and Associate Professor Takayuki Ishii of the Institute for Planetary Materials, Okayama University, has obtained new evidence that oceanic crust subducted into Earth’s interior together with oceanic plates may have reached the core–mantle boundary, located at a depth of approximately 2,900 km.

At Earth’s surface, oceanic plates subduct into the planet’s interior at ocean trenches. These subducted plates are thought to be transported deep into the mantle over hundreds of millions of years. However, directly demonstrating that such plates can reach the core–mantle boundary has not been easy.

The key to this study is silicon dioxide (SiO₂), which is abundant in subducted oceanic crust. Under the extremely high pressures and temperatures of Earth’s deep interior, SiO₂ changes its crystal structure and forms a dense phase known as seifertite in the lowermost mantle. Because this transition has a characteristic effect on how seismic waves propagate, it can serve as a marker for identifying rocks that have subducted deep into Earth’s interior.

In this study, the researchers first precisely determined the pressure and temperature conditions under which SiO₂ undergoes phase transition into seifertite through high-pressure and high-temperature experiments and quantum-beam measurements at SPring-8. They then used atomic-scale quantum theoretical calculations to verify the experimental results and examine the effects of metastable phases, which had previously posed a problem.

Furthermore, by inverting a vast amount of seismic waveform data for seismic velocity structures beneath Central America and Hawaii, the researchers showed that the mineral phase transition observed in the laboratory corresponds to seismic velocity anomalies actually observed in Earth’s deep interior.

This work was supported by JSPS KAKENHI Grant Numbers 19H01989, 23H01277, JP23K25970, JP24K07171, JP23KJ0651, 23K19067, 24K00735, and 24KJ2052. The results were published in Scientific Reports, a Nature Portfolio journal.

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