Laser heating effect on Raman analysis of CO2 co-existing as liquid and vapor in olivine-hosted melt inclusion bubbles

Abstract

Raman spectroscopy has become the tool of choice for analyzing fluid inclusions and melt inclusion (MI) vapor bubbles as it allows the density of CO₂-rich fluids to be quantified. Measurements are often made at ambient temperature (T_amb ~18-25 °C), resulting in reported bulk densities between 0.2 and 0.7 g/mL despite that single-phase CO₂ under these conditions is thermodynamically unstable and instead consists of a liquid (~0.7 g/mL), and a vapor phase (~0.2 g/mL). Here, we present results from experiments conducted at T_amb and 37 °C (above the CO₂ critical temperature) on 14 natural CO₂-rich MI bubbles from Mount Morning, Antarctica. Here, we show that at T_amb, laser power strongly affects the CO₂ Raman spectrum of MI bubbles with bulk densities within the miscibility gap. High-power laser heating and low spectral resolution explain why published measurements have reported such bulk densities at T_amb even when using an instrument-specific calibration.