Emissions of mantle helium and abiotic methane at newly discovered black smokers along the Central Indian ridge (12°S–16°S)

Volatiles emitted from black smokers provide crucial insights into mantle and hydrothermal processes. This study investigated the Cheoeum (CVF) and MIRAE-2 (M2VF) vent fields in the 12-16°S region of the Central Indian Ridge (CIR) using RV ISABU and ROV ROPOS. We measured ³He/⁴He, ⁴He/²⁰Ne, CH₄ concentration, δ¹³C of CH₄, and ²²²Rn activity in 78 seawater samples from vent fluids and the overlying plume. The end-member ³He/⁴He ratio is 7.94 Ra, indicating the MORB mantle source. Vent fluids show high CH₄ concentrations (up to 11,578 nM/kg) and anomalously heavy δ¹³C values (-11.23 to -7.39 ‰), suggesting abiotic near-quantitative CO₂ reduction in closed-system environments such as olivine-hosted fluid inclusions. Principal Component Analysis distinguished the hydrothermal fluids from background waters by analyzing chemical proxies and spatial information. This first volatile-geochemistry dataset from the CIR provides direct isotopic evidence for mantle-derived helium and abiotic methane, offering a key reference for understanding deep-sea carbon cycling and chemosynthetic ecosystems at slow-spreading ridges.