Recently, Wu Zhongqing's research group at the School of Earth and Space Sciences of the University of Science and Technology of China cooperated with Professor Sun Daoyuan and Ni Sidao, a researcher at the Institute of Precision Measuring Science and Technology Innovation of the Chinese Academy of Sciences, and found that the oceanic crust material submerged to the lower mantle has a wave velocity and density that is significantly different from the surrounding mantle, and the wave velocity and density anomalies depend on depth, which can well explain the seismologically observed small-scale scatterers and large-scale high-speed anomalies, indicating that the submersive oceanic crust is an important source of inhomogeneity of matter deep in the earth. The study was published online in the international comprehensive academic journal Nature communications under the title Velocity and density characteristics of subducted oceanic crust and the origin of lower-mantle heterogeneities.
Understanding the material composition of the earth is crucial to understanding the origin of the earth , evolution and dynamics processes. The inner circle of the earth can be roughly divided into the crust, the upper mantle, the lower mantle and the core. The lower mantle is one of the most important circles inside the earth, with a depth range of about 660-2900 kilometers, and its volume and mass account for as high as 55% and 52%. It is the ballast stone of the earth's evolution. Although it is difficult to obtain samples from the lower mantle, seismic wave has the ability to penetrate the entire earth, thus providing a key means for detecting the deep internal structure of the earth. In recent years, with the development of seismic wave observation, analysis and simulation technologies, seismologists have discovered that there are many anomalies of different sizes in the lower mantle, including high-speed and low-speed anomalies at large scales (several hundreds to thousands of kilometers) and scatterers at small scales (several kilometers). The particularly famous low-speed anomalies are large low-shear wave velocity provinces (LLSVPs) located under the African and Pacific plates, with lateral and radial scales up to thousands of kilometers. The causes of these anomalies play a key role in understanding the internal structure and material composition of the earth, and are also important in restricting the origin and dynamic evolution of the earth. They are hot topics and cutting-edge issues in earth science research.
Compared with other Earth-like planets, the Earth has a very unique feature - plate motion and subduction. Plates can subdue to the deep earth and even the boundary of the core mantle, thus forming chemical inhomogeneity in the lower mantle. The subduction crust is the uppermost layer of the subduction oceanic lithosphere. It is richer in SiO2 than the normal mantle and is likely to be the main source of inhomogeneity of the lower mantle. At present, the abnormalities in the lower mantle are mainly characterized by seismic wave velocity. Therefore, it is of critical significance to carry out research on the wave velocity characteristics of the subduction oceanic crust under the temperature and pressure conditions of the lower mantle.
The pressure and temperature of the lower mantle can reach 130GPa and 3000K. Under high temperature and high pressure conditions in the lower mantle, the subduction crust is mainly composed of britishmantite, SiO2 phase, aluminum-containing phase and calcium- perovskite . It is difficult to accurately measure the wave velocity of minerals under temperature pressure conditions of lower mantle through high pressure experiments, so the data is very limited. The first principle-based mineral physics calculation has the advantages of flexibility and efficiency in studying the wave speeds of different elements and multiple mineral combinations. In the early years, Wu Zhongqing's research team developed a high-temperature and high-pressure elasticity and wave speed calculation method based on first principles, and the calculation efficiency was improved by orders of magnitude. After several years of research and accumulation, the research team determined the wave velocity and density characteristics of the subduction oceanic crust under the temperature pressure conditions of the lower mantle (Figure 1). The research results show that the wave velocity of the oceanic crust changes with depth: in the depth interval when the quartz phase becomes CaCl2-type quartz, the oceanic crust is shown as low wave velocity, with the transverse wave velocity abnormality as high as -10%; deeper, the oceanic crust is shown as high wave velocity, with the transverse wave velocity abnormality about 1.5-2.5%. This means that the low shear wave velocity of LLSVPs is unlikely to be caused by the oceanic crust, and the lower mantle high-speed bodies found by seismic tomography are likely to reflect the accumulation of oceanic crust. Therefore, subduction crust does not simply manifest as high or low speed in the lower mantle, but depends on depth. Subduction crust can well explain high-speed anomalies in the lower lower mantle and scatterers found throughout the lower mantle (Fig. 2), and this result provides a key understanding for further study of the abnormalities and dynamics of lower mantle matter.
co-first authors of the paper are Wang Wenzhong and Xu Yinhan, who recently graduated from Wu Zhongqing's research group with a doctorate; Wang Wenzhong and Wu Zhongqing are co-corresponding authors of the paper; University of Science and Technology of China is the first unit of the paper.This work is funded by the Pioneer Project of the Chinese Academy of Sciences, the "973" project of the Ministry of Science and Technology, and the National Natural Science Foundation Project .
Fig. 1 (a) (b) (c) Wave velocity and density of subduction oceanic crust; (d) (e) (f) Wave velocity and density of subduction oceanic crust relative to normal mantle (PREM)
Fig. 2 Schematic diagram of the relationship between subduction oceanic crust and lower mantle wave velocity anomaly
This work is funded by the Pioneer Project of the Chinese Academy of Sciences, the "973" project of the Ministry of Science and Technology, and the National Natural Science Foundation Project .Fig. 1 (a) (b) (c) Wave velocity and density of subduction oceanic crust; (d) (e) (f) Wave velocity and density of subduction oceanic crust relative to normal mantle (PREM)
Fig. 2 Schematic diagram of the relationship between subduction oceanic crust and lower mantle wave velocity anomaly
