Download GEOFLUID PROCESSES IN SUBDUCTION ZONES AND MANTLE

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
o
C 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 (seis-
mic 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 PressureTemperature Research: Properties of Earth and
Planetary Materials, M. H. Manghnani and T. Yagi,
Eds. (American Geophysical Union, Washington,
DC, 1998), pp.249-260.