Geochemical controls on hydrothermal fluids from the Kairei and Edmond Vent Fields, 23°–25°S, Central Indian Ridge

Prior to 2000, no mid‐ocean ridge hydrothermal systems were identified and sampled in the Indian Ocean. The Kairei field, Central Indian Ridge, was discovered and sampled in 2000 (Gamo et al., 2001; Hashimoto et al., 2001). This paper reports the chemical composition of hydrothermal fluids collected in 2001 from both the Kairei and newly discovered Edmond hydrothermal systems. Data are used to infer subsurface processes and conditions at the two sites, as well as to place them within the global context of known hydrothermal sites on the mid‐ocean ridge system. While both sites are located on the intermediate spreading Central Indian Ridge, their hydrologic systems have affinities with those observed on slow spreading ridges. Although the compositions of fluids from the four individual vents sampled at Edmond and the three individual vents sampled at Kairei vary respectively, our interpretation is that a single source fluid at each site is subsequently modified by processes including phase separation, subsurface mixing with seawater, and deposition and/or remobilization of metal‐sulfide deposits to generate the observed range of compositions. Both fields are located ≥6 km from the neovolcanic zone, on steps on the east wall of the axial valley, with the Edmond field ∼160 km north of Kairei and almost 1000 m deeper (3300 versus 2450 m). The Edmond fluids are extremely hot, with maximum measured temperatures of 382°C compared to 365°C for Kairei. All of the fluids sampled have chlorinities greater than local ambient seawater due to phase separation of seawater at supercritical conditions. Visual observations suggest both sites have been active for a substantial period of time, again similar to observations of slow spreading ridges. Our chemical data suggest that the substrate underlying Kairei is highly altered, while that at Edmond is not. Significant albitization appears to be occurring below the Edmond field, but not at Kairei. The Edmond fluids are by far the hottest brines yet observed venting from a mid‐ocean ridge hydrothermal site, resulting in unusually high concentrations of several transition metals.