Determination of Mineral-Specific Clumped Isotope Acid Digestion Fractionation Factors Using Heating Experiments and Mass Spectrometry

Abstract:

Clumped isotope thermometry involves the determination of formation temperatures of carbonates from the fraction of isotopologues containing multiple rare isotopes (oxygen-18 and carbon-13). At high temperatures, the abundance of these isotopologues should be stochastic. At lower temperatures, there is a tendency for heavy isotopes to form bonds with each other.

However, spectroscopic determination of isotope ratios with high precision is difficult in solids, and so ¹³C-¹⁸O bond abundance is not measured in the solid phase. Instead, analysis of carbonates is performed using gas source mass spectrometry, by reacting the carbonate samples with phosphoric acid and measuring the evolved CO₂ gas. As an oxygen atom is lost during the conversion of CO₃^2- groups to CO₂, this reaction is hypothesized to result in mineral and acid digestion temperature-dependent fractionation. In order to quantify this fractionation between CO₃^2- and CO₂, this experiment seeks to determine acid fractionation factors for carbonate samples of varying composition by randomizing samples through intense heating and comparing analyte CO₂ measured composition to the expected composition for a stochastically distributed sample. From this analysis, future carbonate measurements can be calibrated to account for acid digestion fractionation.