Abstract:
The concept that aqueous CO2 can be reduced to hydrocarbons abiotically during serpentinization of olivine has become widespread in the earth and planetary sciences. This process has been invoked to explain the occurrence of hydrocarbons in crystalline igneous rocks and proposed as a source of prebiotic organic compounds for the origin of life. We reevaluate this scenario through an experimental study of the reaction of dissolved CO2 in the presence of olivine under hydrothermal conditions (300°C, 350 bar). Reduction of CO2 to formate (HCOO-) was found to proceed rapidly, with H2 generated from hydrothermal alteration of olivine serving as the reductant. The reverse reaction, decomposition of formic acid to CO2 and H2, was also found to proceed rapidly. Although dissolved hydrocarbon concentrations increased throughout the experiments, isotopic labeling of dissolved CO2 with 13C indicated that these compounds were primarily generated from reduced carbon compounds already present in olivine at the beginning of the experiment rather than by reduction of CO2. The only hydrocarbon product from reduction of CO2 observed in the experiments was a small amount of methane (<0.04% conversion of dissolved CO2 in more than 2500 h of heating). Comparison of the reaction products with thermodynamic data indicates that reactions between dissolved CO2 and formate rapidly achieved metastable equilibrium at the experimental conditions, suggesting that similar reactions could control the concentration of formate in geologic fluids. The results indicate that the potential for abiotic formation of hydrocarbons during serpentinization may be much more limited than previously believed, and other mineral catalysts or vapor phase reactions may be required to explain many occurrences of abiotic hydrocarbons in serpentinites and igneous rocks.