Experimental Studies on K-Rich Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 395

Ol þ Liq + Vap ! Ol

þ Phl + Liq + Vap ! Ol

þ Phl + Opx + Liq + Vap ! Ol

þ Phl + Opx + Ap + Chr + Liq + Vap ! Ol

þ !

Phl + Opx + Ap + Chr + Carb + Rut + Liq + Vap Ol

þ Opx + Phl + Ap + Chr + Rut + Carb + Vap.

Subsolidus assemblage once again is a phlogopite-bearing harzburgite contain-
ing chromium spinel, appatite, rutile and a carbonate phase. Orthopyroxene appears
once again as an important phase because of high CO 2 fugacity in the volatiles.
Priderite does not appear in the case of Satyanarayanpur lamproite. Absence of
K-feldspar, which is present as a minor phase in all the three lamproites can be
explained by the reaction with (Mg, Fe)CO 3 to produce phlogopite (Rice 1977).
The La 2 O 3 (0.40) and Ce 2 O 3 (0.83) contents of apatite crystallized at 950 ° C

from the Satyanarayanpur lamproite are once again high.

13.2.7 Experimental Investigation on a Natural Wolgidite

Arima and Edgar (1983) made experimental studies at variable temperatures and
pressures (up to 4.0 GPa) on a wolgidite collected from Mt. North, West Kimberley,
Australia. Analysis of the rock is given in Table 13.2 . They made two sets of
experiments; in the first set of experiments 13 wt% H 2 O and 0.84 wt% CO 2 [X

(CO 2 ) = 0.03] were added. The second set of experiments were conducted in
absence of excess water [3.22 wt% H 2 O, 0.93 wt% CO 2 , X(CO 2 ) = 0.11]. Results
of the experiments are summarized in Fig. 13.12 a, b. In their study under P
(H 2 O) = P(Total) condition,

liquidus temperature increases from 1,065 ° Cat

1.0 GPa to 1,165 C at 4.0 GPa (Fig. 13.12 a). Olivine is the liquidus phase up to

2.0 GPa, above which orthopyroxene crystallizes in the liquidus. Phlogopite
appeared about 50

°

° C below the liquidus at 1.0 GPa and its temperature of crys-

tallization is C at 4.0 GPa. The liquidus increased with increase in

pressure. They observed that clinopyroxene crystallized between 1,000 and
1,025

1,150 °

*

C
below the liquidus. Below 1.7 GPa, rutile is predominant at above 2.0 GPa also.
Crystallization of rutile is close to the temperature of appearance of phlogopite.
Priderite crystallized at low temperature and pressure.
In case of experimental study under P(H 2 O) < P (Total) condition, olivine is the
liquidus phase at 1.0 GPa, but orthopyroxene appears as the

° C at 1.0 to 3.0 GPa. Rutile, armalcolite and priderite crystallized 30 to 40 °

rst phase at 2.0 and
3.0 GPa (Fig. 13.12 b). When experimental results under vapour-absent conditions
are compared with those studied in presence of excess water, it is observed that the
primary liquidus volume of orthopyroxene decreases relative to that of olivine.
Phlogopite is the second phase appearing 30

°

C below the liquidus at 3.0 GPa,