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Schmidt, 2011). These papers show that reduced
forms of carbon (diamond or carbide) are stable
across most of the upper mantle. Carbonatite
melt or carbonate can be stable only at depths
less than 150 km or in highly oxidized regions
(Figure 2.5). These experiments were performed
under nominally hydrogen-free conditions.
We performed experiments in peridotite and
eclogite systems with a reduced C-O-H fluid
buffered at MMO (Mo-MoO 2 ) and IWat 3-16GPa
and 1200-1600 C (Litasov, 2011; Litasov et al .,
2013b). Roughly, MMO is located one log unit
above the IW buffer at T- fO 2 diagram. The exper-
iments were performed using a modified double
capsule method. The solidus temperature in the
system peridotite-reduced C-O-H fluid for both
buffers is substantially higher than the solidi in
systems with H 2 OandCO 2 (Figure 2.10), but still
400-500 C below the melting curve of ''dry'' peri-
dotite at 16GPa. The eclogite solidi are 50-100 C
below the peridotite solidus. Within the studied
pressure interval (3-16GPa), the solidus does not
flatten out, unlike in systems with H 2 OandCO 2 .
In both systems, the first melt generated near the
solidus has 40-50 wt % SiO 2 in dry residue.
The fluid composition was not determined in
the experiments. The pressure dependence of the
fluid composition at 1200 C was calculated from
the equations of state in (Zhang and Duan, 2009)
(Figure 2.11). The major component at the fO 2
defined by the MMO buffer is H 2 O, which con-
tent increases with temperature and pressure. The
major component at the fO 2 defined by the IW
buffer is CH 4 ,butH 2 O becomes predominant
with rising temperature and pressure. Additional
components of the fluids are ethane or hydrogen.
Fig. 2.9 Phase compositions in the carbonatite
systems at Ca-(Mg
K). N - and K - are
starting compositions of Na- and K-bearing
carbonatite, respectively. The dashed arrow shows the
main trend of melt composition with increasing
temperature (Litasov et al ., 2013a). Reproduced with
permission of the Geological Society of America.
+
Fe)-(Na
+
double carbonate phases were observed only in
the Na-carbonatite system and include the ap-
pearance of Na 2 Mg 2 (CO 3 ) 3 in a single experiment
at 3GPa and 750 C. Na 2 Ca 2 (CO 3 ) 3 (shortite com-
position) was observed at 10GPa and 1100 C, and
Na 2 Ca(CO 3 ) 2 (nyerereite composition) at 16GPa
and 1100 C. Na-Ca-carbonates were reported to
be stable in a carbonated pelite system (Grassi &
Schmidt, 2011). The high-temperature stability
limit of these phases as well as the solidus of the
system were higher than for alkali carbonatite
(Figure 2.8).
2.6.4 Systems with a reduced C-O-H fluid
Recently, carbonated peridotite systems were
studied along a range of fO 2 conditions at
pressures up to 23GPa (Stagno & Frost, 2010;
Rohrbach & Schmidt, 2011) in order to model fO 2
dependence of carbonate stability and melting
temperatures. It was shown that magnesite is sta-
ble at fO 2 =
2.7 Melting Behavior in Different Mantle
Systems with Volatiles
Experimental studies of peridotite and eclogite
systems with C-O-H volatiles show a wide vari-
ation in the position of the solidi depending on
fO 2 as well as on the bulk composition of the sys-
tem. In general, the solidi in the eclogite systems
are below those in the peridotite ones or coin-
cide with them, as is the case with CO 2 -bearing
2-3 log units below the FMQ buffer
and with pressure its stability slightly expands
towards more reduced conditions (Stagno &
Frost, 2010) or remains constant (Rohrbach &
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