The research team led by Prof. WU Zhongqing from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences updated the pyrolite model by adding to it the lateral temperature heterogeneity in the lower mantle transition zone (MTZ). The result was published in Fundamental Research.
The Earth’s mantle is a layer inside the planet and can be divided into the upper mantle, the transition zone, and the lower mantle. The pyrolite model, a widely used one to characterize the composition of the Earth’s mantle, fits well with the velocity and density in the upper and lower mantle but deviates from the observed properties of the lower MTZ.
Previous calculations only considered the mean temperature and ignored that the temperature at the same depth may vary. Generally, the neglect did not affect the result. However, the conventional method becomes inappropriate when the temperature heterogeneity affects the mineral phases, which just occurs at the depth range of the lower MTZ. The majorite-perovskite-akimotoite triple point lies near the temperature range in lower MTZ, indicating that denser akimotoite with higher speed may exist and hugely affect the measured density and seismic velocity.
In this study, researchers modeled the mantle temperature with a Gaussian distribution and investigated the elastic properties of iron-bearing akimotoite at mantle conditions using first principles calculations. It was observed that the calculated volume of akimotoite agreed with the previous experimental measurements under the pressure of MTZ.
Combining their consequences with the elastic properties of other minerals, researchers then estimated the velocities and density of the pyrolite model in the lower MTZ with the effect of temperature heterogeneity. The results of the calculation explained well with seismic velocity, which implied that the pyrolite model could adequately explain the velocities and density of the lower MTZ under the heterogeneous temperature.
In support of the above observations, researchers suggested that there is no need to introduce a novel component into lower MTZ for the pyrolite model to meet with experimental data. The same composition of the upper mantle, the lower mantle, and the lower MTZ supports the whole mantle convection.
This work optimizes the pyrolite model with temperature heterogeneity in the lower MTZ. It introduces a deeper understanding of the composition and the temperature distribution in the lower MTZ and provides evidence for the whole mantle convection.
Elastic properties of Fe-bearing Akimotoite at mantle conditions: Implications for composition and temperature in the lower mantle transition zone - ScienceDirect
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