Publication Date:
2017-04-04
Description:
Methane (CH4) emanating from a continental volcanichydrothermal
system in Nisyros, Greece, is processed through
the abiogenic reduction of mantle- and marine limestonederived
CO2 [1]. Evidence for the occurrence of abiogenic
hydrothermal reduction of CO2 is from the chemical and
carbon isotopic equilibrium patterns. We have further
characterized this abiogenic methane (C1) source for the
concentrations of ethane (C2) and propane (C3), as well as for
the hydrogen isotopic composition of CH4, H2O, H2 and H2S.
C1/C2+ ratios are significantly higher than those typically
observed for purely thermogenic sources. Hydrocarbon
distribution ratios for other continental-hydrothermal sources
rich in CO2 are comparable to those of the Nisyros fumaroles
implying that abiogenic methane might be significantly more
widespread than previously assumed [2]. Relative
concentrations of hydrocarbons in continental-hydrothermal
discharges are even indistinguishable from those measured in
ultramafic hydrothermal emissions. The fact that redox
conditions do not seem to exert any control on the relative
concentrations of hydrocarbons in hydrothermal emissions in
general, implies that the same two sources account for
hydrocarbon production in continental and ultramafic
environments. One source generates methane exclusively
through the selective abiogenic reduction of CO2 (Sabatierreaction).
The other source produces minor amounts of
methane, ethane and propane by a random process and
represents either the thermal cracking of organic matter or the
polymerization starting from methane.
Hydrogen isotope partitioning between H2O, H2S, H2 and
CH4 in Nisyros fumaroles reveals that isotopic exchange rates
are highest for H2O-H2S followed by H2O-H2. In contrast to
H2 and H2S, the hydrogen isotopic composition of methane
exhibits almost no local variations. This is in agreement with
its predominantly abiogenic hydrothermal origin and with the
low temperature sensitivity of the hydrogen isotope
fractionation factor between water vapor and methane.
Description:
Published
Description:
Davos, Switzerland
Description:
1.2. TTC - Sorveglianza geochimica delle aree vulcaniche attive
Description:
open
Keywords:
hydrothermal gases
;
methane
;
ethane
;
propane
;
04. Solid Earth::04.08. Volcanology::04.08.01. Gases
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
Oral presentation
