Chemistry of hydrothermal fluids from the 17°S active site on the North Fiji Basin Ridge (SW Pacific)

Abstract

In June 1989 a diving cruise of the French submersible Nautile was made along the North Fiji Basin spreading ridge. The objective was the geological and geochemical study of the active spreading axis and of the hydrothermal processes discovered during the Kaiyo cruise in 1988. Active anhydrite chimneys associated with underlaying sulfide deposits were studied in the central spreading ridge. The transparent venting fluid showed significant salinity depletion. The extrapolated concentrations of all the chemical species in the pure end member showed a signature similar to that of the Cl-depleted hydrothermal fluid from the ashes vent site in the Juan de Fuca Ridge. These features suggest that the present hydrothermal end member resulted from subcritical phase separation, which generated a low-salinity vapor phase followed by condensation and mixing with normal seawater in the last part of the plumbing system of the vents.