Discovery of ancient and active hydrothermal systems along the ultra‐slow spreading Southwest Indian Ridge 10°–16°E

Abstract

[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 16°E, where the effective spreading rate for mantle upwelling is the slowest of any ocean ridge explored to date (8.4 mm yr À1 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.

Mineralogical and geochemical features of sulfide chimneys from the 49°39′E hydrothermal field on the Southwest Indian Ridge and their geological inferences

“…Among these, smooth terrain appears specific to ultraslow-spreading ridges and the mode of seafloor spreading may be analogous to processes at the ocean-continent transition of continental margins. The rocks exposed on the seafloor are mainly basalts, with some gabbros, pyroxenites and serpentinized peridotites [12]. Sauter et al [18,19] and Cannat et al [20] discussed the melt supply processes based on gravitational, magnetic and petrological data.…”

“…Jourdanne" was located at 27°51′S/ 63°56′E during the Indoyo cruise. During R/V Knorr cruise 162 in 2000, eight sites with hydrothermal anomalies were discovered in the west of the SWIR [12,13], and in the next year, a hydrothermal deposits site in a peridotite-hosted field was discovered between 10° and 16°E [14]. However, active hydrothermal vents were not documented until 2007.…”

Diverse styles of submarine venting on the ultraslow spreading Mid-Cayman Rise

“…But the distribution of slow or fast-spreading ridge systems is not uniform throughout the globe: slow and ultraslow ridges dominate the Arctic, Atlantic, and southwest Indian oceans, while fastspreading ridges are only found in the Southern and Pacific oceans (1). To date, only ∼10% of the global ridge system has been explored systematically for hydrothermal activity (2), and it has only recently been recognized that the world's slowest-spreading ridges can host hydrothermal activity (3)(4)(5)(6). To a first approximation, the incidence of hydrothermal venting is greatest on fast-spreading ridge systems (2).…”

“…The Lost City style of venting is associated with long-lived detachment surfaces that play an important role in extension along all slow spreading ridges (22). Although plume data for Lost City are not available, the vent fluids contain CH 4 and H 2 at levels comparable to Type 2 vents but with much lower dissolved metal concentrations (10,23). Plumes that have previously been attributed to such a source-at the 15°20′N fracture zone, MAR-exhibit extremely high CH 4 ∶Mn ratios and an absence of particulates (24).…”

Methane- and Sulfur-Metabolizing Microbial Communities Dominate the Lost City Hydrothermal Field Ecosystem

“…Carbon-14 radioisotopic dating indicates that hydrothermal activity has been ongoing for at least 30,000 years (17). A large percentage of exposed seafloor on and near slow-and ultraslow-spreading ridges is likely to contain ultramafic rocks similar to those that host the LCHF (4,12,14). Therefore, this system offers a unique opportunity to study an ultramafic-rock-hosted submarine ecosystem that may be both widespread and stable over thousands of years.…”