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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 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.