PACROFI VI - Electronic Program

Quantitative Analysis of H2O-CO2 Fluid Inclusions by Raman Spectroscopy

Robert J. Bodnar*, Cs. Szabó*, S.N. Shilobreeva** and S. Newman***

* Fluids Research Laboratory, Department of Geological Sciences, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061

** Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia

*** Division of Geology and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125

Laser Raman spectroscopy (LRS) has been used very successfully in recent years to obtain quantitative analyses of the non-aqueous volatile components in fluid inclusions. LRS has also been used with less success to identify and quantify dissolved species in aqueous inclusions, and to identify daughter minerals in fluid inclusions. However, quantitative determinations of the H2O/(non-aqueous volatiles) ratios have not been reported because (1) H2O is a very poor Raman scatterer and, (2) at ambient temperatures the inclusion contains multiple phases, with H2O and the other volatiles of interest occurring in different phases.

Using synthetic H2O-CO2 inclusions as standards, a preliminary empirical calibration curve for determining the H2O/CO2 ratio of individual fluid inclusions has been developed. During analysis, the inclusions were heated above their homogenization temperatures using a Chaixmeca heating stage, and the intensities of the H2O (3219 cm-1) and CO2 (1388 cm-1) peaks were measured using a Dilor X-Y system equipped with a CCD detector. For compositions ranging from 0 to about 25 mole percent CO2, the relative intensity of the CO2 peak to the water peak [ICO2/(ICO2+IH2O)] increases linearly with increasing CO2 content according to the relation:

	mole fraction CO2   =   0.4  x  R	(1)

where R is equal to the ratio [ICO2/(ICO2+IH2O)], and is valid for R values <0.66 (which corresponds to a CO2 content of approximately 26 mole percent). For higher CO2 contents, the composition is given by the relation:

	mole fraction CO2   =   0.87  -  2.96 (R)  +  3.09 (R2)	(2)

For CO2 contents less than 26 mole percent, equation (1) fits the analytical data within a few mole percent CO2. For higher CO2 contents, equation (2) fits the data within about 5 mole percent.

As a test of the calibration results, a synthetic rhyolite glass containing H2O-CO2 bubbles of known composition was analyzed. The resulting Raman spectra gave a composition of 87 mole percent CO2, which is identical to the value obtained from manometric analysis of the vapor phase trapped in the capsule with the glass (melt) sample. The vapor was released by piercing the capsule in vacuum. The effect of salts or other volatile components on the intensity ratios used to develop the calibration curve is unknown, but is currently under investigation.