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instability to generate numerous hydrous plumes
in the backarc region, redistribute water witin the
mantle transition zone and the upper mantle in a
relatively short time (Richard & Iwamori, 2010).
If the slab penetrates into the lower mantle, sev-
eral 1000 ppm H
2
O may be also transported into
the lower mantle. As in the present-day Earth,
both slab stagnation and penetration might have
occurred depending on the physical conditions
and the history of subduction (Fukao
et al
., 2001).
A part of the lower mantle could be affected by
the subducted components associated with slab
penetration (Figure 13.7), whereas only the man-
tle above 660 km may be affected in the other
regions. A partial correlation between the seis-
mic structure at CMB (including LLSVP) and the
geochemical signature for an aqueous fluid com-
ponent (i.e., IC2) may reflect such variability in
terms of slab stagnation and penetration in time
and space.
lower mantle, since fluid segregation is limited
at a low fluid fraction. Considering that both
stagnant slabs and slab penetration are observed
by the seismic tomography, the subducted H
2
O
is likely to distribute itself in both the mantle
above 660 km and the lower mantle. Finally, the
mantle isotopic variability in terms of Sr, Nd and
Pb has been discussed to detect such subducted
fluid components. Two major differentiation
processes have been argued to be responsible for
most of the mantle isotopic variability sampled
by oceanic basalts, and one of the processes is
accounted for by aqueous fluid-rock interac-
tion. Accordingly, global geographical domains
have been found, which inherit the anciently
subducted aqueous fluid component, possibly
associated with focused subduction towards the
supercontinents.
Acknowledgements
The authors would like to thank Shun Karato and
an anonymous reviewer for helpful comments.
13.5
Concluding Remarks
We have examined water subduction to the deep
mantle through subduction zones, associated
with both slab stagnation and penetration at
660 km phase transition. Based on numerical
models and geophysical observations, several
different stages have been recognized: (1) exten-
sive dehydration of the subducting slab occurs
between the surface and the choke point (i.e., 0
and
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