Detection limits of CO₂in fluid inclusions using microthermometry and Raman spectroscopy and the spectroscopic characterization of CO₂

In many geologic environments, dominantly aqueous solutions contain low concentrations of CO₂. At ambient temperature, in fluid inclusions which trap these solutions, the typical phase assemblage consists of a CO₂-rich vapor (where P_CO₂ ≈ P_internaI) and an aqueous phase containing dissolved salts and CO₂. In this study, the CO₂ minimum detection limits (MDLs) using microthermometry and laser Raman spectroscopy are established in terms of P_CO₂ using synthetic H₂O-CO₂ inclusions. The purpose of the microthermometric experiments was to examine the diagnostic CO₂ phase changes and determine the quantity of CO₂ necessary to result in observable solid CO₂ melting. The results of these experiments show that an observable solid CO₂ melting event requires P_CO₂ ≥ 45 bar at 25°C.

The Raman spectroscopic detection limits were investigated using a multichannel Raman spectrometer. Because the Raman spectroscopic MDLs are a function of counts, the CO₂ MDLs were determined by collecting signal-to-noise ratios for both the upper and lower v₁-2v₂ bands as a function of CO₂ pressure (5-60 bars) and over a range of integration times and incident laser power to predict the optimal instrument settings. The resulting CO₂ MDLs are on the order of 1 bar for our instrument.

The band splitting of the v₁-2v₂ diad as a function of CO₂ pressure was measured up to 500 bar at ambient temperature. The CO₂ pressures were converted to ρ_CO₂ and the results are given in terms of the frequency separation between the upper and lower bands. These results are compared to those of previous studies. An analysis of the estimated errors indicates that the technique can be used to determine CO₂ densities in fluid inclusions containing a homogenous, free CO₂ phase to a precision of approximately ± 0.02 g/cm³.

The temperature dependence of the intensity ratio of the hot bands to the v₁-2v₂ diad was measured from 270 to 315 K. The close agreement between the calculated and observed results indicate that laser induced sample heating is not significant. The intensity ratio can be used to estimate the CO₂ temperature and, combined with the Raman density determination, allows calculation of the CO₂ pressure.