Shulou(Shulou.com)11/24 Report--

As we all know, as a special planet with plate tectonic movement in the solar system, plate tectonic movement has always been the focus and difficulty of scientists' research. The main driving force of plate tectonic movement is magmatic activity, and the material in the interior of the earth can be ejected to the surface through magmatic activity. So is there any way for surface material to enter the interior of the earth?

Plate tectonics provide a bridge for matter to enter the interior of the earth. At the boundaries of some plate movements, some plates can move toward the interior of the earth due to compression and gravity. Scientists call such plates subducted plates.

Subducted plates can transport material near the surface to places below the earth's crust or even below 1000 km. So what is the composition and nature of the subducting plate when it enters the interior of the earth?

First of all, let's give a brief introduction to the subducted plate. The subducted plate can be divided into five layers: sediment layer, basalt layer, harzburgite layer, lherzolite layer and depleted mantle layer from top to bottom.

We can see that each layer of the subducted plate has different compositions, and the main composition of the lower three layers is almost the same as that of the mantle, mainly olivine, clinopyroxene and orthopyroxene. Today we will mainly introduce orthopyroxene.

Orthopyroxene (space group Pbca) is the main mineral in the mantle. With the increase of temperature and pressure, orthopyroxene minerals will undergo complex phase transition. At room temperature, orthopyroxene will undergo several phase transitions with the increase of pressure, and previous studies have shown that different Fe content in orthopyroxene will change its phase boundary. The phase transition pressure of orthopyroxene with different Fe content except pure iron end element is 10-14 GPa when it is transformed from orthorhombic structure (space group Pbca) to monoclinic structure (space group P21). When the pressure continues to increase, a second phase transition (Dera et al., 2013) occurs at 25-30 GPa. Orthopyroxene itself has the property of tetrahedral symmetry of silicon ions. The increase of coordination number after phase transformation under certain pressure will change the symmetry mode, resulting in changes in its structure and properties. Transition metal ions such as Fe2+ have a strong influence on the phase transition pressure and high pressure phase structure of calcium poor pyroxene system.

Orthopyroxene samples, so we selected orthopyroxene containing a certain amount of Fe to carry out high pressure experiments. Diamond exerted force on the sample cavity. Under very small Mesa (diameter 400um, pressure pyrF / S, generally 400um diamond, we can achieve 40 GPa, interested students can calculate how much pressure is needed) to obtain a very high pressure. The following is a schematic diagram of our experiment and a sample diagram of the experiment conducted at Argonne National Laboratory of Advanced Light Source in the United States, including 2 samples, pressure transfer medium argon, pressure calibration material platinum and ruby.

By increasing the pressure on the top anvil of diamond and combining with synchrotron radiation device to obtain the diffraction signal of the sample, we found that orthopyroxene changed from orthorhombic phase to monoclinic structure at 15 GPa.

So where can such a phase transition take place in the interior of the earth? Through the phase diagram of orthopyroxene under high temperature and high pressure, we can see that because the phase transition can only occur at low temperature, only some low-temperature subduction plates have the conditions for the phase transition to occur.

Xu et al., 2018 and we obtained the change of orthopyroxene wave velocity under high pressure by Brillouin scattering (below, Brillouin scattering optical platform). It can be seen that both longitudinal wave and shear wave velocity soften with the increase of pressure.

Through the above experimental data and combined with thermodynamic parameters, we effectively restrict the wave velocity characteristics of orthopyroxene under the internal conditions of the subducted plate, and compare it with the low velocity wedge in the subducted plate obtained by relevant seismological observations (the gray line below represents the low velocity wedge of metastable olivine, and the red line represents the low velocity wedge with orthopyroxene added. When orthopyroxene is added, it is more consistent with seismological observations).

Finally, we think that the low velocity wedge inside the subducted plate is not only composed of metastable olivine, but also composed of metastable olivine and orthopyroxene, which updates the understanding of the abnormal region of low wave velocity in the subducted plate.

Reference

Dera, P.A., Finkelstein, G. J., Duffy, T. S., Downs, R. T., Meng, Y., Prakapenka, V., & Tkachev, S. (2013). Metastable high-pressure transformations of orthoferrosilite Fs (82), Physics of the Earth and Planetary Interiors, 221,15-21, 10.1016/j.pepi.2013.06.006.

Li, L.A., Sun, N., Shi, W., Mao, Z., Yu, Y., Zhang, Y., & Lin, J. Murray F. (2022). Elastic anomalies across the α-β phase transition in orthopyroxene: Implication for the metastable wedge in the cold subduction slab. Geophysical Research Letters, 49, e2022GL099366. Https://doi.org/10.1029/2022GL099366

Ringwood, A. E., & Irifune, T. (1988). Nature of the 650km seismic discontinuity: implications for mantle dynamics and differentiation, Nature, 331 (6152), 131136, 10.1038/331131a0.

Xu, J., Zhang, D., Fan, D., Zhang, J. S., Hu, Y., Guo, X., et al. (2018). Phase Transitions in Orthoenstatite and Subduction Zone Dynamics: Effects of Water and Transition Metal Ions, Journal of Geophysical Research: Solid Earth, 123 (4), 2723-2737, 10.1002/2017jb015169.

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This paper, entitled "Elastic anomalies across the α-β phase transition in orthopyroxene: Implication for the metastable wedge in the cold subduction slab", was published in Geophysical Research Letters, a famous international academic journal in the field of geosciences. The author is Professor Mao Bamboo, School of Earth and Space Sciences, University of Science and Technology of China, and the first author is Li Luo, a doctoral student. The co-authors include Special Associate researcher Sun Ningyu, Dr. Shi Weigang, Master Yu Yingxin, Professor Lin Junfu of the University of Texas at Austin and Dr. Zhang Yanyi. The related experiments were carried out in the High temperature and High pressure Mineralogy Laboratory of University of Science and Technology of China, Shanghai Synchrotron radiation Light Source and Argonne National Laboratory of American Advanced Light Source. This work is supported by the strategic forerunner B project of the Chinese Academy of Sciences (Grant No. XDB41000000), National Natural Science Foundation of China (Grant No. 41590621) and basic scientific research fees of central universities (Grant No. WK2080000144).

This article comes from the official account of Wechat: stone Science Popularization Studio (ID:Dr__Stone), by Li Luo, written by Li Luo, edited by Liu Yuling

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