LIGHT-HYDROCARBONS IN GAS-EMISSIONS FROM VOLCANIC AREAS AND GEOTHERMAL FIELDS

Geochemical investigation of gas emissions at low temperatures in areas of Quaternary volcanism shows a systematic enrichment of aromatic hydrocarbons and alkenes in fumaroles from active volcanic areas (Vulcano and Phlegrean Fields, Italy: S. Miguel, Terceira and Graciosa, Azores; Chichon, Mexico; Uzon and Geyser Valley, Kamchatka) and geothermal systems (Larderello area, Italy) with respect to those pertaining to areas of recent but exhausted volcanism (Vulsini and Sabatini volcanic districts, Torre Alfina volcano, Italy). The general behaviour of hydrocarbons in the investigated gas-emissions, together with thermodynamic and experimental data (taken from the industrial treatment of petroleum) suggest possible abiogenic processes acting on degradation of alkane hydrocarbons when source temperatures exceed 150-degrees and acid species together with high active-surface area clay minerals are available. Benzene shows very similar enrichments in emissions from Vulcano beach and Phlegrean Fields, Kamchatka and Azores, pointing out the possible occurrence of catalytic reactions at the source. Alkenes display significant relative increases in crater fumaroles from Chichon volcano. Moreover, lower carbon-number alkenes (propene and ethene) progressively appear with increasing alkenes/alkanes ratio, suggesting an increasing importance of thermal-cracking reactions. The general similarities in distribution patterns from different active volcanic environments allow us to reappraise the influence of the nature of sources and to emphasize the effects of abiogenic reactions. With reference to areas of active volcanism, the distribution of hydrocarbons can represent a useful parameter to evaluate the prevailing physico-chemical conditions and their time-space variabilities. Accordingly they can give a significant contribution to the geochemical monitoring of active volcanic areas.