Mantle‐Derived Helium in a CO₂‐Poor, N₂‐Dominated Submarine Gas Seep at George Sound, Fiordland, New Zealand

Submarine gas seeps at Anchorage Cove in George Sound document fluid migration along the submerged Australian‐Pacific plate boundary in Fiordland, New Zealand. Gas samples from shallow seafloor vents (< 8 m) are dominated by N₂ (~96%), with minor O₂, Ar and CH₄. He concentrations (~0.27%) are approximately 500 times above air‐saturated equilibrium values. CO₂ concentrations fall below expected atmospheric contributions, and H₂ is effectively absent. Stable isotope values indicate a predominantly atmospheric origin for nitrogen ( δ ¹⁵ N ≈ +0.5‰), whereas corrected N₂/Ar ratios are elevated relative to the atmospheric value, consistent with a minor nonatmospheric nitrogen component. Methane is ¹³ C‐enriched ( δ ¹³ C‐CH₄ ≈ +11‰), potentially reflecting methane oxidation during shallow fluid circulation. Helium isotope ratios reveal a mantle‐derived contribution, with ³ He/ ⁴ He ratios of 1.96–2.12 Ra and elevated ⁴ He/ ²⁰ Ne ratios indicating approximately 20%–30% mantle‐derived helium within the nonatmospheric fraction. ⁴⁰ Ar/ ³⁶ Ar ratios are also above atmospheric values. Despite this mantle helium signal, CO₂ concentrations fall below atmospheric levels, resulting in He‐CO ₂ decoupling, with CO ₂ / ³ He ratios approximately five orders of magnitude below the mantle reference value. This decoupling indicates substantial postsource modification of the gas, potentially through CO₂ dissolution and carbonate precipitation during fluid migration. The distribution of seepage is consistent with structurally controlled fluid discharge, although a direct connection with the George Sound Shear Zone cannot be established from the available observations.