Discovery of ancient and active hydrothermal systems along the ultra-slow spreading Southwest Indian Ridge 10°-16°E -: art. no. 1044

[1] We report the discovery of active and fossil hydrothermal systems during R/V Knorr Cruise 162, Legs VII and IX along a 400 km long segment of the ultra-slow Southwest Indian Ridge (SWIR) between 10 and 16degreesE, where the effective spreading rate for mantle upwelling is the slowest of any ocean ridge explored to date (8.4 mm yr(-1) full rate). Eight of forty-one optical/temperature profiles contain hydrothermal plume characteristics that indicate firm evidence for two active vent sites and tentative evidence for as many as three others. Fossil hydrothermal material was recovered in 6 of the 38 dredge hauls and includes an occurrence of partially oxidized sulfide breccias, four deposits ofsepiolite and silica, as well as Mn-oxide and nontronite cemented breccias. The massive sulfide deposit likely developed during mixing of upwelling hot fluids with cold seawater within a shallow permeable fault breccia along a deep-seated normal fault bounding the southern rift valley wall. Silica and sepiolite deposits were found mostly on the rift valley walls and likely formed during low-to moderate-temperature ultramafic-hosted hydrothermal activity. The abundance of hydrothermal material and the frequency of localized hydrothermal activity is remarkable because the mantle upwelling and magma supply rates, and hence the magmatic heat input, along this section of the SWIR are lower than on any other explored segment of the global mid-ocean ridge system. This observation suggests that high mantle upwelling and magma supply rates are not required to drive mid-ocean ridge hydrothermal systems and that a close relationship between magmatic heat input and hydrothermal activity may not be established at the ultra-slow end of the ridge spreading spectrum. The frequency and distribution of hydrothermal activity in the study area may reflect a largely tectonic control on fluid circulation, with hydrothermal vent sites being preferentially associated with long-lived faults that provide fluid pathways. Our results suggest the contribution of magma-starved ridge segments to the global ocean-lithosphere geochemical budget is potentially much larger than previously thought.