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Biotite M
a
furite
1510
o
C
3GPa
Ol+L
B
1360
o
C
5 wt %
13
5
0
130
0
A
1250
15 wt %
Ol+L
L+V
1160
o
C
C
H
2
O
CO
2
Fig. 13.16 Schematic diagram showing approximate position of liquidus phases in the system
biotite mafurite-H
2
O
CO
2
at 3.0 GPa (after Ryabchikov and Green 1978)
-
phlogopite + olivine + orthopyroxene + clinopyroxene assemblage should be
characterized by approximately the same K
2
O content (
7 wt%) as the investi-
gated biotite mafurite, but it would be impoverished in silica. On the other hand
clinopyroxenes in their experiments contain less Na
2
O than subsolidus clinopy-
roxenes in pyrolite under comparable P-T
*
T conditions (Green 1973). They therefore,
concluded that partially melted phlogopite-bearing pyroxene pyrolite should pro-
duce a silicate liquid having higher Na
2
O content than biotite mafurite. The near-
liquidus clinopyroxenes in the olivine melilitite composition containing H
2
O and
CO
2
at 3.0 GPa are characterized by approximately the same Na
2
O concentrations
as subsolidus pyrolite (Brey and Green 1975); and the melt equilibrated therefore,
with phlogopite-bearing mantle mineral assemblage should contain Na
2
O in similar
amounts to olivine melilitites (3
-
-
4 wt% Na
2
O); and the K/Na ratio in such melts
may be estimated as equal
1.5. Olivine, othopyroxene, clinopyroxene
and liquid are in equilibrium at point B with CO
2
=
CO
2
þ
H
2
ð Þ
0
:
7
:
It may be
noted that phlogopite is absent at this point. It may be concluded therefore, that
silicate melts with CO
2
=
to 1.2
-
5 and in equilibrium with phlogo-
pite + olivine + orthopyroxene + clinopyroxene, should contain more than 7 wt%
K
2
O, and thus have K/Na ratio >1.5 (Fig.
13.16
).
The phase assemblage with volatiles having CO
2
=
ð
CO
2
þ
H
2
O
Þ
[
0
:
05
does not include orthopyroxene, they therefore, concluded that liquids with low
CO
2
=
ð
CO
2
þ
H
2
O
Þ
ratio
0
:
ratios and equilibrated with phlogopite + olivine + orthopyrox-
ene + clinopyroxene, should contain about 7 wt% K
2
O, but they should be more
ð
CO
2
þ
H
2
O
Þ
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