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7-10GPa) the difference between
fO
2
values for
HRM and CRM increases up to several orders
of magnitude (several log units). Redox melting
can operate only at
fO
2
parameters that drasti-
cally change fluid composition. With increasing
pressure, the expanding range of
fO
2
values for
the water maximum may reach nearly to the
IW buffer, thus at pressures above approximately
10GPa, redox melting becomes less and less im-
portant in the mantle. However, modeling fluid
composition is difficult at present since the effect
of melting and the influence of dissolved silicates
are not well known.
in the upper mantle and magmatism on the
surface (Ivanov, 2007; Maruyama
et al
., 2009;
Ohtani & Zhao, 2009; Zhao & Ohtani, 2009).
This model was applied to continental volcanism
in the China Craton (above the stagnant Pacific
plate in the transition zone) (Ohtani & Zhao,
2009) and to the Siberian Traps (Ivanov, 2007).
The Farallon slab, which is subducting under
the North America deep into the lower man-
tle, may provide an example for this model. Here,
a ''wet'' plume may separate from the bottom of
the transition zone approximately at the 660 km
discontinuity (van der Lee
et al
., 2008).
The original model may require some modifi-
cations. First, the H
2
O content preserved when
the plate is subducted below the level of island-
arc volcanism should not exceed 0.1 wt % in the
upper 10-20 km of the plate and may be higher
only in the coldest plates (Kerrick & Connolly,
2001; Poli & Schmidt, 2002). These contents are
close to those in the source regions of OIB and
enriched MORB and cannot cause serious con-
sequences such as large-scale mantle melting.
Second, as discussed in the previous sections,
the transition zone can be considered as a reser-
voir (''sponge'') for H
2
O, and H
2
O-bearing silicate
melts may not be able to cross the 410 km dis-
continuity (Bercovici & Karato, 2003). Third, a
hypothetical H
2
O-bearing fluid/melt separated
from a subducted plate has to react with reduced
mantle rocks and transforms into CH
4
or H
2
,
which, presumably, have very low silicate sol-
ubility and low wetting angles. Therefore, the
ability of this fluid or melt to migrate will be
limited. Some enrichment of transition zone by
H
2
O can be also realized via transport by thin
overlying low-viscous boundary layer above slab-
mantle interface, where 0.1-0.4 wt % H
2
O can
be stored during subduction (e.g. Iwamori, 2007;
Tonegawa
et al
., 2008). However, even if this is
the case, this water would not initiate significant
melting in the deep upper mantle where olivine
can incorporate up to 0.5-0.8 wt % H
2
O.
The situation may change if we take the role
of carbonates in the melting of subducting slabs
and their influence on the behavior of H
2
Ointo
account. Carbonates are preserved in sediments,
2.9 The Big Mantle Wedge Model and
Carbonates
2.9.1 The model
Recent studies suggest the existence of a so-
called ''big mantle wedge,'' when a water-bearing
subducted plate stagnates and dehydrates in the
transition zone (Figure 2.13). When a stagnant
slab is heated, melting takes place and a water-
bearing melt rises upwards, causing local melting
Intracontinental
volcanism
Back
arc
basin
Island
arc
Mid-
ocean
ridge
Lithosphere
Asthenosphere
Big mantle
410 km
wedge
Oceanic crust, 5-7 km
600 km
Fig. 2.13
Schematic model for a ''big mantle wedge''
caused by devolatilization of a stagnant slab in the
transition zone. Carbonate and minor H
2
O can be
transported to transition zone depth, where
decarbonation melting can occur. Segregation of
carbonate-bearing melt can cause ascent of buoyant
carbonate blob-diapirs from transition zone towards
the surface.
