An international research group led by Prof. dr. Tomoo Katsura of the Bavarian Research Institute of Experimental Geochemistry and Geophysics, University of Bayreuth, discovered why the rocks inside the Earth suddenly become more viscous at a depth of 800 to 1,200 km. The reason for this change is the bridgmanite-enriched rocks that make up most of the Earth’s lower mantle below about 1,000 km. These rocks have a larger grain size than the rocks above them, resulting in high viscosity. The new findings are published in the journal NATURE.
Bridgmanite is the most abundant mineral in the Earth’s lower mantle, which ranges from a depth of 660 km to 2,900 km and occupies almost half of the entire Earth. Scientists from Germany, China, France, the UK, and the US discovered that the grain size of bridgmanite increases at about 1,000 km depth, as the lower mantle rocks are bridgmanite-enriched with increasing depth. . As a result, there is a marked increase in viscosity in the shallower part of the lower mantle because viscosity has a positive dependence on grain size.
The shallower part of the lower mantle consists of pyrolite. This rock contains 20 vol% of other minerals in addition to bridgmanite. These additional minerals inhibit bridgmanite grain growth. On the other hand, there is a smaller proportion of such minerals in bridgmanite-enriched rocks, where bridgmanite can grow freely into large grains.
The resulting viscosity jump affects a wide range of geophysical and geochemical processes. “Although the subducted plates sink slowly into the lower mantle, their sinking is slowed down in the shallow part of the lower mantle. It can be fast over 1,000 km depth. Although these observations are difficult to understand, we can now explain it rationally,” said the first author, Prof. dr. Hongzhan Fei, who is a researcher at the Bavarian Research Institute of Experimental Geochemistry and Geophysics and is currently a professor at one of the leading universities in China, Zhejiang University in Hangzhou.
Highly viscous bridgmanite-enriched rocks formed early in Earth’s history. Because they are so viscous, mantle convection cannot mix them with other mantle components. As a result, bridgmanite-enriched rocks have been preserved deep in the lower mantle for billions of years.
Prof. dr. Tomoo Katsura, Chair of Structure and Dynamics of Earth Material at BGI reports on the new research results of seismic observations. “Seismologists show that many subducted slabs are stagnant in the layer between 600 and 1,500 km deep. They also show that, although plumes rise vertically and can be clearly imaged below a depth of 1,000 km, they become difficult to describe above this depth. Our new theory can explain these observations. Because viscosity increases with depth, slabs have difficulties penetrating into regions that are more deeper than 1,000 kilometers. image,” explained Katsura.
Hongzhan Fei et al, Variation in bridgmanite grain size accounts for mid-mantle viscosity jump, NATURE (2023). DOI: 10.1038/s41586-023-06215-0
Provided by Bayreuth University
Citation: Research team solves riddle of viscous jump in Earth’s lower mantle (2023, July 6) retrieved on July 6, 2023 from https://phys.org/news/2023-07-team-riddle -viscosity-earth-mantle. html
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