HYDROTHERMAL PLUMES AT THE RODRIGUEZ TRIPLE JUNCTION, INDIAN RIDGE

Water column anomalies of light transmission, Mn, Fe, Al and CH, conce ntrations were searched in the central, southeastern and southwestern Indian Ridge segments centered on the Rodriguez Triple Junction (RTJ) (similar to 25 degrees 32'S, similar to 70 degrees 02'E), for the purp ose of locating hydrothermally active areas, in July to August 1993. W e found an active zone in the central Indian Ridge segment (25 degrees 18-20'S) approximately 12 miles north of the RTJ, where significant h ydrothermal plumes were observed at 2,200-2,400 m depth. Intensive tow -yo observations using a CTD rosette multi-sampling system equipped wi th a transmissometer revealed that the plumes show temporal as well as spatial variations. Discrete water samples within the plumes were enr iched in Mn, Fe, and CH4, with maximum concentrations of 9.8 nM, 40.2 nM and 3.3 nM, respectively. Judging from the spatial and chemical cha racteristics of the plumes, especially from transmission anomalies and CH4/Mn ratios, we speculate that the hydrothermal venting site might be not in the rift valley but on the eastern off-axis zone, several mi les distant from the rift valley.