Refined protocol for δ¹³ C analysis of calcite and dolomite in carbonate mixture samples

The carbon isotope composition of carbonates is widely used in petrology and paleoenvironments as a proxy for carbon sources and environmental parameters during mineral formation or late diagenesis. However, most carbonate minerals in nature are complicated mixtures (i.e., coexisting calcite and dolomite). Precise separation of individual signals in mixed carbonate minerals is essential to obtain information regarding these indexes because of the diverse isotopic behavior of different minerals.

The conditions for pretreating calcite-dolomite mixtures to determine each of their δ¹³ C values were optimized with a series of experiments varying the purging and reaction conditions using an isotope ratio mass spectrometer equipped with an online GasBench II interface. Sequential phosphoric acid reactions of pure calcite and dolomite were performed to measure the characteristics of single minerals under different temperatures and reaction times. Artificial mixtures with varied proportions of calcite and dolomite were used to verify the precision of the proposed method.

The optimized pretreatment conditions for δ¹³ C analysis of calcite in the mixture involved purging the sample bottle using helium and then reacting it with 99% H₃ PO₄ for 100 min in a cold water bath at 2°C. The optimized pretreatment conditions for δ¹³ C analysis of dolomite in the mixture involved purging the sample bottle using helium gas after δ¹³ C_cal measurement and reacting it with H₃ PO₄ on a heating plate for 10 h at 45°C.

This study introduced a workflow with ultimate precision for the selective δ¹³ C measurements of calcite-dolomite mixtures combining a GasBench II with a continuous flow isotope ratio mass spectrometer.