Geology Reference
In-Depth Information
CaAl 2 Si 2 O 8
anorthite
(plagioclase)
E
R
1553
y
z
(d)
x
An
1444
Fo
Melt
An
Fo
cf. Fig. 2.4
Di
Anorthite
crystallizes
first
R
1317
1475
z
(c)
1270
y
1274
E
melt
An
Fo
Forsterite
crystallizes
first
Di
x
1392
1890
1388
CaMgSi 2 O 6
diopside
(pyroxene)
Mg 2 SiO 4
forsterite
(olivine)
(a)
(b)
Melt
Melt
Fo
Fo
Di
Figure 2.8 Phase diagram for the ternary system CaAl 2 Si 2 O 8 -CaMgSi 2 O 6 -Mg 2 SiO 4 at atmospheric pressure, after Osborne
and Tait (1952). The liquidus surface is represented as isotherms (=temperature 'contours' graduated in °C). V-ticks mark
10% graduations in mass % of each component. ' x ' and ' y ' are illustrative initial melt compositions discussed in the text.
Circular cartoons represent the 'phenocryst' mineralogy at key stages along each crystallization path, as seen under a
microscope. E represents a eutectic point (see text) and R represents a reaction point (cf. Box 2.6). The inset sketch from Gill
(2010) shows how the liquidus would appear as a 3D model (with temperature forming the vertical axis).
melt composition toward the An apex. In fact, because
the Fo-Di boundary is a 'thermal valley', the melt com-
position migrates with falling temperature along the
cotectic toward the point E - changing melt composi-
tion in any other direction would require a rise in its
temperature. Continued removal of Di + Fo eventually
drives the melt composition to the point E where the
three fields meet. Here anorthite begins to crystallize
alongside the forsterite and diopside (cartoon (c)).
Melt y will follow a different path as it crystallizes but
will arrive at the same final melt composition. Initial
crystallization of forsterite drives melt y to intersect the
An-Fo cotectic, at which point anorthite will begin to
crystallize alongside forsterite (see cartoon (d)). Diopside
will only appear when the melt has reached point E
(cartoon (c)). This point represents (a) the composition
at which the liquidus surface reaches its lowest tem-
perature, and (b) the composition toward which all melts
will converge as they crystallize, even melts whose
initial compositions lie in the anorthite or diopside
fields. It is called the ternary eutectic of this system.
Applying the Phase Rule to the composition y in
Figure 2.8 at a temperature above the liquidus yields:
Point y
( T > T liquidus )
melt alone
ϕ = 1
C = 3
CaAl 2 Si 2 O 8 + Mg 2 SiO 4 + CaMgSi 2 O 6
1 + F ʹ = 3 + 1
Therefore
a trivariant equilibrium
F ʹ = 3
The three degrees of freedom calculated here are
(a)  the  two compositional coordinates required to
define the position of y in the ternary diagram, plus (b)
temperature, which (like point m in Figure  2.4) is
unconstrained as long as the temperature remains
above the liquidus.
 
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