Geoscience Reference
In-Depth Information
not only limited by their maximum P,T stability
field, but also by the availability of Na and K. At
normal mantle abundances, most, if not all, Na
and K may be dissolved in clinopyroxene (Harlow,
1997; Gasparik, 2003; Perchuk
et al
., 2002; Har-
low & Davies, 2004), so that these hydrous phases
will not form, even in the pressure range where
they may be stable for suitable bulk composi-
tions. The only hydrous phase that is stable along
an average mantle adiabat is phase X, a silicate
with an unusual layer structure containing Si
2
O
7
groups. The formula of phase X may be writ-
ten as (K, Na)
2
−
x
(Mg, Al)
2
Si
2
O
7
H
x
,wherex
solid solutions in multicomponent systems, each
phase decomposes over a range of pressures and
temperatures, and the decomposition reactions of
these phases overlap. Water released by decompo-
sition of hydrous phases in the slab likely causes
serpentinization of the shallow and cool parts of
the mantle wedge above the slab. Under some
circumstances, particularly for the subduction of
old oceanic lithosphere along a cool geotherm,
some of the serpentine in the ultramafic part of
the subducted lithosphere may escape decompo-
sition and may be able to transport water into the
deep mantle (R upke
et al
., 2004).
0-1
(Yang
et al
., 2001). The oceanic geotherm passes
through the thermodynamic stability fields of
phlogopite and Na-amphibole. The sodic amphi-
boles found in mantle samples are usually rich in
pargasite NaCa
2
Mg
4
Al
3
Si
6
O
22
(OH)
2
component,
often with a significant content of Ti (kaersutites).
However, due to the low abundance of alkalis in
the normal mantle, their occurrence in mantle
xenoliths is probably related to unusual chem-
ical environments affected by subduction zone
processes or mantle metasomatism. The same
applies for phlogopite KMg
3
(OH)
2
AlSi
3
O
10
.
Hydrous phases are stable in some parts of
subduction zones. The subducted slab contains
hydrous minerals in the sediment layer. In addi-
tion, the basaltic layer and parts of the underlying
peridotite may have been hydrated by contact
with seawater to some degree. Anomalies in heat
flow near mid-ocean ridges suggest that the up-
permost 2-5 km may be hydrated (Fehn
et al
.,
1983). However, much deeper hydration may oc-
cur by the development of permeable fractures
related to the bending of the slab when it enters
the subduction zone (Faccenda
et al
., 2009). For a
long time, it was believed that amphibole in the
basaltic layer is the major carrier of water into the
mantle and that the volcanic front in island arcs
is located above the zone of amphibole decompo-
sition. More recent work (Poli & Schmidt, 1995;
Schmidt & Poli, 1998), however, suggests that
several phases in the sedimentary, basaltic and
ultramafic parts of the slab are involved in trans-
porting water, including amphibole, lawsonite,
phengite and serpentine. Due to the formation of
=
1.3.2 Water in nominally anhydrous minerals
Already more than 50 years ago, it was noticed
that chemical analyses of nominally anhydrous
minerals occasionally suggest the presence of
traces of water (Brunner
et al
., 1961; Griggs &
Blacic, 1965; Wilkins & Sabine, 1973; Martin &
Donnay, 1972). However, since water is an ubiqui-
tous contaminant that may occur as mechanical
impurities in samples, the significance of these
observations was uncertain. This has changed by
the application of infrared spectroscopy to the
study of water in nominally anhydrous minerals,
a method that was pioneered by the groups of
Josef Zemann and Anton Beran in Vienna and by
George Rossman at Caltech (e.g. Beran, 1976; Be-
ran & Zemann, 1986; Bell & Rossman, 1992).
Figure 1.2 shows polarized infrared spectra of
an olivine crystal from the upper mantle. All
of the absorption bands in the range between
3000 to 3700 cm
-
1
in such spectra correspond to
OH groups (or, very rarely in a few minerals, to
molecular H
2
O). The intensity of absorption ob-
viously depends on the polarization, i.e. on the
orientation of the electrical field vector of polar-
ized light relative to the crystal structure. This
demonstrates that these samples contain ''water''
in the form of OH defects incorporated into the
crystal lattice. If the bands were due to some
mechanical impurities, a dependence of infrared
absorption on the orientation of the crystal lattice
would not be expected.
Infrared spectra are exceedingly sensitive to
traces of water in minerals and can detect OH
