Composition of the Crust and the Mantle - Quantitative Plate Tectonics - page 17

Fig. 1.11 Large-scale asthenosphere circulation in the
oceanic domain and cycle of water in the upper mantle.
Blue lines represent transport of H 2 O, while numbers in-
dicate the estimated water content (% wt). Red lines show

upwellings, downwellings, flows, and counterflows in the
asthenosphere. Light and dark green regions represent, re-
spectively, mantle lithosphere and crust that independently
carry variable amounts of water

MORB. When the oceanic lithosphere moves
away from a ridge axis, it generates laminar
flows within the upper asthenosphere by exert-
ing viscous drag forces. These shallow flows,
which diverge from mid-ocean ridges, must be
compensated by counterflows within the deep
asthenosphere that are directed towards the man-
tle upwelling zones (Chase 1979 ). Although An-
derson and Bass ( 1986 ) argued that the sources
of MORB reside within the transition zone (be-
tween the 410 and the 670 km discontinuities),
there are three independent classes of observa-
tion suggesting a shallower source. The first one
is that the viscosity in the transition zone is
too high to allow an effective counterflow. In
other words, even assuming a participation of this
layer to the return flow, the viscosity contrast
with the asthenosphere is so high that most of
the counterflow will be concentrated anyway in
the lower asthenosphere, as we shall prove in
Chap. 13 . The second class of observations is
represented by seismic tomography models of the
asthenosphere beneath ridges, which do not show
evidence of vertical motion below 300 km (e.g.,
Gu et al. 2005 ). Finally, the distribution of water
within the mantle and the balance of water flow
between crust and mantle lead to conclude that
the probable source of MORB is just above the

410-km discontinuity in the lower asthenosphere.
This point will be discussed in detail below.
In the Earth's mantle, H 2 O can either be stored
in solid minerals or be present as hydrous fluids
or melts. Both the instantaneous distribution and
the large-scale transport of this element represent
key aspects of plate tectonics. The distribution is
clearly controlled by the flows, but it also depends
from the storage capacity of the mantle minerals.
The storage capacity of the asthenosphere is be-
tween 0.1 and 1 wt.% (Hirschmann 2006 ), that
of the lower mantle is much lower (<20 ppm
H 2 O). Conversely, the transition zone may have a
relevant storage capacity of 0.5 wt.% at 1,600 ı C
and transition zone pressures (Ohtani et al. 2001 ).
Regarding the water transport, diffusion cannot
be a viable mechanism, because the diffusion
distance is only 10 km Gy 1 . Therefore, the
relevant mechanisms of transport are the subduc-
tion of oceanic lithosphere and partial melting.
In general, the former brings water downwards
to the asthenosphere and the transition zone, the
latter drives water upwards to the crust, as shown
in Fig. 1.11 .
Water can be considered as an incompatible
element during partial melting, having strong
similarity with Ce. Therefore, the measured

ratio H 2 O/Ce in undegassed basalts can be