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continuous exchange of Na, Ca, Al and Si, to re-equili-
brate with later, more albite-rich fractions of the melt.
Thus to maintain complete equilibrium as the melt
evolves to point
a
1
on the liquidus, all the crystals that
have so far accumulated must adjust their composition
to
b
1
on the solidus. Such adjustment requires solid-state
diffusion to and from the centre of each crystal, and is a
slow process. Crystal growth during natural magma
crystallization commonly proceeds too quickly to allow
complete continuous equilibrium between crystals and
magma: the surface layers readjust to changing melt
composition, but the crystal interiors fall behind. The
result is a compositional gradient between relatively
anorthite-rich cores and relatively albite-rich margins of
the plagioclase crystals, a phenomenon known among
petrologists as
zoning
(Box 3.1). Zoning is seen in other
mineral groups as well, notably pyroxene.
If crystallization proceeds slowly enough to permit
continuous re-equilibration (an ideal situation known
as
equilibrium crystallization
), the final melt has the
composition
a
2
and the end-product is a mass of crys-
tals all having the composition
b
n
, the same as the
initial melt composition
m
. Imperfect re-equilibration
between crystals and the evolving melt, however, ties
up a disproportionate amount of the anorthite compo-
nent in the cores of early formed crystals owing to
overgrowth or burial, and the melt can then evolve to
compositions beyond
a
n
before it runs out, creating late
zones of plagioclase more albitic than the original melt.
This process, in which isolation of early-formed solids
allows later melts to develop to more extreme compo-
sitions, is called
fractional crystallization
. Crystallization
of natural melts in crustal magma chambers approxi-
mates closely to fractional crystallization, and this pro-
cess contributes a lot to the chemical diversity of
igneous rocks and magmas.
m
1500
Melt
a
b
1400
a
1
b
1
1300
a
n
b
n
Crystals of plagioclase
ss
1200
1100
0
20
40
60
80
100
NaAlSi
3
O
8
(Ab component)
CaAl
2
Si
2
O
8
(An component)
Percent CaAl
2
Si
2
O
8
by mass
Figure 2.5
T-X
diagram showing melting relations in
plagioclase feldspars at atmospheric pressure. Horizontal
ruling represents a two-phase field. The subscript 'ss'
denotes that plagioclase is a solid solution (see text).
example of a univariant field (cf. Figure 2.4):
ϕ
= 2,
C
= 2,
therefore
F
′ = 1. If equilibrium exists between melt and
plagioclase, specifying
T
automatically defines the
compositions of both phases. Conversely, knowing
either phase composition defines the other composi-
tion and the temperature of equilibrium unambigu-
ously. The line
ab
is one of an infinite series of such
tie-lines traversing the two-phase field, as the horizon-
tal ruling symbolizes.
Crystallization in this system leads to a series of con-
tinuously changing melt and solid compositions. Melt
m
, for example, will cool until it encounters the liquidus
curve at
a
, where plagioclase
b
will begin to crystallize.
Because
b
is more CaAl
2
Si
2
O
8
-rich than
a
, its extraction
will deplete the melt in CaAl
2
Si
2
O
6
and thereby enrich it
in NaAlSi
3
O
8
; with continued cooling and crystalliz-
ation the melt composition will migrate down the liqui-
dus curve. The changing melt composition causes a
corresponding evolution in the equilibrium composi-
tion of the plagioclase crystals. Not only will newly
crystallized plagioclase be more albite-rich than
b
, but
there will be a tendency for early-formed crystals, by
The solvus and exsolution
The final
T-X
section to be examined (Figure 2.6b)
shows the phase relations of the alkali feldspars in the
presence of water vapour (at a pressure of 2 × 10
8
Pa = 2 kbars). This diagram can be visualized as a cross-
section of
PTX
HO
2
−− space, coinciding with the plane
210
8
in which
P
HO
=× (Figure 2.6a). One can speak
of the diagram as an
isobaric
T-X section
of phase
relations in
PTX
HO
2
Pa
2
−− space.
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