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Slab Stagnation in Shallow Lower Mantle Linked to Increased Mantle Viscosity

The main process responsible for recycling material from the surface of the Earth and the atmosphere back into the deep mantle is the subduction of oceanic plates. Subducting plates of oceanic lithosphere can stagnate and broaden in the shallow lower mantle, as indicated by seismic images (Fukao & Obayashi, 2013; Li et al., 2008). No structural transitions at these depths are shown by the main phases of the lower mantle bridgmanite and ferropericlase; therefore only moderate and smooth variations of viscosity are expected with depth (Yamazaki & Karato, 2001; Ammann et al., 2010) to at least ~2,500 km. It is therefore unclear why slabs stagnate, and the stagnation may also lead to chemically distinct reservoirs in the deep mantle (Hofmann, 1997). In this paper Marquardt & Miyagi measure in situ the deformation of ferropericlase at pressures up to 96 GPa by the use of synchrotron radial X-ray diffraction. The strength of the ferropericlase was found to increase by a factor of 3 at pressures from 20-65 GPa. The viscosity of the region surrounding the subducting slabs was shown by modelling, based on their experimental data, to possibly increase by 2.3 orders of magnitude throughout the upper 900 km of the lower mantle. The stagnation of slabs that are sinking through the shallow lower mantle can result from such a strong increase in viscosity.

Sources & Further reading

1. Marquardt, H. and L. Miyagi (2015). "Slab stagnation in the shallow lower mantle linked to an increase in mantle viscosity." Nature Geosci 8(4): 311-314.