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Karato & Jung, 2003). Whether water circulates
shallowly with a relatively short residence time,
or subducts deeply with a longer residence time,
or even accumulates to form a domain in the man-
tle, makes a significant difference in the Earth's
dynamics, thermal evolution, and material differ-
entiation.
Subduction zones are important as entrance
of this water circulation, and may control the
subsequent processes by limiting or enhancing
the water flow into the mantle. There have been
widely different models for the water transport in
subduction zones, assuming instantaneous segre-
gation without any reaction with the overlying
rocks (R upke et al ., 2004; van Keken et al .,
2011) or with chemical reactions between the
fluid/melt and the overlying rocks (Arcay et al .,
2005; this study), porous flow with the chemical
reactions (e.g., Iwamori, 1998; Cagnioncle et al .,
2007; Hebert et al ., 2009), and dyke flow with-
out considering such reactions (Furukawa, 1993).
Alternatively, if inefficient fluid segregation is as-
sumed, a solid packet that involves the light fluid
may rise to form a diapir (e.g., Tatsumi et al .,
1983; Gerya & Yuen, 2003) and has been argued
for the major mechanism of fluid-melt transporta-
tion in the wedge, including 3-D Rayleigh-Taylor
(RT) instability (Ligenfelter & Schubert, 1974;
Honda & Yoshida, 2005).
In this study, first the fluid processes and water
transport in subduction zones, especially water
flux to the mantle transition zone, are discussed
to integrate the previous studies. It is noted
here for clarity that, in this chapter, ''fluid'' is
used as a general term for aqueous fluid, melt
and supercritical fluid. We then discuss the wa-
ter transport into the lower mantle. Estimates
on the maximum H 2 O content of the lower
mantle minerals and rocks show a significant
diversity, ranging from 10 ppm to over 1000 ppm
(e.g., Bolfan-Casanova, 2005; Murakami et al .,
2002). For this sake, we have developed numerical
models for water transport associated with slab
penetration into the lower mantle, based on the
dynamic model of mantle convection and plate
subduction (Nakakuki et al ., 2010). The model
results show that there are different regimes for
water transport into the lower mantle, and that
dynamics of fluid segregation from the descend-
ing mantle affects the practical content of water
in the lower mantle.
Various models for the water distribution in the
Earth's mantle have been proposed (e.g., Richard
et al ., 2002; Huang et al ., 2005; Hirschmann,
2006; R upke et al ., 2006; Yoshino et al ., 2008;
Karato, 2011). Geophysical approaches have po-
tential of providing constraints on the spatial
distribution of water. In particular, electrical con-
ductivity is sensitive to water content (e.g., Shito
et al ., 2006; Karato, 2011), yet the global wa-
ter distribution is poorly constrained at present
partly because of poor resolution and insiffucient
global coverage (e.g., Kelbert et al ., 2009). In this
chapter, we discuss the spatial extent and nature
of global geochemical domains in the mantle,
which could represent regions contaminated by
anciently subducted components (Iwamori et al .,
2010). Finally, by synthesizing these arguments
and evidences, global water cycling is discussed.
13.2
Subduction Zone Processes
Water is brought down into the mantle by sub-
ducting slabs as water bound to solid, pore water,
or a fluid inclusion, in the sediments, oceanic
basaltic rocks, and the underlying peridotitic por-
tion (e.g., Plank & Langmuir, 1998; R upke et al .,
2004). Subducted water, however, tends to be re-
leased from the subducting slab associated with
an increase in temperature and pressure (Schmidt
& Poli, 1998; Hacker, 2008): mechanical com-
paction of pore occurs at relatively shallow depths
and low temperatures (You et al ., 1996), whereas
thermodynamical control (i.e., phase relations)
becomes important under relatively high temper-
atures and pressures at which greater entropy and
compressibility of fluid phases relative to solid
phases enhance dehydration reactions (Miyashiro,
1965). In either case, within the pressure range
corresponding to the slabs beneath magmatic and
hydrothermal arcs, the released fluid phase is
lighter than the solid phase to migrate upwards,
hence subduction and upwelling are competitive
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