By causing mantle rock samples to rupture at high pressure and high temperatures, scientists determined how phase transitions in olivine lead to earthquakes in the mantle transition zone.
peridot is a semi-precious large crystalline variety of peridot, and is the main mineral of the mantle
plate tectonics and mantle convection keeps the earth's matter constant tension. In mantle , the temperature is very high and the mineral is in a viscoplastic state, so mantle convection occurs. Mechanical stress causes rocks to deform slowly and continuously on geological time scales. The lithosphere in which
interacts with the mantle has also deformed, but the low temperature makes the rocks in it brittle rather than plastic. They accumulate mechanical stress and then "break" - that's how earthquakes happen. Most of them are located at depths up to 200 km.
Some earthquakes also occur deeper. Many deep earthquakes occur in the crust layer, which sinks into the mantle during subduction and has not yet been heated to a plastic temperature. But the deepest earthquakes cannot be explained: the deeper the pressure on the overlying rocks is, preventing the spread of the cracks and changing dramatically along them.
left picture: During subduction, crust debris submerged into the mantle. The transition of peridot to spinel-like structure at a depth of 410 km and the attenuation at a depth of 660 km are shown. The right picture: Earthquake distribution with a quake depth in kilometers
Scientists at Ehime University , Japan, led by the great man Tomohiro Ohuchi, discovered the mechanism of deep earthquakes through experiments. To do this, they placed samples of the main mineral olivine in the mantle at pressures and temperatures corresponding to the deep seismic zones and applied additional splitting forces to them. What happened in the experimental volume was monitored using X ray diffraction , camera and acoustic sensors. Scientists show the results in the open access journal Nature Communications.
Deep seismic distribution area is located in the transition zone of the mantle - a strata with a depth of about 410 to 660 kilometers. Among them, the usual structure of peridot loses stability and is replaced by more dense high pressure modifications - 525 km of siliceousite and 525 to 610 km of lynwoodite. The transition pressure is approximately 130 and 200,000 atmospheric pressures. To be more deeply rooted, magnesite decomposes into perovskite and ferromagneticite.
In most cases, deep earthquakes occur at a depth of 600 km, while below 680 km they almost disappear, suggesting that they are related to peridot phase transitions. To test this hypothesis, the scientists conducted experiments under a series of overlapping phase transitions: pressures ranged from 1.1 to 170,000 atmospheres and temperatures ranged from 590 to 1080 degrees Celsius.
It turns out that olivine does indeed suffer from brittle deformation at pressures over 130,000 atmospheric pressures, but it only occurs within a narrow temperature range of 830 to 890 degrees Celsius. The fracture strength of olivine drops sharply at these temperatures, resulting in a lower than the plastic deformation threshold, where the plastic deformation threshold remains high, ranging from 20 to 40,000 atmospheres.
X ray diffraction shows that brittle fracture occurs due to the beginning of the phase transition from olivine to silica. The emerging islands of the new phase act as stress concentration areas, which "catalyze" the phase transition in adjacent areas - a "crack-resistant" form in olivine, which consists of a mixture of nanocrystalline olivine and calciumite, with a greater density than the environment.
left: part of the experimental hyperbaric chamber. piston , molybdenum capsules and magnesium oxide seals are displayed in black, and the dividing lines are displayed in red. Middle: Micrograph of cross-section with splitting lines (resistant to cracking) filled with a mixture of olivine and silicaite crystals and surrounded by olivine. On the right are the inclusions of iron particles formed during melting of the shear zone.
The rock part begins to move along the cracks, accompanied by strong sound emission, i.e. crackling sound. Due to high pressure, friction causes heating up to 2000-2200 degrees Celsius. This can cause instant melting and "lubrication" the cracks with a thin layer of melt.Above 890 degrees, the cracks cease completely—the splitting is replaced by plastic deformation, which explains the sharp decline in the number of earthquakes above 680 km.
Previously, scientists linked deep earthquakes to phase transitions of other minerals sinking into the mantle during subduction, but the described experiments confirmed that olivine itself may be the source of the earthquake . The easy propagation and free sliding along the cracks cause it to spread throughout the sample and in the mantle to the entire mechanical stress region. Therefore, large-scale release of seismic energy in the mantle transition zone proved to be truly possible.
