At a depth of 660 kilometres, there is an abrupt change in the speed at which seismic waves travel through Earth's interior. However, for hitherto unknown reasons this striking seismic discontinuity splits into two below cold subduction zones, where the oceanic crust sinks into the mantle below the continental crust. With high-pressure experiments at DESY's X-ray light source PETRA III and at the Japanese X-ray light source SPring-8, a research team led by the Bavarian Geoinstitute at the University of Bayreuth has now found an explanation for this puzzling phenomenon: the transformation of the mineral akimotoite into bridgmanite. The researchers around main author Artem Chanyshev from DESY and the University of Bayreuth present their results in the journal Nature.

The seismic discontinuity at a depth of 660 kilometres is one of the most important structural boundaries in Earth’s mantle and has a generally accepted explanation: In this transition zone from the upper to the lower mantle, the mineral ringwoodite – which is composed of magnesium, iron, silicon and oxygen – breaks down into the minerals bridgmanite and ferropericlase. As a result, seismic waves can propagate faster. However, below cold subduction zones this seismic discontinuity splits into two discontinuities at around 660 to 670 kilometres and 740 to 750 kilometres. The cause was unknown so far. Experiments in which the pressures and temperatures prevailing in Earth's interior were simulated, clearly show that the known dissociation of ringwoodite into bridgmanite and ferropericlase cannot be responsible for this pattern, since this dissociation is independent of the temperature and always occurs at about 660 kilometres depth.

The researchers now found an explanation by studying the phase transition of the mineral akimotoite to bridgmanite under conditions of Earth's mantle with X-rays. Akimotoite exists mainly in the cooler regions of the transition zone. „We loaded mineral samples in our Large Volume Press and analysed them with the bright X-rays from PETRA III while compressing and heating them“, explains co-author Robert Farla, scientist in charge of the beamline P61B where the experiments took place. „This way, we can follow structural changes in the minerals under conditions of Earth's mantle.“

The experiments led to a surprising result: the akimotoite-bridgmanite phase transition exhibits a steep negative Clapeyron slope. This means that the lower the temperature, the higher the pressure has to be for a phase transition into bridgmanite. Higher pressure means a greater depth. Thus, even a comparatively small drop in temperature causes the phase transition from akimotoite to bridgmanite to shift significantly deeper into Earth's interior.

“The very striking seismic discontinuity at a depth of 740 to 750 kilometres below cold subduction zones can be plausibly explained by the akimotoite-bridgmanite phase transition on the basis of our experiments,” explains Chanyshev. The other discontinuity occurring at 660 to 670 kilometres below cold subduction zones is accordingly caused by ringwoodite breaking down into akimotoite and ferropericlase. “Both these transitions can cause the propagation velocity of seismic waves to change abruptly,” says Chanyshev.

Scientists from the Bavarian Geoinstitute at the University of Bayreuth, the Center for High Pressure Science and Technology Advanced Research in Beijing and the Jilin University in Changchun (China), the Japan Synchrotron Radiation Research Institute and the Tohoku University in Sendai (Japan) and from DESY were involved in this work.

 

Reference:
Depressed 660-km discontinuity caused by akimotoite–bridgmanite transition; Artem Chanyshev, Takayuki Ishii, Dmitry Bondar, Shrikant Bhat, Eun Jeong Kim, Robert Farla, Keisuke Nishida, Zhaodong Liu, Lin Wang, Ayano Nakajima, Bingmin Yan, Hu Tang, Zhen Chen, Yuji Higo, Yoshinori Tange & Tomoo Katsura; „Nature“, 2022, DOI: 10.1038/s41586-021-04157-z

Source: press release from the University of Bayreuth