View of the Campus of the University of Bayreuth, with the Bavarian Research Institute of Experimental Geochemistry and Geophysics (BGI) in the centre of the picture. Photo: UBT.
The lithosphere, the outermost shell of the Earth, consists of large-scale plates that are in constant motion. In particular, when an oceanic plate collides with a continental plate, subduction of the plate can occur: The oceanic plate pushes under the continental plate and sinks deeper and deeper into the Earth's interior. The subducting oceanic plate is called “slab”. At a depth of 410 kilometers, it reaches the transition zone between the upper and the lower mantle, which begins at a depth of 660 kilometers. It has long been known that deep earthquakes are associated with subduction processes. However, the exact cause of these earthquakes has remained mysterious until today. This is because a central assumption that would allow for a plausible explanation seemed paradoxical: Whereas a sinking slab is hydrous due to water from the sea, the mineral olivine, which is the main component of the slab, would have to remain dry on its way into the Earth's interior in order to trigger deep earthquakes by phase transitions.
The two researchers have now been able to verify precisely this assumption by using high-pressure and high-temperature experiments at BGI, a research centre of the University of Bayreuth: Down to a depth of about 600 kilometers, olivine actually remains dry, or more precisely: the water content of olivine crystals represents, down to this depth, only a millionth part of their mass and is therefore vanishingly small. The experiments clearly show that the water content of olivine in sinking slabs has been overestimated so far. The reason for this is that experiments in this regard have been carried out elsewhere at higher temperatures than actually exist in the slabs.
Crucial for the newly detected explanation of deep earthquakes is another finding of the study: The hydrous minerals of the sinking slab absorb water on their way down. Apart from the dry olivine crystals, the slab is wet due to the existence of hydrous minerals. Right below 410 kilometers, the slab is exposed to an increasing temperature. As a result, dehydration sets in: The hydrous minerals in the slab release water molecules. The dry olivine crystals, which have persisted until then, now begin to absorb some of these water molecules. And this is exactly what eventually causes phase transitions: The olivine transforms into the minerals wadsleyite or ringwoodite. These phase transitions can lead to a sudden reduction in volume that triggers strong deep earthquakes, up to a depth of about 600 kilometers. The researchers have also found that the phase transitions can effect a softening of the slab due to incorporation of water into the minerals, significantly deforming it. Deformations of this kind have been detected in many sinking slabs by seismic observations.
"We are not suggesting in our study that phase transitions of olivine in the Earth's deep interior are the only triggers of deep earthquakes. There also appear to be earthquakes below 660 kilometers that cannot be explained by phase transitions of olivine. However, in the search for their causes, the temperature-induced dehydration of high pressure hydrous minerals which occurs in the transition zone to the lower mantle, could provide important clues," said Dr. Takayuki Ishii, corresponding author of the study.
Dr. Takayuki Ishii received his PhD from Gakushūin University in Tokyo, Japan. Since he came to the University of Bayreuth in 2015, his research work was funded by the Japan Society for the Promotion of Science (JSPS), the Alexander von Humboldt Foundation, and the German Research Foundation (DFG). Currently he is working at the Center for High Pressure Science and Technology Advanced Research in Beijing. Prof. Dr. Eiji Ohtani from Tohoku University in Japan was a guest at the University of Bayreuth in 2018 as a research award winner of the Alexander von Humboldt Foundation. Already in 2016, on the occasion of the 30th anniversary of the BGI's foundation, he received the appointment as a Distinguished Affiliated Professor of the University of Bayreuth.
Contact for scientific information:
Dr. Takayuki Ishii
Bavarian Research Institute of experimental Geochemistry and Geophysics (BGI)
University of Bayreuth
E-mail: Takayuki.Ishii@hpstar.ac.cn
Original publication:
Takayuki Ishii, Eiji Ohtani: Dry metastable olivine and slab deformation in a wet subducting slab. Nature Geoscience (2021), DOI: https://doi.org/10.1038/s41561-021-00756-7
