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Australia: The Land Where Time Began |
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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.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |