ANALYSIS OF FLUID INCLUSION GASES FROM GEOTHERMAL SYSTEMS, USING A RAPID-SCANNING QUADRUPOLE MASS-SPECTROMETER

Fluid inclusions in samples from geothermal systems were analyzed by a quadrupole mass spectrometer (QMS) and a capacitance manometer. Individual inclusions were analyzed semi-quantitatively by the rapid scanning mode of the QMS. The results of individual inclusion analysis show that the inclusion fluids comprise mainly H2O and a small amount of variable CO2; decrepitation temperatures range mostly between 250 and 450-degrees-C. No significant decrepitation of vapor-rich inclusions occurred at temperatures up to 500-degrees-C. The H2O, CO2, N2, CH4 and Ar contents were determined quantitatively by bulk analysis. The CO2 contents determined by the QMS are consistent with estimation from microthermometry and crushing, while they are generally higher than the CO2 contents of the fluids discharged in each area. This suggests that the inclusions could have trapped hydrothermal fluids at early stages before considerable degassing or dilution. Differences in the CO2/CH4, CO2/N2 and N2/Ar ratios between the inclusion fluids and present fluids in some areas could be explained by boiling or dilution. The gas compositions of fluid inclusions also indicate that the partial pressure of N2 in geothermal fluids is of the same order as CO2, thus also affecting boiling characteristics.