SPring-8, the large synchrotron radiation facility

Release Date

25 Apr, 2011

Tokyo Institute of Technology

1. Background and development of this research
Because liquid magma is generally lighter than the solid mantle, the magma rises towards the Earth's surface and results in the formation of volcanoes. However, liquid magma may be more easily compressed and enriched in iron than in rock; therefore, dense magma is considered to exist deep in the mantle.

Moreover, ultralow-velocity zones have been observed at the bottom of the mantle (Fig. 1). It has been pointed out that dense magma deep in the mantle may be a cause of these ultralow-velocity zones.

However, it is very difficult to experimentally demonstrate that such dense magma exists deep in the mantle because of its very high pressure and temperature. Hence, the existence of dense magma deep in the mantle has remained within the realm of speculation.

2. Achievements
The research group developed a laser-heated diamond anvil cell (Fig. 2), an experimental device that can create environments of very high pressure and temperature that correspond to the deep mantle conditions. Experiments under very high pressure and temperature using this device have been intensively carried out.

In this study, the scientists experimentally produced magma by partially melting mantle material that consists of silicon, manganese, and iron oxides as principal components under high pressures of up to 160 GPa and temperatures of up to 4,000^oC using the SPring-8 Beamline BL10XU for High Pressure Research, which was developed and managed by the Tokyo Institute of Technology. They succeeded in determining the chemical composition of the magma deep in the mantle for the first time in the world. It was found that the content of iron in the remaining solid mantle material markedly decreased, whereas that of iron in the magma as a melt increased at a pressure of approximately 75 GPa, which corresponds to that at a depth of approximately 1,800 km in the mantle.

From the measurements obtained using the Taiwan Contract Beamline BL12XU at SPring-8, it was also found that the arrangement of the electrons of the iron ions in the magma changed under high pressures (spin crossover), causing iron to be more easily taken into the magma.

The density of the magma calculated from the chemical composition determined in the above experiments revealed that the density of the magma is higher than that of the surrounding solid mantle at a depth of 1,800 km or more (Fig. 3). This indicated that the magma sinks down in the deep mantle.

Soon after the Earth's formation, the surface of the Earth was considered to be covered with a magma ocean. The results of this research strongly indicate that the magma ocean extended not only over the surface of the Earth but also beneath the solid mantle (Fig. 4).

According to a previous calculation, the rate of solidification of the magma ocean beneath the solid mantle is considerably low. Therefore, there is a high probability that dense magma, although in a small amount, still remains at the bottom of the mantle. The ultralow-velocity zones currently observed at the bottom of the mantle (Fig. 1) are considered to be residues of such an ancient magma ocean (Fig. 4).

3. Future progress
The experimental technique of creating environments of very high pressure and temperature developed in this study has led to the successful research on magma deep in the mantle.　Detailed investigations of the formation and solidification of the magma ocean are expected to greatly advance the understanding of Earth's early evolution.

<<Figures>>