APPLICATIONS OF RAMAN MICROSPECTROSCOPY TO FLUID INCLUSION PHASE IDENTIFICATION

Raman spectroscopy is a powerful tool that has been applied widely to characterize a variety of fluid and solid materials. Whereas microspot Raman analyses have been used with great success to identify and characterize materials, it has previously been less useful for characterizing minor components in samples and for characterizing sample heterogeneities. Minerals often contain fluids or gases in bubbles (fluid inclusions, FI) that have been trapped during crystal formation and thus can provide information about the conditions existing during mineralization - composition, temperature, and pressure. The size of FI ranges from less than a micrometer to several hundred micrometers in diameter. Many fluid and gas phases that are common in FI, including H₂O, CO₂, CH₄, H₂S etc., are easily recognized by Raman spectroscopy and in some cases quantified using their Raman band intensities and/or positions. Recently, there has been much interest in searching for small amounts of H₂O in FI that previously had been thought to contain only CO₂. Confirming the presence of even small amounts of water in FI is important in understanding some upper mantle processes in which, as a result of exposure to high temperatures and pressures, hydrated minerals lose water that may then occupy FI. At room temperature, the liquid H₂O phase occurs as a thin, sub-microscopic rim on the walls of the inclusion, it has therefore been diffficult, if not impossible, to locate the H₂O film using spot analysis. Raman mapping, however, facilitates the search for H₂O in these inclusions and provides a rapid method to survey large number of FI to see which, if any contain H₂O. Multivariate analysis methods are used to construct the Raman maps and indicate the presence, in this case, of water and other substances.

Several examples of FI have beed studied and spectra from the gas phase a mixture of CO₂ (1285 cm^-1 and 1388 cm^-1), CH₄ (2916 cm^-1), N₂ (2330 cm^-1), solid carbon phase (amorphous carbon) (1585 cm^-1 ) and liquid H₂O phase (3200-3700 cm^-1) will be shown. This technique was applied to CH₄ and CH₄ + H₂O fluid inclusions in quartz from the Box Vein of Lyonsdale, Port Leyden, NY, USA.