Japan Geoscience Union Meeting 2014

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Symbol S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Techtonophysics

[S-IT40_1PM1] Geofluids: their distribution and role in the Earth's dynamics

Thu. May 1, 2014 2:15 PM - 4:00 PM 416 (4F)

Convener:*Michihiko Nakamura(Division of Earth and Planetary Materials Science, Department of Earth Science, Graduate School of Science, Tohoku University), Hiroshi Sakuma(Department of Earth and Planetary Sciences, Graduate School of Science and Engineering, Tokyo Institute of Technology), Masahiro Ichiki(Graduate School of Science, Tohoku University), Tsutomu Takahashi(Institute for Research on Earth Evolution Japan Agency for Marine-Earth Science and Technology), Chair:Tetsuya Yokoyama(Department of Earth and Planetary Sciences, Graduate School of Science and Engineering, Tokyo Institute of Technology), Keika Horiguchi(Crustal Fluid Research Group, The Institute of Geology and Geoinformation, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology)

3:15 PM - 3:30 PM

[SIT40-15] The Li-Cl-Br systematics of saline groundwater: A new indicator for slab fluid

*Kohei KAZAHAYA1, Masaaki TAKAHASHI1, Hikaru IWAMORI2 (1.Geological Survey of Japan, AIST, 2.Geochemical Evolution Research Program, Japan Agency for Marine-Earth Science and Technology)

Keywords:Li-Cl-Br, slab-derived fluid, groundwater, subduction process

In this study, we propose Br/Cl ratio as a new indicator for slab-derived fluids, which is useful to distinguish their sources between pore water and hydrous minerals in subducting slab. The areal distribution of slab-derived fluids and their sources using Li/Cl and Br/Cl as geochemical evidences will provide a view for water circulation in subduction zones.Subducting slab contains waters (originally seawater) as pore water and many kinds of hydrous minerals. Hydrous minerals such as opal, clay or mica will decompose to release water during subsiding, and pore water will be released by compaction. Even though such complex process occurs, behavior of halogen ions in the subducting slab may be simple because they are always enriched in aqueous phase (pore water) and the rest are in minerals as a replacement of OH. Some metamorphic fluids in wedge mantle peridotite with Br-enriched signature have been observed and were indicated to be from pore water in the slab. The mineral dehydration process is supposed to be responsible for Br-depletion in slab-derived aqueous fluid. Therefore, halogens are potentially good indicators concerning with the water behavior in subduction processes.The higher Br/Cl ratios (> 0.0035 in wt.) have been observed in fossil seawater and oil field brines due to the addition of Br from organic matters. The very low Br/Cl waters (<0.0025 in wt.) have feature of 18O-shift to the slab (magmatic) fluid end member, which is quite lower than that in seawater (Br/Cl = 0.0034 in wt.), indicating that these waters originate from dehydration of the slab.