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2016-8-4

The mantle of a terrestrial planet is thought to mainly consist of silicate minerals. Silicate minerals that melt due to collisions between planetesimals in the process of protoplanet formation move in the form of magma, becoming the mantle as they cool down. Their main component is forsterite, and the primary silicate minerals that are found in meteorites are rocks similar to forsterite.

Specially Appointed Professor SEKINE Toshimori at the Graduate School of Science, Hiroshima University, in a joint research project with a group of researchers led by Professor KODAMA Ryousuke and Associate Professor OZAKI Norimasa at the Institute of Laser Engineering, Osaka University, obtained results showing that magma consisting of forsterite, a primary mineral found in the interiors of terrestrial planets and meteorites coming from space, exhibited an abnormal phase transition under extreme conditions.

This group clarified that some parts crystallized out of the magma, which is composed of forsterite, in association with increasing pressure and temperature at high pressures and temperatures, conditions equivalent to the interiors of planets or produced in impact between meteorites. This phenomenon changes the composition and properties of the magma, and later, the properties of the solidified mantle. Differences of the properties of magma are made from differences between ingredients that were and were not easily incorporated by the crystallized minerals. It is thought that this will greatly influence the planet formation process and changes caused by meteorite collisions.

In order to generate a high pressure and high temperature state, which is not easy to achieve through conventional experimental methods, this group carried out high power laser shock experiments. This group used a streak camera, a high-speed camera for measuring a phenomenon that takes place in a very short time (1 billionth of a second). After examining the phenomenon based on measurements of density, pressure, temperature, and emissivity, this group found that a fairly large amount of crystallization occurred and that these crystals later exhibited phase transitions.

It was said that these phenomena would not happen under extreme conditions; however, this group’s research results showed that magma caused various reactions under extreme conditions as well. It was thought that once forsterite melted, it would vaporize as pressure and temperature further increased, but this group obtained totally different findings.

This group’s study suggests that complex chemical responses and phase transitions can occur in the magma under extreme conditions (approximately 10,000 degrees and 300 GPa) that are produced in the interiors of huge terrestrial planets and in collisions between planetesimals in the process of protoplanet formation. This also suggests the possibility of reconsidering the composition and properties of magma created by a large-scale collision as well as its material transfer. This will affect the allocation of elements at the time of formation of a mantle and core of a super-Earth, greatly influencing study about the history of the formation of terrestrial planets in the future.

Abstract

Forsterite (Mg2SiO4) is one of the major planetary materials, and its behavior under extreme conditions is important to understand the interior structure of large planets, such as super-Earths, and large-scale planetary impact events. Previous shock compression measurements of forsterite indicate that it may melt below 200 GPa, but these measurements did not go beyond 200 GPa. We report the shock response of forsterite above ~250 GPa, obtained using the laser shock wave technique. We simultaneously measured the Hugoniot and temperature of shocked forsterite and interpreted the results to suggest the following: (i) incongruent crystallization of MgO at 271 to 285 GPa, (ii) phase transition of MgO at 285 to 344 GPa, and (iii) remelting above ~470 to 500 GPa. These exothermic and endothermic reactions are seen to occur under extreme conditions of pressure and temperature. They indicate complex structural and chemical changes in the system MgO-SiO2 at extreme pressures and temperatures and will affect the way we understand the interior processes of large rocky planets as well as material transformation by impacts in the formation of planetary systems.

To learn more about this research, please view the full research report entitled “Shock Compression Response of Forsterite above 250 GPa” at this page of the Science Advances website.


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