A research group comprising Graduate Student Wu Wentian of the Graduate School of Science and Engineering and Professor Yu Nishihara of the Geodynamics Research Center at Ehime University announced that they discovered that the anisotropy of seismic waves found deep in the Earth's mantle can be explained by the deformation of the phase D, a high-pressure hydrous mineral. This was discovered by conducting deformation experiments on the phase D at the temperatures and pressures of the Earth's deep mantle. This finding is expected to lead to a better understanding of mantle dynamics. The results are published in the Journal of Geophysical Research: Solid Earth, on October 8.
(a, b) Images show deformation cell assemblage and microstructure of shear-deformed Al bearing-phase D aggregate under 20 GPa, 800 ℃, conditions of lower mantle transition zone. Large deviation of strain marker and elongated grains indicate that significant strain was applied to the sample.
(c) The inverse pole figure suggests that the (0001) lattice plane predominantly aligns in the shear plane after deformation.
Provided by Ehime University
When S waves, which are transverse seismic waves, propagate through materials that exhibit elastic anisotropy, they split into two waves with different velocities and oscillation directions. Such S-wave splitting is observed worldwide, and an anisotropic region exists in the Earth's mantle. This anisotropy is generally thought to be caused by the "crystallographic preferred orientation," in which minerals that make up the mantle are oriented in a specific crystal direction.
In addition, universal seismic anisotropy exists in the vicinity of the subducting plate located in the upper part of the lower mantle (depth of 660 to approximately 1200 km). There, horizontally oscillating S waves move faster than vertically oscillating ones. The high-pressure hydrous mineral phase D is stable at temperatures and pressures equivalent to those in the lower mantle transition zone and uppermost lower mantle and is considered to be a mineral that exists in this region of the mantle due to the presence of water brought about by plate subduction. Phase D crystals also exhibit elastic anisotropy, which may contribute to observed seismic anisotropy.
To verify this possibility, the researchers synthesized the high-pressure hydrous mineral phase D and deformed it under high-temperature and high-pressure conditions using a high-pressure deformation experimental apparatus. Deformation experiments were conducted using various methods, including compressive and shear deformation.
Analysis of these results shows that the deformation of the phase D is dominated by slip deformation along the bottom plane (0001 plane) in the crystal structure, which leads to the crystallographic preferred orientation, in which crystals are aligned in a specific direction. This indicates that some anisotropy in seismic wave velocities observed deep in the mantle near the subduction zone can be explained by the presence of this phase D.
Nishihara said, "The experiment required a lot of trial and error, including trying several materials surrounding the sample to prevent water from escaping from the hydrous mineral due to deformation at high temperatures and pressures. This was a difficult experiment, but Doctoral Student Wu Wentian persevered and obtained research results. We believe that we have gained important insights into how things flow deep in the Earth's mantle, which cannot be seen directly with the naked eye."
Journal Information
Publication: Journal of Geophysical Research: Solid Earth
Title: Crystallographic Preferred Orientation of Phase D at High Pressure and Temperature: Implications for Seismic Anisotropy in the Mid-Mantle
DOI: 10.1029/2024JB029734
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.