Direct evidence for the segregation of the trapped oceanic crust in the zo mantle transition

The research group of Professor YAO Huajian of the School of Earth and Space Sciences of the University of Science and Technology of China (USTC), in cooperation with Dr Piero Poli of the University of Grenoble-Alpes from France, combined the unique resolution of reflected body waves (P410P and P660P) extracted from ambient noise interferometry with mineral physics modeling, to shed new light on the physics of transition zones. Relevant work has been published in Nature communications.

Subduction of oceanic plates is an important process in the Earth’s internal material circulation. Studying the recycling of the oceanic crust into the deep mantle can provide crucial clues to understanding mantle dynamics and the circulation of deep materials. However, this is hardly limited by reliable seismic evidence.

The mantle transition zone (MTZ) is delimited by global seismic discontinuities close to discontinuities of 410 km and 660 km. The structure and properties of this zone have a crucial influence on the mantle convection process. Because the basaltic oceanic crust with a lower density than the normal mantle has negative buoyancy near the 660 km discontinuity, so it can be gravitationally trapped in this region. However, the narrow depth ranges of negative buoyancy and the lower temperature and viscosity of the subducted ocean slabs bring many uncertainties to this speculation. It is still controversial whether the subductured oceanic crust can be separated from the oceanic lithospheric mantle and remain in this transition zone.

Traditional methods on the structure of the transition zone rely mainly on the travel time and amplitude information of the natural phases of seismic body waves which were often limited by the temporal and spatial distribution of natural earthquakes.

In this study, the researchers used continuous waveform data from more than 200 stations in northern China to calculate the background noise cross-correlation function. The result is clear reflected seismic phases between 410 km and 660 km. There are significant P660P waveform anomalies on the leading edge of the stagnant Pacific plate, which has been well explained by a simple mineral model that: segregated basaltic oceanic crust is accumulated in the lower transition zone at the edge front of the subduction slab.

This study found that the subducted oceanic plate has long been trapped at the bottom of the mantle transition zone, which can undergo mantle-crust segregation due to increased temperature and decreased viscosity. The segregated oceanic crust may remain at the bottom of the mantle transition zone for negative buoyancy and this may well explain the observed seismic scattering and the P660P week phase. Oceanic plates that penetrate directly through the transition zone are difficult to separate due to the rapid speed and lower temperature (higher viscosity).

In addition, these subductured slabs are heated at the core-mantle boundary, where crust-mantle segregation is more likely to occur. The separated components of the oceanic crust will be accumulated above the core-mantle boundary or transported to the shallow end by the mantle plume.

Therefore, the evolution and cycling process of the components of the oceanic crust are closely related to the model of oceanic plate subduction. The filtering effect of materials at the 660 km interface may play a crucial role in the chemical evolution of our planet.

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