Geoscience Reference
In-Depth Information
0
2
4
6
8
10
H
2
O wt.%
0
100
200
300
400
500
600
700
800 depth (km)
1: ol
opx
±
pl/sp/gt
±
cpx
2: ol
+
opx
+
sp
+
amp
3: ol
±
+
opx
+
gt
+
amp
wd
-out
4: ol
amp
5: ol
+
opx
+
chl
+
cpx
6: ol
+
talc
+
chl
+
amp
7: ol
+
opx
+
chl
+
+
serp
+
chl
+
amp
8: ol
+
serp
+
chl
+
cpx
9: ol
cpx
10: ol
+
serp
+
gt
+
cpx
11: A
+
serp
+
gt
+
cpx
12: A
opx
MgS
+
+
+
26
21
+
opx
+
gt
+
cpx
1
13: chm
cpx
14: ol
+
ed
±
opx
±
gt
±
cpx
15: wd
±
opx/st
±
gt/mj
±
cpx
16: E
+
opx
+
gt
+
15
14
14
14
23
22
25
+
opx
+
gt
+
cpx
17: E
+
st
+
gt
+
cpx
1
17
24
18: E
cpx
19: A
+
D
+
gt
+
cpx
20: br
+
D
+
gt/mj
+
cpx/Ca-pv
21: rg
D
gt
+
+
+
27
16
2
3
1
13
18
±
st/ak
±
mj
±
Ca-pv
22: sB
Ca-pv
23: sB
+
ak
+
mj
+
Ca-pv
24: sB
+
D
+
mj
+
Ca-pv
25: sB
+
st
+
mj
+
5
4
9
10
6
12
19
+
pv
+
gt/mj
+
Ca-pv
20
26: pv
+
pe
+
Ca-pv
±
Al-phase
8
27: pe
D
gt
Ca-pv
choke point
+
+
+
11
0
5
10
15
Pressure (GPa)
20
25
Fig. 13.1
Phase relation of H
2
O-saturated peridotite and the maximum H
2
O content of the solid phases (
C
max
H
2
O
)
(modified after Iwamori, 2007). Phase assemblages of the H
2
O-saturated peridotite (field no. 1 to 27) are shown on
the right-hand side of the diagram. The abbreviations of the phases are as follows: ol
=
olivine; opx
=
orthopyroxene;
cpx
=
clinopyroxene; pl
=
plagioclase; sp
=
spinel; gt
=
garnet; amp
=
amphibole; chl
=
chlorite; serp
=
serpentine;
MgS
=
Mg-sursassite; A
=
phase A; chm
=
clinohumite; wd
=
wadsleyite; rg
=
ringwoodite; st
=
stishovite;
mj
=
majorite; E
=
phase E; D
=
phase D; br
=
brucite; Ca-pv
=
Ca-perovskite; ak
=
akimotoite; sB
=
superhydrous
phase B; pv
Al-rich phase. In the fields of no. 1, 14, 15,
21 and 26, which are above the stability fields of major hydrous phases,
C
max
=
perovskite; pe
=
periclase (or magnesiow ustite); Al-phase
=
H
2
O
is not zero as H
2
O is contained in the
nominally anhydrous phases, although it is not fully resolved by the color scale used in the diagram. In the field no.
26, the minimum estimate of 10 ppm based on Bolfan-Casanova (2005) is shown. Three thick solid lines indicate
geotherms along the subducting slabs beneath Central Japan (Pacific Plate), NE Japan (Pacific Plate), and SW Japan
(Philippine Sea Plate) based on Iwamori (2007). Reproduced with permission of Elsevier. (See Color Plate 17).
transition zone is visible in all cases as a potential
water storage. How and how much water is trans-
ported to the transition zone will be discussed in
the next section. Hydrous mineral phases along
and within the subducting slab, where the tem-
perature is lower compared to the ambient mantle
(Figure 13.2), potentially play an important role
as a water carrier. During the course of water
subduction along the slab, the choke point is rec-
ognized at which the potential hydrous layer is
minimized in its thickness or even pinched out
(arrows in Figure 13.2), indicating that in any
case the slab dehydrates significantly between
the surface and the choke point. This region
