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radius of the diapir of 1 km. Note that diapirs with
r
C
Melt
H
2
O
M
Melt
H
2
O
M
Melt
=
/
(
M
Diapir
+
H
2
O
)
·
100
0.5 km will be completely reduced during the
first ascent. The interaction of initial diapirs with
the reduced mantle will create a network of ox-
idized conduits saturated by carbonatite melt or
carbonate, through which subsequent diapirs can
arise rapidly. As far as the rate of diapir ascent
is one order of magnitude faster than the convec-
tion in the surrounding mantle (Stern, 2002), the
latter could not significantly modify the oxidized
network if consecutive diapirs emerge every 10
4
years. In such a way a continuous CO
2
(carbon-
atite melt) transport from the transition zone to
the shallow mantle may be established. This sys-
tem should contribute to the volatile budget in
the source regions of basalts, kimberlites, and car-
bonatites, which return CO
2
back to the Earth's
surface (Figure 2.15). We also propose that a simi-
lar mechanism governs the delivery of primordial
carbon and hydrogen from the core-mantle bound-
ary or at least from 660 km depth.
In the above discussion we assumed the melt
composition to be anhydrous carbonatite accord-
ing to (Kerrick & Connolly, 2001). We also esti-
mated the possible contamination of carbonatite
melt by H
2
O during ascent through the tran-
sition zone from a stagnant slab surface (
<
=
25
÷
32 wt
.
%
Yet, no or minor hydration can be expected for
subsequent diapirs travelling through a network
of dried conduits. We expect that hydration of car-
bonatite melt will increase the rate of segregation
and diapir ascent rate up to one order of magni-
tude due to the increased diffusivity and solubility
of silicate components in the H
2
O-bearing melt
(Shatskiy
et al
., 2013).
2.10 Concluding Remarks
>
Fundamental differences between deep (
200 km)
mantlemelting for systems containing H
2
O, CO
2
,
and a reduced C-O-H fluid are outlined. Melt-
ing in the H
2
O-bearing systems is controlled
by hydrogen solubility in nominally anhydrous
silicates and occurs when silicates are supersat-
urated with H
2
Oatdefinite
P
,
T
,
X
,and
fO
2
.
Melting in CO
2
-bearing systems is determined by
alkali carbonate stability and controlled mainly
by Na
2
O, K
2
O, and H
2
O. Studies of the peri-
dotite and eclogite systems containing volatiles
show that most solidi flatten out at pressures
above 6-8GPa, which may cause melting when
the solidus intersects the PT-profiles of subduc-
tion and average mantle. Mantle melting in the
presence of volatiles strongly depends not only
on PT-conditions, but also on the redox state.
An increase in
fO
2
causes redox melting in de-
fined parts of the mantle. The stability boundary
of Fe-Ni metal and the 410 and 660 km dis-
continuities are most important for redox and
decarbonation-dehydration melting. We also ar-
gue that subducted carbonates should play amajor
role in the ''big mantle wedge'' model for stagnant
or deeply-sinking slabs and we propose a new
mechanism for generating slab-derived carbonate-
bearing diapirs in the transition zone.
560
km depth). The maximum H
2
O solubility in
wadsleyite and ringwoodite in equilibrium with
hydrous melt/fluid is about 0.35 wt % at transi-
tion zone P-T conditions (Litasov
et al
., 2011b).
As we discussed above, the ascent of carbonatite
diapirs involves extensive silicate recrystalliza-
tion through intergranular carbonatite melt at
the diapir front. Re-equilibration of wadsleyite
with hydrous-carbonatite melt reduces its H
2
O
content by about 70% (Shatskiy
et al
., 2009).
If carbonatite melt was initially dry, this value
would approach to 100%. For a spherical melt
diapir with
r
∼
1 km the maximum H
2
O content
in a carbonatite melt can vary in the range of
25-32 wt %:
=
M
Melt
H
2
O
10
3
=
π
·
r
·
(560
−
410)
·
ρ
Wd
,
Rw
Acknowledgements
C
Wd
,
Rw
H
2
O
10
15
·
/
100
=
4
÷
6
×
g,
The authors wish to thank Hans Keppler and
Hikaru Iwamori for numerous useful comments,
r
3
10
16
M
Diapir
=
/
·
π
·
·
ρ
melt
=
×
4
3
1.3
g,
