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

[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)

キーワード：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.