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 south western Pacific. Hydrothermal fluid emanations were located at two sit es on the Central ridge of the basin. High-temperature fluids (230-290 degrees C) venting from anhydrite chimneys at a 17 degrees S site hav e end-member compositions of: 11.0-14.5 mmol/kg of CO2; 30.4-43.5 mu m ol/kg of CH4; and 2.3-4.5 X 10(-5) cm(3)STP/g of He. Taking phase sepa ration processes into account, the gas abundances are not enriched com pared with the mid-oceanic ridge hydrothermal systems. Isotopic compos itions of CO2 (delta(13)C=-6.2 to -5.7 parts per thousand PDB), CH4 (d elta(13)C=-20 to -18 parts per thousand PDB), and helium (R/R(A)=9.0-1 0.0) 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-temperat ure shimmering fluids (the highest measured temperature was 5.2 degree s C) associated with biological communities at a site at 18 degrees 50 'S show slight chemical anomalies, in some species, in SiO2, Mn, Li, p H and CH4, and helium isotope ratios distinct from ambient seawater. E valuated helium isotopic compositions prior to dilution fall between R /R(A) = 8.4 and 8.9, supporting the suggestion of high-temperature hyd rothermal activity at this site, although this was not observed by div e expeditions. The gas geochemistry of these two different types of fl uids show several similar characteristics to the mid-oceanic ridge hyd rothermal systems. This result is in accordance with previous petrolog ical studies which demonstrated a dominant N-MORB source signature and a co-existing OIB source influence of the North Fiji Basin magmatism.