HR: 1340h
AN: B13A-0212    [Abstracts]
TI: Abiotic Synthesis of Methane Under Alkaline Hydrothermal Conditions: the Effect of pH in Heterogeneous Catalysis
AU: * Foustoukos, D I
EM: fous0009@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive, SE, Minneapolis, MN 55455
AU: Qi, F
EM: fuxx0033@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive, SE, Minneapolis, MN 55455
AU: Seyfried, W E
EM: wes@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive, SE, Minneapolis, MN 55455
AB: Abiotic formation of methane in hydrothermal reaction zones at mid-ocean ridges likely occurs by Fischer-Tropsch catalytic processes involving reaction of CO2-bearing fluids with mineral surfaces. The elevated concentrations of dissolved methane and low molecular weight hydrocarbons observed in high temperature vent fluids issuing from ultramafic-hosted hydrothermal systems, in particular, suggest that Fe and Cr-bearing mineral phases attribute as catalysts, enhancing abiotic production of alkanes. The chemi-adsorption of dissolved CO2 on the catalytic mineral surface, however, might be influenced by a pH dependent surface electron charge developed within the mineral-fluid interface. Thus, a series of experiments was conducted to evaluate the role of pH on rates of carbon reduction in fluids coexisting with Fe-oxides at 390 degree C and 400 bars. At two distinct pH conditions, acidic (pH = 5) and alkaline (pH = 8.8), the abiotic production of isotopically labelled CH4(aq) was monitored during FeO reaction with aqueous NaCl-NaHCO3-H2-bearing fluid (0.56 mol/kg NaCl, 0.03 mol/kg NaH13CO3). Despite the lower H2(aq) concentrations (120 mmol/kg) in the high pH system, concentrations of abiogenic methane attained values of 195 umol/kg and 120 umol/kg respectively, suggesting enhanced catalytic properties of mineral under moderately high pH. X-ray photoelectron spectroscopy (XPS), performed on unreacted and final solid products, reveal the significantly greater abundances of alkyl (C-C-) groups on the surface of FeO oxidized at elevated pH, in comparison with mineral reacted at low pH conditions. Thus, enhanced adsorption of dissolved CO2 and the resulting Fischer-Tropsch formation of alkyl groups likely contributes to methane production observed at alkaline conditions. Introducing the effect of pH in the Fischer-Tropsch mechanism of alkane formation has important implications for the recently discovered Lost City ultramafic-hosted hydrothermal system, where elevated pH and high CH4 concentrations in the moderate temperature vent fluids are observed.
DE: 8424 Hydrothermal systems (8135)
DE: 4820 Gases
DE: 4832 Hydrothermal systems
DE: 3035 Midocean ridge processes
DE: 1055 Organic geochemistry
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting