Carbon-bearing gas geothermometers for volcanic-hydrothermal systems

- We studied the equilibration behavior of C-bearing gases in hydrothermal systems.
- The CO2-CH4 isotope geothermometer reflects deep hydrothermal temperatures.
- The C3H8/C3H6 redox pair exhibits reactivities most similar to CH4/CO2.

The genetic relationship between carbon-bearing species (CO, CO2, CH4, C2H6, C3H8, C2H4 and C3H6) was investigated in volcanic-hydrothermal gases emitted from Nisyros (Greece), Vesuvio, La Solfatara (Campi Flegrei) and Pantelleria (all Italy).Apparent carbon isotopic temperatures of the CH4-CO2 system are ~ 360 °C at Nisyros, 420-460 °C at Vesuvio, ~ 450 °C at La Solfatara and ~ 540 °C at Pantelleria. These temperatures are confirmed by measured propene/propane and H2/H2O concentration ratios. CH4 and CO2 equilibrate in the single liquid phase prior to the onset of boiling, whereas propene and propane attain equilibrium in the saturated water vapor phase. Boiling in these high-enthalpy hydrothermal systems might occur isothermally. Once vapor has been extracted from the parental liquid, CO/CO2 responds most sensitively to the temperature gradient encountered by the ascending gases.Our results imply that the CH4-CO2 isotopic geothermometer can provide reliable information about temperatures of deep hydrothermal liquids associated with volcanism. Propene/propane and H2/H2O concentration ratios should be measured along with the carbon isotopic composition of CO2 and CH4 to provide independent constraints on the geological significance of the apparent carbon isotopic temperatures.