Analysis of individual fluid inclusions by Fourier transform infrared and Raman microspectroscopy

Abstract

This paper addresses several aspects of the analysis of water and volatile species in individual fluid inclusions by infrared (IR) and Raman microspectroscopy. We discuss differences in physical principles, instrumental designs, requirements for sample preparation, spectral information obtained, and spatial resolution achieved for the two vibrational microspectroscopic techniques. Topics addressed for each technique include minimum fluid inclusion size, detection limits, quantification of spectra, and the detection of water. The major limitations of the Raman microspectroscopic technique are 1. 1) the possibilities of laser-induced heating and fluorescence of the sample in the focused laser beam, both of which commonly preclude the Raman analysis of hydrocarbon-bearing fluid inclusions; and 2. 2) the inability to quantify bulk compositions in multiphase fluid inclusions. The potential advantages of the IR technique are to solve both of these problems. However, due to the sampling geometry and diffraction constraints, the IR microspectroscopic technique has very stringent requirements for the minimum fluid inclusion size (at least ∼200 μm 2 in cross-section for detection), the kind of host mineral, and the sample preparation. In addition, an inclusion may be so large and/or dense as to preclude its analysis, due to total IR absorption by the fluid. In general, we conclude that IR analysis is not a panacea for the problems experienced in Raman analysis of inclusions. With the Raman technique, one can 1. 1) identify and quantify the contents of much smaller fluid inclusions (≤3 μm), even aqueous fluid inclusions, than one can with the IR microspectroscopic technique; 2. 2) detect homonuclear species like H 2, O 2, and N 2, which are not IR-active; and 3. 3) readily identify mineral inclusions.