The chemical and isotopic composition of fumarolic gases emitted from
Nisyros Volcano, Greece, and of a single gas sample from Vesuvio, Italy,
was investigated in order to determine the origin of methane (CH,)
within two subduction-related magmatic-hydrothermal environments.
Apparent temperatures derived from carbon isotope partitioning between
CH4 and CO2 of around 340degreesC for Nisyros and 470degreesC for
Vesuvio correlate well with aquifer temperatures as measured directly
and/or inferred from compositional data using the H2O-H-2-CO2-CO-CH4
geothermometer. Thermodynamic modeling reveals chemical equilibrium
between CH4, CO2 and H2O implying that carbon isotope partitioning
between CO2 and CH, in both systems is controlled by aquifer
temperature.
N-2/(3) He and CH4/(3) He ratios of Nisyros fumarolic gases are
unusually low for subduction zone gases and correspond to those of
midoceanic ridge environments. Accordingly, CH4 may have been primarily
generated through the reduction of CO, by H, in the absence of any
organic matter following a Fischer-Tropsch-type reaction. However,
primary occurrence of minor amounts of thermogenic CH4 and subsequent
re-equilibration with co-existing CO2 cannot be ruled out entirely-
CO2/He-3 ratios and delta(13)C(CO2) values imply that the evolved CO2
either derives from a metasomatized mantle or is a mixture between two
components, one outgassing from an unaltered mantle and the other
released by thermal breakdown of marine carbonates. The latter may
contain traces of organic matter possibly decomposing to CH4 during
thermometamorphism. Copyright (C) 2004 Elsevier Ltd.