As an outcome, there is a significant increase in viscosity in the shallower part of the lower mantle due to the fact that viscosity has favorable grain size reliance. The shallower part of the lower mantle consists of pyrolite. This rock consists of 20 vol% of secondary minerals. These secondary minerals prevent the grain growth of bridgmanite. On the other hand, there are much smaller sized proportions of secondary minerals in the bridgmanite-enriched rocks, where bridgmanite can grow freely to large grains.
Viscosity jump in the lower mantle. Credit: © Hongzhan Fei
Puzzling Phenomena of Billion-Years-Old Structures Deep in the Earth
“Although subducted plates sink rather smoothly into the lower mantle, their sinking is slowed down in the shallow part of the lower mantle. On the other hand, the upwelling of mantle plumes, which produce volcanoes in different locations of the Earths surface area, appears to end up being quick above 1,000 km of depth.
The highly viscous bridgmanite-enriched rocks were formed early in the history of the Earth. Since they are so viscous, mantle convection can not blend them with other components of the mantle. As an outcome, the bridgmanite-enriched rocks have actually been protected in the deep lower mantle for billions of years.
Associating Findings With Seismic Observations
Prof. Dr. Tomoo Katsura, Chair of Structure and Dynamics of Earth Material at the BGI relates the brand-new research study results to seismic observations. “Seismologists have actually revealed that numerous subducted pieces are stagnant in the layer in between 600 and 1,500 kilometers deep. They have also revealed that, although plumes rise vertically and can be clearly imaged listed below a depth of 1,000 kilometers, they end up being hard to image above this depth.
” Our new theory can explain these observations. Since the viscosity increases with depth, the slabs are challenging to penetrate in regions deeper than 1,000 kilometers. On the other hand, the plumes increase quicker at this depth, therefore plumes end up being thinner and difficult to image.” Katsura describes.
Recommendation: “Variation in bridgmanite grain size accounts for the mid-mantle viscosity jump” by Hongzhan Fei, Maxim D. Ballmer, Ulrich Faul, Nicolas Walte, Weiwei Cao and Tomoo Katsura, 5 July 2023, Nature.DOI: 10.1038/ s41586-023-06215-0.
The study, published in Nature, resulted from a close partnership between Prof. Dr. Tomoo Katsura (the University of Bayreuth) and Prof. Dr. Hongzhan Fei (the University of Bayreuth and Zhejiang University/China) with Dr. Nicolas Walte (Technical University of Munich), Prof. Dr. Maxim Ballmer (University College London/UK), Dr. Ulrich Faul (Massachusetts Institute of Technology, Cambridge/USA) and Dr. Weiwei Cao (Extreme Conditions and Materials: High Temperature and Irradiation (CEMHTI), Orléans/ France).
The cause of this change is the bridgmanite-enriched rocks that make up many of the Earths lower mantle listed below about 1,000 kilometers. It is the most abundant mineral in the Earths lower mantle, which extends from a depth of 660 kilometers to 2,900 kilometers and occupies about half of the entire Earth. “Although subducted plates sink rather smoothly into the lower mantle, their sinking is slowed down in the shallow part of the lower mantle. Since they are so thick, mantle convection can not mix them with other components of the mantle. As a result, the bridgmanite-enriched rocks have actually been protected in the deep lower mantle for billions of years.
Scientist discovered that the Earths lower mantle becomes more viscous at depths of 800 to 1,200 kilometers, due to bridgmanite-enriched rocks. These rocks, with bigger grain sizes, affect geophysical and geochemical procedures.
Prof. Dr. Tomoo Katsura and his international research team at the Bavarian Research Institute of Experimental Geochemistry and Geophysics, University of Bayreuth, have actually found why rocks in the Earths interior all of a sudden become more viscous at depths of 800 to 1,200 kilometers.
The cause of this modification is the bridgmanite-enriched rocks that make up many of the Earths lower mantle listed below about 1,000 kilometers. These rocks have a much bigger grain size than the rocks above them, resulting in high viscosity.
Lower Mantle Enriched in Bridgmanite
Bridgmanite is named after the Nobel Prize in Physics Winner Percy Bridgman. It is the most abundant mineral in the Earths lower mantle, which extends from a depth of 660 kilometers to 2,900 kilometers and inhabits about half of the entire Earth. Researchers from Germany, China, France, the UK, and the USA have discovered that the grain size of bridgmanite boosts at around 1,000 km depth, as lower-mantle rocks end up being bridgmanite-enriched with increasing depth.
