Year: 2026
Type: article
Abstract: Raman spectroscopy is widely used to quantify CO2 density in geological fluids, yet acquisition parameters, spectral processing methodologies and densimeter calibrations vary between laboratories. The precision of Raman CO2 measurements, and the broader applicability of densimeters generated for a specific instrument, are still debated. This study describes a series of experiments to investigate the effect of different instrument and acquisition parameters on CO2 densities and their errors that can be performed on different Raman instruments, allowing users to determine analytical best practices. First, we discuss the effect of spectral non-linearity, demonstrating that shifts in the selected acquisition window and the choice of Ne lines used for drift correction can generate a diversity of densimeters on a single instrument. However, our repeat measurements of natural fluid inclusion (FI) standards over ~3 years yield a 1σ variation of <0.01 g cm−3, indicating that when consistent methodologies are applied to calibration data and unknowns, Raman analyses are highly reproducible over many years. We then investigate the effect of parameters such as acquisition time, laser power, and sample positioning on peak fitting errors, rebutting recent suggestions that Raman analyses are associated with large uncertainties in Fermi-diad splitting. We show that although high laser powers greatly improve signal:noise ratios (and thus reduce peak fitting errors), natural fluid inclusions hosted in olivine show changes in calculated density outside analytical error with increasing powers (density changing by ~0 to 0.09 g cm−3 per 10 mW of increased power). The amount of heating of the trapped fluid, and thus the change in density, varies drastically on a sample-by-sample basis, with larger amounts of heating in materials hosting the fluid with high absorption coefficients (melt inclusions, and low Fo olivines). We recommend that users assess the degree of laser heating experienced by their samples and select a laser power that limits density changes to a level that will not compromise the objectives of their study. We also offer practical strategies to optimize signal without raising laser power, noting that small changes in focus and x-y position can greatly increase signal intensity.
Source: Volcanica
Language: English
Subfield: Geochemistry and Petrology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: diamond
OpenAlex ID: W7223415702