High-precision analysis of carbon isotopic composition for individual CO2 inclusions via Raman spectroscopy: Addressing issues arising from the laser-heating effects

Abstract

The carbon isotopic composition of carbon dioxide fluid inclusions trapped within minerals reflects the origin and evolution of the CO2-bearing fluids and melts. Recently, we developed a highly accurate method using Raman spectroscopy to determine the carbon isotopic composition of CO2 fluid inclusions, by simultaneously collecting 13CO2 and hot bands at a controlled temperature, and using the peak height ratio of 13CO2 and one of hot bands near 1410 cm−1 (HR) to determine δ13C value. This microanalytical technique enables precise measurements of the δ13C values of different CO2 fluid inclusions within zoned mineral crystals to reveal the multiple-stages evolution of CO2-bearing fluids and melts. However, when studying fluid inclusions hosted within high laser heating coefficient minerals such as olivine and pyroxene in mantle xenoliths, local temperature near the inclusions will be higher than the temperature controlled by heating-cooling stage. To address this issue, we examined the effects of laser heating on CO2 isotopic standard samples sealed in the silica capillary, and developed a novel procedure to determine the δ13C values of natural fluid inclusions by using two CO2 standard samples with known carbon isotopic compositions. The proposed workflow successfully determines the δ13C values of natural CO2 inclusions, with an average deviation of 0.58 ‰ between the measurements and the model.