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combined with known concentrations of Ce in
abyssal peridotites, which are representative
of MORB source regions (Workman and Hart
2005 ), to provide an estimate of the water
content. Such studies indicate that the MORB
source region has water content between 50
and 200 ppm. Some authors have assumed
this concentration as representative of the
whole asthenosphere, with the exception of
the subduction zones, where the concentration
could be as high as 1 wt.% as a consequence
of slab dehydration (e.g., Hirschmann 2006 ;
Karato 2011 ). The best way to estimate the
effective water content across the asthenosphere
is through analysis of electrical conductivity,
because this quantity is much sensitive to the
presence of hydrogen (Karato 1990 ). Studies of
the electrical conductivity of the upper mantle in
the Pacific region (e.g., Shimizu et al. 2010 )
are compatible with the dry asthenosphere
conductivity profile computed by Karato ( 2011 )
and not with the observed H 2 O concentration of
the MORB source region. Below the 410-km
discontinuity, the theoretical and observed
conductivity profiles are in good agreement if the
concentration of H 2 Ois 1 wt.%. Therefore, the
distribution of water in the upper mantle suggests
a MORB source region close to the 410-km
discontinuity. In this instance, the only regions of
the oceanic asthenosphere above 300 km where
the concentration of H 2 O rises to 0.01 wt.%
will be the upwelling zones beneath mid-ocean
ridges. This model predicts a moderately wet
MORB source region confined to the lower
asthenosphere and an essentially dry upper
oceanic asthenosphere (Fig. 1.11 ). It also implies
hydration of the lower asthenosphere counterflow
by advection of wet material across the 410-km
discontinuity (Hirschmann 2006 ).
The high solubility of H 2 O in the transition
zone minerals suggests that this layer plays a
key role in the global water circulation. De-
spite the shallow dehydration of slabs at sub-
duction zones, which determines a large H 2 O
concentration ( 1%wt,Fig. 1.11 ) in the wedge
just above the slab, the rapidly sinking oceanic
lithosphere introduces a considerable amount of
water at greater depths, directly in the transition
zone. Fresh MORBs contain 0.1 % wt H 2 O
(Green et al. 2010 ), whereas the underlying man-
tle lithosphere peridotites are essentially dehy-
drated. However, the water content of both crust
and lithospheric mantle increases progressively
with the age, because of hydrothermal infiltration
of seawater within the oceanic crust or serpen-
tinization of mantle peridotites along fracture
zones and transform faults. Therefore, when the
lithosphere bends and starts sinking its water
content could be as high as 5-6 % wt at a depth of
10-20 km (Schmidt and Poli 1998 ). A consistent
part of this reservoir will be extracted at shal-
low depth, between 90 and 150 km, determining
extensive partial melting of the mantle wedge
and arc volcanism (Fig. 1.11 ). Schmidt and Poli
( 1998 ) estimated that the degree of dehydration
at this stage is between 18 and 37 %. A fraction
of the remaining part of the original reservoir
will be extracted at greater depth, where the
bulk water content of the slab could decrease
to 0.2-0.5 wt.%. Therefore, the subduction pro-
cess continuously injects H 2 O within the tran-
sition zone, where it can be temporarily stored
in high-pressure polymorphs of olivine, wads-
leyite, and ringwoodite. The steady equilibrium
of the Wilson cycle and the geological evidence
of stationary oscillations of the sea level clearly
exclude a progressive decrease of ocean water
at the Earth's surface. Thus, an upward water
flow from the transition zone to the overlying
asthenosphere, where it will be convoyed towards
upwelling flows, is necessary to obtain a global
mass balance.
The subduction process itself anyway requires
the upward advection of possibly wet material
across the 410-km discontinuity. When cold
oceanic lithosphere penetrates the transition
zone, it is generally subject to upward bending
and flattening just above the base of this
layer. These stagnant slabs tend to increase the
total volume of the transition zone, thereby a
corresponding volume will be pushed upwards
and will cross the 410-km discontinuity. In this
instance, the high H 2 O content will exceed the
storage
capacity
of
asthenosphere
minerals,
determining
hydrous
melting
(Hirschmann
2006 ).
This
mechanism
may
explain
the
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