A high-precision CO₂ densimeter for Raman spectroscopy using a Fluid Density Calibration Apparatus

Fluid and melt inclusions rich in CO2 are common in many geological environments and are a powerful tool to provide constraints on P-T-X conditions of fluids resulting in ore deposits as well as pressures of inclusions in volcanic systems. Raman spectroscopy is an in situ, non-destructive method capable of determining the CO2 densities of inclusions for most sample sizes (> 1 mu m) and densities. However, significant discrepancies exist among published CO2 densimeters for Raman spectroscopic measurements, mainly due to inconsistent calibration procedures, hardware differences, and sparse measurements for fluid densities between 0.2 and 0.7 g/mL. In this study, we re-designed a Fluid Density Calibration Apparatus (FDCA) based on one originally described by Lin et al. (2007) with important structural changes such as high accuracy temperature measurements made directly inside the FDCA that significantly reduce the error for critical region measurements. We provide five highly precise new calibration equations for different temperatures and CO2 density ranges. Application of these equations to a set of melt inclusions from a Pico do Fogo eruption from Cabo Verde indicates that the total percent uncertainty in calculated CO2 contents of bubbles derived from our densimeter are always below 5% except for inclusions with densities in the most sensitive part of the critical region (similar to 7.3% for a 0.425 g/mL bubble), while relative percent uncertainty for literature densimeters are always above 10% and up to 28% in the worst-case scenario. In this study, we include all of the new designs, diagrams, and operational procedures with the goal of providing the community a new high-precision and high-accuracy FDCA for Raman spectroscopy.