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without CO
2
, but with 15 % H
2
O at 3.0 GPa, 1,160
C with oxygen fugacity
controlled by hematite-magnetite buffer. The run yielded phlogopite with relatively
high TiO
2
content (5 wt% TiO
2
). This result supports the
°
first of the above
explanations. Their experiments con
rmed the previously-observed (Edgar et al.
1976) positive correlation between TiO
2
contents of phlogopite and temperatures of
equilibration of melt or negative correlation with H
2
O content in the system. Ry-
abchikov and Green found that primary phlogopites crystallized at 3.0 GPa and
1,300
C from compositions with 2.4 wt% H
2
O and 5.9 wt% CO
2
, are characterized
by the highest titanium concentrations (8.1 wt% TiO
2
). Their study demonstrates
that TiO
2
content of phologpite positively correlates with increased temperature,
increased Fe
2+
/Mg ratio and increased Fe
3+
content [higher f(O
2
)] inversely cor-
relates with increased pressure. Of these controls, the temperature and Fe
2+
contents
may be the most important.
Experimental studies of Edgar et al. (1976) demonstrated that over a wide P-T
°
T
range (in presence of water), orthopyroxene did not crystallize within the melting
interval of biotite mafurite. This suggests that the melt of biotite mafurite compo-
sition was not derived by partial melting of a mantle mineral assemblage containing
olivine, clinopyroxene and orthopyroxene and water as the only volatile compo-
nent. In contrast, the study of Ryabchikov and Green demonstrates that, in the
presence of CO
2
and H
2
O, biotite mafurite melt may become saturated with respect
to orthopyroxene thereby allowing the possibility of generation of silicate liquids
similar to biotite mafurite in the course of the partial melting of an upper mantle
peridotite. According to Ryabchkov and Green, the appearance of orthopyroxene in
the melting range of this potassium-rich silica-undersaturated highly calcic rock
results from drastic reduction in the activities of CaO and alkalis due to the
interaction of these components with (CO
3
)
-
complexes. Similar observation was
made in a number of other silicate systems containing H
2
O and CO
2
(Eggler 1974;
Brey and Green 1975). Ryabchikov and Green also anticipated this effect on the
basis of general consideration of the interaction of basic cations with acid volatile
compounds (Kushiro 1975). Ryabchikov and Green constructed a schematic dia-
gram showing the position of the liquidus
-
fields of various minerals in biotite
mafurite as a function of H
2
O and CO
2
content in the melt (Fig.
13.16
). They drew
the vapour-saturation boundary tentatively in the compositional triangle by analogy
with the data of Brey and Green (1977) on olivine melilitite composition at variable
P
fields of the various
crystalline phases as lines, but they are more probably bands in which two
neighbouring phases coexist with liquid (cf. Brey and Green 1977). This diagram
demonstrates that
T conditions. They drew the boundaries between the liquidus
-
where three crystalline phases may
simultaneously coexist with the liquid of biotite mafurite composition: A
there are three
'
points,
'
—
—
phlogopite + orthopyroxene + clinopyroxene + liquid; B
orthopyroxene + oliv-
ine + clinopyroxene + liquid; and C-phlogopite + olivine + liquid. In their diagram,
point A corresponds to CO
2
=
ð
CO
2
þ
Þ
:
15. The crystalline assemblage at
this point does not include olivine, but has phlogopite, orthopyroxene and clino-
pyroxene. On the basis of which, they concluded that if the CO
2
=
H
2
O
0
ð
CO
2
þ
H
2
O
Þ
ratio in the melt is approximately 0.1
0.2, then the silicate liquid coexisting with
-
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