Evidence for metastable equilibrium between hydrocarbons in volcanic gases

摘要

Despite a wide discussion about the relative importance of kinetics and thermodynamics in governing the chemical and isotopic composition of natural hydrocarbons, there was no direct evidence that thermodynamic equilibrium can be attained between hydrocarbons in any natural process. Here we report our data on the hydrocarbon composition of volcanic gases (C_1-C_4 and benzene) from White Island, New Zealand. We found that ratios of ethane/ethylene, propane/propylene and ethylene/benzene are close to their equilibrium values at temperatures of a fumarolic vent and at concentrations of hydrogen controlled by the H_2S-SO_2 "gas" buffer. This result suggests that the redox processes C_nH_(2n)+H_2 reversibleC_nH_(2n+2) and 3C_2H_4reversible 3H_2 + C_6H_6 at high temperature in volcanic vapors are reversible and fast in both directions, similar to reactions of thermal decomposition of water and the reduction-oxidation CO_2 reversible CO.