A research team led by Prof. WU Zhongqing from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS), in collaboration with Assoc. Prof. RUAN Youyi from Nanjing University and NI Sidao from the Innovation Academy for Precision Measurement Science and Technology of CAS, made a breakthrough in constraining the material composition and thermal state of the Earth's lower mantle. Their findings were published in PNAS. 

The Earth's interior can be roughly divided into the crust, upper mantle, lower mantle, and core. The Earth's lower mantle, located at depths ranging from 660 to 2890 km below the surface, comprises a substantial portion of the Earth's volume and mass. It plays a critical role in the planet's structure and dynamics.

Previous seismological studies have revealed variations in seismic wave velocities within the lower mantle, including large-scale low shear wave velocity provinces (LLSVPs) beneath Africa and the Pacific. However, the nature, origin, and implications of these anomalies remain incompletely understood. Therefore, obtaining a comprehensive understanding of the spatial distribution of material composition and temperature within the lower mantle is crucial for unraveling Earth's formation, evolution, and dynamics. 

In this study, the researchers employed a combination of seismic tomography and the elastic properties of minerals to determine the composition and spatial distribution of mantle materials and temperatures. However, experimental measurements of mineral elasticity under the extreme conditions of the lower mantle present significant difficulties.

To overcome this, Prof. WU's team developed a novel first-principles calculation method that is computationally efficient, representing less than one-tenth of the conventional methods. Utilizing this approach, the team extensively studied the elastic properties of key minerals in the lower mantle and achieved results consistent with experimental data obtained under relatively lower temperatures and pressures.

By integrating computed high-temperature and high-pressure elastic data of lower mantle minerals with a 3D tomographic imaging model, the researchers successfully inverted the 3D mineral composition and temperature distribution of the entire lower mantle using the Markov chain Monte Carlo method. They derived a 3D density model of the lower mantle.  

The inversion results revealed that the lateral temperature distribution in the lower mantle follows a Gaussian pattern, with minimal variations within a depth range of 1600 kilometers. As the depth increases, the distribution gradually widens. At the very bottom of the lower mantle, the lateral temperature distribution deviates from the Gaussian pattern, indicating strong lateral heterogeneity, likely associated with the presence of LLSVPs. Further analysis demonstrated that thermal anomalies primarily contribute to velocity anomalies in the upper portion of the lower mantle, while variations in chemical composition predominantly influence velocity anomalies in the deepest part of the mantle. 

Besides, it was disclosed that LLSVPs exhibit higher densities at the bottom of the lower mantle compared to the surrounding mantle, while displaying lower densities above a depth of approximately 2700 kilometers, and that they are characterized by elevated temperatures and enriched concentrations of iron and bridgmanite, supporting the hypothesis that LLSVPs may have originated from primordial basal magma oceans during the early stages of Earth's development. 

This study provides insights into the composition and thermal state of the Earth's lower mantle, significantly advancing the understanding of the planet's deep structure, and it is expected to have a profound impact on research pertaining to the formation, evolution, and dynamics of Earth.