加熱デクレピテーション-四重極質量分析法による流体包有物のガス分析
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概要
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A new computerized mass spectrometric technique for analyzing gases in fluid inclusions has been developed. A quartz sample is subjected to heating under vacuum. The gases liberated by thermal decrepitation of fluid inclusions are instantaneously introduced into a quadrupole mass spectrometer (QMS) and ion currents of gas species are successively measured in the temperature range from 130 to 650°C. After the measurement, the average of ion currents for gas species over the whole temperature range is calculated. Using the average ion currents and the sensitivity of the QMS for the gases, the bulk composition of inclusion fluids is obtained.The pegmatitic quartz sampled from three pegmatite fields in Japan were analyzed by this method. Inclusion fluids in quartz from the Naegi pegmatite have a molar concentration of H2O higher than 98%. Inclusion fluids from the Ishikawa and Sakihama pegmatite fields are rich in CO2. The former has a molar concentration of CO2 ranging from 2 to 24%, and the latter from 8 to 11%. The analytical results are in good agreement with the observations of fluid inclusions under the microscope.Gas release profiles are different between the samples from the Naegi, Ishikawa and Sakihama pegmatite fields. The Naegi pegmatitic quartz has a single peak of release profiles both in H2O and CO2. The gas release profiles of the Ishikawa quartz are comlex, and those of Sakihama are simple. The diversity of gas release profiles observed between the pegmatitic quartz may be ascribed to the number of formation stages of fluid inclusions.A significant extent of fluid release upon heating was observed at the temperature around 573°C. This is due to fluid inclusion rapture caused by the alpha-beta phase transition of quartz. The presence of H2S as a minor component of inclusion fluids was confirmed from the mass decrepitation above.
- 2004-11-30