Non-methane Volatile Organic Compounds (VOCs) at El Chichón volcano (Chiapas, Mexico): Geochemical features, origin and behavior

Geochemical features of the organic gas fraction in fumarolic fluids at El Chichón volcano (Chiapas State, Mexico) are studied to investigate the origin of non-methane volatile organic compounds (VOCs) and the processes governing their behavior. VOCs from fumaroles and bubbling pools mainly consist of C2-C5 alkanes, with total abundance one order of magnitude less than that of methane, and relatively high concentrations of aromatics (benzene and toluene) and C2-C3 alkenes (mainly propene and iso-butene). Significant concentrations of S-bearing compounds, mostly of the heterocyclic group, are also detected. This composition is similar to that of gas emissions of geothermal systems. The production of organic gases at El Chichón may be due to thermogenic processes. Total VOC abundance is relatively low when compared with typical hydrothermal gas discharges, and may be attributed to magmatic-related fluids in the main reservoir feeding the thermal manifestations of the crater. As absence of O-bearing compounds seems to indicate that the hydrothermal fluids are not significantly affected by organic species produced at shallow depth by decomposing vegetation, such as that buried by the volcanic products of the 1982 eruption.