Lower-mantle materials under pressure - Dr. Jiuhua Chen
JANUARY 8, 2016
In today’s issue of Science, Dr. Jiuhua Chen from HPSTAR published his Science Perspective paper entitled Lower mantle materials under pressure. In this paper, he proposed a hybrid mantle convection model: a mixture of layered and whole mantle circulations of minerals inside the Earth based on the most recent experimental data and accumulated observational results of geophysics.
The model of mantle convection has been a topic in debate for decades. A layered circulation is favored by geochemical observations while a whole-mantle circulation is often preferred by geodynamic modeling. New experimental data on rheology of the mineral assemblage of bridgmanite and magnesiowüstite indicates a possibility of shear localization during a large plastic deformation in the lower mantle. This shear localization may separate the lower mantle into a load-bearing framework (LBF) and interconnected weak layers (IWL) according to Handy’s semi-theoretical model for two non-linear viscous phases, and therefore attributes seismic anisotropy variations to local flow in this region of Earth’s interior. On the other hand, a purely layered mantle convection model is insufficient for the heat removal from Earth’s interior. High-resolution seismic imaging indicates that some subduction slabs deflect at the base of the transition zone and some reach the bottom of the lower mantle. All the evidence leads to the hybrid convection model—a mixture of layered and whole mantle circulations.
Hybrid mantle convection model. Subducting slabs bring oceanic plates (blue) into the deep mantle. The slabs deflected at the 660-km discontinuity form layered convection within upper mantle and transition zone. The slabs penetrating into the lower mantle reaching the core-mantle boundary form whole-mantle convection. Plumes (red) rise from the core-mantle boundary, bringing materials that are enriched in incompatible elements relative to the expected mantle average back to the 660-km discontinuity. Some of them penetrate through the discontinuity, whereas others are deflected and may produce secondary upper-mantle plumes. Shear localization induces interconnected weak layers (IWL) along the slabs or plumes as well as the top and bottom of the lower mantle, yielding a less efficient mixing for the central LBF (load-bearing framework) part of the lower mantle (the reason for long-lived geochemical reservoirs).