400°C hot Boiling Fluids From a Hydrothermal Field at 5°S on the Mid-Atlantic Ridge: Results of Meteor Cruise M64/1

HR: 1340h
AN: OS33A-1456 [Abstracts]
TI: 400°C hot Boiling Fluids From a Hydrothermal Field at 5°S on the Mid-Atlantic Ridge: Results of Meteor Cruise M64/1
AU: * Koschinsky, A
EM: a.koschinsky@iu-bremen.de
AF: International University Bremen, School of Engineering and Science, P.O. Box 750561, Bremen, D-28725 Germany
AU: Garbe-Schoenberg, D
EM: dgs@gpi.uni-kiel.de
AF: Universitaet Kiel, Institut fuer Geowissenschaften, Olshausenstrasse 40, Kiel, D-24118 Germany
AU: Seifert, R
EM: seifert@geowiss.uni-hamburg.de
AF: Universitaet Hamburg, Institut fuer Biogeochemie und Meereschemie, Bundesstrasse 55, Hamburg, D-20146 Germany
AU: Strauss, H
EM: hstrauss@uni-muenster.de
AF: Westfaelische Wilhelms-Universitд, Geologisch-Palaeontologisches Institut, Corrensstrasse 24, Muenster, D-48149 Germany
AU: Weber, S
EM: stefan.weber@uni-hamburg.de
AF: Universitaet Hamburg, Institut fuer Biogeochemie und Meereschemie, Bundesstrasse 55, Hamburg, D-20146 Germany
AU: Marbler, H
EM: h.marbler@iu-bremen.de
AF: International University Bremen, School of Engineering and Science, P.O. Box 750561, Bremen, D-28725 Germany
AB: Until recently, hydrothermal activity was unknown from the Mid-Atlantic Ridge (MAR) south of the Equator where the MAR is offset by several 100 km at the St. Paul's and Romanche fracture zones. During the British cruise CD 169 in Feb./March 2005, an active hydrothermal vent field was discovered on the southern MAR at 5°S. During the German follow-up cruise M64/1 in April 2005, the first samples from this vent field were recovered with the ROV QUEST (Univ. Bremen). The high-temperature vent field is situated in a water depth of 2990 m within a large basaltic sheet flow. A diffuse, low-temperature field occurs in few hundred meters to the east. Here we show for the first time that boiling fluids of 400°C are emanating at a depth of about 3000 m at a slow-spreading MAR segment. Although this segment is composed of thick basaltic crust, these fluids are dominated by hydrogen relative to methane, similar to ultramafic-hosted systems. The samples have significantly reduced chloride concentrations with a calculated endmember value of 254 mmol/l Cl compared to a background seawater value of 560 mmol/l Cl. This indicates that the fluids are phase-separated and that the samples collected represent the vapor-type phase of the boiling fluids. Fluids from the neighbouring diffuse vent field lie on the same mixing line of seawater and hydrothermal endmember chlorinity, indicating the same fluid source. The fluids of an additional vent field found only 3 km distant in a similar setting do not show any signs of phase separation but higher metal concentrations and a much lower Fe/Mn ratio compared to the hot vapor-type fluids. Apparently, the temperature conditions in a hydrothermal vent system in combination with phase separation processes may be more important for the fluid composition than the leached rock type. We hypothesize that a very recent intrusion and/or eruption event at the vent field where the hot phase-separated fluids were sampled causes the high heat, gas and Fe fluxes. If so, the system will probably cool significantly over the next years with corresponding changes in the fluid chemistry and related biological associations.
DE: 0450 Hydrothermal systems (1034, 3017, 3616, 4832, 8135, 8424)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005