J. C. Seitz | Geology Reports

USGS1996

Raman spectroscopic characterization of gas mixtures. II. Quantitative composition and pressure determination of the CO2-CH4 system

Raman spectral parameters were determined for the v 1 band of CH4 and the v 1 and 2 v 2 bands (Fermi diad) of CO 2 in pure CO 2 and CO 2 -CH 4 mixtures at pressures up to 700 bars and room temperature. Peak position, area, height, and width were investigated as functions of pressure and composition. The peak positions of the CH 4 and CO 2 bands shift to lower relative wavenumbers as fluid pressure is increased. The peak position of the lower-wavenumber member of the Fermi diad for CO 2 is sensitive to fluid composition, whereas the peak positions of the CH 4 band and the upper Fermi diad member for CO 2 are relatively insensitive in the CO 2 -CH 4 system. The magnitude of the shifts in each of the three peak positions (as a function of pressure) is sufficient to be useful as a monitor of fluid pressure. The relative molar proportions in a CO 2 -CH 4 mixture may be determined from the peak areas: the ratio of the peak areas of the CH 4 band and the CO 2 upper Fermi diad member is very sensitive to composition, whereas above about 100 bars, it is insensitive to pressure. Likewise, the peak height ratio is very sensitive to composition but also to fluid pressure. The individual peak widths of CO 2 and CH 4 , as well as the ratios of the widths of the CH 4 peak to the CO 2 peaks are a sensitive function of pressure and, to a lesser extent, composition. Thus, upon determination of fluid composition, the peak width ratios may be used as a monitor of fluid pressure. The application of these spectral parameters to a suite of natural CO 2 -CH 4 inclusions has yielded internally-consistent, quantitative determinations of the fluid composition and density.

American Journal of Science↗

USGS1990

High-density volatiles in the system C-O-H-N for the calibration of a laser Raman microprobe

Three methods have been used to produce high-density volatiles in the system C-O-H-N for the calibration of a laser Raman microprobe (LRM): synthetic fluid-inclusion, sealed fused-quartz-tube, and high-pressure-cell methods. Because quantitative interpretation of a Raman spectrum of mixed-volatile fluid inclusions requires accurate knowledge of pressure- and composition-sensitive Raman scattering efficiencies or quantification factors for each species, calibrations of these parameters for mixtures of volatiles of known composition and pressure are necessary. Two advantages of the synthetic fluid-inclusion method are that the inclusions can be used readily in complementary microthermometry (MT) studies and that they have sizes and optical properties like those in natural samples. Some disadvantages are that producing H 2 O-free volatile mixtures is difficult, the composition may vary from one inclusion to another, the exact composition and density of the inclusions are difficult to obtain, and the experimental procedures are complicated. The primary advantage of the method using sealed fused-quartz tubes is its simplicity. Some disadvantages are that exact compositions for complex volatile mixtures are difficult to predict, densities can be approximated only, and complementary MT studies on the tubes are difficult to conduct. The advantages of the high-pressure-cell method are that specific, known compositions of volatile mixtures can be produced and that their pressures can be varied easily and are monitored during calibration. Some disadvantages are that complementary MT analysis is impossible, and the setup is bulky. Among the three methods for the calibration of an LRM, the high-pressure-cell method is the most reliable and convenient for control of composition and total pressure. We have used the high-pressure cell to obtain preliminary data on 1. (1) the ratio of the Raman quantification factors for CH 4 and N 2 in an equimolar CH 4  N 2 mixture and 2. (2) the spectral peak position of ν 1 of CH 4 in that mixture, as well as in pure CH 4 , at pressures up to 690 bars. These data were successfully applied to natural inclusions from the Duluth Complex in order to derive their compositions and total pressures.

Geochimica et Cosmochimica Acta↗