Carbon and Oxygen Isotope Analysis of Calcite and Dolomite Mixtures Using Selective Acid Extraction by Gasbench-IRMS

Carbonate cements formed at different stages in sandstone have different mineral compositions, structural characteristics and carbon and oxygen isotope compositions. Carbon stable isotope compositions in cements can be used to indicate the source of carbon in diagenetic fluids, while oxygen stable isotope can be used to estimate the temperature and the source of fluid at the time of crystallization. In order to obtain the carbon and oxygen isotope compositions of different kinds of carbonates in sandstone cements, off-line selective acid extraction method was usually used to extract and test the carbon and oxygen isotope compositions of each carbonate. In order to overcome the shortcomings of the off-line method such as cumbersome process and low efficiency, the temperature and time experiments of the reaction of the mixture sample with 100% phosphoric acid were carried out for the first time by using Gasbench-IRMS with different ratios of quartz, calcite and dolomite mixture samples in this study. When the particle size of the sample is mainly about 63-75 μm, the reaction temperature is 25 ℃ and the reaction time is 2 h, the generated CO₂ represents the carbon and oxygen isotopic compositions of calcite. When the reaction temperature is 25 ℃, there is almost no CO₂ generated in reaction of dolomite with 100% phosphoric acid for 1 h, and the yield of CO₂ is only about 3% for 2 h. The mixture experiment showed that this method is only suitable for the mixture samples with more calcite than dolomite. When the content of calcite is higher than that of dolomite, Gasbench-IRMS can be used to separate calcite and dolomite mixture samples online, and test C and O isotope compositions. The carbon dioxide produced at 25 ℃ for 2 h represents the C and O isotope compositions of calcite. The CO₂ gas is removed after another 2 h of reaction, then the carbon dioxide produced at 50 ℃ for 18 h represents δ¹³C and δ¹⁸O of dolomite. The precision of δ¹³C and δ¹⁸O is better than 0.15‰ (1σ) and 0.20‰ (1σ), respectively.