INSITU GEOLOGICAL AND GEOCHEMICAL STUDY OF AN ACTIVE HYDROTHERMAL SITE ON THE NORTH FIJI BASIN RIDGE

In June 1989 a diving cruise by the submersible Nautile was carried out on the North Fiji Basin spreading ridge. The objective of the cruise was a geological and geochemical study of the active spreading axis and associated hydrothermal processes. This operation was the third cruise of the French-Japanese Starmer project following the two cruises of the R.V. Kaiyo in 1987 and 1988. Six dives along the spreading axis between 16-degrees-58' and 17-degrees-00S show that the axial graben consists of alternating N15-trending horsts and grabens. Extinct hydrothermal sites have been observed all along the graben. These consist of fossil chimneys, oxide staining and dead shells. At 16-degrees-59'S an active chimney has been discovered and called the "White Lady" because of its almost exclusively anhydrite composition. This chimney expels a peculiar water characterized by low chlorinity and a 285-degrees-C maximum temperature. In the northern extremity of the N15 axis a wide fossil hydrothermal site has been explored and sampled. It is located in an area cut by N15, N140 and N60-trending faults and fissures.