Résumé des résultats (Abstract)
(Anglais)
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Nisyros (Greece) represents an island arc volcano with ongoing magmatic-hydrothermal activity. As part of the European project GEOWARN we continuously analyzed fumarolic gas discharges, between 1997 and 2002, both for their chemical and isotopic composition. The major goal of this approach was to identify chemical and isotopic parameters reliably describing the processes occurring at the magmatic-hydrothermal interface. Identification of such parameters is the prerequisite for an evaluation of ongoing volcanic risks.
Within the last year geochemical precursors of volcanic-hydrothermal activity have been identified. The hydrogen- and oxygen isotopic composition of fumarolic water and the sulfur isotopic composition o fH2S are prone to secondary processes such as condensation and variations in oxygen fugacity. Primary processes occurring at depth are best reflected by the chemical and isotopic composition of carbon bearing species within the fumaroles.
Carbon isotope partitioning between CO2 and CH4 has been used in order to derive temperatures of gas formation using recently determined fractionation factors by Horita (2001). Calculated temperatures are independent of the location of fumarolic outlet, at around 350°C. This mean temperature is consistent with temperatures measured directly for the deep aquifer (330-340°C ; Chiodini et al., 1993). It is, therefore, concluded that carbon isotope partitioning between CO2 and CH4 reflects the temperature of the deep aquifer and that all hydrothermal vents are directly fed by the deep aquifer. If apparent isotopic temperatures are compared to apparent chemical temperatures as inferred from the CO-CO2-CH4-H2-H20 gas geothermometer (data from Chiodini), it becomes obvious that -except for Polybotes Mikros- lower temperatures are computed by the CO-CO2-CH4-H2-H20 gas geothermometer. This is due to the fact that CO is reacting in minor amounts to CO2 during the ascent of the gases, finally affecting molar ratios of CO but not the isotopic composition of CO2 (Brombach et al., 2002). Hence, the carbon isotope geothermometer is less prone to re-equilibration effects than the CO-CO2-CH4-H2-H20 gas geothermometer. This makes the C-isotope geothermometer to the most important tool we have in monitoring heat flux variations occurring between magma chamber and deep aquifer. From our data of June 2001 until February 2002 no significant variations in temperature have been detected implying that hydrothermal activity was constant within that period, too. However, data from June 2002 imply that hydrothermal activity increased during spring 2002. For all location higher temperatures are obtained. The highest increase in temperature of up to 20°C is observed for Polybotes Gigantes and Polybotes Mikros.
Carbon isotopic composition of CO2 and CH4 do not vary with relative amounts of CO2 and CH4 , implying an origin of both gases from a single source. Delta 13C of CH4 is very close to the range expected for abiogenic methane (delta 13CCH4>-25%o, Welhan, 1988), and thermodynamic modeling confirms a pure inorganic origin for the evolved CH4.
The results of our work are summarized in the manuscript ' Origin of methane within a subduction-related volcanic-hydrothermal system and its role in risk evaluation ', that has recently been submitted to Geochimica et Cosrnochimica Acta. The whole data set will be presented at the EUG Meeting 2003 in Nice.
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