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to determine water flux into the deeper mantle.
These processes are controlled mainly by the hy-
drous phase relation and the fluid dynamics of
water transport, which will be discussed in the
following.
hydrous phase stable at a higher pressure: e.g., ser-
pentine and phase A around 6.2 GPa. Kawamoto
et al
. (1996) termed this point ''choke point''
(Figure 13.1), since the subducting slab with a
geotherm passing above this point (e.g., SW Japan
in Figure 13.1) liberates water, whereas a slab with
a geotherm below this point (e.g., Central Japan)
may transport a significant amount of water by
hydrous phases.
On the other hand,
C
max
13.2.1 Phase relation
Maximum H
2
O content (hereafter referred to as
C
max
H
2
O
) of peridotite is shown in Figure 13.1 for a
depth range up to 28 GPa (the uppermost lower
mantle). A clear feature is that major hydrous
phases which contribute to high
C
max
H
2
O
of basaltic and sed-
iment systems exhibits rather continuous and
monotonous decrease as temperature and/or pres-
sure increases, owing to more reactions with
solid solution (Schmidt & Poli, 1998; Ono, 1998;
Hacker, 2008). Water content up to 30 wt % in
the initial sediment before subduction (Plank &
Langmuir, 1998) decreases to 2-4 wt % by ex-
tensive mechanical compaction and dehydration
of minerals, including clay minerals, at depth
shallower than 30 km (You
et al
., 1996; Hynd-
man
et al
., 1997), which further decreases slowly
to 1-2 wt % but may be retained in phengite
and hydrous aluminum silicates (topaz-OH and
phase egg [AlSiO
3
(OH)]) at depths greater than
150 km (Ono, 1998). In MORB, after a numerous
H
2
O-bearing reactions associated with a gradual
decrease in H
2
O content from 6 to 1 wt %, there
is no major hydrous mineral after breakdown of
lawsonite at 10 GPa (300 km depth). The liber-
ated water hydrates the overlying peridotite at
the base of the mantle wedge along the slab, and
continues to subduct to a depth that depends on
the geothermal structure (Iwamori, 2007), which
is controlled by phase relation described above
and in Figure 13.1.
The spatial distribution of
C
max
H
2
O
of rocks are
stable at relatively low temperatures. From a low
to high pressure, serpentine, phase A, phase E and
phase D are stable below maximum
∼
600, 1000,
1200 and 1200
◦
C, respectively, with
6wt% to
more than 15 wt % H
2
O. These mineral phases
break down above the upper-limit temperatures,
where nominally anhydrous phases may incorpo-
rate several 1000 ppm H
2
O for the upper mantle
assemblages, or several weight percent H
2
Ofor
the transition zone including wadsleyite and ring-
woodite (e.g., Smyth, 1987; Kohlstedt
et al
., 1996;
Bolfan-Casanova
et al
., 2000). Estimated range for
C
max
∼
H
2
O
of the lower mantle varies from 10 to sev-
eral 1000 ppm, and will be discussed later. In any
case, if the actual H
2
O content exceeds
C
max
H
2
O
,an
aqueous fluid phase occurs and migrates upwards
if the fluid is less dense than the surrounding
material.
Pressure increase may also contribute to de-
hydration. The upper limit curve bounding the
serpentine stability in peridotite changes its pos-
itive
dT/dP
slope to a negative slope at
3GPa,
since above 3 GPa the dehydration reaction causes
an increase in entropy and a volume decrease ow-
ing to high compressibility of the liquid phase.
This holds for the stability fields involving am-
phibole and chlorite, and possibly for hydrous
phases at higher pressures as long as a simi-
lar liquid-solid relation applies. Accordingly, a
cusp at which stability of the hydrous mineral
phases has a minimum upper-limit temperature
may exist when a negative
dT/dP
curve of a hy-
drous phase stable at a relatively low pressure
overlaps with a positive
dT/dP
curve of another
∼
H
2
O
along the sub-
ducting slabs has been calculated according to
the thermal structure in subduction zones as a
function of the slab age and subduction veloc-
ity (Figure 13.2). Depending on the subduction
parameters, the thermal structure differ signifi-
cantly: e.g., the potential thickness of the sub-
ducting slab with hydrous phases is appreciably
thinner when the slab is younger with a slower
subduction velocity. At the same time, several
common features are recognized. Reflecting high
C
max
H
2
O
of wadsleyite and ringwoodite, the mantle
