Volatile-Rich Hydrothermal Plumes Over the Southern Central Indian Ridge, 24 degrees 49'S: Evidence for a New Hydrothermal Field Hosted by Ultramafic Rocks

Water column physico-chemical studies were conducted over the southern Central Indian Ridge between 24 degrees 44'S and 25 degrees 52'S to identify and chemically characterize seafloor hydrothermal activity. High turbidity values were observed between 2300 and 2700 m with two distinct layers, between water depths of 2320-2500 m and 2510-2650 m, at two closely spaced CTD stations at 24 degrees 48.62'S (CTD-17-P5) and 24 degrees 48.68'S (CTD-17-P8). Elevated concentrations of dissolved Mn (DMn: 19-112 nM), dissolved Fe (DFe: 33-88 nM), methane (CH4: 32-246 nM), elevated delta He-3 values (28%-88%), and stable carbon isotope ratios of CH4 confirm the hydrothermal origin. In plume layer-1, the maximum concentrations were observed at 2375m at P8 and in plume layer-2, the maximum concentrations were observed at 2570 m at P5. The stable isotope ratios of methane (delta C-13-CH4) show that heavier isotopes are enriched (-13.2 parts per thousand to -14.7 parts per thousand) in the plume waters and are similar to vent fluids on the global mid-oceanic ridges. Further, morphological and mineralogical studies of plume particles, collected from the plume layer-2 maxima, clearly show the presence of barite, pyrite, chalcopyrite, and indicate possible venting of high-temperature fluids in the vicinity of P5. Enrichment in methane relative to the other tracers and the general geochemical characteristics of these two plume layers, CH4/Mn (1.8-2.2); CH4/Delta He-3 (85-97 x 10(6)), Mn/Delta He-3 (44-46 x 10(6)), Fe/Delta He-3 (52-54 x 10(6)), indicate that these plumes are formed from fluids released at the seafloor that circulated through ultramafic/gabbroic rocks. The high concentrations of dissolved gases and metals combined with the presence of sulfide particles in the water column provide evidence for a new ultramafic/gabbroic-hosted hydrothermal vent field, at 24 degrees 49'S on the southern Central Indian Ridge.