Chemistry Reference
In-Depth Information
anhydrous system/
low water pressure
1553
T°C
T°C
L1
F1
L1
L2
F1
F2
L3
F3
L2
L4
F4
F2
F¹2
Ld
Fd
L¹3
F¹3
1120
high water pressure
L¹4
F¹4
NaAISi
3
O
8
CaAI
2
Si
2
O
8
CaAI
2
Si
2
O
8
NaAISi
3
O
8
anhydrous system
Figure 3.5
Albite - anorthite system (after Tuttle and Bowen, 1958).
In practice, we commonly observe oscillatory zonations and even (par-
tially) reverse zonations. This indicates that the system has not crystallized
in a closed system. Variations in water pressure, in particular, induce such
reverse recurrent zoning (Figure 3.5). Suppose that at L2, occurs an increase
in water fugacity (high f
H
2
O
). Liquidus and solidus are lowered so that the
plagioclase in equilibrium with the liquid L2 will be F
2 plagioclase that is
much more calcic than the plagioclase F2. Strictly speaking, if the change in
water fugacity is sudden, the liquid L2 is no longer at the liquidus and resop-
tion of the already crystallized plagioclase may occur. The temperature must
again decrease so that this liquid reaches in L
′
′
3 the liquidus at high water
pressure: it then F
3 crystallizes feldspar. In fact, during the fractional crys-
tallization of magma, water fugacity increases gradually producing a reverse
zoning of the plagioclase which becomes more and more calcic toward its
rim. Then, the evolution of the liquid occurs along the liquidus, thus pro-
ducing a normal zonation F
′
4. On the other hand, abrupt departure
of fluid, and thus abrupt decrease of water pressure, can occur during the
crystallization of magma.
′
3-F
′
Binary system of alkali feldspars: albite (Ab) -
potassium feldspar (FK) system
Under low water pressure, the field of the liquid (:melt) in the phase diagram of
the system albite - potassic feldspar is entirely located above the solvus. Such
evolution is called
hypersolvus
crystallization. In this binary system, the albite
that crystallizes, is enriched in K-feldspar constituent; If potassium feldspar
crystalizes, it is enriched in Na-constituent. Fractional crystallization produces
