HR: 1340h
AN: OS33A-1451    [Abstracts]
TI: Geochemistry of Hydrothermal Fluids and Fe-Mn Crusts From the Logatchev Hydrothermal Field, 15°N at the Mid-Atlantic Ridge: Time Series Study in an Ultramafic System
AU: * Schmidt, K
EM: k.schmidt@iu-bremen.de
AF: International University Bremen IUB, School of Engineering and Science, Institute of Geosciences and Astrophysics, Campus Ring 8, Bremen, 28759 Germany
AU: Garbe-Schoenberg, D
EM: dgs@gpi.uni-kiel.de
AF: University of Kiel, Institute of Geosciences, Olshausenstr. 40, Kiel, 24098 Germany
AU: Koschinsky, A
EM: a.koschinsky@iu-bremen.de
AF: International University Bremen IUB, School of Engineering and Science, Institute of Geosciences and Astrophysics, Campus Ring 8, Bremen, 28759 Germany
AU: Seifert, R
EM: seifert@geowiss.uni-hamburg.de
AF: University of Hamburg, Institute for Biogeochemistry and Marine Geochemistry, Bundesstr. 55, Hamburg, 20146 Germany
AB: Hydrothermal systems at mid-ocean ridges are of special interest concerning the elemental and isotopic input to the oceans and the close relationship between hydrothermal fluids and related ecosystems. Until now, only scarse data are published about the temporal variability of chemical and physicochemical parameters in hydrothermal fields. Changes in salinity, gas and metal concentration, mainly caused by magmatic or tectonic events, have a first order effect on the microbial and faunal association.
Within the Special Priority Program 1144, funded by the German Research Foundation DFG, the temporal variability of hydrothermal fluids in an ultramafic environment is investigated at different time scales (within years and thousands of years, respectively). The target area of this work is the Logatchev Field located at the Mid-Atlantic Ridge near 15°N, in a slow-spreading ridge segment dominated by tectonic spreading and characterized by outcrops of serpentinized mantle rocks. The water depth ranges between 2950m and 3050m. Geochemical data obtained for hydrothermal fluids sampled during our first two research cruises in 2004 and 2005 (out of 6 planned), show high concentrations of CH₄ and H₂, higher concentrations of certain trace metals and a lowered silica concentration, compared to a basaltic system. This signature is caused by the alteration of ultramafic rocks. The fluids show a slightly lowered chlorinity compared to seawater (10-20%), indicating phase separation near the critical point of seawater and the emanation of the vapor phase. Spatial variations in the geochemistry of fluids between different vent sites were observed in samples obtained in 2004. A first evaluation of a temporal variability between 1996 (Douville et al., 2002), 2004 and 2005 show slight changes in the chlorinity, silica, gas, and metal concentrations.
Fe-Mn deposits are used for the reconstruction of long-term changes in the Logatchev vent field. They were sampled in different settings and exhibit different textural and geochemical features. The deposits are influenced by hydrothermal plume fall-out, rising diffuse fluids, porewater and seawater. Iron-rich crusts were formed during oxidation of sulfides. Results for the concentrations of REE, HFSE, transition metals as well as for ¹⁴³Nd/¹⁴⁴Nd and ¹⁸⁷Os/¹⁸⁸Os in different crust types will be presented. These data were used to get information about the hydrothermal component in the deposits. Osmium isotopic data for pure Mn crusts show a strong hydrothermal influence. Small-scale elemental profiles and age dating in these crusts will help to define changes in the geochemistry of the hydrothermal fluids over time. Variations in the concentrations and elemental ratios of certain elements (Zr, Hf, Nb, Ta) between crust types provide informations about the mobility of these elements and their fractionation process in a hydrothermal system.
Douville, E. et al. (2002) Chem. Geol. 184, 37-48.
DE: 0450 Hydrothermal systems (1034, 3017, 3616, 4832, 8135, 8424)
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1065 Major and trace element geochemistry
DE: 4825 Geochemistry
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005