Relict hydrothermal events along the super-slow Southwest Indian spreading ridge near 63° 56′ E -: mineralogy, chemistry and chronology of sulfide samples

During the Indoyo cruise in 1998 with Shinkai 6500 a massive sulfide field was located in a water depth of about 2940 m, near 27 degrees 51 'S/63 degrees 56 'E, east of the Melville fracture zone. The newly discovered hydrothermal field is situated at the summit of an axial volcanic ridge, informally called, Mount Jourdanne. Within the field, inactive chimneys as well as hydrothermal mounds were observed on the ocean floor but no indications such as shimmering water, chemical anomalies in the water column or faunal relicts. which might point to an active vent site, were observed. Our discovery proves, for the first time, that high-temperature massive sulfide chimneys can also develop at a super-slow spreading ridge such as the Southwest Indian Ridge. This discovery of massive sulfides will possibly open new aspects in the understanding of hydrothermal processes along super-slow spreading ridges, since the sulfide samples from Mt. Jourdanne show interesting differences in their mineralogical, as well as chemical compositions, when compared to sulfide samples from faster opening ridge environments. To reconstruct the history of the evolution of this field, age dating studies were carried out on the recovered sulfide samples. These indicate activity between 70 000 and 13 000 years ago. (C) 2001 Elsevier Science B.V. All rights reserved.