Helium and carbon geochemistry of hydrothermal fluids from the North Fiji Basin spreading ridge (southwest Pacific)

The North Fiji Basin is one of the active marginal basins in the southwestern Pacific. Hydrothermal fluid emanations were located at two sites on the Central ridge of the basin. High-temperature fluids (230-290°C) venting from anhydrite chimneys at a 17°S site have end-member compositions of: 11.0-14.5 mmol/kg of CO₂; 30.4-43.5 μmol/kg of CH₄; and 2.3-4.5 × 10^-5 cm³STP/g of He. Taking phase separation processes into account, the gas abundances are not enriched compared with the mid-oceanic ridge hydrothermal systems. Isotopic compositions of CO₂ (δ¹³C = -6.2 to -5.7‰PDB), CH₄ (δ¹³C = -20 to -18‰PDB), and helium ( R R_A = 9.0-10.0) are comparable to the mid-oceanic ridge signature. Together with basalt helium data, the helium isotopic signature may be attributed to the incorporation of a hotspot-like primitive component. Low-temperature shimmering fluids (the highest measured temperature was 5.2°C) associated with biological communities at a site at 18°50′S show slight chemical anomalies, in some species, in SiO₂, Mn, Li, pH and CH₄, and helium isotope ratios distinct from ambient seawater. Evaluated helium isotopic compositions prior to dilution fall between R R_A = 8.4 and 8.9, supporting the suggestion of high-temperature hydrothermal activity at this site, although this was not observed by dive expeditions. The gas geochemistry of these two different types of fluids show several similar characteristics to the mid-oceanic ridge hydrothermal systems. This result is in accordance with previous petrological studies which demonstrated a dominant N-MORB source signature and a co-existing OIB source influence of the North Fiji Basin magmatism.