First evidence for high-temperature off-axis venting of deep crustal/mantle heat: The Nibelungen hydrothermal field, southern Mid-Atlantic Ridge

During segment-scale Studies of the southern Mid-Atlantic Ridge (MAR), 7-12 degrees S, we found evidence in the water column for high-temperature hydrothermal activity, off-axis, east of Ascension Island. Extensive water column and seafloor work using both standard CTD and deep submergence AUV and ROV deployments led to the discovery and sampling of the Drachenschlund (Dragon Throat) black smoker vent at 8 degrees 17.87' S/13 degrees 30.45'W in 2915 m water depth. The vent is flanked by several inactive chimney structures in a field we have named Nibelungen. The site is located 6 kin South of a non-transform offset between two adjacent 2nd-order ridge-segments and 9 km east of the presently-active, northward-propagating A2 ridge-segment. on a prominent outward-facing fault scarp. Both vent-fluid compositions and host-rock analyses show this site to be an ultramafic-hosted system, the first of its kind to be found on the southern MAR. The thermal Output of this single vent, based on plume rise-height information, is estimated to be 60 15 MW. This value is high for a single black smoker vent but small for an entire field. The tectonic setting and low He content of the vent fluids imply that high-temperature off-axis venting at Drachenschlund is driven not by magmatic processes, as at the majority of on-axis hydrothermal systems, but by residual heat mined from the deeper lithosphere. Whether this heat is being extracted from high-temperature mantle peridotites or deep crustal Cumulates formed at the duelling non-transfrom offset is unclear, in either case the Drachenschlund vent provides the first direct observations of how cooling of deeper parts of the lithosphere, at least at slow-spreading ridges, may be occurring. (c) 2008 Elsevier B.V. All rights reserved.