Geoscience Reference
In-Depth Information
T
T
T
critical point
melt
critical point
melt
+
fluid
Qtz
melt
melt
fluid
Qtz
melt
melt
+
fluid
fluid
fluid
Qtz
+
fluid
Qtz
+
fluid
Qtz
+
fluid
SiO
2
SiO
2
SiO
2
H
2
O
H
2
O
H
2
O
Fig. 1.7
Melting relationships in the SiO
2
-H
2
O system, schematic. Left: Melting below the critical curve; right:
melting above the critical curve. After Keppler and Audetat (2005). Shown are the phase relationships for three
different pressures, with pressure increasing from the left to the right diagram. Reprinted with permission from the
European Mineralogical Union.
of an excess fluid phase occurs at a defined
water-saturated solidus temperature, at which
hydrous melt, fluid and solid silicate coexist.
This is the situation on the left-hand side of
Figure 1.7. A miscibility gap between the melt
and the fluid allows both phases to coexist. How-
ever, with increasing pressure, the miscibility gap
is getting smaller and finally disappears. In the
absence of a miscibility gap (Figure 1.7, right di-
agram), a fluid of variable composition coexists
with a solid at any temperature. The fluid com-
position is very water-rich at low temperature;
with increasing temperature, the solubility of sil-
icate in the fluid increases until it finally reaches
the composition and properties of a hydrous sil-
icate melt. Due to the continuous change from
a ''fluid-like'' to a ''melt-like'' phase, the solidus
temperature cannot be defined any more. In any
binary silicate-H
2
O system, a critical curve can
be defined, which for a given pressure specifies
the maximum temperature under which a melt
and a fluid phase may coexist. The intersection of
this line with the water-saturated solidus curve
gives a critical endpoint at which the solidus
curve ceases. Beyond the pressure of this crit-
ical endpoint, a water-saturated solidus cannot
be defined any more and melting phase rela-
tionships resemble the situation shown on the
right-hand side of Figure 1.7. Bureau and Keppler
(1999) have mapped out critical curves for several
felsic compositions; they suggest that in these
systems the water-saturated solidus terminates
at about 1.5-2 GPa. However, the location of the
critical endpoint in basic to ultrabasic systems
is not yet well constrained; it may occur some-
where in the 4-6 GPa range (Mibe
et al
., 2004;
Kessel
et al
., 2005).
1.3.4 Hydrous fluids
Hydrous fluids are important agents of mass
transport in subduction zones (e.g. Manning,
2004) and they are responsible for the formation
of many economically important hydrothermal
ore deposits (Hedenquist & Lowenstern, 1994).
The properties of water under typical subvolcanic
hydrothermal conditions, e.g. at 0.1 0.2 GPa
and at 700-800
◦
C are very different from the
water known at ambient conditions (Eugster,
1986; Franck, 1987). Liquid water under standard
conditions is an extremely good solvent for ionic
species; salts such as NaCl are highly soluble
in water and they are practically completely
dissociated into Na
+
and Cl
−
ions. Similarly,
HCl dissolved in water is a very strong acid
with nearly complete dissolution into (hydrated)
protons and Cl
−
anions. This is due to the high
dielectric constant (e
80) of water under stan-
dard conditions. According to the Coulomb law,
the attractive force between a pair of cation and
=
