HR: 09:55h
AN: OS21C-07 INVITED     [Abstracts]
TI: The Rainbow Hydrothermal System: Experimental and Theoretical Controls on Vent Fluid Chemistry and Subseafloor Alteration Processes
AU: * Seyfried, W E
EM: wes@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN 55455 United States
AU: Fu, Q
EM: fuxx0033
AF: University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN 55455 United States
AU: Foustoukos, D I
EM: fous0009@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN 55455 United States
AB: Recent discoveries of high temperature vent fluids issuing from ultramafic rocks along slow-spreading sections of the Mid-Atlantic Ridge have provided information on hydrothermal alteration processes for compositional systems distinct from basalt-hosted hydrothermal systems. For example, the Rainbow hydrothermal system at 36N on the Mid-Atlantic Ridge is associated with vent fluids at temperatures as high as 364C. These fluids reveal unusually high dissolved Fe concentrations and significant dissolved silica, which is surprising considering the ultramafic composition of the host-rocks. Phase equilibria calculations indicate that in addition to reducing conditions (high dissolved H2) and moderately high chloride, a result of phase separation effects, acidic conditions of fluids in the subseafloor reaction zone are needed to account for the high Fe concentrations. The association of acidic conditions and ultramafic rocks is surprising, but can occur when dissolved SiO2 concentrations exceed amounts predicted for full equilibrium in the MgO-FeO-SiO2-H2O-HCl system at elevated temperatures and pressures. The source of dissolved SiO2 at Rainbow may involve non-stoichiometric dissolution of pyroxene minerals or may be derived from associated gabbroic intrusions. In effect, dissolved SiO2 enhances the stability of Fe-bearing olivine, which together with talc and tremolite, provides the requisite buffer to fix pH at values sufficiently low (4.9) to increase Fe to the levels observed. The reducing nature of ultramafic-hosted hydrothermal systems, however, is not surprising. High dissolved hydrogen is not only critical for the observed dissolved Fe at Rainbow, but when coupled with the observed low dissolved H2S, enhances dissolved concentrations of Cu owing to constraints imposed by magnetite-bornite-chalcocite-fluid equilibria. Indeed, some of the highest dissolved Cu concentrations of vent fluids are from ultramafic-hosted hydrothermal systems, with attendant implications for seafloor ore-mineralization and the composition of metal-bearing sediment. Recent experimental data have also linked the reducing nature of fluids issuing from these systems with hydrocarbon formation by mineral-catalyzed Fischer Tropsch Type (FTT) synthesis. The reducing nature of vent fluids, together with their acidity and dissolved chloride concentrations, may account for the unusually high levels of REEs that characterize these fluids, in general agreement with experimental and theoretical data.
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005