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15%
5%
Dry
5%
H
2
O
15%
H
2
O
30
20
10
1200
1400
1100
1300
1100
1300
T
o
C
T
o
C
T
o
C
Fig. 13.13 P(H
2
O)-T diagram of a synthetic biotite mafurite. The left hand diagram shows
subsolidus (open symbols) and above-solidus runs for water-saturated and anhydrous conditions
and the positions of the liquid for 5 and 15 % H
2
O. Olivine is the liquidus phase, shown as solid
lines. The centre diagram shows experimental points and incoming of phases for 5 % H
2
O and the
right diagram shows similar data for 15 % H
2
O. The olivine-out boundary at high pressure, due to
reaction with liquid and other solid phases with decreasing temperature, is shown as a dashed
boundary. Below 2 GPa olivine crystallization is followed by phlogopite and clinopyroxene. In
case of 15 % H
2
O ilmenite appears as the last phase (after Edgar et al. 1976)
Table
13.2
. The results of
experiments and runs with various amounts
of H
2
O and/or CO
2
added are summarized in Fig.
13.14
a, b. The phase relations on
the system biotite mafurite-H
2
O at 3.0 GPa is shown in Fig.
13.14
a, b. In this
“
anhydrous
”
gure,
they took the data on 5, 15 and 40 % H
2
O from Edgar et al. (1976). They had two
sets of runs on the biotite-mafurite at 3.0 GPa. In one set of runs H
2
OorCO
2
was
not added and in case of another set 30 wt% CO
2
was added. Comparison of both
sets of run products, show that in both cases olivine is the
first phase crystallizing
from the melt. However, the liquidus temperature for the CO
2
-saturated composi-
tion is approximately 120
o
lower than that for volatile-free biotite mafurite, sug-
gesting substantial solubility of carbon dioxide in the silicate liquid. They analysed
quenched glasses from biotite mafurite + 30 wt% CO
2
runs, and found that it
contains 10.4 wt% CO
2
at 1,380
C and
3.0 GPa giving an average of 10.6 wt% CO
2
. This value is only slightly higher than
the CO
2
solubility in olivine melilite melts (Brey and Green 1976; Brey 1976). Both
magmas contain high Mg and Ca have but very different K and Na contents.
Ryabchikov and Green established that olivine was the liquidus phase for both
volatile-free and CO
2
-saturated compositions, but the second crystallizing phase was
clinopyroxene in the
°
C and 3.0 GPa and 10.9 wt% CO
2
at 1,370
°
first case and orthopyroxene in the second. Similar observation
was noted by them in the runs with both CO
2
and H
2
O. They further established that
at suf
ciently high concentration of CO
2
, clinopyroxene is replaced by orthopy-
roxene. They concluded from their experiments that approximately 6 wt% CO
2
is
necessary to bring orthopyroxene to the liquidus in biotite mafurite composition at
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