Geoscience Reference
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transition zone. Empirical profiles of cold and hot
subducting slabs, stagnant in the transition zone
and penetrating into lower mantle are shown in
Figure 2.3.
The oxidation state of the mantle is discussed
in detail by Frost & McCammon (2008). Most
estimates of
fO
2
for basaltic rocks and shallow
mantle peridotites plot close to the FMQ (fayalite-
magnetite-quartz) oxygen buffer. However, data
for garnet peridotite from kimberlite indicate
a well-pronounced decrease in
fO
2
with depth
(Woodland & Koch, 2003). These data along with
experimental studies (Frost
et al
., 2004; Rohrbach
et al
., 2007) showed that the Fe
3
+
contents of sili-
cates increased with pressure even at equilibrium
with Fe-metal. This increase is due to the sta-
bilization of Fe
3
+
in some high-pressure phases,
particularly in Mg-perovskite. At
8 GPa, the
curve of the average
fO
2
from mantle peridotites
would cross the stability line of Fe-Ni metal
(Figure 2.4). In the presence of a small amount
of this alloy (
∼
Fig. 2.2
PT-profiles in the shallow mantle. Mantle
geotherms based on xenoliths in continental basalts of
Southeast Australia (O'Reilly & Griffin, 1985) and in
kimberlites of the Udachnaya pipe, Siberia (Boyd
et al
.,
1997) and the Jericho pipe, Canada (Kopylova
et al
.,
1999) are shown as well as a MORB adiabat with
potential temperature of 1315
◦
C constrained using a
model of McKenzie
et al
. (2005). The PT-profiles of
subduction zones under Northeast and Southwest
Japan are shown after Peacock and Wang (1999).
Symbols show PT-data for subduction-related rocks in
ophiolite and high-pressure complexes of different age
(in Ma) (Maruyama & Liou, 2005). The gray field shows
the maximum pressure and temperature of ultra-high
pressure metamorphism in the Kokchetav complex
(Korsakov & Hermann, 2006). The gray line is the
solidus of wet basalt (Poli & Schmidt, 2002). The
graphite-diamond phase boundary is after Kennedy and
Kennedy (1976).
∼
0.1 wt % in the upper mantle at
10-14GPa and
1.0 wt % in the lower mantle,
Frost
et al
., 2004; Rohrbach
et al
., 2007), the sys-
tem would be buffered near the IW (Iron-wustite)
equilibrium (or 1-2 log units below this buffer)
and the average
fO
2
corresponds to the bold curve
in Figure 2.4. As yet, it is difficult to accurately
determine
fO
2
below the intersection with the
stability line of the Fe-Ni alloy, because it de-
pends not only on the disproportionation of Fe
2
+
in silicates, but also on the original heterogeneity
in oxygen distribution in the bulk Earth. Oxi-
dized material of subducting slabs (and of the
mantle wedge entrained to convective mixing)
may significantly affect the redox state and melt-
ing regime of the upper and maybe lower mantle.
The consequences of this processes is discussed
in detail in the next sections.
∼
modern subduction slabs are based on resent
estimates by Syracuse
et al
. (2010). Examples of
cold subducting slabs are Tonga and NE Japan.
Representative hot subduction zones include
SW Japan, and North and South American slabs
(van Keken
et al
., 2002; Syracuse
et al
., 2010).
The temperature profiles of subducting slabs are
difficult to constrain at depth below 250-300 km,
especially if
2.4 An Outline of Experimental Studies at
Pressures below 6-7 GPa
Most studies of peridotite and eclogite systems
with a C-O-H fluid were carried out at pressures
below 6-7GPa, where most basaltic melts and
the slabs are stagnant
in the
