Refined protocol for δ¹³C analysis of calcite and dolomite in carbonate mixture samples

Abstract

RationaleThe 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.MethodsThe conditions for pretreating calcite–dolomite mixtures to determine each of their δ13C 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.ResultsThe optimized pretreatment conditions for δ13C analysis of calcite in the mixture involved purging the sample bottle using helium and then reacting it with 99% H3PO4 for 100 min in a cold water bath at 2°C. The optimized pretreatment conditions for δ13C analysis of dolomite in the mixture involved purging the sample bottle using helium gas after δ13Ccal measurement and reacting it with H3PO4 on a heating plate for 10 h at 45°C.ConclusionsThis study introduced a workflow with ultimate precision for the selective δ13C measurements of calcite–dolomite mixtures combining a GasBench II with a continuous flow isotope ratio mass spectrometer.

Determination of Vanadium Isotope Compositions in Carbonates Using an Fe Coprecipitation Method and MC-ICP-MS

“…GBW07217a and GBW07214a are dolostone and limestone, respectively, purchased from the National Research Centre of Geoanalysis (NRCG), the former of which has been analyzed for C and Mg isotopes as a reference. 39,40 COQ-1 is the Cretaceous igneous carbonatite collected from the Oka carbonatite complex (Canada), produced by the United States Geological Survey (USGS). The contents of major matrix elements (Ca, Mg, Al, and Mn), isobaric interfering elements (Cr and Ti), and V of these reference materials are given in Table S3.…”

“…JDo-1 and JLs-1 are the Permian gray dolostone collected at the Kuzuu area (Tochigi Prefecture) and Upper Triassic dark gray limestone collected at the quarry of Hekireji Deposit (Kamiisocho, Hokkaido), respectively, commercially available from the Geological Survey of Japan (GSJ). GBW07217a and GBW07214a are dolostone and limestone, respectively, purchased from the National Research Centre of Geoanalysis (NRCG), the former of which has been analyzed for C and Mg isotopes as a reference. , COQ-1 is the Cretaceous igneous carbonatite collected from the Oka carbonatite complex (Canada), produced by the United States Geological Survey (USGS). The contents of major matrix elements (Ca, Mg, Al, and Mn), isobaric interfering elements (Cr and Ti), and V of these reference materials are given in Table S3.…”