LIGHT-HYDROCARBONS IN HYDROTHERMAL AND MAGMATIC FUMAROLES - HINTS OF CATALYTIC AND THERMAL-REACTIONS

Volcanic gaseous mixtures emitted from active volcanoes frequently sho w variable amounts of saturated (alkanes), unsaturated (alkenes) and a romatic volatile hydrocarbons. Three major patterns of distributions c an be recognized, apparently related to the chemical-physical environm ent of formation of the gas exhalations: alkane-rich, low-temperature gas emissions from recently active volcanic areas; aromatic-rich hy dr othermal manifestations; and alkene-rich, 'magmatic' fumaroles on acti ve volcanoes. Thermodynamic data, together with theoretical and practi cal findings from the petroleum industry, point to two main types of r eactions occurring in these volcanic environments: cracking and reform ing. Cracking processes, mainly caused by thermal effects, occur when hydrocarbon-bearing hydrothermal fluids enter and mix with a hot and d ry, rapidly rising magmatic gas phase. The most probable products are light alkenes with carbon numbers decreasing with increasing reaction temperatures. The presence of aromatic species in hydrothermal fluids can be linked to reforming processes, catalysed by several possible ag ents, such as smectites and zeolites, generally present in the hydroth ermally altered Volcanic terranes, and facilitated by long residence t imes in a hydrothermal envelope.