RADIOCHRONOLOGICAL INVESTIGATION OF HYDROTHERMAL DEPOSITS FROM THE MESO ZONE, CENTRAL INDIAN RIDGE

Hydrothermal activity in the Indian Ocean has hitherto only been poorl y documented. First indications of hydrothermal mineralization were ob served in 1987 during the GEMINO-cruise over the 4th ridge segment of the Central Indian Ridge, 270 km north of the Rodriguez Triple Junctio n. Subsequent cruises in 1993 (HYDROTRUNC) and in 1995 (HYDROCK) were carried out in order to obtain detailed information about the geologic al setting and the extent and mineral distribution of the field, but p rimarily to sample the first massive sulfides of the Indian Ocean floo r. This hydrothermal field, which was called the MESO zone, was studie d by detailed mapping and by ocean bottom photography. Hydrothermal pr ecipitates were recovered with a TV-grab sampler at four sampling loca tions, two in the northern part of the mineralized field, called the T alus Tips Site, and two in the central part, called the Sonne Field. T he sulfide samples were age-dated using the Th-230/U-234 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 t he deep-seated hydrothermal circulation is continuous but that the dis charge of hot fluid occurs alternately at the Sonne Field or at the Ta lus 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 fr om a low-temperature hydrothermal event dated at around 11 ka is prese ntly the youngest hydrothermal formation in the entire MESO zone. (C) 1998 Elsevier Science B.V. AU rights reserved.