Acid Fractionation Factor of Oxygen Isotopes in Mixed Calcite-Dolomite Samples: Implications for Data Correction

The conventional method for measuring carbon and oxygen isotopes in carbonates involves the reaction of carbonate minerals with phosphoric acid (PPA) to generate CO₂, followed by purification and isotopic analysis. During this reaction, a temperature-dependent oxygen isotope acid fractionation factor (AFF) is introduced, as not all oxygen is released in CO₂. Although the temperature dependence of AFF has been extensively studied in pure carbonate minerals, little research has been conducted on AFF variations in mixed carbonate minerals with diverse chemical and mineralogical compositions. This study aims to investigate the range of AFF variations in carbonates containing multiple mineral phases.

Oxygen isotope compositions of CO₂ produced from CaCO₃-MgCO₃ mixed minerals reacting with PPA at 25°C, 50°C, 70°C, and 90°C were analyzed using a GasBench II system coupled with a Thermo Finnigan Delta V Plus isotope ratio mass spectrometer.

The AFF values for CaCO₃-MgCO₃ mixed minerals fall between those of pure calcite and dolomite, generally aligning with theoretical predictions. Additionally, the fractionation gradient (dδ¹⁸O/dT^-1) increases with rising mole Mg and dolomite content, indicating a systematic trend in fractionation behavior.

These findings provide a framework for AFF correction based on mole Mg or dolomite content, enhancing the reliability and precision of oxygen isotope measurements in natural impure carbonates.