Experimental Studies on K-Rich Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 385

Arima and Edgar (1983) made experimental study on a katungite (for analysis of
the rock see Table 13.2 ) collected from the western branch of the African Rift
Valley. They determined phase relations in that rock between 0.8 and 4 GPa under
“

and wet conditions (in presence of 5 and 15 wt% H 2 O) Fig. 13.5 a, b. They
also carried out experiments in presence of both H 2 O and CO 2 (corresponding to
CO 2 =

dry ”

75 molar ratio) under NNO buffer condition. They
observed that under H 2 O-undersaturated conditions, the liquidus temperature for
'

ð CO 2 þ H 2 O Þ 0 : 5 and 0 :

dry ' runs (Fig. 13.5 a) range from 1,375 ° C at 1 GPa to nearly 1,510 ° C at 3 GPa

(Fig. 13.5 b).
In case of experiments with 5 % H 2 O (Fig. 13.5 a) at 1 GPa, the liquidus
temperature is 225 C lower than those obtained under dry condition at the same

pressure. Likewise the liquidus temperature for the H 2 O-undersaturated runs at
3 GPa is 220

°

C lower. It may be observed that above 2 GPa the phases obtained
with lowering of temperature are as follows: (1) liquid, (2) clinopyroxene + Ti-
magnetite + liquid and (3) clinopyroxene + phlogopite + perovskite + Ti-magne-
tite + apatite + liquid. At 1 GPa however, the phase assemblages observed at
successively lower temperatures are, (1) liquid, (2) olivine + liquid, (3) oliv-
ine + chromite + liquid and (4) olivine + melilite + perovskite + apatite + Ti-
magnetite + liquid.

°

(a) (b)

15 wt % H 2

35 35

5 wt % H 2

“Dry”

30 30

Cpx
+phl
+Ti-mt
+ap
+L

25 25

B L

Ol
+
L

Ol + Cpx
+Ti-mt
+ L

20 20

L

L

Cpx+phl
+Ti-mt + ap
+ pv + L

Cpx+phl ± pv
+Ti-mt ± ap
+L

Ol
+Ti-mt
+ L

15 15

Ol + cr
+ L

Ol
+ cr
+ L

A
Ol
+ cr
+ap
+ L

10 10

Ol + mel + pv
+ ap + Ti-mt
+ L

Ol+mel+ap
+Ti-mt
- pv + L

5 5

900 1000 1100 1200 1300 1400 1500 900 1000 1100 1200 1300

o o

Temperature C Temperature C

Fig. 13.5 Pressure-temperature diagram for phase relations in katungite composition under
(a) ' dry ' and 5 wt% H 2 O-added experiments, and under (b) 15 wt% H 2 O added experiments.
Arrows joining experimental points indicate reversal runs. Full lines represent experimentally
determined boundary curves, dashed lines inferred boundary curves. Line A - B represents reaction
relationship of olivine with liquid to produce phlogopite (after Arima and Edgar 1983). TiMt:
Titanomagnetite, Pv: perovskite, Cr: chromite, Mel: melilite