Geoscience Reference
In-Depth Information
silicate melt of unknown water activity. This is
because in the deep mantle, silicate melts and
aqueous fluids are completely miscible, so that
the composition of a fluid phase coexisting with
minerals changes continuously from fluid-like to
melt-like as a function of temperature (e.g. Shen
& Keppler, 1997; Bureau & Keppler, 1999; Kessel
et al
., 2005). Only in situations where water activ-
ity is buffered by phase equilibria (e.g. Demouchy
et al
., 2005) a thorough thermodynamic treatment
of the data is possible. In the deep mantle, the be-
havior of water is therefore often best described
by the partition coefficient of water between solid
phases and melts.
In the transition zone, most water is stored
in wadsleyite and ringwoodite, the two high-
pressure polymorphs of olivine. Compared to
these two phases, majorite-rich garnet appears to
dissolve less water (Bolfan-Casanova
et al
., 2000).
The prediction by Smyth (1987) that wadsleyite
should be able to dissolve up to 3.3 wt % of
water was confirmed by later experimental work
(Inoue
et al
., 1995: 3.1 wt % H
2
O measured by
SIMS; Kohlstedt
et al
., 1996: 2.4 wt % measured
by FTIR). Polarized infrared spectra (Jacobsen
et al
., 2005) are consistent with the protonation
of the electrostatically underbonded O1 site, as
originally proposed by Smyth (1987). In contrast
to wadsleyite, the observation by Kohlstedt
et al
. (1996) that spinel-structure (Mg, Fe)
2
SiO
4
ringwoodite also dissolves up to 2.6 wt % of H
2
O
was unexpected, in particular considering that
aluminate spinel appears not to dissolve any
water. The dissolution of water in ringwoodite
may be related to Mg vacancies and/or to
partial disordering between Mg
2
+
and Si
4
+
,with
some Mg
2
+
substituting for Si
4
+
and charge
compensation by two protons (Smyth
et al
., 2003;
Kudoh
et al
., 2000). Several studies have shown
that the solubility of water in both wadsleyite
and ringwoodite decreases with temperature
above approximately 1300
◦
C, presumably due
to reduced water activity in the coexisting melt
phase. Litasov
et al
. (2011) estimate that the
maximum water solubility in wadsleyite along an
average mantle geotherm is about 0.4 wt %. The
pressure effect on water solubility in both phases
appears to be small. In equilibrium between wad-
sleyite and ringwoodite, water usually appears
to partition preferentially into wadsleyite with
D
wadsleyite/ringwoodite
of approximately 2 at 1450
◦
C;
however, this partition coefficient may decrease
with temperature (Demouchy
et al
., 2005).
Chen
et al
. (2002) report a partition coefficient
D
wadsleyite
/
olivine
5 for directly coexisting phases,
which is roughly in agreement with predictions
from solubility data. Deon
et al
. (2011) measured
D
wadsleyite
/
olivine
=
3
.
7andD
wadsleyite
/
ringwoodite
=
2
.
5. Similar data were reported by Inoue
et al
.
(2010) and by Litasov
et al
. (2011).
The two main phases of the lower mantle,
ferropericlase (Mg, Fe)O and MgSiO
3
perovskite
appear to dissolve very little water, suggesting
that the lower mantle may be largely dry
(Bolfan-Casanova, 2005). The solubility of water
in ferropericlase was systematically studied by
Bolfan Casanova
et al
. (2002). A maximum water
solubility of about 20 ppm H
2
O by weight was
observed at 25 GPa, 1200
◦
CandRe-ReO
2
buffer
conditions.
Both the work by Bolfan-Casanova
et al
. (2000)
and by Litasov
et al
. (2003) suggest that pure
MgSiO
3
perovskite dissolves very little water,
with maximum water solubilities in the order of a
few or at most a few tens of ppm. Bolfan-Casanova
et al
. (2003) observed hydrous (Mg, Fe)
2
SiO
4
ring-
woodite to coexist with nearly dry (Mg, Fe)SiO
3
perovskite, yielding a water partition coefficient
D
ringwodite
/
perovskite
=
1050.
There is some controversy surrounding wa-
ter solubility in Al-bearing MgSiO
3
perovskite.
Murakami
et al
. (2002) reported about 0.2 wt %
of water in magnesiumsilicate perovskite and
0.4 wt % water in amorphized calcium silicate
perovskite synthesized from a natural peridotite
composition at 25 GPa and 1600-1650
◦
C. Litasov
et al
. (2003) found up to 1400 ppm H
2
Oinper-
ovskites containing up to 7.2 wt % Al
2
O
3
,with
the water content increasing with Al. However,
in all these studies, the infrared spectra of the per-
ovskites show one very broad band centered near
3400 cm
−
1
, sometimes with some superimposed
=
