A REVISED METHODOLOGY TO ACQUIRE STABLE AND CLUMPED ISOTOPIC COMPOSITIONS FROM CALCITE-DOLOMITE MIXTURES: CASE STUDY INVESTIGATING EOCENE DIAGENESIS IN THE BOHAI BAY BASIN, CHINA

Clumped isotope paleo-thermometry is widely used in carbonate-diagenesis study recently because it allows an independent temperature measurement. However, calcite and dolomite with different growth histories often coexist in limestones and are difficult to physically separate, making the determination of clumped isotope values of individual minerals difficult. This difficulty prevents straightforward study of the diagenetic history of the Bohai Bay Basin, China, which hosts important oil and gas reservoirs where dolomite and calcite layers are often inter-fingered. Phosphoric acid extraction at different temperatures has been used to separate CO₂ evolved from calcite and dolomite in the same sample. However, previous methods require either a large amount of sample or labwork. We present a new and efficient way to separately collect CO₂ generated from mixtures of calcite and dolomite, and apply the method to study carbonates from the Bohai Bay Basin. Calcite-dolomite mixtures are reacted at 50°C for 10 minutes to digest calcite while the liberated CO₂ gas is collected continuously in a liquid nitrogen trap (reaction step I). The acid bath is then isolated from the rest of the vacuum line and the acid temperature is increased to 90°C to digest the dolomite (reaction step II). The liberated CO₂ gas from reaction step II is collected continuously in a liquid nitrogen trap for 70 minutes, after the CO₂ from reaction step I is purified and collected for isotope analysis, and the purification system is baked out. The CO₂ from reaction step II is then purified and collected for dolomite isotopic analysis. We tested the method with pure carbonate standards and calcite-dolomite mixtures. After calibration for isotopic mixing and acid fractionation, the results show that the pure calcite and dolomite isotopic values are recovered from mixed-sample reaction step I and II CO₂ isotopic values, respectively. The estimated contamination from one phase to another is less than <8 %. Using this method, we systematically studied the carbonate-diagenetic history of Eocene Es1 formation of Bozhong sag, Bohai Bay Basin, China, which is an important sedimentary stage when carbonates deposit in this basin. Results show distinct isotopic values and diagenetic temperatures for different stages of calcite and dolomite growth in samples.