In Situ Quantification of Carbonate Species Concentrations, pH, and pCO₂ in Calcite Fluid Inclusions Using Confocal Raman Spectroscopy

Carbonate minerals are globally distributed on the modern and ancient Earth and are abundant in terrestrial and marine depositional environments. Fluid inclusions hosted by calcite retain primary signatures of the source fluid geochemistry at the time of mineral formation (i.e., pCO₂) and can be used to reconstruct paleoenvironments. Confocal laser Raman spectroscopy provides a quick, nondestructive approach to measuring the constituents of fluid inclusions in carbonates and is a reliable method for qualitatively determining composition in both the aqueous and gas phases. Here, we demonstrate a method for accurately quantifying bicarbonate and carbonate ion concentrations (down to 20 mM) and pH (7-11) from calcite fluid inclusions using confocal Raman spectroscopy. Instrument calibrations for carbonate (CO₃^2-) and bicarbonate (HCO₃^-) concentrations and pH were performed using stock solutions. We show that the calcite host mineral does not affect the accurate quantification of carbonate solution concentrations and that these parameters can be used to estimate the pH and pCO₂ of a solution entrapped within a fluid inclusion. We apply the technique to Icelandic spar calcite and find a [CO₃^2-] = 0.11, [HCO₃^-] = 0.17, pH = 10.1, and CO₂ parts per million = 2217. The presence of gaseous Raman bands for CO₂, CH₄, and H₂S suggests that the mineral precipitated in a reducing environment.