Life on Earth is significantly affected by the exchange of mass and energy between the Earth’s interior and its surface. Hot upwellings under mid-ocean ridges and hot-spot regions (e.g. Hawaii) are balanced by downward plunges of cold crust at subduction zones (e.g. the Andes range along the West Coast of South America).
To directly “observe” the structure of the Earth’s interior, scientists analyze the travel times of earthquake shock waves. One puzzling finding from this data is that subducted slabs of crust penetrate the sharp density boundary between the upper and lower mantle at 660 km (~410 miles), but tend to stall in their descent at around 1000 km (~625 miles), where they thicken and buckle.
Researchers have now found a possible explanation. They performed experiments on the presumed weakest mineral of the lower mantle, ferropericlase [(Mg,Fe)O]. They subjected the material to stress in a diamond-anvil cell up to a pressure of 100 GPa (~14 million psi) and probed its structure and strength using radial x-ray diffraction at ALS Beamline 12.2.2. The results showed that ferropericlase becomes 100 times more viscous at pressures corresponding to the depth at which the slabs stall. This dramatic change is assumed to be linked to a change in the deformation mechanism of ferropericlase, which had previously been predicted.
The accumulation of crust material at certain zones within the lower mantle creates large-scale chemical variations within the mantle. This finding could thus finally offer an explanation for variations in the chemical compositions of volcanic lava around the globe.
Work performed on ALS Beamline 12.2.2.
Hauke Marquardt and Lowell Miyagi, “Slab stagnation in the shallow lower mantle linked to an increase in mantle viscosity,” Nat. Geosci. 8, 311 (2015).