Radiochronological investigation of hydrothermal deposits from the MESO zone, Central Indian Ridge

Abstract Hydrothermal activity in the Indian Ocean has hitherto only been poorly documented. First indications of hydrothermal mineralization were observed in 1987 during the GEMINO-cruise over the 4th ridge segment of the Central Indian Ridge, 270 km north of the Rodriguez Triple Junction. Subsequent cruises in 1993 (HYDROTRUNC) and in 1995 (HYDROCK) were carried out in order to obtain detailed information about the geological setting and the extent and mineral distribution of the field, but primarily to sample the first massive sulfides of the Indian Ocean floor. This hydrothermal field, which was called the MESO zone, was studied by detailed mapping and by ocean bottom photography. Hydrothermal precipitates were recovered with a TV-grab sampler at four sampling locations, two in the northern part of the mineralized field, called the Talus Tips Site, and two in the central part, called the Sonne Field. The sulfide samples were age-dated using the 230 Th/ 234 U method. They show different periods of activity at each site: 52±5, 22.7±3 and 16±2 ka for the main events at the Talus Tips Site and 18±2 and 12.5±1.5 ka for the Sonne Field. We conclude from these data that the two sites were not active at the same time. If it is assumed that there is a complete cessation of activity between 52 and 23 ka, for more recent periods (from 23 to 10 ka) it may be postulated that the deep-seated hydrothermal circulation is continuous but that the discharge of hot fluid occurs alternately at the Sonne Field or at the Talus Tips Site. Comparing our results with those obtained previously at the different hydrothermal sites, we favor the conclusion of episodic activity. Moreover, in the Sonne Field, jasper formation resulting from a low-temperature hydrothermal event dated at around 11 ka is presently the youngest hydrothermal formation in the entire MESO zone.