Carbon and Hydrogen Isotope Compositions of Geothermal Gases

Abstract

Carbon and hydrogen isotopic compositions are compared for the gases methane, hydrogen and carbon dioxide from Ngawha, Wairakei ,Ohaaki-Broadlands and Tikitere Springs). The different areas have different isotopic compositions with some general relationships to reservoir temperature and inferred origins. The isotopic exchange of hydrogen with water was found to indicate reservoir temperatures from most spring samples but often, higher than measured temperatures in well samples. Indicated temperatures assuming equilibria between and are higher than measured maxima. This difference may be due to ei ther partial isotopic equilibration or to source conditions of methane. Possible sources for carbon dioxide and methane are shallow or deep crustal (inorganic or organic) material with the 1it y of some juveni1e contribution.