Carbon and oxygen isotopic analyses of calcite in calcite–dolomite mixtures: Optimization of selective acid extraction

Abstract

Rationale Carbon and oxygen isotopic compositions of individual minerals in calcite–dolomite mixtures could be measured using a selective acid extraction technique based on the different reaction rates of calcite and dolomite with phosphoric acid. However, poor accuracies of calcite are usually obtained when mixtures of low calcite content are analyzed, which may result in incorrect conclusions. To overcome this shortcoming, improvements are needed in accuracy and precision when using the technique with mixtures of low calcite content. Methods Calcite and dolomite were analyzed under different reaction conditions using a GV Isoprime™ II isotope ratio mass spectrometer equipped with a dual inlet (DI) and an automatic sampler, to study the relationships between isotopic compositions and CO2 yield, and reaction time and temperature. Artificial mixtures with varied calcite:dolomite ratios were prepared, and their calcite isotopic compositions were determined with the aim of optimizing analytical conditions. Results The CO2 yields and isotopic compositions of calcite and dolomite were obtained on‐line under different experimental conditions using DI‐isotope ratio mass spectrometry (DI‐IRMS). Based on the analysis results of artificial mixtures, the reaction conditions for analyzing isotopic composition of calcite in mixtures were optimized as follows. Samples should have grain sizes of 75–80 μm; and samples with calcite contents of >50%, 30–50% and < 30% should be reacted with acid for 10–15 min at 50°C, 6 min at 50°C and 45 min at 25°C, respectively. Conclusions Optimized analytical conditions are suggested for the determination of the isotopic compositions of calcite in mixtures with improved accuracy by an automatic selective acid extraction technique.