Experimental Studies on K-Rich Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 400

1600

above liquidus
olvine
clinopyroxene

phlogopite
ilmenite

3 GPa

X

1500

X
X
X

1400

Ol+
Cpx+
L

Ol+l+L

1300

L

X L+V

Ph+L

1200

Cpx+Ph+L

X

O l+L

1100

Ol+l+L

X

X

1000

0 10 20 30 40

Wt % H 2 O

Fig. 13.15 Phase relations for biotite mafurite at 3.0 GPa as a function of water content. Date for
5 wt% H 2 O, 15 wt% H 2 O and 40 wt% H 2 O are taken from Edgar et al. (1976) and for anhydrous
biotite mafurite from Ryabchikov and Green (1978). For abbreviations see Table 13.1

(5 wt% H 2 O, Fig. 13.15 ). They found that in case of the composition, biotite
mafurite with 2.4 wt% H 2 O and 5.9 wt% CO 2 , the highest temperature of crys-
tallization for primary phlogopite (above 1,300

C at 3.0 GPa) was observed in
experimental runs. Phlogopite is not a liquidus phase in these runs and it crystallizes
from liquid enriched in potassium compared with biotite mafurite due to the pre-
ceding crystallization of olivine or pyroxene.
In their runs liquidus phlogopites are characterized by slightly lower Mg/
(Mg + Fe) values than coexisting olivines or orthopyroxenes, whereas previously
published experimental results on biotite mafurite with H 2 O (Edgar et al. 1976)
show the opposite relation. Ryabchikov and Green (1978) concluded that these
differences are possibly due to the fact that their experimental runs were carried out
under much more oxidizing conditions than the previous study, and part of iron in
the runs of Ryabchikov and Green may be present in trivalent state. They therefore,
concluded that Mg 2 þ =

°

Fe 2 þ values should be higher than the measured
Mg =ð Mg þ Fe 2 þ þ Fe 3 þ Þ values, obtained from microprobe analyses. In case of
runs of Edgar et al. (1976), the lower f(O 2 ) values should cause little difference
between Mg 2 þ =

Mg 2 þ þ

and Mg

Fe 2 þ Fe 2 þ þ Fe 3 þ Þ values.

The TiO 2 contents of liquidus phlogopites in case of runs of Ryabchikov and
Green (1978), are somewhat higher than in phlogopites crystallizing from biotite
mafurite with H 2 O (Edgar et al. 1976) crystallized under similar P, T and P(H 2 O)
conditions. This may be related to either the difference in oxidation states or to the
effect of CO 2 dissolved in the melt or the activity of titanium. They carried out a run

Mg þ =ð Mg þ