Tm. Mccollom et Js. Seewald, A reassessment of the potential for reduction of dissolved CO2 to hydrocarbons during serpentinization of olivine, GEOCH COS A, 65(21), 2001, pp. 3769-3778
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.