Oxidation of methane at the CH4/H2O-(CO2) transition zone in the external part of the Central Alps, Switzerland: Evidence from stable isotope investigations

Détails

ID Serval
serval:BIB_ED3A7FB4B922
Type
Article: article d'un périodique ou d'un magazine.
Collection
Publications
Titre
Oxidation of methane at the CH4/H2O-(CO2) transition zone in the external part of the Central Alps, Switzerland: Evidence from stable isotope investigations
Périodique
Chemical Geology
Auteur(s)
Tarantola A., Mullis J., Vennemann T.W., Dubessy J., De Capitani C.
ISSN-L
0009-2541
Statut éditorial
Publié
Date de publication
2007
Peer-reviewed
Oui
Volume
237
Pages
329-357
Langue
anglais
Résumé
With the aim of understanding the mechanisms that control the
metamorphic transition from the CH4- to the H2O-(CO2)-dominated fluid
zone in the Helvetic domain of the Central Alps of Switzerland, fluid
inclusions in quartz, illite ``crystallinity'' index, vitrinite
reflectance, and the stable isotope compositions of vein and whole rock
minerals and fluids trapped in quartz were investigated along four
cross-sections. Increasing temperature during prograde metamorphism led
to the formation of dry gas by hydrocarbon cracking in the CH4-zone.
Fluid immiscibility in the H2O-CH4-(CO2)-NaCl system resulted in
cogenetic, CH4- and H2O-dominated fluid inclusions. In the CH4-zone,
fluids were trapped at temperatures <= 270 +/- 5 degrees C. The end of
the CH4-zone is markedby a sudden increase of CO2 content in the gas
phase of fluid inclusions. At temperatures > 270 +/- 5 degrees C, in the
H2O-zone, the total amount of volatiles within the fluid decreased below
1 mol% with no immiscibility. This resulted m total homogenization
temperatures of H2O-(CO2-CH4)-NaCl inclusions below 180 degrees C.
Hydrogen isotope compositions of methane in fluid inclusion have delta D
values of less than -100 parts per thousand in the CH4-zone, typical for
an origin through cracking of higher hydrocarbons, but where the methane
has not equilibrated with the pore water. delta D values of fluid
inclusion water are around -40 parts per thousand., in isotopic
equilibrium with phyllosilicates of the whole rocks. Within the CH4 to
H2O(CO2) transition zone, delta D(H2O) values in fluid inclusions
decrease to -130 parts per thousand interpreted to reflect the
contribution of deuterium depleted water from methane oxidation. In the
H2O-zone, delta D(H2O) values increase again towards an average of -30
parts per thousand which is again consistent with isotopic equilibrium
with host-rock phyllosilicates. delta C-13 values of methane in fluid
inclusions from the CH4-zone are around -27 parts per thousand in
isotopic equilibrium with calcite in veins and whole rocks. The delta
C-13(CH4) values decrease to less than -35 parts per thousand at the
transition to the H2O-zone and are no longer in equilibrium with the
carbonates in the whole rocks. delta C-13 values of CO, are variable but
too low to be in equilibrium with the wall rock fluids, compatible with
a contribution of CO2 from closed system oxidation of methane.
Differences in isotopic composition between host-rock and Alpine fissure
carbonate are generally small, suggesting that the amount of CO2
produced by oxidation of methane was small compared to the C-budget in
the rocks and local pore fluids were buffered by the wall rocks during
precipitation of calcite within the fissures. (c) 2006 Elsevier B.V. All
rights reserved.
Création de la notice
29/09/2012 16:23
Dernière modification de la notice
20/08/2019 16:15
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