HR: 14:10h
AN: V23C-02 INVITED     [Abstracts]
TI: Processes and Rates of Mass Transfer in Ultramafic-Hosted Hydrothermal Systems: An Experimental Study with Implications for Dissolved Inorganic and Organic Components in High-Temperature Vent Fluids
AU: * Seyfried, W E
EM: wes@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455 United States
AU: Fu, Q
EM: Fuxx0033@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455 United States
AU: Foustoukos, D I
EM: fous0009@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455 United States
AU: Allen, D E
EM: Alle0167@umn.edu
AF: Department of Geological Sciences, Salem State College, Salem, MA 01970 United States
AB: Recently discovered ultramafic-hosted hydrothermal systems at mid-ocean ridges reveal high temperature vent fluids with relatively high SiO2, Ca, H2, and methane and other hydrocarbons. Dissolved Fe concentrations are the highest of any vent systems yet discovered and require a relatively low pH and reducing conditions in subseafloor reaction zones from which the fluids are derived. This, together with the SiO2 concentrations of the vent fluids, strongly indicates fluid buffering by silica-rich phases possibly produced during pyroxene dissolution, the likely abundant presence of olivine notwithstanding. Theoretical predictions of olivine dissolution kinetics at elevated temperatures and pressures, however, suggest relatively rapid conversion of olivine to talc and serpentine with corresponding lowering of dissolved SiO2 and Fe, and increase in pH. Thus, to test this, we performed a series of experiments at 400øC, 500 bars involving olivine (Fo89) alteration in SiO2-bearing systems containing dissolved Na and chloride concentrations approximately equivalent to the Rainbow hydrothermal system. Time series fluid samples indicate steady state conditions. Results confirm unusually slow olivine reaction kinetics, even when coexisting with moderate to high dissolved SiO2. XPS and SEM analysis indicate Fe-enrichment on olivine surfaces, and formation approximately 14 percent talc. The olivine to talc conversion rate suggests a log rate of olivine hydrolysis of -11.97 (moles/cm2/sec), well below that predicted from available rate data extrapolated from lower temperatures and pressures. The relative enrichment of Fe on olivine surfaces may decrease the thermodynamic and kinetic drive for olivine dissolution; effectively precluding pH increases predicted assuming full equilibrium. Rates and processes of mass transfer involving Fe-bearing minerals may also help to catalyze Fisher-Tropsch synthesis of complex hydrocarbons reported for vent fluids issuing from ultramafic-hosted hydrothermal systems, as suggested by results of a companion series of hydrothermal experiments.
DE: 8424 Hydrothermal systems (8135)
DE: 4832 Hydrothermal systems
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting