Geology Reference
In-Depth Information
10
CaAl
2
Si
2
O
8
anorthite
(plagioclase)
1553
Grossular
+ quartz
820°C
7. 7×10
8
Pa
1444
5
Anorthite
crystallizes
first
R
1317
1475
z
50% anorthite
Anorthite
+ wollastonite
1270
1274
E
Forsterite
crystallizes
first
520°C
1×10
5
Pa
0
400
600
800
1000
T
/°C
1392
1890
1388
CaMgSi
2
O
6
diopside
(pyroxene)
Mg
2
SiO
4
forsterite
(olivine)
2.4
(a) 1400°: liquidus to vertical dashed line is
4.0 mm.
Solidus to vertical line is 23 mm.
Liquid/crystals = 23/4.0 = 5.8
The composition plots in the forsterite field,
meaning this will be the first mineral to crystal-
lize from a melt of this composition as it cools to
the liquidus.
2.6
The required diopside-plagioclase mixture is
d:
23 100
23
×
+
or
percent liquid
=
)
=
85%
(
4.0
Liquid composition = An
35
. Crystals An
73
.
(b) Liquid/crystals = 0.55. Percent melt = 36%.
Liquid An
15
. Crystals An
54
.
(c) Liquid/crystals = 0. Liquid An
7
. Crystals An
40
.
40
Percent diopside in
d
=
×
100
=
47%.
46
+
40
46
Percent plagioclase
c
=
×
100
×
53%
46
+
40
2.5
A rock consisting of plagioclase, diopside, nephe-
line and olivine can be plotted in a CaAl
2
Si
2
O
8
-
CaMgSi
2
O
6
-Mg
2
SiO
4
ternary diagram,
provided that
the percentages of plagioclase (represented by
CaAl
2
Si
2
O
8
), diopside (represented by CaMgSi
2
O
6
)
and olivine (represented by Mg
2
SiO
4
) are recalcu-
lated to add up to 100% (Box 2.7).
Using the data given, plag + diopside + olivine =
only 85%. Therefore, eliminating nepheline (not
represented in this diagram), the values to plot are
calculated as follows: plag = 42.55% × 100/85 = 50%.
Diopside = 25.5% × 100 / 85 = 30%. Olivine = 17% × 100/
85 = 20% (check total = 100%). The composition plots
as shown:
Solid mixture
a
consists of plagioclase (average
composition
f
= An
31
) and diopside. The lever rule
gives 41%
f
and 59% diopside.
a
lies on the boundary of the three-phase triangle
melt-diopside-plagioclase at 1220 °C.
The equilibrium assemblage is melt
b
58%, diop-
side 42% and plagioclase
c
0%.
2.7
20 × 10
8
Pa ≡ 67 km from Equation 2.1.
(a) Di
69
Fo
07
En
24
(b) Di
69
Fo
07
En
24
The first melt to form has the composition E in
each case.
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