A reassessment of the potential for reduction of dissolved CO2 to hydrocarbons during serpentinization of olivine

The concept that aqueous CO2 can be reduced to hydrocarbons abiotically dur ing serpentinization of olivine has become widespread in the earth and plan etary sciences. This process has been invoked to explain the occurrence of hydrocarbons in crystalline igneous rocks and proposed as a source of prebi otic organic compounds for the origin of life. We reevaluate this scenario through an experimental study of the reaction of dissolved CO2 in the prese nce of olivine, under hydrothermal conditions (300 degreesC, 350 bar). Redu ction of CO2 to formate (HCOO-) was found to proceed rapidly, with H-2 gene rated from hydrothermal alteration of olivine serving as the reductant. The reverse reaction, decomposition of formic acid to CO2 and H-2, was also fo und to proceed rapidly. Although dissolved hydrocarbon concentrations incre ased throughout the experiments, isotopic labeling of dissolved CO2 with C- 13 indicated that these compounds were primarily generated from reduced car bon compounds already present in olivine at the beginning of the experiment rather than by reduction of CO2 The only hydrocarbon product from reductio n 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 betw een dissolved CO2 and formate rapidly achieved metastable equilibrium at th e 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 catal ysts or vapor phase reactions may be required to explain many occurrences o f abiotic hydrocarbons in serpentinites and igneous rocks. Copyright (C) 20 01 Elsevier Science Ltd.