Geoscience Reference
In-Depth Information
13
Fluid Processes in Subduction
Zones and Water Transport
to the Deep Mantle
HIKARU IWAMORI
1
AND TOMOEKI NAKAKUKI
2
1
Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan.
2
Department of Earth and Planetary Systems Science, Hiroshima University,
Higashi-Hiroshima, Japan.
Summary
which may explain an extensive hydrous plume
magmatism in the Japanese backarc and the
mainland Eurasia. On the other hand, when the
subducting slab penetrates into the lower mantle,
the hydrous boundary layer releases fluids at
660 km depth since the maximum H
2
Ocontent
of the lower mantle is low. However, considering
a critical segregation velocity of fluid from the
descending mantle flow, an appreciable amount
(several 1000 ppm) of H
2
O may subduct into the
lower mantle. Geochemical observations and
their statistical analyses suggest that the mantle
compositional variability of five-dimensional
space with Sr-Nd-Pb isotopes has been created by
only two independent differentiation processes,
and that one of the two processes is related to
aqueous fluid-rock interactions. Based on this
analysis, global geographical domains inherited
with subducted aqueous fluid components have
been
Water may play an important role in the Earth's
evolution, yet its distribution and circulation are
poorly constrained. Subduction zones are impor-
tant as entrance of the water circulation, where
H
2
O-bearing phase relations and mechanisms
of fluid migration are key to understanding the
water flux to the mantle transition zone and
the lower mantle. The phase relation coupled
with predicted thermal structure suggests that
extensive dehydration of the subducting slab,
including a serpentinite layer just above the
slab, occurs at the depths shallower than the
choke point (a cusp between the stability fields
of serpentine and phase A in a peridotitic bulk
composition at
200 km depth). The overlying
corner region are heavily hydrated to form
a magmatic-hydrothermal arc, where porous
and channel flows seem dominant over the
Rayleigh-Taylor instability or diapiric ascent,
based on the scaling analysis as well as the
seismic observations. The solid corner flow
passing through this extensive hydrated region is
predicted to form a hydrous boundary layer at the
base of the mantle wedge, even after breakdown
of the major hydrous phases (
>
200 km depth).
This layer is seismologically observed as a
low
V
S
layer, transporting 0.1 to 0.4 wt % H
2
O
to the mantle transition zone, and becomes
gravitationally unstable along the stagnant slab,
∼
detected,
indicating
focused
subduction
beneath the past supercontinents.
13.1
Introduction
Water within the Earth, including H
2
O, OH and
H in fluids and mineral phases, affects signif-
icantly the physico-chemical properties of the
Earth's interior, such as density, elastic proper-
ties, transport properties, and in particular, melt-
ing temperature and rheology (e.g., Green, 1973;
