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 CO. At ambient temperature, in fluid inclusions which trap these solutions, the typical phase assemblage consists of a CO-rich vapor (where P CO P internaI ) and an aqueous phase containing dissolved salts and CO. In this study, the CO minimum detection limits (MDLs) using microthermometry and laser Raman spectroscopy are established in terms of P CO using synthetic HO-CO inclusions. The purpose of the microthermometric experiments was to examine the diagnostic CO phase changes and determine the quantity of CO necessary to result in observable solid CO melting. The results of these experiments show that an observable solid CO melting event requires P CO 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 CO MDLs were determined by collecting signal-to-noise ratios for both the upper and lower v-2v bands as a function of CO pressure (5-60 bars) and over a range of integration times and incident laser power to predict the optimal instrument settings. The resulting CO MDLs are on the order of 1 bar for our instrument. The band splitting of the v-2v diad as a function of CO pressure was measured up to 500 bar at ambient temperature. The CO pressures were converted to CO 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 CO densities in fluid inclusions containing a homogenous, free CO phase to a precision of approximately ± 0.02 g/cm³. The temperature dependence of the intensity ratio of the hot bands to the v-2v 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 ...