Geoscience Reference
In-Depth Information
anion is reduced proportional to 1/e; therefore,
in a solvent with e
changes of fluid properties are best rationalized
as a function of temperature and fluid density
(Burnham
et al
., 1969; Pitzer & Sterner, 1984),
rather than of temperature and pressure. This
implies that the most profound changes in fluid
properties actually occur in the 0-1 GPa range,
with more subtle changes at higher pressures.
SiO
2
is the most important solute in hydrous
fluids of the upper mantle. The solubility of SiO
2
in water has been well calibrated in several stud-
ies (e.g. Manning, 1984). The solution mechanism
was studied by in-situ Raman spectroscopy in a
hydrothermal diamond anvil cell by Zotov and
Keppler (2000; 2002). These data show that silica
is initially dissolved as orthosilicic acid H
4
SiO
4
at low concentrations, which then polymerizes
first to pyrosilicic acid dimers H
6
Si
2
O
7
and then
to higher polymers, as solubility increases with
pressure. This increasing polymerization of silica
in the fluid, together with the depolymerization
of silicate melts by water, provides an atomistic
explanation for the occurrence of complete misci-
bility in silicate-H
2
O systems. Viscosities in such
systems increase continuously with silicate con-
centration (Figure 1.5, above); however, most of
the increase occurs on the very silicate-rich side of
the system, so that fluids with moderately high
silicate content still retain very low viscosities
(Audetat & Keppler, 2005).
The composition of aqueous fluids coexisting
with MgSiO
3
enstatite and Mg
2
SiO
4
forsterite in
the system MgO-SiO
2
changes profoundly with
pressure; while at 1 GPa, the solute appears to
be rich in SiO
2
,theMgO
/
SiO
2
ratio of the fluid
as well as bulk solute concentration strongly in-
crease with pressure (Ryabchikov
et al
., 1982;
Stalder
et al
., 2001; Mibe
et al
., 2002).
High field strength elements (HFSE), i.e. ele-
ments that form cations with a high charge and
relatively small ionic radius, such as Ti
4
+
,Zr
4
+
,
Hf
4
+
,Nb
5
+
,andTa
5
+
, are generally poorly sol-
uble in water, even at upper mantle pressures
(Manning, 2004; Audetat & Keppler, 2005). As
an example, Figure 1.8 shows experimental data
on the solubility of TiO
2
in water. In the pres-
ence of silicates and aluminosilicates solutes, the
solubility of these elements increases, but it still
80, the attractive force
between the two ions is just about 1% of the force
expected in vacuum. This effect is responsible for
the strong dissociation and the associated high
solubility of many ionic solutes in water. At an
atomistic level, the high dielectric constant is
related to the polar nature of the angular H
2
O
molecules, which forms oriented layers around
ions in such a way that the preferred orientation
of the water molecules shields the electrical
fields. However, at 0.1-0.2 GPa and 700-800
◦
C,
the density of water (Burnham
et al
., 1969; Pitzer
& Sterner, 1994) is reduced to 0.2-0.4 g
/
cm
3
so that much less water molecules per volume
unit are available to shield electrical charges;
moreover the high temperature counteracts the
formation of oriented dipole layers. For this
reason, the dielectric constant of water under
these typical magmatic hydrothermal conditions
in the crust is only about 5, which is similar
to a chlorinated hydrocarbon under standard
conditions. Accordingly, the solvent behavior of
water changes dramatically. Salts, such as NaCl,
are not fully dissociated any more; rather they are
dissolved as ion pairs or as multiple ion clusters
and immiscibility between a water-rich vapor and
a salt-rich brine may occur (Quist & Marshall,
1968; Bodnar
et al
., 1985; Brodholt, 1998). HCl
is only weakly dissociated and only a weak acid
under these conditions (Frantz & Marshall, 1984).
Since ionic substances are primarily dissolved as
ion pairs, solubilities of cations strongly depend
on the availability of suitable counter ions that
allow the formation of stable ion pairs. This is
the reason why under magmatic-hydrothermal
conditions, the solubility of ore metals, such as
Cu or Au, very much depends on the availability
of halogens as ligands. The fluid/silicate melt
partition coefficients of Cu, for example, in-
creases by orders of magnitude in the presence of
a few wt % of NaCl or HCl in the fluid (Candela
& Holland, 1984; Keppler & Wyllie, 1991).
When pressure increases by several GPa to typ-
ical upper mantle conditions, dielectric constants
and ionic dissociation increase again, with
profound changes in the behavior of fluids. The
=
