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.