Source conditions and degradation processes of light hydrocarbons in volcanic gases: an example from El Chicho´n volcano (Chiapas State, Mexico)

Since the 1982 plinian eruption, El Chicho´n volcano (Chiapas State, Mexico) has exhibited intense hydrothermal activity. Fumaroles, bubbling gases, hot springs, and acid water pools discharge at both the crater floor and the volcano flanks. Sixteen different hydrocarbon species were identified in fumaroles and bubbling gases, including alkanes, alkenes, and aromatic hydrocarbons. Among the possible reactions, dehydrogenation involving light alkenes and alkanes pairs (C2 up to linear C4) approaches metastable equilibria when redox state is controlled by the rock mineral assemblage. Reequilibration conditions of the C3 and n-C4 alkanes with their unsaturated counterparts appear to be established at temperature and redox conditions, similar to those of CH4/CO2 pair. Conversely, C2 and branched C4 alkene/alkane ratios, although controlled by a similar rock redox buffer system, appear to reflect equilibrium at different temperature and/or redox conditions, probably due to the slow kinetics of the reactions involved. Therefore, a full chemical equilibrium simultaneously involving CH4 and heavier hydrocarbons is not attained. Thermodynamic evaluation indicates that cracking reactions involving the CUC bond breakage do not achieve a thermodynamic equilibrium. However, their relative distribution appears to be consistent with a thermal—rather than a biological—origin. Catalytic processes are also invoked in the production of benzene, whose distribution appears to be related to the measured H2 concentration, other than to possible nonoxidative and oxidative conversion of methane.