Discovery of hydrothermal plumes at the Rodoriguez segment, Mid-Indian Ridge

Geochemical surveys for the Rodriguez Segment of the Central Indian Ridge have been quite few so far. One of the principal objectives of this cruise is to locate hydrothermally active sites along the plate spreading center of the segments 15 and 16 of the Central Indian Ridge. We conducted water column observations using a CTDT (a package of Conductivity, Temperature, Depth, and light transmission sensors) + CMS (Carousel Multi-Sampling system) attached with an in situ Mn analyzer GAMOS (Okamura et al., 2001) in order to detect any anomalies due to hydrothermal plumes. Summary of some highlighted results: (1) Two typical hydrothermal active stations have been recognized; one is located east of East Brigitte Protrusion in the Roger Plateau (on the segment 15), and the other is close to Beak Rocks on the Great Dodo Lava Plain (on the segment 16). (2) Both sites are characterized by light transmission (LT) anomalies of ∼0.2 % at maximum, suggesting the existence of black (or white) smoker activity. (3) While the DMn(nM)/DLT(%) at the former station is ∼100, in a similar trend as that observed during KH-93-3 cruise (Kairei Field), the ratio at the latter station is only 10, an order of magnitude lower than that at station 20. There may be a significant difference in hydrothermal fluid chemistry at these two locations.