Earlier this month, Nature Communications published a geological research done mainly by USTC researchers which determine the velocities and density of subducted oceanic crust under lower-mantle conditions. The results reveal that the velocity anomalies produced by subducted oceanic crust strongly depend on depth, the presence of which can explain some seismic heterogeneities in the lower mantle, and thus provide a deeper look into the origin of lower-mantle seismic heterogeneities.
the Earth inside
The lower mantle is the largest continuous region inside our Earth, occupying the volume for around 55%. It was previously regarded as homogeneous, except for some certain parts, or “provinces”. With the advancements in seismology, however, various heterogeneities have been detected in the lower mantle.
Such heterogeneities were first proposed to be related to subducted oceanic crust. Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another, and is forced to sink due to gravity into the mantle.
Subduction of crust. (Free Image)
Nevertheless, the velocity and density of the subducted crust at lower-mantle conditions remained unknown.
Subducted oceanic crust in the lower mantle. (Image in the paper, by WU Zhongqing et al.)
Here, USTC researchers led by WU Zhongqing and SUN Daoyuan discover that subducted oceanic crust shows a large negative shear velocity anomaly at the phase boundary between stishovite and CaCl2-type silica, which is highly consistent with the feature of mid-mantle scatterers. Using ab initio calculations, they obtain the elastic properties of CF-type phase at high pressure and temperature, and determine the velocities and density of the problem mentioned above.
The results reveal that the velocity anomalies by subducted oceanic crust strongly depend on depth, which serves to explain lower-mantle seismic heterogeneities.
The first authors of the paper are WANG Wenzhong and XU Yinhan, who graduated recently at USTC as doctoral students. The work was supported by CAS and National Natural Science Foundation.
See the paper: https://www.nature.com/articles/s41467-019-13720-2
(Written by QIANG Jiaxuan, edited by YE Zhenzhen, USTC News Center)
