DISCOVERY OF PERIDOTITE-HOSTED HYDROTHERMAL DEPOSITS ALONG THE ULTRASLOW-SPREADING SOUTHWEST INDIAN RIDGE

We report the discovery of hydrothermal deposits recovered by dredging from an oblique segment of the Southwest Indian Ridge (SWIR) between 10° and 16°E, recently surveyed by Knorr Cruise 162, Legs VII to IX. The SWIR falls at the ultraslow end of the spreading spectrum and represents the low magma budget end-member of mid-ocean ridges. The section surveyed is exceptional because for most of its length, only partially serpentinized peridotite and scattered basalt is exposed on the seafloor. This makes it an ideal area to study ultramafic-hosted hydrothermal systems. The hydrothermal deposits consist of concretions of opaline silica, basalt breccias cemented by smectite and Mn-oxides (birnessite), and partially oxidized massive sulfides. Preliminary XRD analyses indicate a range of silica polymorphs in the deposits from opal-A to opal-CT to quartz. Sepiolite, a rare Mg-rich clay mineral in the deep sea, has also been identified that likely formed during seawater/ultramafic rock interaction. We believe the massive sulfides represent a fossil site of high-temperature discrete venting. A plume-mapping program was conducted concurrently that collected temperature, pressure, and light back scattering data from 67 lowerings. Ten casts produced anomalous profiles characterized by sudden deviations from background values. Together, the recovery of massive sulfides and plume data indicate that high-temperature hydrothermal convection may exist in the absence of volcanic activity at slow-spreading ridges and that it may be taking place at present. This is remarkable because of the extremely low magma budget and hence minimal magmatic heat input into the system. Instead, “non-magmatic” heat sources (e.g., conductive heat loss and exothermic mineral reactions) might be the driving force of these systems and long-lived normal faults along the rift valley walls may provide pathways along which deeply circulating fluids rise to the seafloor. Our preliminary data suggest that hydrothermal input from ultraslow spreading ridges might be larger than expected. Determining the driving forces and structural controls of hydrothermal systems at ultraslow spreading ridges and the frequency of active hydrothermal sites will be important in determining their role in lithospheric accretion and global geochemical budgets.