Hydrothermal plumes over the Carlsberg Ridge, Indian Ocean

Indian Ocean ridges north of the Rodriguez Triple Junction remain poorly explored for seafloor hydrothermal activity, with only two active sites confirmed north of 25°S. We conducted water column surveys and sampling in 2007 and 2009 to search for hydrothermal plumes over a segment of the Carlsberg Ridge. Here we report evidence for two separate vent fields, one near 3°42′N, 63°40′E and another near 3°41.5′N, 63°50′E, on a segment that is apparently sparsely magmatic. Both sites appear to be located on off‐axis highs at the top of the southern axial valley wall, at depths of ∼3600 m or shallower (∼1000 m above the valley floor). At the 63°40′E site, plume sampling found local maxima in light scattering, temperature anomaly, oxidation‐reduction potential (ORP), dissolved Mn, and3He. No water samples are available from the 63°50′E site, but it showed robust light‐scattering and ORP anomalies at multiple depths, implying multiple sources. ORP anomalies are very short‐lived, so the strong signals at both sites suggest that fluid sources lie within a few kilometers or less from the plume sampling locations. Although ultramafic rocks have been recovered near these sites, the light‐scattering and dissolved Mn anomalies imply that the plumes do not arise from a system driven solely by exothermic serpentinization (e.g., Lost City). Instead, the source fluids may be a product of both ultramafic and basaltic/gabbroic fluid‐rock interaction, similar to the Rainbow and Logatchev fields on the Mid‐Atlantic Ridge. Key Points Exploration for hydrothermal plumes over slow spreading Carlsberg Ridge We report evidence for two vent fields on a segment that is sparsely magmatic Fluid‐rock interaction similar to Rainbow/Logatchev fields is inferred