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Water circulation in the mantle at a depth of 2900 km
Kei Hirose (Professor, Department of Earth and Planetary Science)
It is well known that subducting plates cause major earthquakes. In addition, plates that were originally at the bottom of the ocean partially dehydrate as they sink, and the water melts mantle rocks to form magma. This is the reason why there are so many volcanoes in Japan. It has been believed that the plate brings water to the bottom of the mantle, where it is completely dehydrated, creating a large chemical heterogeneity at the core-mantle boundary region at a depth of around 2900 km. However, when we conducted experiments under ultrahigh pressure and high temperature, which correspond to this region, we found that this theory is incorrect.
The properties of the Earth's mantle, which is composed of rocks, change drastically depending on the presence of a small amount of water. For example, the melting temperature of the mantle is greatly reduced, magma is formed more easily, and rocks become softer and more fluid. Therefore, it has been an important research question how much water is carried by oceanic plates, which are subducting into the Earth's interior with large amounts of water, and at what depths the water is dehydrated.
Such studies have been conducted mainly through high-pressure experiments, in which high-pressure and high-temperature conditions in the Earth's interior are realized in the laboratory. Recent studies have shown that water is transported to the lowest part of the mantle by silicon dioxide minerals contained in subducting plates. Quartz is a well-known silicon dioxide mineral, but deep in the mantle it is transformed into a different, denser crystal structure. The lowest part of the mantle is located directly above the metallic core. It has been believed that the silicon dioxide minerals are completely dehydrated by the heat of the high-temperature core, and that the water melts the lowermost mantle and chemically reacts with the metallic core to form iron oxides and other compounds. However, it has not been confirmed whether dehydration actually occurs at ultrahigh pressure and temperature just above the core.
We have been investigating the behavior of materials deep in the mantle and core using diamonds in high-pressure experiments (Figure). The reason for using diamond is that it is the hardest material known to man. The core-mantle boundary is under ultrahigh pressure and temperatures of 136 GPa and about 3500°C (actually, the temperatures are not yet well understood). Experiments conducted under such conditions revealed for the first time that, contrary to the prevailing theory, silicon dioxide minerals do not dehydrate even at temperatures exceeding 3500°C. In other words, plates that sink to the bottom of the mantle with water do not dehydrate, but rather retain the water, which eventually rises due to convective movement of the mantle, and is thought to be responsible for volcanic activity in Hawaii and other hot spots.
Seismic wave observations have shown that rocks and metals at the core-mantle interface are chemically diverse, as is the case with rocks on the earth's surface. Based on the results of this study, it is necessary to consider causes other than dehydration from subducting plates. Approximately 4.5 billion years ago, the Earth was covered by a "magma ocean" several thousand kilometers deep. We suspect that part of the magma remained in the deep mantle, and that the process of slow crystallization over billions of years may have created a large chemical heterogeneity at the core-mantle boundary region.
The results of this study were published in Y. Tsutsumi et al., Nature Geoscience, 17, 697 (2024).
Published in The Rigakubu News, September 2024
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