The rheological properties of olivine affect many geological processes of the earth and other planets in the solar system. Previous experimental studies have found that the rheology of olivine depends not only on thermodynamic parameters such as stress, temperature, confining pressure, oxygen fugacity, melt content and water content, but also on particle size and microstructure. In the past, the research on this problem mainly focused on forsterite rich in magnesium, which is similar to the natural olivine in the earth's mantle. As a solid solution, olivine can be transformed from pure magnesium silicate (Mg _ 2SiO4, Fo) to pure iron silicate (Fe _ 2SiO4, Fa). The average composition of olivine in the earth's mantle is Fo 90 (90 mol. % pure forsterite), while the composition of olivine in the Martian mantle is about Fo75 to FO 77. In addition, Fo 50, which is rich in fayalite, is often used to simulate natural olivine because it is more easily deformed under laboratory conditions. However, the influence of iron content on the microstructure evolution of olivine is lack of in-depth study.

Qi Chao, an associate researcher in the Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, China Academy of Sciences, together with Professor Zhao Yonghong of Peking University, Professor kolster of the University of Minnesota and researcher zimmermann, and researcher Kim Dae-kun of the Korean Polar Research Institute, conducted high-temperature and high-pressure deformation experiments on a series of olivine samples with different iron contents, and analyzed the microstructure of the deformed samples, in order to reveal the influence of iron content on lattice preferred orientation (CPO) and grain size.

It is found that with the increase of strain, the CPO changes from (0 10)[ 100] and (00100] to (0 10) [65438+]. The seismic wave anisotropy analysis based on CPO also found that the samples with different iron contents will reach the same maximum radial anisotropy ~ 1. 15 after the shear strain reaches 3.5. This result shows that it is feasible to simulate the seismic anisotropy of mantle olivine with CPO rich in fayalite. At the same time, it also shows that the research method of seismic wave anisotropy applied to fayalite-rich earth can be applied to iron-rich Martian mantle. This study supports the use of research methods on earth to study the seismic data of Mars.

Fig. 2 (a) evolution of CPO strength and (b) variation of radial anisotropy with shear strain. Orange is Fo 70, green is Fo 50 and blue is Fo 0. The solid circle and solid line are anhydrous conditions, and the hollow circle and dotted line are water-containing conditions.

The achievement was published in the international authoritative academic journal JGR: Solid Earth (Qi Chao, Zhao Yonghong, zimmermann, M E, KimD, Kohlstedtd L. Evolution of microstructure properties of shear-rich fayalite [J]. Journal of Geophysical Research: Solid Earth, 202 1,126 (3): E2020JB. Doi:10.1029/2020jb019629). This research is supported by the key independent deployment project of Institute of Geology and Geophysics, Chinese Academy of Sciences, the national key research and development project, the research project of Korean Polar Institute and the American Natural Science Foundation.

Proofreading: Zhang Song and Wang Haibo