Environmental Engineering Reference
In-Depth Information
In conclusion, the crushing experimental method is available for a convenient and semi-
quantitative gas analysis of liquid-rich inclusion with CO
2
content lower than about 2.0 mol%
in geothermal fields.
3.2. Quadrupole Mass Spectrometry
Gas chemistry of fluid inclusions in minerals provides us useful information of the
evolution of ore-forming fluid and geothermal fluid, and furthermore the exploration for
mineral deposit and geothermal field. For these purposes, various gas analytical methods such
as gas chromatography, mass spectrometry, Raman spectrometry, FT-IR have been
researched by several investigators (Shepherd et al., 1985). Although it is desirable to obtain
the gas composition of individual inclusion, the volume of gases released from individual
inclusion is generally small to obtain the quantitative value. But, the QMS can be obtained
gas composition semi-quantitatively and quantitatively. The analytical system and method
using ANELVA AGA-360 were reported in detail by Sasada et al. (1992). The gas chemistry
of fluid inclusion performed by the equipment at the Institute of the Geological Survey of
Japan (AIST) has been reported for several Japanese geothermal fields by Sasada et al.(1992),
Sawaki et al. (1997, 1999), Sawaki and Muraoka (2002), Muramatsu et al. (1997, 2000,
2006).
3.2.1. Analytical Method
Quartz and anhydrite samples were treated with concentrated HCl, organic solvent and
H
2
O
2
to eliminate contamination by organic matter and other minerals such as calcite which
produce excess gases by their decompositions. The samples were also checked
microscopically to verify that they did not include other minerals. As calcite and wairakite
decompose during heating for gas analysis and produce excess gases and water, the fluid
inclusions have been previously studied only in quartz and anhydrite. Equipment for fluid
inclusion gas analysis is shown in figure 7 (Sasada et al., 1992). The spectrometer was
connected by stainless steel tubing to a capacitance manometer (MKS315), a cold trap, an
infrared furnace and a vacuum system. Individual and bulk analyses of the inclusions were
performed following the methods.
Individual Gas Analytical Method
Because a mineral has commonly trapped the various fluids progressed from its
precipitation stage to the present in a inclusion, the gas analysis of individual inclusion is
ultimate purpose to clarify fluid evolution in detail in a geothermal reservoir. Individual gas
analysis determines semi-quantitatively the major gaseous species in the mass range from 2 to
70 for individual inclusion. About 10 mg of each sample was placed in a quartz glass tube and
heated to 500 ºC at a slow rate of 10 ºC/min. Water vapor and other gases, released when an
individual inclusion decrepitated during the heating run, were analyzed by the QMS run in a
rapid-scanning mode. The analyzed fluids were mainly derived usually from the liquid-rich
inclusions because the vapor-rich inclusions did not decrepitate at <500 °C.
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