Marine Ecosystems Research Department - jamstec japan agency ...

Japan Marine Science and Technology Center
Institute for Frontier Research on Earth Evolution (IFREE)
Research Program for Geochemical Evolution
1. Research Overview
The crust and mantle, which are composed of silicate
minerals, represent % of the Earth's mass.
Although the crust is volumetrically insignificant in
the solid Earth, it contains a large fraction of the elements
that preferentially enter the melt when a silicate
is melted. High concentrations of these elements in
such a small volume mean that the Earth is an extensively
differentiated body. While the Earth's mantle is
much more homogeneous in compositions than the
crust, it has been well established that at least four,
compositionally distinct components are required to
explain the isotopic compositions of mid-ocean ridge
basalts (MORBs) and lavas of ocean islands that are
built by the activity of deep-seated hotspots.
Understanding the origin of such geochemical endmembers
in the crust and mantle is essential to document
the evolution of the solid Earth. The Research
Program for Geochemical Evolution aims to seek an
understanding of the evolutionary processes of the
solid Earth based on comprehensive studies of solid
earth materials.
2. Some Research Results for Fiscal year 2002
2.1. The role of the subduction factory - the evolution
of the Earth's mantle
Subduction zones, where oceanic lithosphere is
foundering into the Earth's interior, have been working
as factories and have contributed significantly to the
evolution of the solid Earth. Raw materials, such as
pelagic or terrigenous sediment, oceanic crust, and
mantle lithosphere, are supplied into the factory
(Fig.). In the process of transportation and processing
of these raw materials, the factory causes vibrations
as earthquakes. The major products of the factory
are arc magmas and their solidified materials, continental
crust. The waste materials processed in the subduction
factory, such as chemically modified lithosphere
and delaminated lower continental crust, sink
into the deep mantle (Fig.).
continental
crust
mantle
wedge
residual
materials
volatiles
volcanoes
earthquake
oceanic
materials
Raw Materials
- oceanic material
- mantle material
Products
- magma/volcanoes
- volatiles
- continental crust
Residues
- chemically modified slab
- delaminated lower crust
Fig.13 The processes occurring in the subduction factory. Raw
materials, such as oceanic sediments, oceanic crust, and
mantle lithosphere, are fed into the factory and are manufactured
into arc magmas and continental crust. The
waste materials processed in this factory, such as chemically
modified oceanic crust/sediments and delaminated
lower continental crust, sink into the deep mantle and are
likely to have greatly contributed to the mantle evolution.
Dehydration reactions within subducting hydrated
basaltic crust occur continuously from very shallow
levels to over km depth, but experimental studies
on trace element behavior during dehydration are
generally limited to those related to the amphiboliteeclogite
transformation and element partitioning
between aqueous fluids and garnet/clinopyroxene.
A notable feature demonstrated by these experiments
is that Pb is more preferentially partitioned into
H
O fluids than U and Th, leaving the residue, after
dehydration, with higher U/Pb and Th/Pb than its
original composition. It has also been demonstrated
that Rb and Nd are released from the subducting crust
more readily than Sr and Sm. Thus, residual basaltic
crust after the amphibolite-eclogite transformation
will have lower Sr/ Sr, higher Nd/ Nd, and
higher Pb/ Pb values than hydrated basaltic crust
(Fig.). Pb/ Pb ratios of the dehydrated residue
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