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University of Bayreuth, Press Release No 004/2022 - 11 January 2022

In "Nature": Bayreuth researchers discover cause of previously puzzling seismic discontinuities

At a depth of 660 kilometers, there is an abrupt change in the speed at which seismic waves propagate in the Earth's interior. However, this striking seismic discontinuity shifts to a depth of 750 kilometers below cold subduction zones, where the oceanic crust sinks into the mantle below continental crust. A research team at the Bavarian Geoinstitute (BGI) of the University of Bayreuth has found an explanation for this previously puzzling phenomenon through high-pressure experiments: the transformation of the mineral akimotoite into bridgmanite. The researchers present their results in "Nature".

A research team at the Bavarian Geoinstitute (BGI) of the University of Bayreuth has found an explanation for previously puzzling phenomenons in the Earth's interior.

In geoscientific research, there is consensus on how to explain the seismic discontinuity at a depth of 660 kilometers: 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 bridgmanite and ferropericlase. As a result, seismic waves can propagate faster. However, the seismic discontinuities that can be observed below cold subduction zones at depths between 660 and 750 kilometers have been unclear until now. In these much cooler regions of the Earth's interior, the seismic discontinuity splits to discontinuities at around 660-670 kilometer and 740-750 kilometer. Experiments at the BGI, in which the compressive pressures and temperatures prevailing in the Earth's interior were simulated, clearly show that the dissociation of the ringwoodite into bridgmanite and ferropericlase cannot be responsible for these discontinuities, since this dissociation is independent from temperature and occurs at about 660 kilometers depth.

Dr. Artem Chanyshev at the IRIS press at BGI used in the study recently published in "Nature".

In geoscientific research, there is consensus on how to explain the seismic discontinuity at a depth of 660 kilometers: 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 bridgmanite and ferropericlase. As a result, seismic waves can propagate faster. However, the seismic discontinuities that can be observed below cold subduction zones at depths between 660 and 750 kilometers have been unclear until now. In these much cooler regions of the Earth's interior, the seismic discontinuity splits to discontinuities at around 660-670 kilometer and 740-750 kilometer. Experiments at the BGI, in which the compressive pressures and temperatures prevailing in the Earth's interior were simulated, clearly show that the dissociation of the ringwoodite into bridgmanite and ferropericlase cannot be responsible for these discontinuities, since this dissociation is independent from temperature and occurs at about 660 kilometers depth.

The Bayreuth researchers found an explanation by studying the akimotoite-bridgmanite phase transition under the same conditions. The mineral akimotoite exists mainly in the cooler regions of the transition zone. The experiments in the BGI laboratories 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 compression pressure has to be for a phase transition – i.e. the transformation into bridgmanite – to occur. However, the higher pressure is only given at a greater depth. Thus, even a comparatively small drop in temperature causes the phase transition from akimotoite to bridgmanite to shift significantly deeper into the earth's interior.

This finding now offers the possibility of solving two unexplained geoscientific enigmas: "The very striking seismic discontinuity at a depth of 740-750 kilometers below cold subduction zones can be plausibly explained by the akimotoite-bridgmanite phase transition on the basis of our experiments. Another discontinuity occurring at 660-670 kilometers below cold subduction zones is caused by the ringwoodite breaks down into akimotoite and ferropericlase. Both these transitions can cause the propagation velocity of seismic waves to change abruptly," explains Dr. Artem Chanyshev, research associate at BGI and lead author of the study now published in Nature.

The research results have emerged from a close cooperation of the BGI with the German Electron Synchrotron DESY in Hamburg; the Center for High Pressure Science and Technology Advanced Research in Beijing and the Jilin University in Changchun (China); and the Japan Synchrotron Radiation Research Institute and the Tohoku University in Sendai (Japan).

Dr. Artem Chanyshev

Bayerisches Geoinstitut, University of Bayreuth

E-Mail: artem.chanyshev@uni-bayreuth.de

Prof Dr. Tomo Katsura

Bayerisches Geoinstitut, University of Bayreuth

E-Mail: tomo.katsura@uni-bayreuth.de

Anja-Maria Meister

PR Spokesperson University of Bayreuth

Phone: +49 (0) 921 / 55-5300
E-mail: anja.meister@uni-bayreuth.de