Japan Geoscience Union Meeting 2025

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

[S-IT19] Coupling of deep Earth and surface processes

Tue. May 27, 2025 3:30 PM - 5:00 PM 104 (International Conference Hall, Makuhari Messe)

convener:YoungHee Kim(Seoul National University), Jin-Oh Park(Department of Ocean Floor Geoscience, Atmosphere and Ocean Research Institute, The University of Tokyo), Takehi Isse(Earthquake Research Institute University of Tokyo), Hyunwoo Lee(Seoul National University), Chairperson:YoungHee Kim(Seoul National University), Jin-Oh Park(Department of Ocean Floor Geoscience, Atmosphere and Ocean Research Institute, The University of Tokyo), Takehi Isse(Earthquake Research Institute University of Tokyo), Hyunwoo Lee(Seoul National University)

4:42 PM - 5:00 PM

[SIT19-11] Geochemistry of volatiles emitted from Segments 2-5 of the Central Indian Ridge (9°44’ to 15°33’S)

*Wonhee Lee1, Heejun Kim2, Jeongyeon Yu1, Jung-Hun Song1, Hyunwoo Lee1, Wonnyon Kim2, Jonguk Kim2, Urumu Tsunogai3, Naoto Takahata4, Yuji Sano5 (1.Seoul National University, 2.Korea Institute of Ocean Science & Technology, 3.Nagoya University, 4.Atmosphere and Ocean Research Institute, University of Tokyo, 5.Marine Core Research Institute, Kochi University)

Keywords:Central Indian Ridge, Hydrothermal vent fluids, Volatiles, Stable isotopes

Hydrothermal fluids from segments from 2 to 5 of Central Indian Ridge (CIR; 9°44’S to 15°33’S) were collected during the research expedition aboard R/V ISABU in March 2023, conducted by Korea Institute of Ocean Science & Technology (KIOST). We report dissolved gas concentrations (e.g., H2, He, CH4, and CO2) and isotopic compositions (3He/4He, δ13C-CH4, δD-CH4, and δ13C-CO2) in the hydrothermal fluid samples obtained from eight vent fields. The measured helium isotope ratios (3He/4He = 1.17 to 10.23 Ra, where 1 Ra = 1.4 x 10-6) indicate dominant contributions from the mantle plume (>9 Ra) in segments 2 and 3 and the upper mantle (~8 Ra) in segments 4 and 5. This is supported by the negative correlation between the 3He/4He ratios and seismic S-wave velocity anomalies (dVs; -3.47 to -2.91%) in the underlying mantle. The δ13C-CO2 values (-13.5 to -7.4‰ vs. V-PDB) also point to the presence of mantle-derived CO2 (δ13C = -6.5 ± 2.5‰). The hydrothermal fluids contain high concentrations of H2 (1 to 86 mol.%) and CH4 (0.3 to 4.5 mol.%), with δ13C-CH4 (-21.6 to -8.1‰) and δD-CH4 (-196.0 to -87.0‰) values consistent with abiotic methane signatures observed at Mid-Atlantic Ridge (MAR). This implies that reductive hydrothermal conditions have fostered substantial hydrogen and methane formation. Our results suggest that volatiles and heat supplied from the mantle could facilitate hydrothermal reactions within the overlying oceanic lithosphere, whereby seawater-crust-mantle interactions may drive secondary hydrogen production, with potential implications for resource development.