Why was garnet identified as the key mineral shaping the 660-km discontinuity?

Because high-temperature and high-pressure experiments simulating coexisting ringwoodite and garnet showed that garnet's phase transition triggers the decomposition of ringwoodite in a coupled reaction.

What mantle composition model does this discovery support?

It supports a homogeneous pyrolite composition model rather than a mixture of heterogeneous rocks.

How does this differ from the previous ringwoodite-only theory?

The prior theory could not explain the irregular topography of the discontinuity, whereas incorporating garnet consistently explains it across all temperature conditions.

Garnet Dominates the Formation of the 660-km Boundary in the Earth's Interior

Published Jun 15, 2026 8:00 AM ・ Updated Jun 14, 2026 11:22 PM ・ 5 min read ・ Source: PR TIMES

A joint research team from Okayama University and Gakushuin University revealed through high-temperature and high-pressure experiments that the phase transition of garnet, a major mantle mineral, dominates the formation of the 660-km discontinuity. They discovered a 'coupled reaction' where the phase transition of garnet triggers the decomposition of ringwoodite, supporting the theory that the mantle has a homogeneous pyrolite composition.

A joint research team consisting of Associate Professor Takayuki Ishii from the Institute for Planetary Materials at Okayama University's Institute for Advanced Research, Professor Hiroshi Kojitani, and Professor Emeritus Masaki Akaogi from Gakushuin University, has clarified through high-temperature and high-pressure experiments that the phase transition of the major mantle mineral garnet dominates the formation of the 660-km discontinuity. The research results were published on May 25, 2026, in the British earth science journal Nature Communications.

Near the depth of 660 km in the Earth's interior, there is a '660-km discontinuity' where seismic wave velocities change abruptly. Previously, this was thought to be caused by the decomposition of the major mineral ringwoodite (post-spinel transition), but this could not fully explain the complex irregular topography of the observed 660-km discontinuity.

In this study, focusing on garnet, the second most abundant mineral in the mantle, experiments were conducted under more realistic conditions where ringwoodite and garnet coexist. As a result, the team discovered a 'coupled reaction' in which the phase transition of garnet occurs first, triggering the decomposition of ringwoodite.

By taking the presence of garnet into account, it became clear that the irregular topography of the observed 660-km discontinuity can be explained consistently under cold subduction zones, warm hot plumes, and average mantle temperatures.

Associate Professor Takayuki Ishii commented: 'This achievement is the culmination of 15 years of work on a question I have had since my student days. It is deeply satisfying to see what has been built over many years take shape. During this time, deep Earth science has progressed significantly, and questions whose importance was underestimated back then are now attracting attention as major themes. I believe the true joy of research is that science always moves forward, accumulates, and reveals new horizons the more we challenge ourselves. I encourage students and young researchers to cherish their own questions. Even a small doubt, if pursued, can lead to discoveries that change the way we see the world.'

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Source: PR TIMES
Category: Survey

Garnet Dominates the Formation of the 660-km Boundary in the Earth's Interior

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