Scientists from a joint laboratory at the Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences and Novosibirsk State University, used an electron-beam welding machine to reproduce the conditions in which rocks are found beneath active volcanoes.
This research brought together scientists from the V.S. Sobolev Institute of Geology and Mineralogy SB RAS (IGM SB RAS), Institute of Theoretical and Applied Mechanics SB RAS (ITAM SB RAS), G.I. Budker Institute of Nuclear Physics SB RAS (INP SB RAS), Novosibirsk State University (NSU) and Novosibirsk State Technical University (NSTU).
The results of the study were published in the journal Geochemistry International. The primary author of the article Victor Sharapov, Head Scientific Advisor IGM SB RAS and Professor at the NSU Department of the Geology and Geophysics, talked about this work. It is impossible to study directly the processes taking place in the bowels of the Earth. The length of the deepest, the Kola, is not 13 km. Scientists have to assess what happens deeper on the basis of indirect evidence.
One of the methods for obtaining information is the study of fragments of deep rocks, xenoliths. They are brought to the surface by lava during volcanic eruptions. For geologists, xenoliths serve as a source of information on the mineral composition and structure of the earth's crust and upper mantle. The objects studied by Novosibirsk scientists were samples of xenoliths collected on the surface of Avachinsky hill, a functioning volcano on Kamchatka. Researchers were interested in cracks, more precisely cracked cavities inside xenoliths.
"Imagine, from depths of 70-40 km to the surface of the Earth, crystal fragments of rocks with open cavities are excavated: crystals grow and dissolve in them. Why is this happening? According to theory, not only heat fluxes but also flows of liquid and gaseous fluids move to the surface from the depths of the Earth's upper mantle. As a result, in seismically active horizons of the Earth's lithosphere, some elements are replaced by others, and the original pattern of mineral distribution is replaced by another, because the minerals dissolve in one place and the sedimentation of the dissolved substances in another," said Victor Sharapov.
A mathematical model describing these processes was created by Anatoly and Vera Cherepanov, staff members at the Institute of Theoretical and Applied Mechanics SB RAS. An experimental verification of the proposed model was conducted at the joint educational and scientific Electron Beam Welding Laboratory at INP SB RAS and NSU. In the laboratory, scientists created conditions close to those in which xenoliths are located inside the Earth during a period of volcanic activity. A special installation for electron-beam welding, equipped with an electron gun, was developed for this at INP SB RAS.
"The installation forms a focused electron beam, whose diameter can reach 1 mm," says Yuri Semenov, a researcher at INP SB RAS. The power of the installation makes it possible for it to be used to work with tungsten, the toughest refractory metal. It is also used to produce thin films, nanopowders and in the military industry.
With the help of the installation, scientists worked on a sample of xenolith from Avachinskaya hill with a powerful electron beam 3-4 cm in diameter. The melting point on the surface of the sample was about 2500 degrees. This is approximately twice as high as lava in the hottest Hawaiian volcano.
"The melting edge slowly collapses inward and boils on the surface just as lava is boiling on the volcano. Emerging gas flows are filtered through cracks and dissolve the minerals that are inside. Under laboratory conditions this process takes less than 45 minutes and in nature it takes several days during strong earthquakes," explained Sharapov.
According to scientists the results obtained, in particular the dissolution rate, are in agreement with data from the mathematical model.
Further work on this will allow scientists to make significant progress studying physical processes taking place in the bowels of the Earth and that will lead in the future to improvements in the forecasting of seismic activity.
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