Methane, manganese, and helium-3 in newly discovered hydrothermal plumes over the Central Indian Ridge, 18°-20°S

[1] We have investigated newly discovered submarine hydrothermal plumes over the Central Indian Ridge, 18 degrees-20 degrees S. Onshore chemical analyses of methane, its carbon isotope, manganese, and helium-3 in seawater samples obtained from the plumes revealed their detailed geochemical characteristics. One of the newly discovered hydrothermal plumes located over the western wall of the axial valley at Segment 15B (19 degrees 33'S), called the Roger Plateau, showed constant CH(4)/Mn and CH(4)/(3)He ratios throughout the plume. The CH(4)/(3)He ratio (4 x 10(6)) and delta(13)C (-17.5 parts per thousand) are consistent with those of basalt- hosted sediment-free hydrothermal systems, although the CH(4)/Mn ratio ( similar to 1) is moderately higher. These features are thought to indicate a metal- depleted fluid chemistry. The other hydrothermal field is located within the axial valley at the northern part of the smooth lava plain at Segment 16 (18 degrees 20'S), called the Dodo Great Lava Plain, where several plumes were detected. The CH(4)/Mn and CH(4)/(3)He ratios showed large variation through the plumes while delta(13)C values were almost constant. Geochemical characteristics of venting fluid estimated from those of the plumes were apparently high CH(4)/Mn (>6) and CH(4)/(3)He (>60 x 10(6)) ratios and low delta(13)C values (<-27.5 parts per thousand), suggesting possible influences of several methane input processes to the fluid chemistry.