HR: 14:40h
AN: V43C-05    [Abstracts]
TI: Experimental and Thermodynamic Constraints on Serpentinization: Implications for Fluid Chemistry, Abiotic Synthesis of Hydrocarbons, and Subsurface Microbial Processes
AU: * McCollom, T M
EM: mccollom@lasp.colorado.edu
AF: CU Center for Astrobiology and Laboratory for Atmospheric and Space Physics, Box 392, University of Colorado, Boulder, CO 80309-0392 United States
AB: Serpentinized rocks have received a great deal of attention in recent years, primarily because they are believed to play significant roles in supporting autotrophic, hydrogen-based microbial communities and in the abiotic synthesis of methane and other organic compounds. Both of these processes are a consequence of the high H₂ concentrations generated by reaction of water with ultramafic rocks during serpentinization. The amount of H₂ produced during serpentinization is dependent on both thermodynamic and kinetic constraints, which can be evaluated through numerical thermodynamic models and laboratory experiments. Because olivine is thermodynamically stable at temperatures above ~375°C, alteration of ultramafic rocks at higher temperatures generates only minor amounts of H₂. Even at lower temperatures olivine may equilibrate with the fluid after only partial reaction, so that olivine may persist stably in partially serpentinized rocks. Olivine only becomes completely unstable at ~315°C, so that the maximum potential for H₂ production only occurs at or below this temperature. At high temperatures, serpentinization proceeds rapidly, so that thermodynamic constraints provide the primary control on H₂ production. However, the rate of serpentinization becomes sufficiently slow below ~150°C that the residence of time of fluids may prevent high concentrations of H₂ from accumulating. Since H₂ generation during serpentinization is primarily attributable to conversion of ferrous Fe in olivine to ferric Fe in magnetite, another factor that may limit H₂ production is the sequestration of ferrous Fe in brucite or serpentine. Some serpentines and brucites are reported to contain significant amounts of Fe, but these data are somewhat uncertain because the analyses may include microcrystalline magnetite. Although the thermodynamic properties of solid solutions of these minerals are highly uncertain, the available data suggest that the Fe content of serpentines and brucite may increase with decreasing temperature. Together, these factors indicate that maximum H₂ production may occur during serpentinization at temperatures around 300°C. Production of H₂ in many cases may exceed its solubility in water, leading to the exsolution of H₂-rich vapor, which may promote abiotic organic synthesis if the reactions are more favorable in a vapor phase. Each kg of ultramafic rock undergoing serpentinization can supply as much as 90 kJ of energy to H₂-oxidizing microbes, which is sufficient to produce about 3 grams of biomass.
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1055 Organic and biogenic geochemistry
DE: 4840 Microbiology and microbial ecology (0465)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005