Geoscience Reference
In-Depth Information
Table 1.3 Thermodynamic models for water solubility in minerals.
Mineral
V solid
H 1bar
A
n
Reference
(ppm/bar n )
(cm 3 / mol)
(kJ / mol)
Olivine
0.0066
1
10.6
-
Kohlstedt et al . (1996)
0.0147 a
1
10.2
-
Mosenfelder et al . (2006)
0.54
1
10.0
50
Zhao et al . (2004) b
MgSiO 3 enstatite
0.0135
1
12.1
4.56
Mierdel & Keppler (2004)
Aluminous enstatite c
0.042
0.5
11.3
79.7
Mierdel et al . (2007)
Jadeite
7.144
0.5
8.02
-
Bromiley & Keppler (2004)
Cr-diopside d
2.15
0.5
7.43
-
Bromiley et al . (2004)
Pyrope
0.679
0.5
5.71
-
Lu & Keppler (1997)
Ferropericlase
0.0004
0.5
4.0
-
Bolfan-Casanova et al . (2002)
Notes
Tabulated parameters refer to Equation (1.2). Where no value for H 1 bar is given, the enthalpy term is missing because the temperature
dependence of water solubility was not calibrated and the equations are strictly valid only at the temperatures they were calibrated.
a Recalculated from a value of A = 2 . 45 H / 10 6 Si / MPa which in the original publication is probably misprinted as A = 2 . 45 H / 10 6 Si / GPa.
b The equation by Zhao et al . (2004) also includes a term exp ( α x Fa / RT), where a is 97 kJ/mol and x Fa is the molar fraction of fayalite.
c This equation gives the water solubility couples to Al of an Al-saturated enstatite. In order to get the total water solubility in Al-saturated
enstatite, the water solubility in pure MgSiO 3 according to Mierdel et al . (2007) has to be added.
d These data may reflect metastable equilibria.
Since water fugacity increases with pressure,
one would normally expect that at higher pres-
sures, more water is dissolved in minerals. How-
ever, the term V solid in Equation (1.2) is usually
positive and therefore counteracts the effect of
increasing water fugacity. For this reason, water
solubility in minerals usually first increases with
pressure, and then decreases again. Depending on
the sign and magnitude of H , water solubility
may either increase or decrease with temperature.
The partition coefficient of water between two
phases α and β can be described as the ratio of the
water solubilities in the two phases (Keppler &
Bolfan-Casanova, 2006):
An important consequence of Equation (1.3) is
that the partition coefficient may depend on water
fugacity at given P and T if the two exponents n
for the two phases are not equal. In other words, in
such a case the partition coefficient may vary with
bulk water content (e.g. Dai & Karato, 2009a).
Water storage in the upper mantle is largely
controlled by olivine and orthopyroxene. Water
solubility in garnet appears to be relatively low
(Lu & Keppler, 1997) and no trace of water has ever
been detected in the MgAl 2 O 4 spinel phase of the
upper mantle. Clinopyroxenes from mantle xeno-
liths may contain twice more water than coexist-
ing orthopyroxenes (Skogby, 2006), but the very
low modal abundance of clinopyroxene implies
that it is not a major repository of water in the
mantle. Clinoyroxene (omphacite) may, however,
be very important for recycling water deep into
the lower mantle in subducting slabs; this idea is
consistent with the observation that the highest
water contents from xenolith samples are usually
found in eclogitic omphacites (Skogby, 2006).
Water solubility in olivine has been extensively
studied (Kohlstedt et al ., 1996; Zhao et al ., 2004;
c water
c water =
A α
A β
f n α n β
H 2 O
D α/β
water
=
H 1 bar
α
H 1 bar
β
+
( V solid
α
V solid
β
) P
×
exp
.
RT
(1.3)
Search WWH ::




Custom Search