Geotermical

Geochemical Journal Vol. 27, pp. 7 to 17, 1993


Light hydrocarbons in gas-emissions from volcanic areas and
geothermal fields





Bruno Capaccioni1, Marino Martini2, Filippo Mangani3, Luciano Giannini2, Giovanni Nappi1 and Franco Prati2

Institute of Mineralogy and Petrography, University of Urbino, via Oddi 14, Urbino1, Department of Earth Sciences, Universityof Florence, via La Pira 4, Florence2, and Institute of Chemical Sciences, University of Urbino, P. Rinascimento 6, Urbino3, Italy

(Received June 29, 1992; Accepted December 10, 1992}


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; Chichdn, 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° 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 Chichdn 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 indistribution 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. Accordinglythey can give a significant contribution to the geochemical monitoring of active volcanic areas.


Introduction

Natural gaseous emissions have been currently investigated for geothermal purposes or geochemical surveillance of active volcanoes, and significant information is available on the chemical composition of fumarolic gases in a wide range of temperature (Martini et al., 1980; 1984;1991; Carapezza et al, 1981; Le Guern et al, 1982; Gerlach, 1982; Giggenbach et al., 1986; Matsuo, 1988). With reference to light hydrocarbons, however, only few data have been provided. Ellis (1957) and Matsuo (1961), who firstly discussed the chemical composition of fumarolic gases in terms of equilibrium thermodynamics, suggested that methane detected at Showanshinzan (Japan) and Wairakei (NewZealand) might have an extra-magmatic origin. Giggenbach et al. (1986) also agreed with this conclusion and hypothesized the occurrence of two main end members mixing into a cooler two-phase hydrothermal zone surrounding the magmatic system: a magmatic component characterized by the presence of H2S and S02, low contents of methane and an hydrothermal component low in S02 but high in methane. Thekinetics of reactions involving methane are very slow, and equilibrium is approached only within the slow moving two-phase hydrothermal zone, where redox conditions are controlled by rock

composition. Equilibrium is on the contrary quenched within a magmatic gas phase rapidly ascending towards the earth's surface. Tedesco and Sabroux (1987) suggested a re-examination of the influence of...