Geology Reference
In-Depth Information
10.3.4 The Join Forsterite
Akermanite Studied
at 2.3 GPa and Variable Temperatures
Leucite
-
-
The phase relations in the join Fo
Ak were studied at 1 atmospheric pressure by
Gupta (1972) (Fig.
10.17
a). Nag et al. (2007), made 68 runs to investigate the join
(Fig.
10.17
b, c) which shows that the join intersects the primary phase volumes of
forsterite
ss
, phlogopite
ss
, kalsilite, K-feldspar
ss
, diopside
ss
, and merwinite
ss
. There
are 4
-
Lc
-
five-phase points occurring at following compositions and temperatures: (1)
'
C), where phlogopite
ss
, forsterite
ss
- and diop-
side
ss
coexists with liquid and vapour, (2)
N
'
(Fo
7
Lc
42
Ak
51
and 1,165
±
10
°
C),
where a liquid is in equilibrium with phlogopite
ss
, K-feldspar
ss
, diopside
ss
and
vapour, (3)
'
O
'
(Fo
3
Lc
49
Ak
48
and 1,140
±
10
º
C), where merwinite
ss
forsterite
ss
and diopside
ss
are in equilibrium with liquid and vapour, and (4)
'
M
'
(Fo
13
Lc
21
Ak
66
and 1,255
±
10
°
'
P
'
(Fo
5.5
L-
c
73.5
Ak
21
, and 775
C), where kalsilite, phlogopite
ss
and K-feldspar
ss
coexist
with liquid and vapour (Fig.
10.17
b).
When the phase relationships in the join Fo
±
10
°
Ak at one atmospheric pressure
(Gupta 1972 (Fig.
10.17
a) is compared with those at 2.3 GPa (Fig.
10.17
a, b), it may
be noted that there is a
Lc
-
-
field of monticellite
ss
occurring near the join Fo
Ak. In
-
contrast, Fig.
10.17
b, c in set shows the presence of a
field of merwinite
ss
as sug-
gested by Yoder (1973). The piercing point
(Fo
ss
+Ak
SS
+Lc
ss
+ Liq) observed in
this join at one at atmospheric pressure (Gupta 1972) occurs at 1,286
'
G
'
°
C, whereas the
temperatures of the
five-phase points
'
N
'
(1,165
°
C),
'
O
'
(1,140
°
C),
'
M
'
(1,255
°
C),
and
cantly lower, because the present study at 2.3 GPa was
conducted under a condition of 2.3 GPa [P(H
2
O) = P(Total)]. The results are sum-
marised in Fig.
10.17
b. The comparison of the tow
'
P
'
(775
°
C) are signi
figures (Fig.
10.17
a, b) shows that
in case of the 2.3 GPa isobaric diagram there are such additional phased as phlog-
opite
ss
, K-feldspar
ss
, kalsilite and merwinite
ss
, which are absent in the one atmo-
spheric phase diagram of the same join (Figs.
10.3
and
10.17a
). On the basis of
microprobe analyses of liquid equilibrium with vapour phases, the trends of liquid
composition (bold lines with solid squares Fig.
10.17
c) are projected on to the
Fo
Ak join from the H
2
O apex (Fig.
10.17
b). Although the trends of liquid
compositions do not coincide with the exact phase boundaries, the course of crys-
tallization is consistent with the observed sequence of crystallization.
Lc
-
-
10.3.5 The Paragenetic Sequence in the
Kalsilite
CaO
MgO
SiO
2
-
H
2
O System
-
-
-
If a small amount of alumina present in diopside as CaAl
2
SiO
6
molecule is ignored,
the system may then be treated as quinary having the following components,
kalsilite
CaO
MgO
SiO
2
-
H
2
O.
It
is further assumed that
the vapour phase
-
-
-
observed in the glasses should combine under elevated P-T
fluid
phase (f) and the remaining water is dissolved in the crystalline phases. Because of
T condition as a
-
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