Geoscience Reference
In-Depth Information
(e.g. Gavrilenko
et al
., 2010) - is associated with
the dissolution of water as OH point defects in
silicates. As such, the effect of increasing water
contents is qualitatively similar to the effect of in-
creasing temperature. However, the ratio of P and
S wave velocities v
p
/
v
s
appears to be particularly
sensitive to water as water in minerals affects
S wave velocities much more than P wave ve-
locities. The v
p
/
v
s
ratio may therefore be able to
distinguish the effects of water from temperature,
although the effects may be subtle (Karato, 2011).
A property that is particularly sensitive to
water is electrical conductivity, which may be
greatly enhanced by proton conduction (Karato,
1990) (see also Chapter 5 of this topic). Therefore,
numerous studies on the effect of water on
minerals such as wadslyeite and ringwoodite
have recently been carried out, since these two
main constituents of the Earth's transition zone
areabletodissolvemorethan2-3wt%ofwater
(Smyth, 1987; Kohlstedt
et al
., 1996). Although
there are some significant discrepancies between
the available studies (Huang
et al
., 2005; Yoshino
et al
., 2008), it appears that a fully hydrated transi-
tion zone containing several wt % of water can be
ruled out. The data may, however, be compatible
with water concentrations up to 1000 or 2000 ppm
by weight (Huang
et al
., 2005; Dai & Karato,
2009b). Such concentrations, if confirmed, would
imply that the transition zone is the most
important reservoir of water in the mantle,
containing roughly 0.5 ocean masses of water.
Experimental data on the stability range of
hydrous mantle minerals (Frost, 2006) are com-
piled in Figure 1.1. At low temperatures and
high pressures, a variety of dense hydrous magne-
sium silicates (DHMS) are stable, including phase
A, D, E, and superhydrous phase B. However,
Figure 1.1 also shows clearly that the stabil-
ity range of these phases is far away from a
normal mantle adiabat; they may perhaps exist
in cold subducted slabs, but not in the nor-
mal mantle. Of all the DHMS phases, phase D
with composition MgSi
2
O
4
(OH)
2
is stable at the
highest pressures. A new aluminum-rich version
of phase D has recently been described (Boffa-
Ballaran
et al
., 2010). All other hydrous phases
shown in Figure 1.1 also contain some Na and/or
K. Both alkali elements occur in normal mantle
peridotite only at a minor or trace element level
(typically about 0.3 wt % Na
2
O and 0.03 wt %
K
2
O). The occurrence of these phases is therefore
25
700
D
SB
Phase X out
600
20
DHMS
Phase X
500
E
15
400
A
10
300
K-richterite
1.3
Water
K-
richterite in
200
1.3.1 Hydrous minerals in the mantle
5
Phlogopite
According to available phase equilibria studies
(e.g. Gasparik, 2003), the bulk of the Earth's man-
tle is made up by nominally anhydrous minerals,
i.e. silicates and oxides that do not contain any
H
2
O or OH in their formula. Mantle xenoliths,
however, while they mostly consist of olivine,
pyroxenes, and garnet or spinel, sometimes do
contain some hydrous minerals such as amphi-
boles or phlogopite-rich micas (Nixon, 1987),
indicating that these phases may be stable in
the mantle under some circumstances.
100
Na-amphibole
0
900
1000
1100 1200 1300 1400 1500
1600
1700
Temperature(
°
C)
Fig. 1.1
Stability range of hydrous minerals in Earth's
mantle. AMA is an average mantle adiabat, OL is a
geotherm for a 100 million year old oceanic
lithosphere. The thick black line gives the tentative
location of the water-saturated peridotite solidus.
Modified after Frost (2006).
