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(a)
(b)
1400
1400
3 GPa
(Equiv to 15 wt % H
L
+
V
L
2 O)
Ol
X
X
X
X
?
?
L+V
L+Xtals
1300
1300
X
X
X
X
L
?
X
L+Xtals+V
Ph
1200
1200
X
L+Xtals
30 Kb
(Equiv to 5 wt % H
X
2 O)
1100
1100
0.25
0.50
0.75
0.25
0.50
0.75
CO 2
H 2 O
CO 2
H 2 O
CO 2 / CO 2 + H 2 O mole
fraction in charge
CO 2 / CO 2 + H 2 O mole
fraction in charge
12.2 wt %
5 wt %
36.7 wt %
15 wt %
Fig. 13.14 a Phase relations for biotite mafurite at 3.0 GPa as a function CO 2 /(CO 2 +H 2 O) values
in the charge in the presence of an external MH-buffer. Data for compositional joins equivalent to
5 wt% H 2 O. (after Ryabchikov and Green 1978). For abbreviations see Table 13.1 , Xtals: crystals.
b Data for the biotite mafurite- H 2 O side were taken from Edgar et al. (1976). Note that at low
CO 2 /(CO 2 +H 2 O), experiments were carried out under vapour- undersaturated condition, but were
vapour-oversaturated for high CO 2 /(CO 2 +H 2 O) values. Xtals: crystals. For other abbreviations
see Table 13.1
3.0 GPa. This result is similar to that obtained by Brey and Green (1975) for olivine
melilitite.
Ryabchikov and Green conducted a series of runs at various pressures with
biotite mafurite containing 6.8 wt% H 2 O and 16.5 wt% CO 2 (H 2 O: CO 2 = 1:1
molar ratio, mixture equivalent to 15 wt% H 2 O). They observed that orthopyroxene
persists at the liquidus with decreasing pressure down to at least 2.0 GPa. They did
not
find phlogopite as a liquidus phase at 3.0 GPa in their experiments containing
CO 2 . They however, speculated that it should appear as the first crystallizing phase
in H 2 O-rich compositions as is evident from Fig. 13.14 a, b, which combine their
experimental data with those for the biotite mafurite + H 2 O composition (Edgar
et al. 1976). From Fig. 13.14 , it is apparent that at 10
11 wt% H 2 O, phlogopite
-
should start to crystallize at 1,240
C, whereas in their experiments with
10.7 wt% H 2 O and 8.7 wt% CO 2 , primary phlogopite precipitates from the melt
between 1,160 and 1,140
1,250
°
-
C. This observation suggests that large amount of CO 2
strongly suppresses the stability of phlogopite. They however noted that this effect
is, not so evident in the series of runs with lower CO 2 concentration. In the mixture
with 3.7 wt% H 2 O and 3.0 wt% CO 2 and 2.4 wt% H 2 O and 5.9 wt% CO 2 , they
found that crystallization temperatures of phlogopite are higher than that observed
for composition in the biotite mafurite-H 2 O join with equivalent water content
°
 
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