(ORDO NEWS) — By subjecting samples of mantle rocks to fracture under high pressures and temperatures, scientists have determined how phase transitions in olivine cause earthquakes in the transition zone of the Earth’s mantle.
Plate tectonics and mantle convection keep the Earth’s substance in constant tension. In the earth’s mantle , temperatures are quite high, and the minerals are in a viscoplastic state, due to which mantle convection occurs. Mechanical stresses lead to slow and continuous deformation of rocks on geological time scales.
Lithosphere , interacting with the mantle, is also deformed, but low temperatures make the rocks in it brittle, not plastic.
They accumulate mechanical stresses and then “break” – this is how earthquakes occur. Most of them are localized at depths up to 200 kilometers.
Some earthquakes also occur at much greater depths. Many deep earthquakes occur in layers of the earth’s crust that have sunk into the mantle during subduction and have not yet had time to heat up to the plasticity temperature.
But the deepest earthquakes could not be explained: the deeper, the stronger the pressure of the overlying rocks prevents the propagation of cracks and sharp shifts along them.
Scientists from the Japanese University of Ehime, led by Tomohiro Ohuchi (Tomohiro Ohuchi) found out the mechanism of deep earthquakes experimentally.
To do this, they subjected samples of olivine , the main mineral of the mantle, to pressures and temperatures corresponding to the zone of deep earthquakes, and applied an additional splitting force to them.
What happened in the experimental volume, the researchers monitored using X-ray diffraction, videography and acoustic sensors. The scientists presented the results in the open access in the journal Nature Communications.
The area of distribution of deep earthquakes is located in the transition zone of the mantle – a layer with a depth of approximately 410 to 660 kilometers.
In it, the usual structure of olivine loses stability and is replaced by denser high-pressure modifications – wadsleyite at a depth of up to 525 kilometers and ringwoodite from 525 to 610 kilometers.
The transition pressures are about 130 and 200 thousand atmospheres. Even deeper, ringwoodite breaks up into perovskite and ferropericlase.
Most often, deep earthquakes occur at a depth of 600 kilometers, and they practically disappear below 680 kilometers, which suggests their connection with olivine phase transitions.
Testing this assumption, scientists conducted experiments in a range of conditions overlapping phase transitions: at pressures from 110 to 170 thousand atmospheres and temperatures from 590 to 1080 degrees Celsius.
It turned out that at pressures of more than 130 thousand atmospheres, brittle deformation is indeed possible in olivine, but it occurs only in a narrow temperature range from 830 to 890 degrees Celsius.
The fracture strength of olivine at these temperatures dropped sharply and turned out to be below the plastic deformation threshold, which at these temperatures is still quite high and ranges from 20 to 40 thousand atmospheres.
X-ray diffraction showed that brittle fracture occurs due to the onset of a phase transition from olivine to wadsleyite.
The emerging islands of the new phase serve as areas of stress concentration, and this “catalyzes” the phase transition in neighboring areas – an “anti-crack” is formed in olivine, consisting of a mixture of nanocrystalline olivine and wadsleyite, which is denser than the environment.
Rock sections begin to move along the crack, which is accompanied by strong acoustic emission, in other words, crackling. Due to the high pressure, friction forces cause heating up to 2000-2200 degrees Celsius. This leads to instantaneous melting and “lubrication” of the crack with a thin layer of the melt.
Above 890 degrees, the cracking completely stopped – the split was replaced by plastic deformation, which explained the sharp decrease in the number of earthquakes deeper than 680 kilometers.
Previously, scientists associated deep earthquakes with phase transitions in other minerals that sink into the mantle during subduction, but the described experiment confirms that olivine itself can be a source of earthquakes.
The easy propagation of a crack and free sliding along it lead to its propagation to the entire sample, and in the mantle, to the entire zone of mechanical stress. Thus, a large-scale release of seismic energy in the transition zone of the Earth’s mantle turned out to be really possible.
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