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GEOFLUID PROCESSES IN SUBDUCTION ZONES AND MANTLE DYNAMICS
E. Takahashi1*, T. Yokoyama1, M. Kanzaki2 and T. Okuchi2 1Department of Earth and Planetary Sciences, Tokyo
Institute of Technology, Tokyo 152-8551, Japan. 2Institute for Study of the Earth’s Interior, Okayama University,
Misasa, Tottori 682-0193, Japan. *email: [email protected]
Introduction: Fluids within the Earth, such as
slab-derived fluids in subduction zones, migrate and
spread widely in the mantle and crust as various forms
of aqueous fluid, gas-rich fluid, melt and supercritical
fluid (Fig. 1). Recent progress in observational, experimental and theoretical research concerning
"Geofluids" highlights their importance in a variety of
geodynamic processes. This symposium aims to integrate a wide range of scientific approaches concerning
geofluids and their related processes (e.g. earthquakes,
magmatism, metamorphism, hydrothermal activity) in
subduction zones and other geodynamic settings. The
goal is to better understand the origin and physical/chemical properties of geofluids as well as their
roles in geodynamic processes [1,2].
Experimental: We have precisely measure Os
isotopes in bulk samples of carbonaceous (Renazzo,
EET92042, Tagish Lake, Murchison and Allende),
enstatite (Yilmia and Pillistfer) and ordinary (Dhajala
and Allegan) chondrites. These samples were processed using an alkaline fusion total digestion technique (AF) which decomposes diamond and SiC, as
well as major mineral phases. We also digested five
bulk carbonaceous chondrites with aqua regia by applying the conventional Carius tube technique (CT). In
addition to bulk chondrites, we examined acid residues
of Tagish Lake, Murchison and Allende as well as two
nanodiamond fractions from Allende. To access the Os
in these fractions, we applied a newly developed combustion technique (COMB) which combusts diamond
and SiC at 1000 oC with extremely low blanks. In some
cases, aliquots of acid residues were combusted at up
to 800oC. The objective of precombustion was to further concentrate Os in the highly-refractory SiC present in the acid residues. Os isotopes were measured
by a TIMS (ThermoElectron Triton) at the Carnegie
Institution of Washington with negative ionization
mode.
Results and Discussion: The Japanese island arc
is one of the most tectonically active belts on the Earth
where more than four lithospheric plates interact with
each other. Deep fluids liberated from the subducting
plates migrate upward, playing vital roles in various
subduction zone phenomena, e.g., magmatism, seismicity, crustal deformation, metamorphism, hot
springs activity and ore formation, etc. In order to understand the nature and dynamics of these fluids and
clarify their roles in the subduction zone processes, we
organized a research team consisting of more than 60
scientists working in the area of geophysical observation of deep-seated rocks (seismic tomography and MT
imaging), material science of fluids including
high-pressure experiments and molecular dynamics on
chemistry and physical properties of fluids and microstructure of fluid-bearing rocks, and forward modeling
coupled with geochemical inversion on fluid flow,
magma genesis and ore formation. As a model area for
very fine 3D imaging of fluid distribution in the crust
and uppermost mantle, we selected the central part of
Northeastern Japan and set up a dense network of both
seismic and MT stations. At the same time, we aim to
construct the "geofluid map" underneath Japanese island arc based on our interdisciplinary collaboration, in
order to have a big picture of fluid distribution in subduction zones. To this end, we present a "preliminary
reference rock model (PROM)", which provides a
testbed for physical properties of fluid bearing rocks in
a coherent manner. The final goal of our project is to
establish a universal model for geofluid dynamics that
explains the circulation of fluids throughout subduction zones and its effects on the active Earth.
Fig. 1 Geofluids: Nature and dynamics of fluids in subduction zones.
References: [1] T. Yokoyama, C. M. O. Alexander, R.
J. Walker, Earth Planet. Sci. Lett. 291, 48 (2010). [2] T.
Okuchi and E. Takahashi, in High Pressure- Temperature Research: Properties of Earth and Planetary
Materials, M. H. Manghnani and T. Yagi, Eds.
(American Geophysical Union, Washington, DC,
1998), pp.249-260.