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

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

Dec 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994e%26psl.128..183i&link_type=abstract

Earth and Planetary Science Letters (ISSN 0012-821X), vol. 128, no. 3-4, p. 183-197

Other

5

Basalt, Bathymeters, Carbon, Geochemistry, Helium, High Temperature Fluids, Hydrothermal Systems, Mid-Ocean Ridges, Pacific Ocean, Structural Basins, Carbon Dioxide, Coulometers, Methane, Ocean Surface, Temperature

Scientific paper

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 deg site have end-member compositions of: 11.0-14.5 mmol/kg of CO2; 30.4-43.5 micro mol/kg of CH4; and 2.3-4.5 x 10-5 cu cm STP/g of He. Taking phase separation processes into account, the gas abundances are not enriched compared with the mid-ocean ridge hydrothermal systems. Isotopic compositions of CO2 (delta C-13 = -6.2 to -5.7 per mil PDB), CH4 (delta C-13 = -20 to -18 per mil PDB), and helium (R/RA = 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 deg 50 min 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/RA = 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 Mid-Ocean ridge basalts (MORB) source signature and a co-existing Ocean-island basalts (OIB) source influence of the North Fiji Basin magmatism.

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