Experimental Studies on K-Rich Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 404

Fig. 13.18 Results of
experiments with H 2 O+CO 2
at X(CO 2 ) * 0.75 Symbols:
◊ - garnet, + orthopyroxene.
Open circle: all glass, Open
square: olivine, Cross:
chromite. (after Edgar et al.
1980)

4.0

X CO

A

~0.75

2

+ + +

+

3.0

+ +

B

+ ++

2.0

C

1100 1200 1300 1400

o

T C

variable H 2 O contents. It is apparent that under dry condition there is an increase in
the liquidus temperature and the curve has a positive slope, whereas with the
increase of the H 2 O content there is a gradual change in the slope of the liquidi, and
under H 2 O-saturated condition (>25 wt% H 2 O), there is a decrease in the liquidus
temperature with the increase in pressure.
Figure 13.17 b shows that in presence of 5 wt% H 2 O, chromite is in the liquidus
phase up to 2.0 GPa, above which olivine is the primary phase. Above 3.6 GPa
clinopyroxene appears as the first phase. Figure 13.17 c shows the appearance of

olivine as the primary phase up to 3.2 GPa followed by chromite. Clinopyroxene is the
liquidus phase above 3.2 GPa, and phlogopite precipitates as the fourth phase with the
progressive decrease of temperature. In their experiments with added H 2 O and
CO 2 XCO 2

at 2.0, 3.0 and 3.5 GPa (Fig. 13.18 ), Edgar et al. noted that at 3.0
and 3.5 GPa, garnet and orthopyroxene appear as the liquidus phase followed by the
appearance of clinopyroxene as the third phase. The curve A

½ ð Þ 0 : 75

C (Fig. 13.18 ) is the
liquidus curve for the ugandite established in presence of volatile X CO 2

B

- -

½ ð Þ ¼ 0 : 75

under variable P-T T conditions. Garnet does not appear at all in the 2.0 GPa runs but

chromite and orthopyroxene are liquidus phases at this pressure. Clinopyroxene
appears as the third phase with decrease in temperature. They did not encounter
phlogopite and olivine at all in runs equilibrated in presence of

-

uid with high X CO 2 ð Þ

content. Their runs at lower X CO 2 show that at 2.0 GPa and X(CO 2 ) = 0.5, stability

of orthopyroxene increases but that of clinopyroxene decreases.With further lowering
of X(CO 2 ) = 0.23

ð Þ

½ °

C and 2.0 GPa both pyroxenes are eliminated and
olivine becomes the liquidus phase. They did not observe the presence of phlogopite
in any of their runs, suggesting that presence of high CO 2 content in the volatile

at 1,200