An international research team has discovered why rocks in Earth’s lower mantle suddenly become more viscous at depths between 800-1,200km.
The cause of this change is the bridgmanite-enriched rocks that make up most of the Earth’s lower mantle below about 1,000 km. These particular rocks have a much larger grain size than the rocks above them, which results in their high viscosity.
A paper detailing the research, ‘Variation in bridgmanite grain size accounts for the mid-mantle viscosity jump,’ is published in Nature.
Bridgmanite dominates Earth’s lower mantle
Bridgmanite is the most abundant mineral in the mantle, which extends from a depth of 660km-2,900km and occupies about half of the entire Earth.
Scientists from Germany, China, France, the UK, and the US have discovered that the grain size of bridgmanite increases at around 1,000km depth as lower-mantle rocks are bridgmanite-enriched with increasing depth.
As a result, viscosity is markedly increased in the shallower part of Earth’s lower mantle because viscosity has positive grain size dependence. The shallower part of the lower mantle consists of pyrolite, containing 20 vol% of other minerals in addition to bridgmanite.
These additional minerals prevent the grain growth of bridgmanite; however, there are much smaller proportions of such minerals in the bridgmanite-enriched rocks, where bridgmanite can grow freely to large grains.
How is viscosity affected by these minerals?
The resulting viscosity jump affects a wide range of geophysical and geochemical processes in Earth’s lower mantle.
“Although subducted plates sink relatively smoothly into the lower mantle, their sinking is slowed down in the shallow part of the lower mantle.”
Professor Hongzhan Fei, who led the research, explained: “On the other hand, the upwelling of mantle plumes, which produce volcanoes in different areas of the Earth’s surface, seems to become rapid above 1,000km depth.”
Fei added: “Although these observations were difficult to understand, we can now explain them rationally.”
The highly viscous bridgmanite-enriched rocks were formed in the Earth’s early history. Because they are so viscous, mantle convection cannot mix them with other mantle components.
As a result, the bridgmanite-enriched rocks have been preserved in the deepest part of Earth’s lower mantle for billions of years.
“Seismologists have shown that many subducted slabs are stagnant in the layer between 600-1,500km deep. They have also shown that, although plumes rise vertically and can be clearly imaged below a depth of 1,000 km, they become difficult to image above this depth,” said Professor Tomoo Katsura, Chair of Structure and Dynamics of Earth Material at the BGI.
Katsura concluded: “Our new theory can explain these observations. Because the viscosity increases with depth, the slabs have difficulty penetrating into Earth’s lower mantle regions deeper than 1,000km.
“On the other hand, the plumes rise faster at this depth, and so plumes become thinner and hard to image.”
