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when no excess water is added to the system. The assemblages obtained from the
same bulk composition with lowering of temperature between 2.0 and 3.0 GPa (in
presence of excess water, Fig.
13.12
a are as follows: orthopyroxene + phlogo-
pite + rutile + liquid and othopyroxene + phlogopite + clinopyroxene + liquid. At
pressure below 1.5 GPa, assemblages obtained in presence of excess water are: (1)
olivine + liquid; (2) olivine + armalcolite + liquid; (3) olivine + phlogopite + ar-
malcolite + liquid; (4) phlogopite + clinopyroxene + armalcolite + pride-
rite + liquid. Since the experiments were conducted in presence of excess water, a
vapour phase should have been present in association with the above assemblages.
It is interesting to note that under both vapour-absent and excess water condition,
orthopyroxene is the liquidus phase whereas below 1.5 GPa olivine is the liquidus
phase. According to Arima and Edgar except for the presence of orthopyroxene,
Ti
Ti-K-richterite and leucite, the mineralogy of natural wolgidite is similar to that
found by them. Leucite is absent because the P-T
-
T range of study of these two
-
investigators is higher than the stability
field of leucite. Barton and Hamilton (1982)
did not observe the presence of leucite from an orendite composition above
0.4 GPa. They also obtained Ti
-
Ti-K-richterite below the subsolidus.
13.2.8 Phase Relations on a Biotite Mafurites under High
P
T Conditions
-
13.2.8.1 Investigation on a Synthetic Biotite Mafurite Glass
A synthetic biotite mafurite glass was studied by Edgar et al. (1976) between 1.0
and 3.0 GPa under dry and wet conditions. A natural biotite mafurite of similar
composition, was studied by Holmes (1942), who found that the rock consisted of
olivine, kalsilite, perovskite, and biotite with xenoliths of glimmerite. Edgar et al.
1976 made a synthetic glass of biotite mafurite compositions see Table
13.2
. They
noted that the liquidus range from 1,490
C at 3.0 GPa under
dry condition (Fig.
13.13
). Clino-pyroxene is the primary phase at higher pressures,
and olivine is the liquidus phase at lower pressures. The liquidus, however was
lowered by 200
°
C at 1.0 GPa to 1,625
°
C, in presence of 5 % water, olivine being the liquidus phase.
Their runs with 15 % water, show a further lowering of temperatures, olivine still
remaining as the liquidus phase up to a pressure of slightly lower than 3.0 GPa,
above which phlogopite is a liquidus phase.
300
°
-
13.2.8.2 Investigation on a Biotite Mafurite by Ryabchikov and Green
A synthetic glass of a biotite mafurite composition very similar to that used by
Edgar et al. (1976) was investigated by Ryabchikov and Green (1978) in presence
of variable amounts of H
2
O, and different concentration of CO
2
and H
2
O. The
composition of the glass used by them is same as that used by Edger et. al. (1976),
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