Experimental investigations on the origin and fate of carbon species in hydrothermal systems

A series of experiments were conducted to study mineral catalyzed carbon reduction reactions producing methane and other hydrocarbons of abiotic origin in subseafloor hydrothermal systems, and silicate mineral dissolution and precipitation kinetics at elevated temperature and pressure conditions.; To understand reaction pathways and isotope systematics during abiotic synthesis of hydrocarbons under hydrothermal conditions, experiments involving magnetite and CO2 and H2-bearing aqueous fluids were conducted at 400°C and 500 bars. Time-series fluid samples indicated significant concentrations of dissolved CO and C1-C3 hydrocarbons and relatively large changes in dissolved CO2 and H2 concentrations, consistent with formation of additional hydrocarbons beyond C3. The existence of relatively high dissolved alkanes in the experiment involving non-pretreated magnetite, suggests a complex catalytic process, likely involving reinforcing effects of mineral-derived carbon with newly synthesized hydrocarbons at the magnetite surface. The "isotopic reversal" trend was not observed for 13C values of dissolved alkanes with increasing carbon number. This may relate to the specific mechanism of carbon reduction under hydrothermal conditions at elevated temperatures and pressures.; The role of iron-nickel sulfide on hydrothermal carbon reduction reactions was evaluated in a hydrothermal experiment using a 13C-labeled carbon source at conditions (T, P, and fluid chemistry) close to Rainbow/Logatchev vent systems. Pentlandite ((Fe2Ni7)S8), a common alteration mineral in subseafloor hydrothermal systems, was used. Dissolved 13C-bearing alkanes species were produced with relatively low H 2(aq)/CO2(aq) ratio and the existence of dissolved H 2S. Hydroxymethylene is the key intermediate in the hypothesized reaction mechanism, which was consistent with XPS and ToF-SIMS analysis results on the mineral product.; Perthite dissolution was studied in a batch reactor to examine dissolution and precipitation processes in an initially acidic fluid at 200°C and 300 bars. Non-stoichiometric K-feldspar dissolution suggests formation of an unidentified aluminous mineral coexisting with muscovite. Its metastable existence is permitted by the slow rates of K-feldspar dissolution, especially under near equilibrium conditions, and accounts for the observed discrepancy between experimental and theoretical data for the time to achieve full equilibrium. Analysis of mineral reactants (SEM, TEM and XPS) following the experiment confirmed the existence of abundant secondary mineralization associated with K-feldspar surfaces.