Pletchov P.Yu. and Gerya T.V. Effect of
H2O on plagioclase-melt equilibrium.
key words [plagioclase
crystallization.]
melt equilibrium magmatic
The plagioclase-melt equilibrium is well
investigated in anhydrous systems (Drake, 1976;
Ariskin, Barmina, 1990; Pajasawatwong et al., 1995).
Many authors mention that an increase in the water
content in the melt results in a considerable (by 20-40
numbers) shift of compositions of crystallizing
plagioclases to a more Ca region (for example, Housh,
Luhr, 1991; Danyshevsky et al., 1997).
This model is based on the exchange equilibrium of
Ca and Na between co-existing plagioclase and melt
under both dry and water-saturated conditions.
The exchange equilibrium constant (KD) is:
KD 
This model describes only the exchange
equilibrium of Na and Ca between plagioclase and melt
and does not include the crystallization reaction of
plagioclase from the melt. At specified intense
parameters (T, P, ZH2O) by the Pl composition, the
model allows one to calculate Ca# of the melt and vice
versa. In addition, the water content in the melt can be
estimated from the co-existing melt-plagioclase. This
exchange equilibrium does not allow one to estimate
with a sufficient accuracy the temperature and pressure.
Other models based on equations of crystallization
equilibrium (for example, Ariskin and Barmina, 1990)
can be used to estimate the crystallization temperature
of plagioclase.
The convergence of the calculated and experimental
data is presented in Fig. 1.
1.0
Dry
Pl
L
X Ca
 (1  X Ca
)
L
Pl
X Ca
 (1  X Ca
)
On the other hand, the equilibrium of the reaction is
achieved when is minimum of the free Gibbs energy.
The latter conditions can be presented in the simplified
form as follows:
ΔGo + ΔGH2O + GeAn - GeAb = -RTln(KD),
Water-saturated
0.8
measured X(Ca)melt
Experiment in GeoSciences,1998, V7 N2, pp.7-9
Bindeman,1998
0.6
0.4
0.2
(1)
0.0
where ΔGo= ΔH – TΔS + PΔV
ΔGH2O= ZH2O (ΔHH2O - ΔSH2OT + ΔVH2OP),
(2)
0
0.2
-0.40
0.00
0.20
0.4
0.6
0.8
1
0.40
0.60
0.80
1.00
calculated X(Ca)melt
0.40
where ZH2O = XH2O/XH2O(sat)
The excessive energies for a solid solution of
plagioclases (Perchuk et al., 1990) are:
GeAn = 2W21XAn(XAb)2 + W12·[(XAb)3 - XAn(XAb)2]
(3)
X(calc)-X(real)
0.30
0.20
0.10
0.00
-0.10
-0.20
-0.30
GeAb = 2W12XAb(XAn)2 + W21·[(XAn)3 - XAb(XAn)2]
(4)
W12= 1980-1.526T, W21= 6860-3.877T.
(5)
X(Ca)Plag
We obtained the general equation for this reaction
with unknown coefficients ΔH (reaction enthalpy), ΔS
(reaction entropy), ΔV (volume effect of the reaction),
ΔHH2O, ΔSH2O, and ΔVH2O (effect of water to enthalpy,
entropy, and volume, respectively). The authors
obtained the following coefficients by the least-squares
method from a set of 198 experimental melt-plagioclase
pairs (INFOREX, Ariskin et al., 1997):
ΔH=-21.91 cal
ΔHH2O=-4610 cal
ΔS=0.0807 cal/K
ΔSH2O=-1.084 cal/K
ΔV=-0.0392 cal/bar
ΔVH2O= 0.182 cal/bar
Fig.1. Convergence of calculated (equations 1-5)
and experimental data.
To check calibration of the exchange reaction, Fig.
1a was supplemented with additional data (Bindeman et
al., 1998), which were not included in the data set
according to which the model was calibrated. As can be
seen in Fig. 1, these data agree well with the calculated
data. Absolute errors for the calculation of Ca#
(Ca/Ca+Na) of the melts are shown in Fig. 1b. It is well
seen that a calculation error is constant for a wide
spectrum of compositions of plagioclases.
Calculated lines of the plagioclase-melt equilibrium
for P = 1 kbar and T = 1000oC in dry and watersaturated systems are shown in Fig. 2. The composition
of plagioclase crystallizing from the water-saturated
system is strongly enriched in the An component as
compared to the dry system (to 40 numbers of Al at Ca#
of
the
melt
0.4-0.6).
The model calculated makes it possible to estimate
the plagioclase-melt equilibrium under dry and watersaturated conditions and to calculate the calcium
content in the melt equilibrium to the specified
plagioclase. This is necessary, for example, in studying
melt inclusions in plagioclases when their equilibrium
state with the mineral-host is estimated.
P=1 kbar, T=1000 C
1
X(Ca) melt
0.8
dry
system
0.6
0.4
References :
watersaturated
0.2
0
0
0.2
0.4
0.6
0.8
X(An)
Fig. 2. Calculated lines of the plagioclase-melt
equilibrium for P = 1 kbar and T = 1000oC in the
dry and water-saturated systems.
The calculated diagram for P = 1 kbar and natural
co-existing plagioclase-melt pairs are compared in
Fig.3. The data of Danyushevsky and co-authors
(Danyushevsky et al., 1997) represents homogenized
melt inclusions in plagioclase of high-calcium boninites
of the Tonga channel. Temperatures (1040-1080oC),
H2O content (close to water-saturated ones: ~1.5%), and
crystallization pressures (0.25 kbar) were determined
for them.
P=1 kbar, T=600,800,1000,1200 C, watersaturated system
1
Pl melt inclusions from Tonga boninites,
Danyushevsky et al.,1997
X(Ca) melt
0.8
Pl melt inclusions from Klyuchevskoy basalts,
Pletchov et al., 1998
0.6
0.4
0.2
0
0
0.2
0.4
0.6
X(An)
0.8
1
Fig. 3. Natural data on melt inclusions in plagioclase
on the calculated (equations 1-5) diagram.
The data of Danyushevsky are arranged in the
diagram along the isotherm of 1000oC and agree well
with the model discussed. The data of Pletchov and coauthors (Pletchov et al., 1998) reflects the compositions
of naturally quenched melt inclusions in plagioclase
from basalts of the Klyuchevskoy volcano. The lowestcalcium group of the points represents co-existing
interstitial glass-microlyte of plagioclase in the
groundmass.
The differential trend of basalts with a gradual
decrease in Ca# of the melt and An of plagioclase as the
temperature decreases is well seen in diagram 3.
1
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boninites:
implications
for
plagioclase-melt
equilibria at low P(H2O).” Canadian Mineralogist
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