Geology Reference
In-Depth Information
(1980, p. 397, Fig. 3), Yoder (1989) discussed various assemblages in the system
Fo-Ks-Qz at 950
C, which are displayed in Fig.
8.1
. As the incongruent melting of
sanidine persists to 2.0 GPa (Lindsley 1966), the join forsterite
°
sanidine does not
become a thermal barrier until the pressure exceeds 2.0 GPa. Yoder further stated
that as the incongruent melting of enstatite terminates at about 0.14 GPa, the
principal thermal barrier in the system is forsterite
-
leucite up to 1.9 GPa. It is
essential to establish the thermal barrier in order to ascertain the limits of compo-
sition within which K-rich magmas may be generated. Figure
8.2
demonstrates that
leucite
-
enstatite-bearing assemblage is stable below <0.3 GPa.
In this connection mention may be made about the study of Yoder (1989) on the
system forsterite
sanidine
-
-
H
2
O (Fig.
8.1
). It may be observed that the stable
assemblages in the silica-undersaturated part of the system are phlogopite + leu-
cite + liquid, phlogopite + liquid, phlogopite + enstatite + liquid, leucite + liquid,
leucite + sanidine + liquid or sanidine + liquid (Fig.
8.1
). Figure
8.2
a
kalsilite
SiO
2
-
-
-
d demon-
-
strates stable assemblages under dry conditions. This
gure suggests a narrow range
of stability for leucite and enstatite (Fig
8.2
b).
The above discussion shows that in volatile-free system leucite and enstatite may
co-exist at low pressure within a limited P-T
-
T
field, but at lower temperatures
°
<1,000
C, it is eliminated by a reaction to produce sanidine and olivine. Under
vapour-present condition leucite reacts with ma
c components to produce phlog-
opite + enstatite + liquid or phlogopite + leucite + liquid (depending on bulk
composition) and they do not coexist in equilibrium.
Fig. 8.1 Phase relation
in the system
forsterite
Ks
0
kalsilite
quartz at
950 C
-
-
950
C under 0.2 GPa in
presence of excess water
(after Yoder 1989)
°
P
H 0
= 2 kbar
2
Ph+Ks+Lc
Lc
Lc+L
Lc+Sa+L
Sa
Sa+L
L
Ph
Ph+En+L
Qz+L
En+Qz+L
Ph+Fo+En
VV V VV
VV
V
En
Fo
Qz
Weight Percent
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