Volatiles under High Pressure - Physics and Chemistry of the Deep Earth - page 13

weak and sharp peaks. A number of observations
suggests that the broad band, which accounts
for most of the water found in aluminous per-
ovskites, is due to some mechanical impurities
(i.e. inclusions of hydrous phases) and that the
true water solubility in aluminous perovskites is
very low. In particular, Bolfan-Casanova et al .
(2003) showed that the broad infrared absorp-
tion band is only observed in perovskite samples
that appear milky under the microscope. These
samples also show Raman bands of superhydrous
phase B and the infrared peaks of this phase co-
incide with the main infrared bands reported for
aluminous perovskite. If only perfectly clear parts
of aluminous perovskite are analyzed by infrared
spectroscopy, observed water contents are very
low and comparable to those observed for pure
MgSiO 3 perovskite.

with pressure and at low pressures, the solubility
of often proportion to the square root of water
fugacity (McMillan, 1994). There are subtle differ-
ences in water solubility between felsic and basic
melts and even for felsic melts (e.g. Dixon et al .,
1995; Holtz et al ., 1995; Shishkina et al ., 2010),
parameters such as the Na/K ratio can affect wa-
ter solubility (Figure 1.4). In general, however,
the solubility at given pressure and temperature
is broadly similar for a wide range of melt compo-
sitions. Water solubility in melts can be measured
with high precision, if melts can be quenched to
bubble-free glasses, which may be analyzed by a
variety of methods, including Karl-Fischer titra-
tion, infrared spectroscopy or SIMS (e.g. Behrens,
1995). However, for ultrabasic melts, for basic
melts with high water contents and for any water-
saturated melt beyond about 1 GPa, quenching to
a homogeneous glass is usually not possible any
more. Accordingly, while water solubility under
subvolcanic conditions in the crust is very well
known, it is only poorly constrained for melts in
the deep mantle.
Because of the square root dependence of wa-
ter solubility on water fugacity, it was believed

1.3.3 Water in silicate melts

Water is highly soluble in silicate melts and the
associated melting point depression is essential
for melting in subduction zones and at other lo-
cations in crust and mantle (e.g. Tuttle & Bowen,
1958; Kushiro, 1972). Water solubility increases

16

14

12

Haplogranite, 800 ° C

10

8

6

Tholeitic basalt, 1250 ° C

4

2

0

0 100 200 300 400 500 600 700 800 900

Pressure (MPa)

Fig. 1.4 Water solubility in haplogranitic melt at 800 ◦ C(Holtz et al ., 1995) and in tholeitic basalt at 1250 ◦ C
(Shishkina et al ., 2010).