Spatially resolved CO₂ carbon stable isotope analyses at the microscale using Raman spectroscopy

Measuring the carbon stable isotope ratio (¹³C/¹²C, expressed as δ¹³C_CO2) in geogenic CO₂ fluids is a crucial geochemical tool for studying Earth's degassing. Carbon stable isotope analysis is traditionally performed by bulk mass spectrometry. Although Raman spectroscopy distinguishes ¹²CO₂ and ¹³CO₂ isotopologue bands in spectra, using this technique to determine CO₂ isotopic signature has been challenging. Here, we report on in-situ non-destructive analyses of the C stable isotopic composition of CO₂, applying a novel high-resolution Raman configuration on 42 high-density CO₂ fluid inclusions in mantle rocks from the Lake Tana region (Ethiopia) and El Hierro (Canary Islands). We collected two sets of three spectra with different acquisition times at high spectral resolution in each fluid inclusion. Among the 84 sets of spectra, 58 were characterised by integrated ¹³CO₂/¹²CO₂ band area ratios with reproducibility better than 4‰. Our results demonstrate the determination of δ¹³C_CO2 by Raman spectroscopy in individual fluid inclusions with an error better than 2.5 ‰, which satisfactorily matches bulk mass spectrometry analyses in the same rock samples, supporting the accuracy of the measurements. We thus show that Raman Spectroscopy can provide a fundamental methodology for non-destructive, site-specific, and spatially resolved carbon isotope labelling at the microscale.